anatomy & physiology series · the special senses introduction in this instalment of your...

ANATOMY & PHYSIOLOGY SERIES

LECTURE 6 – A&P

THE NERVOUS SYSTEM / SPECIAL SENSES

Lecture 6 – The Nervous System and Special Senses

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CONTENTS

THE SPECIAL SENSES

INTRODUCTION

GENERAL SENSE ORGANS

OVERVIEW Sense of Smell, Sense of Tast e, Sense of Hearing,

Sense of Balance, Sense of Vision

CONCLUSION

INSIGHT Optical Illusions

PATHOPHYSIOLOGY

THE NERVOUS SYSTEM

INTRODUCTION

CELLS OF THE NERVOUS SYSTEM Glial, Neurons

NERVE IMPULSES

NEURONAL FUNCTIONAL ZONES

MAIN GROUPS OF TRANSMITTER SUBSTANCES Acet ylcholine, Amines, Amino Acids, Neuroact ive Pept ides,

Lipids, Nucleosides, Soluble Gases Inactivation of Neurotransmitters

DIVISION OF THE NERVOUS SYSTEM

The Central Nervous System – The brain

FUNCTIONS OF THE BRAIN Cerebrum, Diencephalon, Brain Stem, Cerebellum,

COVERINGS OF THE BRAIN AND SPINAL CORD

SUMMARY

CONCLUSION

PATHOPHYSIOLOGY


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THE SPECIAL SENSES

INTRODUCTION

In this instalment of your A&P lectures we are going to be viewing a fascinating area

of study: the special senses. These comprise the sense of smell, taste, hearing,

balance and vision. We will take a close look at each of these special senses, where

‘special’ denotes the fact that they have receptors grouped closely together or within specialized organs. Having considered these different sense organs, we will be

in a good position for the next lecture that explores the nervous system as a whole.

All you really need to know for now about the nervous system in general is that

sensory nerve fibres (neurones) take information gathered by the sense organs into

the central nervous system (the spinal cord and brain) for processing. This information

is transmitted along the nerve fibres (axons) in the form of nerve impulses (also called

action potentials – ‘they have the potential to bring about action’) and the job of the special sense organs is to translate (the technical term is ‘transduce’) sensory stimulation into nerve impulses that the central nervous system can interpret in a

useful way. So, for example, the eyes convert visual information from light focused on

the back of the eye into potentially meaningful nerve impulses that the brain can use

to construct a version of the visual world around us. An analogy is found with cable

TV- the camera converts light it receives into digital impulses that are ultimately sent

down the wires as simple impulses that the TV set uses to reconstruct a version of

what the camera received.

As a matter of fact, our body has millions of sense organs, subdivided into two main

groups: general sense organs and special sense organs. Although in this session we

shall concentrate on the second category, I would like to mention the first one to put

you in the picture, so to speak. Before doing so, it is important to realise that these

div isions largely arise from the anatomical study of the body – eyes are different to

ears and different nerves and brain areas deal with these different senses. But our

inner experience of the senses is integrated – senses complement each other but are

not isolated from each other – seeing the food on a plate will help us smell that food.

This ‘sensorium’ of all the combined senses is only beginning to be explored by

mainstream science and some counter-intuitive findings are emerging.

For example, it seems that how information allows us to act is the most important

thing, not what was the source of that information. There is a device (the BrainPort

developed by a researcher called Bach-y-Rita) that stimulates the tongue with tiny

(not painful) electric shocks using electrodes arranged in a grid pattern. Information

from a v ideo camera strapped to the forehead can be fed into the device so that a bright spot causes more shocks than a dark area. Within a very short time, people

have reported a sense of actively ‘looking out into the world’ and not noticing the tongue sensation at all – it is as if the areas of the brain that deal with vision can use

any information sources that can be interpreted meaningfully in a visual way and will

use this to construct a sense of a visual outer world. If the same device is coupled to

sensors that indicate how the head is positioned, someone who has lost their sense of

balance can find they just ‘know’ where their head is and perform movements they have been unable to do so for years.

This illustrates how we need to be careful when studying these sense organs not to

view them in a simplistic way like biological cameras or microphones or other such

devices – the sense organs are just a part of an incredibly elaborate sensory web

that generates a sense of the world around us and that enables us to interact in that


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world in a way that promotes our survival – and surv ival doesn’t care about the information being ‘true’ – it just wants what is useful. This voyage we will go on

starting with the special senses and finishing (if it ever does) with the brain in the next

lecture will challenge many beliefs we have about ourselves and the world – you

certainly will not be able to say that ‘seeing is believing’ with any confidence!

READ CHAPTER 15, THIBODEAU PATTON.

GENERAL SENSE ORGANS

There are millions of general sense organs, or receptors, widely distributed throughout

our body, which are constantly providing v ital information concerning both our

external and internal environment. The ultimate goal of our sense organs is to keep

our homeostatic mechanism informed of changes in both our internal and external

environment, to alert our body of any factor that may prejudice our survival. Our

sense organs trigger general, or somatic (‘soma’ meaning ‘body’), senses such as touch, pain and temperature, which, in turn, will initiate several reflex responses –

such as withdrawing automatically from a painful stimulus.

Sensory receptors respond to external and internal stimuli by converting them to

nerve impulses. Most sensory receptors are specialized, in as much as they will

respond to one type of stimulus only, to avoid confusion in the ensuing reaction. The

receptor’s response to a stimulus relies on the sensory stimulation that sense organ is

‘tuned’ to, (comma here) generating changes in the balance of electrolytes (ions such as sodium and potassium) across the membranes that enclose the receptor

cells. The greater the stimulation the greater the change in the electrolytes. If the

stimulation is above a certain threshold level, the disturbance in the electrolytes can

trigger a set of special channels in the sensory nerve cell membrane to open up –

and this sets a wave of channel opening sweeping up the nerve fibre (or axon) as a

nerve impulse. When this reaches the central nervous system it is processed and

forms the basis of our perception of the world at the simplest reflex level to the

highest cognitive level, and may trigger responses based on this processing.

Thousands of such nerve impulses are produced when just one sensory organ at the end of a single sensory neurone is stimulated briefly. The generation of nerve impulses

will be explored further in the next lecture – but for now, be aware that the concept

of these electrolyte changes being able to produce a nerve impulse (or action

potential) is referred to as the ‘receptor potential’ – as the sensory organ is

stimulated the receptor potential rises. If the stimulation is strong enough it will trigger

an action potential in the nerve fibre (axon) that the sensory organ is, in effect,

plugged in to.

However, if constantly stimulated, the magnitude of the receptor potential will

eventually decrease as a result of what is called adaptation. This will result in a

decrease in the intensity of the associated sensation. Adaptation is as important to

our survival because without it we would constantly be overwhelmed by sensorial

information – the ability to ignore what is not important is perhaps more vital than to

be aware of what is important! By ignoring those sensory impulses that have been

processed and classified as harmless, our body can then concentrate on being alert

for the real potential dangers.


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PHYSI-FUN-OLOGY

Place a light object on your palm and make sure neither you nor the object will

move, close your eyes and notice how long you can maintain any awareness of the

object.

A fun version of this is to have fun with someone snoozing near by, if there is a group,

take it in turns to place objects on the sleeping person’s body. The one who disturbs

them looses. If they get completely covered the game can then change to taking

turns to retrieve the objects. If they object tell them that this was SCIENCE

demonstrating adaptation and not human Buckaroo!

As opposed to the special senses, whose receptors are clustered in specific areas

and organs, the general sense receptors can be found virtually anywhere in the

body: in the skin, mucosa, tissues, muscles, tendons, joints and viscera. Their

distribution is varied, according to functionality.

Receptors can be classified according to:

1. their location

2. the stimulus detected

3. their structure

Of the three, the classification by stimulus detected is more relevant to us within the

context of this lecture – the main types of sensory receptor are:

mechanoreceptors: activated by mechanical stimulation e.g. pressure.

chemoreceptors: activated by the change in concentration of certain

chemicals.

thermoreceptors: activated by changes in temperature

nocireceptors: activated by intense stimuli of any receptor resulting in

tissue damage.

photoreceptors: activated by light. Only found in the eye.

For a summary of the different types of somatic sensory receptors, their locations and

functions, there is a table in your A&P text book. Needless to say, you do not have to

memorize this table: I draw your attention to it for purely informative purposes!


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THE SPECIAL SENSES - OVERVIEW

These have long been seen as the royalty of the senses – and they do produce a

very rich sensory input. But the brain weaves these into our sensorium without

prejudice – touch can be just as powerful as v ision in influencing our actions. The feel

of food can completely alter the associations generated by taste, smell or vision – if

you have ever drooled over a luscious fruit and then been surprised to find it was

plastic when you touched it, you will be aware of the sudden change in how you

relate to the fruit.

THE SENSE OF SMELL As it is quite normal in every day life, the ability to smell is generally taken for

granted. And unless your profession relies upon your olfactorial abilities, then

only on few occasions may you become aware of this wonderful capability.

You may, for example, pass a bakery during the first hours of the morning

and find yourselves being seduced by the aroma of freshly baked bread. This

is more than just a pleasant smell! I t invades and corrupts your thinking,

causing an inner battle over your commitment to a wheat free diet and

‘how well you are feeling’ since you gave it up. Sometimes, the wheat wins.

But, a mental note is always made to drink some extra water on top of the

camel quantities that you are currently consuming

The point I am, rather circuitously, trying to make here is that the sense of

smell is quite obviously linked to our brain and the information it sends is more

often than not involved in triggering physiological, mental and even

emotional responses. In times gone by we would have heavily relied upon

our sense of smell for our surv ival, to sense an approaching predator,

whether a food was poisonous or not and whether a mate was ovulating –

some argue that it was our early domestication of dogs that allowed us to

cease to rely on our own sense of smell –and instead rely on a dog’s much more developed sense of smell to warn of predators etc. Nowadays, thanks

to the wonders of modern science, all these ‘tedious’ subtleties are no longer required: after-shaves and perfumes now cover our natural smells, to create

a new, sophisticated ‘attraction factor’. Equally, the alluring and seductive scents of modern day food are there to remind us that eating is more often

than not a pleasure rather than a necessity. As far as predators go, we tend

to rely on our other senses, as well as on our instinct, of course. Needless to

say, all along goods manufacturers have taken the point of smell very

seriously, as they know that it has far reaching effects upon a person.

Naturally, we still have the ability to smell our food and at the same time

distinguish what is good and what is bad for us. However, sadly, most of us

have been literally fed a mass of contradictory information. In fact, one of

the fascinating and pleasantly rewarding gifts that the application of

Nutritional Healing brings to your life is the return of a keener sense of taste and smell. one could say that rather than re-educating your special senses

one is remembering what sensory receptors were initially designed for.

Anyhow, let’s have a close look at the nose, and see what happens when we inhale. I f you look inside the nasal cavity (preferably in a book!), you will

see the nasal cavity has several borders, one being created by the hard

palate, which is considered the ‘floor’ of the cavity. Incidentally, it is this structure that is absent in a ‘cleft palate’, causing obvious complications with

eating, breathing, and smelling.


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THE SENSE OF SMELL Continued……

The lining of the nasal cavity has the ability to produce large amounts of

mucous to help capture and transport pathogens out of the cavity and

protect the lungs. Also you will find many tiny hairs, called cilia, which not

only help to maintain the direction of flow of mucous away from the lungs

but also help to mix and distribute the chemicals that create smells around

the olfactory receptor neurons.

If you look more closely at the nasal cavity you will see that the fibres of the

olfactory nerve at the top of the picture, and its bulbous end, are in fact

located in a small recess called the olfactory recess. This would at first seem

perhaps illogical as most of the smells would simply pass by this cove-like

recess unperceived. However, as the primary role of the nose is to inhale air

for respiration purposes, it is not surprising that we do not perceive every

inhaled smell. In fact we need to sniff to detect more subtle smells, a

voluntary and specific action for the purpose of information. The design of

the nasal cavity ensures that a sudden influx of air would spiral around the

cavity and cause a great deal of the scented chemicals to pass through

the olfactory recess and linger whilst being classified.

Because the scents that we perceive are in fact chemicals, the olfactory

receptors are said to be chemoreceptors. These receptors respond

according to the previously mentioned action potential, given that the

stimuli received by the mucous surrounding the olfactory cilia reach a

threshold level. The action potential will be passed to the olfactory nerves in

the olfactory bulb to then travel along the olfactory pathway to be finally

interpreted, classified and stored in the brain. There is great debate as to

exactly how smell molecules are perceived – we have genes for hundreds

of different smell receptors and these vary enormously between people –

perhaps part of the reason why one person’s perfume is another person’s stink. Whatever the mechanism, it is subtle – the molecule that gives us the

scent of caraway has a mirror image molecule (an optical isomer that has

exactly the same arrangement of atoms in a mirror image form – like a right and left hand) that smells of spearmint. And yet two different molecules

can produce the same scent – many molecules smell of rotten eggs.

Optical Isomers

The olfactory nerves feed directly into the limbic system (or emotional brain) giving

the strong emotional reactions to smell – and as the limbic system is closely

associated with memory functions, perhaps this is why smell is so evocative of past

experiences. Smells exert a powerful physiological effect and can set up strong

conditioned reflexes – some hypnotherapists have used particular scents to help

people with epilepsy postpone fits by associating a particular scent with post-

hypnotic suggestions – the oil is sniffed when an attack is imminent.


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THE SENSE OF TASTE If you recall we have already mentioned the tongue in a previous lecture so I

will take a cursory look through this subject. I t has been taught for many years

that we perceive different tastes according to different areas of the tongue:

sweet on the tip, salty and sour on the flanks, and bitter on the base of the

tongue. While true to some extent, more recent experiments seem to

indicate that most tastes are perceived by most areas of the tongue – there

is just a predominance of one type in certain areas.

The well known taste buds are located along the sides of raised projections

that cover the tongue called papillae. There is a type of pit formed where

the saliva can settle with its dissolved contents. Tiny cilia hairs project from the

surrounding gustatory cells that form the taste bud which draw the saliva and

it’s dissolved contents into the gustatory cell membranes. Here, special

receptor sites are triggered and initiate the now familiar action potential that

is sent via three cranial nerves into the medulla oblongata. From here they

are sent to the thalamus and interpreted in the gustatory cortex of the brain.

The five primary tastes are sweet, sour, salty, bitter and umami. We are more

responsive to bitter tastes, probably because most poisons are generally

bitter tasting. On the other hand, organic compounds tend to be sweet and

this would explain the reason why there are a lot of sweet receptors on the

tip of the tongue. Interestingly, we lose taste buds as we age – perhaps why

children are so easily enthralled to sweet foods as bitter foods will taste so

much more bitter than when adults taste the same food with their diminished

numbers of taste buds. There are also great variations in how many taste

buds we start out with – ‘super-tasters’ have many more taste buds per square millimetre than ordinary folk – and therefore will be appalled at the

bitter compounds in say cabbage that an ordinary taster may barely notice.

If you come across someone who seems to be a super-taster, have a laugh

and send them to a Chinese herbalist! Gustatory receptors are also

chemoreceptors, being stimulated by the chemicals dissolved in the saliva.

Like the olfactory receptors, they adapt rather quickly, so, whatever Wrigley tell you about their longer lasting gum, don’t believe them!

Our appreciation of food is a combination of smell and taste receptors –

block the nose and we barely taste our food.

THE SENSE OF HEARING The ear is a highly complex sense organ that could easily put students off

learning due to the many different areas of interest connected with the

workings of the ear, hearing and balance. For this reason I would like to keep

this section as simple as possible, creating, with the aid of your text book,

pictures in your mind as to how the ear works.

Open your text book (and look at the diagram overleaf) and notice the

three sections of the ear. First, and most obvious is the external ear, which

comprises of the ear itself. It is actually called the auricle or pinna and its

fleshy, cartilaginous design is to capture sound waves and direct them

inwards to the middle ear. I t does this by channelling the sound waves along

the ear canal, which not only leads to the middle ear but also is the site for

the secretion of a substance called cerumen, or ear wax. The ear canal

terminates at the tympanic membrane, more commonly known as the

eardrum, on the other side of which the mechanism of hearing begins.


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THE SENSE OF HEARING continued…

In the diagram, the middle and inner ear are highly exaggerated in size for clarity of

description. In the middle ear section you will find possibly the three cleverest,

certainly the smallest, bones in the body. If you studied biology at school, you may

recall the words ‘hammer’, ‘anvil’ and ‘stirrup’: these three names were given to describe these three bones according to their shape. The correct names for these

auditory ossicles are in fact the Latin translation: malleus, incus and stapes. These

three bones form a continuum that connects the eardrum to the oval window,

another eardrum, if you like, but separating the middle ear from the inner ear. When

sound waves enter the ear canal, they cause the eardrum to v ibrate back and forth.

This moves the hammer (malleus), attached to the eardrum (tympanic membrane),

which in turn sets in motion the other two bones, the anvil (incus) and stirrup (stapes),

with the overall effect of transferring the original sound wave along the auditory

ossicles to the inner oval window. This is a brilliant mechanism, as long as there are

three joined bones to exaggerate the original movement of the eardrum. In this way

the wave is amplified twice: first the actual ear concentrates the sound waves into

the ear canal and then the auditory ossicles exaggerates them further.

Beside the two openings in the middle ear that we have already mentioned, there

are the round window, which also has a membrane covering it and separates the

middle from the inner ear, and the auditory or Eustachian tube, a tube that runs

down into the nasopharynx.

The Eustachian tube is a structure that helps equalize the pressure between the

external and internal parts of the ear. Without this, if the atmospheric pressure were

different to that of the middle ear, the eardrum would become distorted in shape or

rupture. However, having access to the middle ear via the nasopharynx and the

Eustachian tube allows equalization of the pressures. This is why holding your nose

and blowing air creates pressure in your ear, a useful measure when taking off in a

plane or diving in deep water. Swallowing and yawning may achieve the same

result, hence the tradition of sucking barley sugars during air travel – lots of

swallowing and a boring taste to make you yawn!


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THE SENSE OF HEARING continued… So, to re-cap, sound waves are captured and directed into the ear canal by

the external ear. They then travel along the ear canal as an amplified version

before they strike the eardrum causing it to vibrate. The ear ossicles

(hammer, anvil and stirrup) form a continuum which connect the eardrum to

the oval window, which leads to the middle ear. Effectively, this also amplifies

the sound waves and is reproduced into the inner ear for interpretation. In

here, two very important structures are involved in the two functions of the

ear, hearing and balance. The semi-circular canals are concerned with

balance, which we will discuss later. The structure that interprets the

amplified sound waves is the cochlea, so named because of its snail-like

appearance. The cochlea itself is a bony structure which houses the

cochlear membranous duct. The duct is sectioned into an upper and lower

duct, respectively called the scala vestibuli and the scala tympani. Between

these two is the cochlear duct which contains the all important organ of

Corti, with associated hair cells and supporting cells. These ultimately

interpret the sound waves into action potentials along the cochlear nerve.

I think, however good this drawing of the inner ear is, that to most it will

appear to be just an elaborate network of tubes. Hence I would like for you

to keep in mind the picture of the sound waves being channelled along the

middle ear! What I hope is very clear is that the cochlear duct is itself

enveloped in a loose membrane which separates it from the inside of the

cochlea. Between these is in fact a fluid similar to the cerebrospinal fluid that

circulates around the brain and is called perilymph. Inside the membrane

that encloses the cochlear duct there is again another fluid filling this cavity,

called endolymph and this is a potassium-rich lymph fluid. When the sound

wave is reproduced upon the oval window leading to the cochlea, the oval

window vibrates causing the sound wave to be reproduced in the perilymph,

much the same as a wave is created in a swimming pool complex with a

wave machine.

Imagine the movement of the wave from striking the eardrum all the way to creating an actual mini-wave in the perilymph that fills the cochlea. This

wave has the effect of causing an indent in the membrane that surrounds

the cochlear duct, which in turn causes movement of the endolymph

surrounding the cochlear duct. The culminating effect of these movements is

ultimately to recreate the existing sound wave at the level of the cochlear

duct, where the receptive hair cells are located. This causes an action

potential to be started at the root end of the cochlear nerve. From here the

nerve impulse travels through the medulla, pons, midbrain and thalamus

before being interpreted in the auditory centres of the temporal lobe.

Another interesting point here is that the frequency of sound waves, or the

number of sound waves recorded in a certain length, are perceived at

different points along the cochlear duct, with the higher frequencies being

detected at the beginning of the duct. The same is true for pitch or loudness,

as the amount of movement that the membrane is exposed to determines

how much the receptive hair cells will be bent and thus the degree of

perceived amplitude.


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SUMMARY As with all modes of sensory perception, the overall aim is to in some way take the external

stimulus, enhance it and ultimately transport that stimulus along to a neural point where an

action potential can be set about to be sent to the brain for interpretation.

As far as hearing is concerned, the actual sound wave is taken and amplified and recreated

until it becomes a fluid wave. This has the effect of altering the fluid pressures inside another

sealed duct, so that tiny hairs, connected to neurons, are bent. This will cause a stimulus to

start up an action potential which is then sent to the brain for interpretation.

Due to the mechanisms involved in accurately reproducing a sound wave in a fluid, the

ability to hear has the added advantage of interpreting amplitude as well as pitch. This is

possible due to the different ways in which hair cells become bent by the frequency and

pitch of the original stimulus.

THE SENSE OF BALANCE Here, again, we find ourselves with a highly complex set of canals, fluids and

nervous tissues. The sense organs involved in our sense of balance are in the

vestibule and semicircular canals. The vestibule deals with static equilibrium,

i.e. sensing the position of our head in response to gravity, or acceleration and deceleration of our body in response to external movement. On the

other hand, the semicircular canals deal with dynamic equilibrium, i.e.

maintaining balance when our head or body is suddenly moved or rotated.

The vestibule is subdivided in two sections, the utricle and saccule, and it is

here that we find the all important maculae. Here, two yellow nerves exit

from the vestibule, and thus are named vestibular nerves. There are two oval-

shapes that are the utricle and the saccule, very similar to the hairy cells

found in the cochlea. There is a vase-like cell projecting upwards. This has a

gelatinous matrix deposited on the hairy projections, called microvilli, as in

the intestines. This gelatinous matrix is more or less fixed to the hairs and has

the effect of partially sliding, causing the hairs to become bent when the

angle of the head is altered in any way. This is the area that reacts to the

position of the head and tends to cause reflex muscle contractions to help

support the head when it finds itself in less than an upright and balanced

position. For example, you are probably leaning forward whilst reading this

lecture and so the matrix would have slid downwards, bending the hairs and

sending a message to the brain that your head is tilting forward. The brain

would send a message to your posterior neck muscles to contract, to

counteract the movement. If you were to tilt your head to one side then the

matrix on the horizontal plane would shift, sending information on its relevant

positional to the brain for interpretation and relevant commands.

Three semicircular canals are arranged on three dimensional planes. Where

each canal joins the vestibule is a swelling called ampulla, which in turn

contain the crista ampullaris and cupula. It is these cone-shaped cupulae

that sway according to the movement of the fluid inside the canals. The

cupulae are once again attached to hair cells which are in turn attached to

the nerve fibres of the vestibular nerve. If you rotate your head to the left, the

fluid in the canals moves in the opposite direction causing the cupulae to

bend and thus stimulate a nerve potential which is sent to the brain for

interpretation. The same applies to each of the canals which means that any

movement in any direction is recognised by the movement of the fluid

bending the relevant cupulae in the opposite direction. This way, messages

can be sent to the muscles of the body so that balance can be maintained.


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SUMMARY The inner ear contributes two particular aspects to our sense of balance: the position of our

head relative to gravity – utricle and saccule’s static sense that detects how tilted our head is with respect to the pull of gravity. Our body follows our head – so when we stand in a bus

and tilt as it goes around a corner – it is the change in the apparent pull of gravity as we

corner that alters the static balance sense in the inner ear and causes our body to realign.

The semicircular canals detect rotation of the head in the 3 dimensions – front/back, side-to-

side and left-to-right. This is a dynamic sense and only picks up movement.

Infection of the inner ear can be profoundly disturbing to our sense of balance and cause

terrible vertigo. The inner ear is housed in bone and is called the labyrinth – hence an

infection of this area is called labyrinthitis.

Sea- and motion-sickness arise from a mis-match between the visual cues for balance (e.g.

perpendicular lines) and the inner ear’s sense of balance.

We make our selves dizzy by spinning round so much that the fluid in the inner ear starts to

move. When we stop it keeps moving for a while – triggering the sensory cells to make us

think we are still moving. The world spins because the brain normally keeps the eyes steady

and fixed on particular objects using inner ear information. Because the inner ear is reporting

movement, our brain makes the eyes move to supposedly keep them steady – but is

actually making them flick from side-to-side (a movement called nystagmus)

inappropriately. This makes the world seem to spin. Alcohol in excess shuts down the part of

the brain that uses inner ear information (the cerebellum) and stops the eye being stabilised

– hence the world spins.

PHYSI-FUN-OLOGY

Need revenge on someone? While not to be recommended, you can

induce temporary vertigo by running hot water (but not too hot) into one

ear canal and ice-cold water into the other. This sets up convection

currents in the semicircular canals and can induce profound, vomit-making

vertigo for a short time.

THE SENSE OF VISION As we all know, our vision relies on a pair of sense organs, the eyes, which

although incredibly complex, act very much like an organic camera. The

eye is a ball shaped structure that is responsible for converting light into

nerve impulses, which are then interpreted by the brain as images. I t is

predominantly a hollow organ, filled with a watery solution called aqueous

humour, which allows the eye to change shape uniformly to aid the

process of vision.

Three layers of tissue form the eye, the retina being the innermost layer,

followed by the choroid layer, and finally the white outer layer, the sclera. It

is the outer layer that travels all around the eye to become the transparent

cornea, pictured at the front of the eye. It is this bulbous frontal extension of

the sclera that contact lenses are placed upon. I t is the cornea which is the

main structure that focuses the light onto the retina – the inner lens just fine-

tunes this.


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THE SENSE OF VISION Continued……

There is an extensive network of blood vessels which supply the highly vascular and

nervous, light-sensitive retina – and these are great ev idence for the random process

of evolution as they run over the surface of the retina, reducing the light falling on

the retina. Once the early structure of the eye evolved with these vessels on the top,

the structure was committed and it would be too big a leap to place all the vessels

underneath the retina.

The lens is suspended from suspensory ligaments, separating the anterior chamber

from the posterior chamber in the anterior cavity. The lens alters its shape to refract

the light accordingly via the tiny ciliary muscles. It is an extension of these muscles,

arranged in a doughnut shape, that form the coloured part of the eye, the iris,

responsible for controlling the amount of light into the eye by closing or opening the

aperture of the eye. The hole in the doughnut is the black circular spot named pupil.

The lens has a tendency to become more rigid with age – and the cilary muscles

cease to be able to distort it and so fine-focus light. Hence the increased use of

spectacles with age to achieve focus across near and far objects.

At the back of the eye is the optic nerve which branches out, along with the central

retinal artery and vein, to cover extensively the retina. Here there is a ‘blind spot’ (or macula), due to the lack of two important structures that would otherwise enable us

to see: the rods and cones. Most people have heard about these little light receptive

nerve endings. They differ from indiv idual to individual in numbers, distribution and

function, with cones usually being less numerous. Cones radiate from the densest

and central part of the retina, called the fovea centralis. Rods, on the other hand,

tend to be completely absent from this area and instead proliferate around the

periphery of the retina.


16

THE SENSE OF VISION continued…… Rods and cones both contain light-sensitive pigmented compounds called

photopigments. These compounds physically change when stimulated by

light, causing a nerve action potential to be generated. With rods the

photopigment is called rhodopsin and is sensitive even to dim light. Once

stimulated the rhodopsin breaks down into opsin, a protein, and retinal, a

vitamin A derivative, which then expand and separate to cause an action

potential to be generated. Energy is then required to reconnect these

substances and form rhodopsin once more. Due to their photopigments, the

image is perceived as grey shades, and are thus more suitable for night v ision

where clarity is more important than colours. Furthermore, the image is best

perceived if not stared at directly, due to the fact that the rods are on the

periphery of the retina – you will often see things in dim light in the corner of

the eye that disappear when looked at full on. The rods are also much more

sensitive to changes in light than the cones – this is why you can perceive

screens flickering from the corner of your eye but not the centre. This makes

rods ideal for picking up movement threats in the periphery of the vision. The

cones, on the other hand, are stimulated by bright light and are responsible

for daylight vision and colour perception. There are three types of cones,

erythrolabe, chlorolabe and cyanolabe, each sensitive to the different light

waves of the colours red, green and blue respectively. This mixture makes up

a full spectrum of colour in the brain. This means that our sense of different

colour is dependent on how light waves of a particular colour differentially

stimulate different cones. Thus our sense of the rich colours of the world is an

illusion created by the brain – we don’t see orange – we invent orange from

the partial stimulation of red and green cones.

The optic nerves are linked from both eyes to the optic chiasma. The nerves

not only cross over, to be interpreted on the opposite sides of the brain, but

there is also a branch leading to the same side – this separates v ision into the

right and left visual fields – the right eye receives images from the world to

the left of the nose and this is joined with the information coming from the left eye’s view of the what lies to the left of the nose. The same happens for the right visual field. This means that, whatever the visual symptomatology, one

should be able to accurately predict where interference on the optic nerve

may be coming from: pressure on one side only before the chiasma will

mean that v ision is still possible from the unaffected eye, whereas pressure

after the optic chiasma results in loss of one visual field – left sided loss will be

demonstrated by losses in the left and right eyes.

CONCLUSION

Quite simply the purpose of all the special sense organs is to receive a stimulus, be it

light, sound, smell or taste, and to then turn that stimulus into a nerve action potential

so as it may be sent to the brain for interpretation. The way the body has done this is

to develop highly specialized receptors that are suitably designed to enhance these

stimuli and thus cause, physically or chemically, a nerve impulse to be generated.

These specialized receptors are what essentially make up the special senses: they

have served us well in the need of survival of our species and now serve us more

readily in the pursuit of the many pleasures that our special senses can bring!

Because of this, any indication of a loss or disturbance of v ision or hearing should be

immediately investigated.


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INSIGHT – OPTICAL ILLUSIONS

If one stares fixedly at any object certain visual phenomena arise that can be quite

dramatic. The eyes have light sensitive cells (rods and cones) in the retina at the

back of the eye. These amplify the contrast between bright and dark areas in order

to help to distinguish the edges of objects. The eyes also continually scan an object

with tiny movements (called saccades) – these movements are too small to normally

notice but they give liv ing eyes their sparkle and their loss, in death, make the eyes

look dull. This is similar to the sparkle of sunlight on moving water compared to the

relative dullness of sunlight on still water. As a consequence of these saccadic

movements, if one stares at a dark object against a bright background the retinal

cells receiving light from the edge of the object have to cope with rapidly altering

contrasts on top of the amplification effects that heighten contrast differences. This

results in a bright ‘halo’ or fringe around the dark object. This becomes more pronounced the longer and more fixedly you stare at the object. See Box 1 for a

simple demonstration of these contrast effects.

Box 1: A Hermann Grid

Stare at the grid below – a Hermann Grid (Spillmann, 1994). As one stares, illusory

patterns and contrast differences start developing around and between the black

squares in the pattern below.


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A way in which the ‘creativity’ of the brain’s processes impacts on waking life is encountered in blind spot phenomena. The blind spot or macula is an area within

the retina at the inner side of the central colour v ision area that is devoid of light-

sensitive cells. See Box 2.

Box 2

To find the blind spot, cover your left eye and stare at the black spot on the diagram

below with the right eye. With the diagram about a foot away and the black spot

directly in front of the right eye, keep the right eye fixed on the black spot and move

the page forward until the white spot disappears. Note how the space where the

blind spot lies is ‘patched’ by the brain with the gr id pattern.

When familiar with the position of the blind spot, try the following rather unnerving

experiment. Hold a small-sized coin at arms length, pinched between the right

thumb and finger. Cover the left eye. Fix the right eye on a distant point straight in

front on a level with the eyes. With the coin in the right hand, move the hand so the

coin lines up with the distant point. Keep fixing on the point and move the hand

slowly out to the side (laterally) keeping it level with the fixed point. When the coin is

about 15-20 degrees away from the starting point it should disappear. I t is essential to

keep the right eye staring unwaveringly at the fixed point in front. If the blind spot is

hard to find, try slowly raising and lowering the coin as it moves out to the side (but it

is much harder to keep the eye fixed forward when doing this) or try again with the

diagram above as a reminder of where the blind spot is located.


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INSIGHT – OPTICAL ILLUSIONS continued……

This exercise can make you feel a little uncomfortable – it is as if the brain does not

like to be caught out with evidence of how it constructs our v isual world. The brain is

continually creating a ‘patch’ covering up the hole in each visual field – if one stares

at some tartan the brain creates a tartan patch.

Such an exercise can lead us to the profound realisation that all we see is a creation

of the visual systems of the brain that interpret light information falling on the retina. I t

is as if we are looking at a television screen that is fed images processed by a

production team (the v isual areas of the brain) that may come from a v ideo camera

(the eye) but may also be independent creations of a computer graphics team

(dreams) or a superimposition of the two (waking dreams, v isions or visual

hallucinations).

Despite the illusions it can induce, the human visual system is very impressive. In

common with other vertebrates, the human retina, whether or not we are conscious

of these abilities, can be shown to distinguish a single photon of light from

background noise (Reike & Baylor, 1998), it can determine the direction of a

magnetic field (Thoss et al, 2000) and can detect the polarisation of light (Houd-

Walter, 1990). Even destruction of the visual cortex and the consequent cortical

blindness does not preclude some individuals with ‘blind sight’ to have the ability to avoid thrown objects or to accurately ‘guess’ whether a light is on or off due to the processing by non-cortical v isual centres (Ramachandran & Blakeslee, 1999). Given

that the visual systems are capable of perceiving and processing retinal information

without conscious awareness, it is therefore possible that subtle cues regarding

health status or vitality are apprehended unconsciously. Some indiv iduals may then

be able to access this information by entering more inward or reflective states and

may use some of the above visual phenomena to do so.

(The above is extracted from: Duerden, T. (2004) An Aura of Confusion: seeing auras

– vital energy or human physiology. Part 1. Complementary Therapies in Nursing and

Midwifery. 10, 22-29 )

PATHOPHYSIOLOGY

It is important to familiarise yourself with common pathological processes.

The following websites offer helpful resources for the study of clinical conditions and

their treatments.

NHS Direct: http://www.nhsdirect.nhs.uk – click on the encyclopaedia. UK based

health information and most conditions.

Medline Plus: http://medlineplus.gov - click on the encyclopaedia. American so

there are differences in some names of diseases and drugs.

British National Formulary: http://www.bnf.org – free registration. Explore the

respiratory drug section. Excellent for finding out what drugs are and what official

prescribing practice is.

The following common conditions should be investigated by using these sites:

Ageing effects on vision, impaired vision, ageing effects on hearing and

balance, ear infection, tinnitus.


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THE NERVOUS SYSTEM

INTRODUCTION While the Nervous System is often considered the most complex system that we as

human beings are bestowed with, it is very easy to get carried away with all this

complexity and start seeing the brain as the sole seat of our consciousness and the

control centre for the whole body. This is counter to an holistic view of the body that

emphasises links between each element of the body across all levels of complexity.

A possibly more useful model is illustrated below:

This model considers how the functioning of all our body cells depends largely on the

sea of signalling molecules (also called chemical messengers or, technically, ligands)

that includes hormones from endocrine glands, neurotransmitters from nerve cells

but also a vast range of other molecules released by v irtually all body cells. This

chemical chatter endlessly modifies the activity of all the body cells. The sense of

being we have at any one moment – how we think (our cognition), feel (our affect)

and sense ourselves to be in our body (our somatisation) is profoundly influenced by

these messengers. If we have an abscess, the white blood cells release traces of

potent molecules that spread through the circulation and, on reaching the brain,

cause fever, tiredness and behavioural changes associated with infective illness. Thus the immune cells in the abscess are ‘controlling’ our nervous system. I f we also include external factors: nutrition, elimination, vitality, we begin to see just what an

immensely complex web of interactions influence our state of being. These

interactions are beginning to be teased out by the new overlapping disciplines of

psychoneuroimmunology and psychoneuroendocrinology.


21

The advent of various brain imaging techniques are revolutionising our knowledge of

the central nervous system (brain and spinal cord) – in particular techniques called

functional NMR (fNMR) which is now becoming somewhat outdated as it involves

exposure to radioactivity, is being replaced by the brain imaging PET scan.

The level of research and documentation on this system alone could easily fill a

lifetime’s study – so to give you a chance to avoid this – we will instead concentrate

on the broader functions and generally accepted components of this system.

This module will explore:

The cells of the Nervous System

The div ision of the Nervous system

The brain and its functions, the spinal cord and their coverings

Taking each in turn and exploring how they all play their part in maintaining

homeostasis.

READ CHAPTERS 12 - 14 IN THIBODEAU & PATTON

CELLS OF THE NERVOUS SYSTEM

There are two main types of nerve cells in the body: neurons and glia. Neurons are

mainly responsible for conducting the electrical impulses and are therefore, the main

communication nerve cells. Glial cells, or neuroglia, on the other hand, do not

normally conduct information themselves but instead support the functions of the

neurons in several ways.

GLIAL CELLS

The predominant difference between glial cells and neurons are that glial

cells have kept their capacity for cell division. There are an estimated 900

billion glial cells within the body, about nine times the number of stars in our

own galaxy! There are five of the main types of glia: the Astrocytes,

Microglia, Ependymal Cells, Oligodendrocytes and the Schwann Cells.

Astrocytes are star-shaped cells (from the Greek astron = star) only found in

the CNS and are also the most numerous of the glial cells. These cells tightly

surround the numerous blood capillaries of the brain and form what is

known as the blood-brain barrier. The tightly packed astrocytes only allow

very small molecules to pass through into the brain by diffusion out of the

capillaries, molecules such as oxygen, carbon dioxide, water glucose and

alcohol. These glial cells have recently been shown to pass their own

version of nerve impulses (based on glutamate release) from cell to cell,

moderating in as yet unclear ways the activity of neurons. I t is as if we have

just discovered a brand new nervous system.

Microglia are very small and predominantly stationary nervous cells,

generally found within the CNS. It is understood that one of their main

functions is phagocytosis (engulfing of small particles) when brain tissue is

inflamed or damaged such as after a stroke. It is thought that, in these

conditions, microglia enlarge themselves and then become mobile so as to

be able to engulf any microorganism or cellular debris.


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GLIAL CELLS Continued…… Ependymal Cells are the epithelial cells of the CNS. They form the linings

for the cavities of both the brain and the spinal cord. I t is believed that

some of these cells actually produce the fluids that fill the spaces that the

linings create. Some of the ependymal cells have cilia on their borders

that help maintain the movement of the fluids.

Oligodendrocytes: The name means ‘cell with few branches’. The most important function of these cells is that they help support nerve fibres and

produce the fatty myelin sheath, that surrounds nerve fibres in the CNS.

Schwann cells are similar to oligodendrocytes. These cells however, are

only found outside the CNS in the PNS. Their main function is to support

and protect the nerve fibres and sometimes they form a myelin sheath

around them. The latter is formed by layer upon layer of Schwann cells,

wrapped around each single nerve fibre. Myelin is a white fatty insulating

substance that gives the appearance of a white nerve – en masse they

look white-ish, hence ‘white matter’ refers to collections of myelinated fibres. ‘Grey matter’ is made up of the cell bodies and the short unmyelinated fibres of neurons where interconnections tend to occur.

If you were to look at a single myelinated neuron it would have the

appearance of a string of sausages. This is because the Schwann cells

form microscopic gaps called nodes of Ranvier, essential in the effective

conduction of impulses along the PNS. We will refer to this point in more

detail later on. Similar gaps are found between oligodendrocyte cells.

NEURONS About 100 billion, or 10% of all nerve cells form the more widely known

neurons in the brain. I ts main components are that of a rather large cell

body with at least two processes. One of these will be the efferent axon,

transmitting information away from the cell body, whilst the other will be

the dendrite, (normally more than one) and with the appearance of tree

branches. Dendrites transmit incoming sensory information to the cell

body. Both afferent sensory dendrites and efferent axons are often known

collectively as nerve fibres.

The neuron is similar to many cells, with all the normal components of a

typical cell that you would expect.Extending through the axons body are

numerous fine strands called neurofibrils, which are just bundles of even

finer filaments called neurofilaments. Also contained within the neurons

are many microtubules and again even smaller microfilaments that all

help provide a network for transporting important molecules to and from

the far ends of the neuron. Only the axons are myelinated and not the

sensory dendrites. The axons can vary in length and may be as long as 2

meters in a tall person (running from the toe to brain stem) and may have

many branches. Take a look at a diagram of a ‘myelinated axon’,you can see the way in which a myelinated axon is formed by many Schwann

cells. At the end of the axon (the synaptic knob) the nerve impulses

received cause chemical compounds, called neurotransmitters, to be

released from the numerous small sacs, or vesicles, which then cross the

very narrow synaptict cleft and connect onto the post-synaptic

membrane of the new neuron. This chemically signals the impulse to

continue, to begin again, or even to stimulate another type of response.


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NEURONS continued……

But before we can really look in more detail at this mechanism, let us first take a

closer look at the way in which a nerve transmits an impulse.

Cell Body: metabolic centre of nerve cell containing the nucleus.

Sustains the cytoplasmic processes (nerve fibres) leaving the cell

body. If this dies the whole nerve cell dies.

Dendrites: short fibres with many branches. Often receive nerve

impulses from other nerves and pass these onto the cell body.

Around 1000 other nerve fibres from other parts of the nervous

system may synapse with the dendrites.

Axons: longer fibres with fewer branches that tend to take impulse

away from the cell body. (The start of the axon by the cell body

is called the axon hillock.) They have many terminal branches

that connect the nerve cell to around 1000 other nerve cells.

Branches off the main axon are called collateral fibres.

Synaptic knob, nerve endings or axon terminals: enlarged end of

nerve fibres.

Synapse: gap between end nerve fibre and a second cell (the gap of

the synapse is also called the synaptic cleft). The second cell

could be a nerve, muscle or gland cell. Chemical messengers

called neurotransmitters are stored in membrane bound sacs

(vesicles) in the synaptic knob. These are released into the

synapse when the nerve impulse arrives and they cause

changes in the second cell: in nerves they usually trigger a nerve

impulse, muscle are stimulated to contract, glands to secrete.

(The first cell is called the pre-synaptic cell, the second the post-

synaptic cell).

NERVE IMPULSE The nerve impulse is a wave of electrochemical activ ity that passes through the

membrane of the nerve fibres.

It is produced by the controlled opening of special ion channels found in the

neurone membrane that cause a change in the electrical voltage (also called the

potential difference) between the inside and outside of the cell: i.e. across the

membrane. (This change in voltage is called depolarisation). Sodium and potassium

are the most important ions involved.

Optional detail: The nerve impulse in an adjacent area of the nerve fibre causes

sodium to enter the nerve fibre through special sodium channels. This now causes a

depolarisation of this area of membrane and so the nerve impulse travels to this

area. This in turn triggers the next area to depolarise. In this way the nerve impulse is

propagated.

The wave of electrochemical activity normally travels in only one direction and is

called an Action Potential. It can only travel in one direction because after

depolarisation in an area of membrane it takes time to recover and get back to the

resting potential which is the state needed for a nerve impulse to be able to trigger

another depolarisation. This recovery period (called the refractory period) stops the

impulse going backwards.


24

When the action potential reaches the synaptic knob at the end of the nerve fibre it

causes the release of chemical messengers from vesicles stored in the axon ending.

These messengers (neurotransmitters) cross the synapse and bind to receptors in the

membrane of the nerve, muscle or gland cell on the other side of the synapse. This

typically causes a second action potential (AP) in a nerve cell, a muscle cell to

contract or a gland cell to secrete. Nor-adrenalin (nor-ephinephrine in USA) is found

in the sympathetic nervous system and acetyl choline is found in the para-

sympathetic, voluntary and sensory nervous systems. There are hundreds of

neurotransmitters found in the brain. Some neurotransmitters inhibit the generation of

an action potential and this kind of synapse is an inhibitory synapse. Where a

synapse promotes an action potential it is called an excitatory synapse.

The action potential either happens or does not happen: nerves are either firing or

not firing. The fact that there is no half-way house is called the ‘all or nothing

response’.

Nerves require a huge amount of energy to function: the brain uses 25% of the

body's energy intake. They are therefore, very vulnerable to a break in oxygen or

glucose supply and can die within 4 minutes of such a break.

Nerves carrying very precise information are often covered in a special fatty layer

called myelin forming a myelin sheath. It is here that we refresh our acquaintance

with the Schwann cells in the peripheral nervous system and the oligo-dendrocytes in

the CNS. There are gaps between the cells forming the myelin sheath called nodes

(or the nodes of Ranvier). The effect of these nodes is to speed the transmission of the

nerve impulse. This occurs because the impulse jumps from node to node rather than

having to travel through all the nerve fibre's membrane. These fast neurones are

used to transmit important sensory or motor information. In multiple sclerosis, damage

to the myelin sheath causes the disturbances to sensation and muscle action.

Contrary to popular belief, it is the frequency of impulses that affects the strength of

the impulse, as the actual intensity of the electrical impulse is always the same.

It is the overall way in which a neuron conducts the impulse that is the most

important concept to grasp here. And whether or not a neuron actually conducts

an impulse is determined by the balance of inhibitory and excitatory

neurotransmitters that are showered onto the dendrites and cell body from the

synapses formed from incoming neurones.

The diagram overleaf attempts to summarise this.


25


26

NEUROTRANSMITTERS

There are over a hundred compounds presently identified as

neurotransmitters, and quite possibly many more. They are the way in which

neurons speak to each other and instruct the next neuron, whether it be as

‘go-between’, excitors or inhibitors. They are commonly described according to their function or their chemical structure, i.e. excitatory or inhibitory,

although it seems that some neurotransmitters can have opposite effects in

different areas of the nervous system. As the functions of neurotransmitters

change from place to place, it is generally preferred to classify them

according to their chemical structure.

While general physiology tends to focus on the faster acting

neurotransmitters involved in sensory and motor pathways it tends to ignore

the fact that most neurons secrete 2 types of neurotransmitter, fast-acting

molecules such as acetylcholine, and slower acting neuropeptides.

Surprisingly, in v iew of their widespread distribution, not much is known about

the factors that regulate the secretion of neuropeptides. Neuropeptides play a crucial but subtle role in modifying the responsiveness of neurones to faster

acting neurotransmitters and have been linked to longer term brain functions

such as mood.

MAIN GROUPS OF TRANSMITTER SUBSTANCES It can be useful to be vaguely aware of the names of the neurotransmitters below,

especially the amines as many antidepressant drugs are thought to influence levels

of these in the brain. So many drugs act on neurotransmitters, it is useful to have at

least encountered the main types

1. ACETYLCHOLINE

This is very common in the PNS it is released at synapses on skeletal muscles

and has an excitatory effect on skeletal muscles and inhibitory effect on

heart muscle. I t is widespread in the brain.

2. AMINES (AKA MONOAMINES OR BIOGENIC AMINES)

In the brain these don’t transmit specific information but modulate brain function, acting like a volume control, increasing/decreasing activities of particular brain

regions. In the PNS the catecholamines act as neurotransmitters for the SNS.

Catecholamines – the group name of adrenalin, noradrenalin, dopamine

o Dopamine - can be excitatory or inhibitory. A decrease in dopamine is

related to Parkinson’s disease, an increase related to schizophrenia. Dopamine also stimulates the pleasure centres and is released by most

addictive substances or activities. o Adrenalin and Noradrenalin - alertness, wakefulness; excess brings

manic states while deficiency brings depression

Serotonin - mostly inhibitory; to do with mood, eating, sleep/arousal,

dreaming. LSD interferes with serotonin, causing dreaming while awake. Low

levels brings severe depression, hence many more recent anti-depressants

aim to raise serotonin levels.

Histamine – mainly excitatory, involved in emotions, temperature regulation

and the inflammation response.


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MAIN GROUPS OF TRANSMITTER SUBSTANCES Continued……

3. AMINO ACIDS:

Glutamic acid/glutamate - excitatory; it lowers the threshold for getting a

nerve impulse – the food additives MSG can lead to excessive levels of

glutamate and neurological disturbance.

GABA - inhibitory, it raises the threshold for excitation; it keeps all neurons from

becoming excited, as in a seizure. Epilepsy may be related to abnormality in

GABA-secreting neurons; GABA binding sites are sensitive to benzodiazepines

(valium, librium), barbiturates and alcohol.

Glycine - inhibitory, found in the spinal cord and lower portions of the brain. The bacteria that causes tetanus (lockjaw) blocks the activity of glycine

synapses, and the removal of its inhibitory effect causes muscles to contract

continuously.

4. NEUROACTIVE PEPTIDES

There are over a hundred of these short chains of amino acids with ever more being

discovered, often molecules encountered as hormones or signalling molecules

elsewhere in the body double as brain neurotransmitters.

Endogenous opioids – These are perhaps the most useful to know about. These

include enkephalins, endorphins, dynorphins and are what opiate drugs (e.g.

morphine or heroin – known as diamorphine in medical circles) mimic. These

opioides suppress pain and cause dissociation from stressful events.

5. LIPIDS

E.g. anandamide - the natural ligand (the name for the molecule that binds

onto a receptor) for cannabis (THC) receptors is a lipid. This is why cannabis

takes so long to clear from the body, it persists in body fat. Heroin is water

soluble and is rapidly cleared. Random testing for drugs in some schools and

prisons therefore, drives people to heroin use as the odds of it being

detected are much reduced.

6. NUCLEOSIDES

For example, adenosine; released by glial cells as well as neurons. When the

oxygen supply is low it is released causing nearby blood vessels to dilate. It

also works as a neuromodulator resulting in neural inhibition. Caffeine blocks

adenosine receptors therefore, preventing inhibition thus increasing

excitation.

7. SOLUBLE GASES

For example, adenosine; released by glial cells as well as neurons. When the

oxygen supply is low it is released causing nearby blood vessels to dilate. It

also works as a neuromodulator resulting in neural inhibition. Caffeine blocks

adenosine receptors therefore, preventing inhibition thus increasing

excitation.

These include carbon monoxide and nitric oxide (this diffuses out of cells as

soon as it is created, rapidly affecting other cells, and is very potent).


28

INACTIVATION OF NEUROTRANSMITTERS

This is essential – if neurotransmitters were not inactivated after they were released,

they would cause a prolonged inhibition or excitation of the post-synaptic cell.

The action of neurotransmitters can be stopped by four different mechanisms

1. Diffusion: the neurotransmitter drifts away, out of the synaptic cleft where it

can no longer act on a receptor.

2. Enzymatic degradation (deactivation): a specific enzyme changes the

structure of the neurotransmitter so it is not recognized by the receptor.

3. Glial cells: astrocytes (a type of glial cell, remember?) remove

neurotransmitters from the synaptic cleft.

4. Reuptake: the whole neurotransmitter molecule is taken back into the axon

terminal that released it. This is a common way by which the action of

noradrenalin, dopamine and serotonin is stopped...these neurotransmitters

are removed from the synaptic cleft so they cannot bind to receptors. Many

common antidepressant drugs (e.g. Prozac) work by inhibiting re-uptake so

prolonging the action of the neurotransmitter/s released.

DIVISION OF THE NERVOUS SYSTEM

There are no true divisions within this system as such (as there are complex inter -

relations across all divisions), but we find that we can organise the Nervous System

into several distinct but interrelated areas. These divisions are based on structure and

function, and do reflect some of the key organisational hierarchies within the nervous

system. These hierarchies are a consequence of the bolt-on nature of evolution. The

simple nervous system of a worm evolves into the more complex system to

coordinate the simple motions of a fish. It does this by bolting on a new system on

top (from an organisational point of view) of the earlier system. As the nervous system

evolved in reptiles, early mammals and primates, we find further new systems

controlling the earlier ones. Generally, the newer system inhibits simple responses of

the earlier system to allow more complex responses to be made. Ev idence of this is

seen when there is damage to the controlling system – and the normal inhibition is

lost giving what are called ‘release’ phenomena. Spinal damage, for example, can remove the normal control of bladder reflexes so that the bladder empties without

regard to normal higher controls that can inhibit emptying to an appropriate time.

So the way we divide the nervous system up often does reflect natural organisational

differences.

The first major division we will explore is that of the central nervous system (CNS) and

the peripheral nervous system (PNS).

The central nervous system is the ‘central processing unit’ of the nervous system. I t is mostly abbreviated to CNS and this is how we will refer to it in this lecture. The CNS

consists predominantly of the brain and spinal cord, and is primarily responsible for integrating incoming sensory information, evaluating or interpreting its significance

and then initiating an outgoing response. The peripheral nervous system is defined as

any nerve tissue that lies outside of the CNS and is abbreviated to PNS.


29

DIVISION OF THE NERVOUS SYSTEM continued……

The PNS brings sensory information into the CNS v ia sensory neurons whilst motor

neurons take instructions out to the tissues that respond to nerve impulses – mainly

nerve, muscle and gland cells but also immune cells and other body cells. Really try

to get a feel of these incoming and outgoing tides of sensory and motor information.

Within the CNS we can follow the routes (often called tracts) taken by sensory and

motor neurons to and from areas of the CNS that are involved with the information

these neurons carry.

The incoming information or outgoing messages are also described as afferent

nerves or efferent nerves. It is quite easy to remember these distinctions as ‘afferent’ is like ‘affectionate’ – and we come closer/move in when we are being

affectionate, whereas ‘efferent’ may remind the foul-minded of ‘eff-off’ – said when

you want someone to go away! Based upon these principles, we have an afferent

division/sensory division and an efferent division/motor division of the nervous

system.

Another standard division of the nervous system is based on the type of tissues (also

called effectors) that are controlled – this gives us the autonomic nervous system

(ANS) and the somatic nervous system.

The autonomic nervous system is the part of the nervous system that is responsible for

carrying information to the autonomic or visceral effectors, mainly the smooth or

involuntary muscles (such as we find in the walls of blood vessels or the digestive

tract), cardiac muscles and glands (for example the salivary glands or adrenal

glands). I t controls all the organs and structures of the body that do not come under

the direct voluntary control of the human being – you may like to thank whatever

God or Gods or Goddesses you wish, that this system is in place – life would be so

tedious if we had to consciously control every heart beat, gut contraction, opening

or closing of a series of blood vessels etc. Although the name autonomic actually

stems from the word autonomous or self governing, advanced yoga practitioners,

people in hypnotic trances or using biofeedback devices can exert influence over

this system. Some yogis, for example, can virtually stop their hearts, and then speed them up again consciously.

There are sensory (afferent) nerves that pick up autonomic sensory information

(usually unconsciously), this information is used by the body to regulate inner

processes. Generally though, the focus of discussions relating to the ANS is on the

motor (efferent) pathways of the ANS that can be further subdivided into the

sympathetic division (SNS) and the parasympathetic division (PSNS). We can

remember this as the sympathetic division is sympathetic to the body’s increased needs and so it will produce the ‘fight or flight’ response, a survival mechanism. In

complete contrast we have the parasympathetic division which ‘calms’ the body down. It is responsible for returning the body back to a resting state so that it may

recover and repair.


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DIVISION OF THE NERVOUS SYSTEM continued……

The somatic nervous system carries information to the voluntary skeletal muscles (the

somatic effectors). I t is therefore, sometimes called the voluntary nervous system, the

system that allows us to move muscles consciously. The somatic nervous system also

includes the sensory (afferent) pathways, with the somatic sensory division that send

back information from the skin, joints, tendons and muscles to the CNS.

If you look in Thibodeau & Patten, you will find a summary of all the systems within the

nervous system. Notice that they all return to the CNS where information is ultimately

interpreted and then passed back out along efferent nerves to effector centres that

then exert a response on the effectors (organs, glands or muscles).

Let’s look at the CNS in more detail.

THE CENTRAL NERVOUS SYSTEM

The Central Nervous System (CNS) is comprised of the brain and the spinal cord

(these are wrapped up in the meninges – see later) and its main function is to

interpret incoming information and then send out appropriate responses. The

constant stream of incoming sensory information communicates to the CNS what is

happening throughout the body, this enables it to generate appropriate responses,

crucial to maintaining the overall and specific balance of the body, or homeostasis.

THE BRAIN Take some time to familiarise yourself with a diagram of the brain in your text

book.

The brain offers more evidence of the bolt-on nature of evolution – structures

that were neat little bundles in a mouse brain have enormously expanded

and stretched giving the complex and folded structures we see in a human

brain. The main areas of the brain are the cerebellum, diencephalon and

cerebrum that make up the brain proper, and the medulla oblongata, pons

and midbrain make up the brain stem which is the transitional point from the

spinal cord to the brain. The brain stem is often considered to be responsible

for ‘vegetative function’ the housekeeping that keeps the body ticking over at different activity levels – so it is here we find vital and non-vital control

centres for the respiratory and cardiovascular system as well as the controls

that produce sneezing, coughing, swallowing, vomiting and even

hiccupping. Various eye movements are also controlled here, such as the

tracking movements that flick the eyes to moving objects.

The cerebellum at the base of the main brain and behind the brain stem,

performs three general functions, all of which have to do with the control of

skeletal muscles. Firstly, it acts with the cerebral cortex to produce skilled

movements by co-ordinating the activities of groups of muscles. Secondly, it

helps to control posture by making movements smooth and controlled

instead of jerky and awkward, and thirdly, it acts to control skeletal muscles

concerned with maintaining balance. The importance of the cerebellum is

revealed every time you see the poor coordination of a drunk, i.e. slurred

speech, spinning world, staggering gait, lack of balance, intension tremor (a

tremor that gets worse the more precisely you try to control your movements

– like putting a key in a lock, the drunk finds their tremor is worse the closer

they get the key to the keyhole.)


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THE BRAIN Continued……

The diencephalon consists of several structures, of which three are very

important. The thalamus recognises crude sensations of touch, pain and

temperature as well as relaying information to other parts of the brain. The

hypothalamus, which helps regulate and co-ordinate the autonomic nervous

system and endocrine system, is part of the emotional brain or limbic system.

As such it can be seen as the linking structure for many ‘mind-body’ phenomena, if we wish to consider such a split. The blush of embarrassment

or the pallor of fear or anger are physiological reactions in response to

emotions, mediated by the hypothalamus, acting on autonomic nerve

centres. I t controls the ANS by sending regulatory connections down into the

parts of the brain stem and spinal cord that give rise to the SNS and PSNS. It

controls much of the endocrine system through its influence over the pituitary

gland, often seen as the master gland of the body. The hypothalamus also

receives a rich sensory input allowing it to make appropriate responses to

maintain homeostasis as well as having a crucial role in many body cycles

and instinctive activities e.g. sleep/wake, hunger, thirst, sexual maturation.

Finally, we have the pineal body, also involved in regulation of the body’s biological clock and it also produces some hormones, notably melatonin.

Melatonin secretion is inhibited by light – interestingly there are probably

special retinal cells devoted to picking up day-length signals (at least there

are in rats and so are probably in humans) – and through this means our

body rhythms are reset each day with a morning light signal. Jet lag is

produced by the mismatch between our body clock and the daylight signal.

Note that daylight is typically 10,000 times as bright as interior lighting – and

at high summer may be 50,000 times as bright – and a blast of this intensity is

needed to properly reset the body clock.

The Cerebrum is responsible for higher level processing. The cerebrum is

subdiv ided in the right and left cerebral hemispheres. The surface portion of

the cerebrum is called cerebral cortex. The cerebrum is div ided into lobes (frontal, parietal, temporal and occipital) and the various functions of the

cerebrum are typically seen as being the function of these lobes or the

structures lying deep within them.

Motor control is an important function of the frontal lobe, vision of the

occipital, hearing of the temporal and the parietal lobe houses the

somatosensory cortex that processes information from skin (and to a lesser

extent muscles, tendons and bones).

The right and left hemispheres are connected by the structure called the

corpus callosum.


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FUNCTIONS OF THE BRAIN

Having taken our tour though basic brain anatomy and neuronal function, lets

consider the function of the different brain areas. There is so much going on in the

brain I have rev isited some of the functions reviewed but in a little more detail.

1. CEREBRUM. The largest portion of brain divided into the two cerebral hemispheres by the

longitudinal fissure. The 2 hemispheres have many independent functions but are

connected by a band of white fibres: the corpus callosum. The surface has grooves

(sulci) separating raised areas (gyri).

Each hemisphere has a number of lobes:

Frontal: Personality, behaviour (e.g. aggression hence pre-frontal lobotomies),

intellectual functions (e.g. reasoning, abstract thought), motor functions

(speech in Broca’s area, skilled and postural movement, initiation of movement).

Parietal: Separated from frontal lobe by the central sulcus or fissure. Sensation:

somatic sensation, pattern re-cognition, awareness and discrimination of pain,

temperature, touch (fine and coarse), pressure, vibration, position sense

(proprioception), taste. Spatial awareness.

Temporal: smell, hearing, language comprehension – this latter function is

located in Wernicke’s area.

Occipital: v ision and visual interpretation.

The grey matter of the cerebrum is mainly on the surface in the cerebral cortex and it

is here that the higher cortical processes such as memory, language, reasoning,

intelligence and personality occur. It contains almost 75% of all the cell bodies in the

entire nervous system.

The white matter in the cerebrum contains the enormous number of nerve fibres that

connect all the various areas together. (There are 3 types of fibre: association fibres

connect areas of grey matter within the same hemisphere; commissural fibres

connect one hemisphere to the other. The corpus callosum is a large band of these.

Projection fibres connect the cerebrum with different parts of CNS.)

BASAL GANGLIA

These are areas of grey matter within the lower regions of cerebrum that

form part of the indirect motor system. They are responsible for producing

fine movements, controlling muscle tone, initiating movement and co-

ordinating the responses of different muscle groups so they co-operate with

each other. One particular ganglia, the substantia nigra is damaged in

Parkinson’s disease and does not produce enough of the neurotransmitter dopamine. This results in the characteristic movement disturbances of

Parkinson’s disease and is mainly treated by various dopamine substitutes.


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CEREBRUM Continued……

FUNCTIONAL REGIONS OF CEREBRUM

Various regions of cerebrum have particular functions that have been

identified. The parietal, temporal and occipital lobes have regions specific

for each of the main senses. Of particular importance is the primary

somatosensory cortex located in the parietal lobe on the postcentral gyrus

which is posterior to the central sulcus. This receives all the sensory

information from the skin and skeletal muscles. It is very organised so that

nerve fibres from a particular part of the body all go to the same area within

the somatosensory cortex. In addition, nerve fibres from the left side of the

body terminate in the right sensory cortex and vice versa. Behind the sensory

cortex lies a less well defined area: the somatosensory association area. This

receives fibres from the sensory cortex and interprets the crude sensory

information that reaches the sensory cortex, allowing patterns to be

recognised and the meaning of sensations to be understood. It sends fibres

to the pre-motor area of the frontal lobe.

Motor areas have also been identified. Anterior to the central sulcus is the

primary motor area located on the precentral gyrus. This controls the skeletal

muscles and each area controls specific muscles.This area contains the cell

bodies of the upper motor neurones that will travel down the cord, crossing

over to the opposite side at some point, and finally synapse with lower motor

neurones. The lower motor neurones then leave the spinal cord and travel to

the skeletal muscle fibres. The area anterior to the motor cortex is another

association area: the premotor cortex. This receives input from other brain

areas and generates appropriate learned motor responses e.g. the

sequence of muscle movements to write a letter of the alphabet. These

complex movement patterns are then fed into the motor cortex where the

relevant upper motor neurones are stimulated.

There are a number of other association areas which are regions that

interpret sensory input or generate learned responses. Humans have

proportionately more cortex given over to association areas than any other

animal.

There are primary sensory areas for vision, hearing, taste, smell and

association areas are linked to these.

SPEECH AND LANGUAGE

Motor Speech Area or Broca's Area: located in the frontal lobe at bottom of

the premotor area. It controls the motor side of speech - it generates

the patterns that result in the mechanical production of sound.

Auditory Association Area or Wernicke's Area: found at the junction of the

parietal and temporal areas beneath the primary auditory area. It governs

comprehension of speech, music, singing or else classifies it as noise. It

translates words that are heard into thoughts. I t also is important for visual

meaning: e.g. comprehending written words.


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SPEECH AND LANGUAGE Continued……

Speech. The meaning of a desired sequence of speech is assembled in

Wernicker's area and then sent to Broca's area for its physical production.

Broca's area then sends the required muscle patterns on to motor cortex

where they are sent on to the muscles.

Words we hear and see are fed to the Wernicker's area for comprehension

by the auditory and visual areas of the cortex.

When the Broca's area is damaged, words are understood but speech is

garbled: (Non-fluent aphasia). When the Wernicker's area is damaged,

speech is often fluent but lacks meaning (fluent aphasia). The connection

between these two areas can also be damaged.

Speech tends to be generated in the dominant hemisphere (usually the

left). Singing is governed by the opposite hemisphere and as such

sometimes people can sing words after a stroke but cannot speak them.

DOMINANCE

Genetic and environmental factors cause one half of body to be favoured

for physical activity. The brain reflects this with a dominant side which

controls the dominant side of the body. Most left and right handers (90%)

are left brain dominant. Only a few left handers are right brain dominant. It

has been noticed that certain functions predominate in one or the other

hemisphere. However, this is not clear cut and both sides are involved in all

activities to some extend.

Dominant hemisphere = categorical = language / speech / analysis /

reasoning / mathematics,

Non-dominant hemisphere = representational = v isuo-spatial / motor skills /

intuitive / emotion / art / singing / music. Poetic and creative side.

2. DIENCEPHALON (THALAMUS, HYPOTHALAMUS AND EPITHALAMUS)

The thalamus is a sensory relay for most incoming sensory information. The

thalamus gives a crude awareness of sensation even if sensory cortex is

damaged. Thalamic pain is an occasional problem in which a disturbance

in thalamus generates pain sensation that has no obvious physical cause.

The hypothalamus is an incredibly important area despite its small size. It

receives impulses from most internal and external sensory receptors. I t also

monitors blood temperature, concentration, various chemicals and

hormone concentrations. I t therefore plays a crucial role in homeostasis.

The epithalamus gives rise to the pineal gland which is involved in the onset

of puberty and rhythmic cycles in the body. I t produces melatonin, a

hormone that is associated with Seasonal Affective Disorder: SAD.


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3. BRAIN STEM. Containing the medulla, pons and midbrain. These areas are a thorough-

fare for the ascending and descending fibres running between the spinal

cord and higher areas of the brain. The midbrain specifically contains

motor tracts coming down from the cortex and contains reflex centres for

vision and hearing. The pons is below the midbrain and contains vital

centres that regulate breathing.

The medulla oblongata is below the pons and merges with the top of the

spinal cord. It is here that most descending fibres cross to the opposite side

(decussate). I t contains three v ital centres (the cardiac centre, the

vasomotor centre and the respiratory centre). It has non-vital centres that

regulate coughing, sneezing, swallowing, vomiting (the vomiting centre

can be triggered by certain drugs or toxic chemicals in the blood). I t also

has receptors for blood carbon dioxide and oxygen levels.

The reticular formation is a scattered set of neurones found throughout the

brain stem. I t maintains alertness and filters out repetitive stimuli and plays

an important role in sleep/wake cycles. I t also helps co-ordinate muscle

activity and maintains muscle tone.

4. CEREBELLUM.

This is the finely folded structure at the rear of the cerebrum. It is involved in the

subconscious control of posture, balance and muscular co-ordination.

THE LIMBIC SYSTEM Is a connected system of regions of grey matter in the cerebral

hemispheres and diencephalons and includes the hypothalamus. It is

involved in arousal, memory, smell, emotional responses and pain. It is our

emotional brain and helps generate motor patterns for facial expression.

BLOOD SUPPLY OF THE BRAIN. The brain receives blood from the 2 common carotids (right and left) and 2

vertebral arteries (right and left) that enter the head from the neck. The

common carotids split into 2: the external carotids which supply the skin

and muscles of the head and the internal carotids which enter the skull and

supply the brain. The vertebral arteries join together and enter the skull

where it joins the internal carotids the cerebral arterial circle (circle of Willis).

This is an arterial circle at the base of the midbrain – while it is anatomically

circular, blood cannot flow from the one side to the other because the

joining arteries are too narrow in most people. Three arteries come off this

circle on each side: the anterior, middle and posterior cerebral arteries.

Each brain area has a functionally unique blood supply: it is dependant on

one artery for its needs and has poor connections to other arteries. This

means if the artery became blocked the area will go ischaemic (injury due

to hypoxia) and then finally infarct (die due to hypoxia) if the blood supply

doesn't return.

This occurs in cerebrovascular accidents or strokes. As each brain area has

its function, death to that area disturbs/stops that function i.e. damage to

the speech area will stop accurate speech. The middle cerebral is the one

most often affected in strokes.


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BLOOD SUPPLY OF THE BRAIN Continued…… The capillaries in the brain have tightly interlocking cells making up their

walls. This prevents any water soluble substances apart from simple nutrients

and salts and other very small chemicals from entering brain tissue. This is

the blood-brain barrier. This makes it difficult to get many drugs into the

brain. Lipid soluble substances pass with ease – this includes blood gases

(oxygen, carbon dioxide), lipid-soluble drugs (including alcohol!) and

anaeasthetic agents (including alcohol!!).

The brain is very dependent on constant supplies of oxygen and glucose. In

starvation it can use ketone bodies (from fat breakdown) but this

adaptation takes days to occur. The brain can take 10 to 20 seconds of

hypoxia before you lose consciousness due to impaired brain functioning.

THE SPINAL CORD Before reading any further, take a quick look at a diagram of the spinal

cord and it’s major tracts in Thibodeau & Patton.

The spinal cord is the part of the central nervous system which runs inside

the vertebral canal from the medulla in the brain stem down to the level of

spinal vertebrae L1 or L2 (L3 in children). Below this, spinal nerve roots fan

out in the vertebral canal forming the cauda equina. The cord is about 2.5

cm in diameter and is covered in the 3 layers of the meninges (we will look

at these in more detail later). Cerebral spinal fluid circulates through the

meninges and through the central canal within the cord.

The cord is divided into 31 segments, each segment giving off a pair of

spinal nerves that come out between the spinal vertebrae through spaces

called the intervertebral foramina. It is these spinal nerves, along with

cranial nerves, that form the PNS. Apart from the cervical spinal nerves, the

spinal nerves are named after the vertebra directly above the exit point of

the nerve. The cervical nerves are named after the vertebra below their

exit points as the first cervical nerve leaves between the skull and first

cerv ical vertebra. There are 8 cervical nerves (C1-C8), 12 thoracic (T1-T12),

5 lumbar nerves (L1-L5), 5 sacral nerves (S1-S5) and 1 coccygeal nerve (C0).

The cord acts as conduit to take sensory nerves and sensory nerve impulses

from peripheral nerves to the brain and to take motor nerves and motor

nerve impulses from the brain to the peripheral nerves.

The cord has a ‘butterfly-shaped’ area of grey matter (largely nerve cell bodies –nerve cell bodies means information processing occurs) in the

centre which is surrounded by white matter (largely nerve fibres going up or

down the cord – remember, the white colour comes from myelin sheaths).

The nerves ascending and descending between cord and brain are

grouped into sensory or motor nerves with similar origins and destinations.

These groups are called tracts. The white matter is divided into posterior,

lateral and ventral columns through which these tracts run.

The grey matter has 3 ‘horns’ which are linked to 3 types of peripheral nerves.

1. Dorsal (or Posterior or Sensory) Horns. Sensory nerves receive stimuli at their nerve

endings and carry the impulses that result to the cord. They enter v ia the dorsal or posterior nerve root. Their cell bodies are also found here in the

dorsal (or spinal) ganglia. The dorsal root takes the sensory nerves into the


37

dorsal horn where they may then go straight up the cord to the brain or they

may synapse (interface) with a second sensory nerve which takes the

impulses up to the brain. Both these incoming nerves usually give off small

branches that synapse with accessory nerves or interneurons which connect

to motor nerves. These are very important, as we shall see, in spinal reflex arcs.

Some sensory nerves have branches that synapse directly with outgoing

motor nerves – it is these that give us tendon reflexes, the classic one being

the patella tendon reflex that is elicited by tapping below the kneecap.

The sensory input from the skin goes to the cord in a very orderly fashion so

that all the skin from one area will give sensory information that enter one

spinal nerve. Thus the skin from the little finger and inner side of the arm travels

via the ulnar nerve and the C8 spinal nerve. A skin area linked to a particular

spinal nerve is called a dermatome. This information can be used to identify

nerve injury – if an area of skin is numb that corresponds to a dermatome, it is

possible that there is damage along the course of the spinal nerve.

2. Ventral (or Anterior or Motor) Horns. Here motor nerves leave the cord via

the ventral or anterior nerve roots to innervate the skeletal muscle that they

will help stimulate to contract. The cell bodies of these lower motor neurons

(LMNs) are found in the ventral horn where they synapse with upper motor

neurons (UMNs). These UMNs come from the motor areas of the brain and

descend to synapse with LMNs. In this way the brain controls the contraction

of skeletal muscles.

The accessory nerves or spinal interneurons also synapse with the LMNs to form

the reflex arcs.

The dorsal and ventral roots combine after leaving the cord to form the spinal

nerves. Some of these spinal nerves form plexi in which several spinal nerves

combine and form nerves that contain nerve fibres from a number of spinal

nerve roots. E.g. the ulnar nerve passing behind the elbow contains nerve

fibres from C8 &T1 – in this case, this neuronal interchange occurs in the

brachial plexus found in the armpit. There are 4 main plexi that control the

main regions of the body.

3. Lateral Horns. These are found in the thoracic and lumbar cord and contain

autonomic nerve cell bodies for the sympathetic nervous system (SNS) whose

nerves leave v ia the ventral horns. On leaving the ventral root these SNS

nerves leave the spinal nerves and form a chain of what are called ‘ganglia’ running along both sides of the cord. This ‘sympathetic chain’ of ganglia then sends its nerves back into the spinal nerves or produces its own autonomic nerves. Via both of these routes the SNS innervates its target organs.

Be aware that the right side of the body receives motor control from and gives

sensory inputs to the left side of the brain and v ice versa. This means that all nerves

passing between the brain and peripheral nervous system must cross from one side

of the central nervous system to the other.

There are also 12 cranial nerves that connect directly to the brain and brain stem

exerting control over many of the senses and also the ANS. Most of the sensory and

motor functions of the face and much of the head and neck are controlled by these

nerves. The tenth cranial nerve, the vagus nerve (the name means wanderer, like

vagrant) is the main PSNS nerve of the upper body, controlling the organs of the

chest and upper abdomen. Get heartburn – thank your vagus. See Thibodeau &

Patton, for a table showing the structure and function of the cranial nerves.


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COVERINGS OF THE BRAIN & SPINAL CORD

The v ital and delicate nature of the CNS requires it to be heavily protected, and this

is achieved firstly by the outer bony coverings of the cranial bones of the skull and

the spinal vertebrae, which not only form a stable structure on which muscles are

attached to and organs are suspended from, but also provides a canal where the

spinal cord runs safely through.

Secondly, the brain and the spinal cord are both covered with three layers of

membranous tissue called the meninges. The tough, outer layer is called the dura

mater (dura = hard substance – think durable) and is a strong, white fibrous tissue

that not only is the outer layer of the meninges but also acts as the inner layer of

tissue that lines the bony cavities of the skull and vertebral canal, and for this reason

it is also called periosteum - periosteum is the membrane that covers bones. The

second layer is called the arachnoid membrane (= spidery membrane, like

arachnids) and is a delicate, cobwebby layer that separates the outer and inner

meninges. Finally we have the pia mater (= clear substance), an almost transparent

membrane rich with blood vessels and is stuck tightly to the brain and spinal cord.

The spinal cord actually ends in the spinal canal at about the second lumbar

vertebrae. This spinal cord does not grow as fast as the spine that surrounds it, so the

spinal nerves leaving the lower segments of the cord must travel down the vertebral

canal to get out of their appropriate spinal vertebral exit. This is clinically significant as

it allows lumbar punctures to be performed (in which a needle is introduced into the

meninges to drain cerebro-spinal fluid for testing) with relative safety as it can be

done below the level at which the cord terminates. Injury to the high lumbar or lower

thoracic spine may damage the lower end of the cord which contains the reflex

centres controlling urination and defecation.

The meninges are very important structures as they have several spaces within the

membranes that contain fluid and act as shock absorbers. The first of these spaces is

the epidural space (= on the dura) that lies between the tough, fibrous dura mater

and the dense bony coverings of the skull and vertebral canal. This space is mainly

filled with fat and other connective tissues and its main role is to insulate and cushion

the delicate CNS.

Next we have the subdural space (= below the dura) that is located between the

dura mater and the arachnoid membrane. Here we find a very ‘oily’ lubricating fluid, the serous fluid. This prov ides lubrication, allowing free movement of the cord and to

some degree the brain.

The final layer is called the subarachnoid space (= below the arachnoid) and is

found between the arachnoid membrane and the pia mater. This space is filled with

the very important cerebrospinal fluid (CSF) that allows the brain and cord to almost

float in this space, helping reduce the effective weight so that it is not crushed under

its own weight.

The cerebrospinal fluid also provides an additional route to circulate messenger

molecules, nutrients and gases and pick up waste products. CSF is continually

produced in the hollow chambers of the brain (the ventricles), it then circulates

around the brain and spinal cord through the meninges before finally draining into

cerebral veins. Walking is considered by some therapists to create the cranial-sacral

pump which aids the circulation of the cerebrospinal fluid: any impairment to the

production of the fluid, the consistency of it or its ability to circulate will have some

effect upon the CNS and thus the system as a whole.


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THE AUTONOMIC NERVOUS SYSTEM The Autonomic Nervous System’s (ANS) main function is to control the

cardiac muscle, smooth muscles of blood vessels and the organs and glands.

It achieves this by having two efferent div isions: the sympathetic division,

which generates the immediate alarm reaction in response to stress, basically

stimulates or causes excitement (e.g. increases the heart rate) and the

parasympathetic division, which has the opposite effect of inhibiting or

relaxing (e.g. slowing the heart rate).

The ANS, structurally, is different to the CNS as it uses two nerves to

communicate with the desired organs whereas the CNS uses one.

The immediate alarm reaction (fight or flight response) is governed by the

sympathetic nervous system (SNS) and the adrenal medulla of the adrenal

glands that sit on top of the kidneys. I t gives immediate mobilisation of

resources required for immediate physical activity. This reaction starts and

finishes quickly.

Origin of SNS: From special cells found in the spinal cord, these special cells

are the lateral horn cells of the spinal cord in segments T1 to L2 or L3 as

mentioned above. These form the sympathetic chains running each side of

the spinal cord. These chains are the main meeting point of neurons from the

spinal cord synapsing with the neurons that will go to the organ. Technically,

these collections of nerve tissue are called ganglia and the neurons entering

the ganglia are called pre-ganglionic neurons, those leaving the ganglia are

called post-ganglionic cells. This is worth knowing only because certain drugs

(nicotine being a good example) act on the post-ganglionic cells in the

ganglion. So, from the sympathetic chain arise the nerves that go to the

various tissues controlled by the SNS. The nerves of the sympathetic nervous

system that go to the abdomen all come together again to form the coeliac

plexus and mesenteric plexus. These plexi are crossing points for the nerves

before they go to their final destination. The coeliac plexus is popularly called

the solar plexus because it looks vaguely like a disc with nerves forming rays.

The sympathetic nervous system has a special gland associated with it: the adrenal gland. This releases the hormone adrenalin. This hormone is the same

as the neurotransmitter of the sympathetic nervous system and causes global

activation of the sympathetic nervous system. Noradrenalin is the other main

neurotransmitter of sympathetic nervous system. (Dopamine is yet another

one and all these types of neurotransmitter are called adrenergic

neurotransmitters: they are all arousing in their effects on the brain. They are

also called catacholamines). In America these hormones are known as

epinephrine and nor-epinephrine

The alarm reaction is moderated by the activity of the parasympathetic

nervous system (PSNS) – this promotes resting, digesting and restoration of

energy reserves.

Origins of the PSNS: From the brain stem and lower end of the spinal cord

(specifically from the lateral horn cells of S2-4 and the brain stem with nerves

leaving with various cranial nerves). Of the cranial nerves, the Vagus nerve

supplies the throat, thorax and abdomen.

The neurotransmitter of the parasympathetic nervous system is acetylcholine.

This is derived from the nutrient choline.


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STUDY EXERCISE

Complete this table – tick the correct column in the first 4 rows, then use the correct word

from the pair of words offered in each column. For some there is no parasympathetic

response. See what makes sense to you if you consider that the SNS is trying to get you to

survive a crisis situation… answers at the end.

ACTIVITY SYMPATHETIC

NERVOUS SYSTEM

PARASYMPATHETIC

NERVOUS SYSTEM

Fight, Fright, Flight (tick whether SNS or PSNS) [tick]

Vegetative, digesting, assimilating, recuperative (tick

whether SNS or PSNS) [tick]

Arousing, Breakdown of energy reserves (tick whether

SNS or PSNS) [tick]

Calming, rebuilding energy reserves (tick whether SNS

or PSNS) [tick]

Effect on heart rate (increase or decrease)

Effect on force of contraction of heart (increase or

decrease)

Effect on ventilation rate (increase or decrease)

Effect on airway diameter (dilate or constrict)

Effect on blood sugar level (increase or decrease)

Effect on digestive activity (increase or decrease)

Effect on saliva production (increase or decrease)

Effect on blood flow to muscles (increase or decrease)

Effect on tension (muscle tone) of muscles (increase or

decrease)

Effect on blood flow to skin (increase or decrease)

Effect on sweating (increase or decrease)

Effect on adrenalin release from adrenal gland

(increase or decrease)

Effect on urine production (increase or decrease)

Effect on urge to pass urine (increase or decrease)

Effect on pupil diameter (dilate or constrict)

Effect on decision making (more reactive or less

reactive)

Effect on arousal level (increase or decrease)


41

Now, both the sympathetic and parasympathetic nervous divisions are constantly

and simultaneously sending information to excite and relax. Whichever is the

predominating signal has the affect of either stimulating or suppressing. The balance

is called the autonomic tone. If the information received by the CNS indicates a

need for stimulation, then the signal sent via the sympathetic division becomes

stronger, or more correctly, more frequent, and v ice versa. This means that the rate

of the impulses being transmitted to the viscera is constantly adapting in response to

the sensory information being supplied to the CNS.

SUMMARY

By way of a summary, we can consider the generation of a simple reflex – an

automatic nervous response

THE 3 MAIN FUNCTIONAL NERVE TYPES INVOLVED IN A REFLEX: Some nerves have special receptors on the end of an axon that detect changes in

the environment: these are sensory nerves. The different receptors can be seen as

specialised sockets that fit on the end of the standard nerve fibre. This means all

sensory information is transmitted as nerve impulses and the only way the brain is

able to know what sensory information is arriving down a particular sensory nerve is

through ‘knowing’ what type of sensory receptor is on the end of it and where it

comes from.

Some nerves can make a muscle cell contract or gland secrete: these are called

motor nerves.

Other nerves connect these two, allowing anything from a simple connection to

complex analysis and interpretation. These are association, connector nerves or

inter-neurones. These nerves are found in the CNS and many association nerves may

be connected together, especially in the cerebral cortex. (Sensory nerves and motor

nerves run in the peripheral and central nervous systems).

A way of looking at this is as follows:

SENSORY

ORGANS

———>

SENSORY

INPUT

ANALYSIS AND

INTEGRATION IN

CNS

—————>

MOTOR OUTPUT

MUSCLES OR

GLANDS


42

REFLEX ARC. In the simplest form, these cells can be connected to form a reflex arc. A reflex is a

fast response to a change in the internal or external environment that attempts to

restore homeostasis. Reflexes are usually out of our conscious control but may be

consciously initiated or inhibited. A simple reflex arc involves:

1. A sensory receptor that is stimulated.

2. A sensory neurone that carries the sensory information. (Also called the

afferent nerve.)

3. An association neurone in central nervous system (CNS: the brain and

spinal cord) that connects the sensory nerve to the appropriate motor

nerve.

4. A motor neurone to carry the impulse to the muscle or gland. (Also

called efferent nerve.)

5. The effector: the muscle or gland that produces the balancing

(homeostatic) effects.

CONCLUSION

In this lecture we have taken a broad look at the subdivisions of the nervous systems

and how they inter-relate with each other for the sole purpose of maintaining

homeostasis.

For all the complexity, the brain is part of the holistic web of inter-relationships that

emerge from the study of the body, it helps us react immediately. In the next lecture

we will consider the endocrine system and discover that this is far more adaptive in

its controls, and in some ways, it has a more profound control of the body than the

nervous system, although both are intrinsically linked in the overall communication,

control and integration of the entire body.

PATHOPHYSIOLOGY

It is important to familiarise yourself with common pathological processes. The

following websites offer helpful resources for the study of clinical conditions and their

treatments.

NHS Direct: http://www.nhsdirect.nhs.uk – click on the encyclopaedia. UK based

health information and most conditions.

Medline Plus: http://medlineplus.gov - click on the encyclopaedia. American so

there are differences in some names of diseases and drugs.

British National Formulary: http://www.bnf.org – free registration. Explore the Central

Nervous System drug section. Excellent for finding out what drugs are and what

official prescribing practice is.

The following common conditions should be investigated by using these sites:

Cerebrovascular accidents (stroke, cerebral haemorrhage)

Convulsions and epilepsy

Facial pain and facial weakness (trigeminal neuralgia, shingles, Bell’s palsy) Headache: cluster, migraine, tension

Chronic degenerative disease: Parkinson’s disease, Multiple Sclerosis, Motor Neurone Disease

Brain tumours

Neuropathy

Dementia, Alzheimer’s disease

Depression


43

STUDY EXERCISE ANSWERS

ACTIVITY SYMPATHETIC

NERVOUS SYSTEM

PARASYMPATHETIC

NERVOUS SYSTEM

Fight, Fright, Flight (tick whether SNS or PSNS) [tick]

Vegetative, digesting, assimilating, recuperative (tick

whether SNS or PSNS) [tick]

Arousing, Breakdown of energy reserves (tick whether

SNS or PSNS) [tick]

Calming, rebuilding energy reserves (tick whether SNS

or PSNS) [tick]

Effect on heart rate (increase or decrease) increase decrease

Effect on force of contraction of heart (increase or

decrease) increase

Effect on ventilation rate (increase or decrease) increase decrease

Effect on airway diameter (dilate or constrict) increase decrease

Effect on blood sugar level (increase or decrease) increase

Effect on digestive activity (increase or decrease) decrease increase

Effect on saliva production (increase or decrease) decrease increase

Effect on blood flow to muscles (increase or decrease) increase

Effect on tension (muscle tone) of muscles (increase or

decrease) increase

Effect on blood flow to skin (increase or decrease) decrease

Effect on sweating (increase or decrease) increase

Effect on adrenalin release from adrenal gland

(increase or decrease) increase

Effect on urine production (increase or decrease) decrease increase

Effect on urge to pass urine (increase or decrease) increase decrease

Effect on pupil diameter (dilate or constrict) dilate constrict

Effect on decision making (more reactive or less

reactive) more reactive

less reactive

Effect on arousal level (increase or decrease) increase decrease

anatomy & physiology series · the special senses introduction in this instalment of your...

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