peripheral nervous system messages via spinal and...
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Biol 067: Section 13 - Nervous System
A. Overview of the nervous system:
1. 2 parts of the Nervous System:
2. How CNS and PNS are divided and interconnected:
Central nervous system
Sensory nerves -
carry sensory info to brain and spinal cord
somatic sensory nerves - FROM skin, muslces, joints, special
senses
visceral sensory nerves - FROM
body organs
Motor nerves -
from CNS to effectors
Somatic motor nerves -
(controls movement)
TO skin, skeletal muscles, tendons
-voluntary
Autonomic motor nerves -(controls body
function)
To smooth muscle, cardiac muscle, organs,
glands -involuntary
Parasympathetic division
"rest and digest"
"normal state"
Sympathetic division
"Fight or flight"
Brain Spinal
cord
Peripheral Nervous system – messages via spinal and cranial nerves
Nervous system
Central Nervous
System (CNS)
Peripheral Nervous
System (PNS)
B. Neurons and how they work:
Nervous tissue: made up of 2 types of cells
1) Neurons – transmit nerve impulse
2) Neuroglia cells – support and service neurons
1. Types of neurons: classified by function
i. Sensory neuron -takes info from sensory receptor (detects changes in the environment)
to CNS
ii. Motor neuron – takes info away from CNS to an effector (i.e. muscle fiber, gland, etc)
iii. Interneuron – convey info between neurons – sum up info before passing it along to
motor neurons inside CNS
2. Structure:
1. dendrites – receive signals from sensory receptors or other neurons and send signals
towards cell body from axon
2. cell body – contains nucleus and organelles
3. axon – conducts nerve impuls along its length
3. Myelin sheaths
Cover long axons
Protective layer – contains myelin (a lipid substance) that insulates axon
Has gaps – nodes of Ranvier – used in neuron transmission
In Peripheral Nervous system (PNS) – act as insulator – sheath formed by Schwann cells
(a type of neuroglia)
In CNS – different kind of neuroglia but still contains myelin
Myelin sheath in long axons not short
In Central nervous system (CNS) – white matter –white because of myelin/ grey matter
– no myelin
MS (multiple sclerosis) and leukodystrophies (degeneration of white matter in brain)
caused by loss of myelin –( generally caused by defect in genes involved in growth and
maintenance of myelin, and/or environmental – not really understood)
C. Nerve impulse
Is the way a neuron transmits info
2 states: 1. resting potential
2. action potential
Can be measured with an oscilloscope by measuring voltage difference between inside
and outside of axon.
1. Resting potential: =potential energy of neuron
Is when an Axon is not conducting an impulse
-65 to -70millivolts mV inside of membrane is negative compared to outside of axon
Charge difference is related to ion concentration difference across axon membrane.
There is relatively more large negatively charged ions on the inside of the nerve cell
during resting potential than outside the nerve cell.
Even though it is called ‘resting’ the nerve cell is busy keeping the potassium at a
relatively even concentration –it drifts out of potassium channel but then gets attracted
back in by the negative ions on the inside – and the Na+ ions are being pumped to the
outside through the Na+potassium pump.
Therefore inside more negative than outside of nerve cell at resting potential (due to
large negative ions that stay inside the cell)
***Unequal distribution due to sodium – potassium pump = membrane protein that actively transports Na+ out and K+ into axon thru separate channels
Conctrn of
K+ inside
higher then
outside –
larger
negative
ions keep
inside
relatively
more neg.
then
outside
Conctrn
of Na+
greater
on
outside
2. Action Potential (pg 345 of text shows graphic)
When axon is conducting an impulse
Rapid change in polarity or charge across membrane
Nerve impulse consists of electrochemical change across membrane
a) Due to message from chemoreceptor, gates of sodium channels open first and Na+
moves into axon
Membrane potential changes from -65 mV to +40mV = depolorization
Change inside of axon from – to +
This is because now the inside is more positive due to all the positive Na ions coming in.
Almost immediately after depolarization, the Na channels close and the K+ channels open…
However, before action potential can be reached, must depolarize enough to cross
threshold of ~-40 mV – once crossed it will continue all the way through action
potential - If it doesn’t reach threshold impulse will not go anywhere.
b) Positive repel positive – so the K+ ions now get pushed out the open gates of potassium
channel, K+ flows to outside of axon
=40mV -65 mV = repolorization
Change in action potential
Inside of axon resumes negative charge as positive K+ ions exit – now the inside is
relatively negative again due to the relative number of the larger negative ions
relative to the remaining positive ions (which is the Na+ this time)
After the impulse has passed the nerve cell gets busy returning to resting potential
because the Na K pump moves K+ back to inside and Na+ to outside - then its ready
for another stimulus
E. Propogation of action potential
As it travels down axon = successive depolarization and repolarization of axon occurs
Refractory period, Na+ gates unable to open as soon as repolarization occurs (while
Na+/K+ pump is putting it back to original state)
Ensures action potential moves forward (not backward) and towards its axon branches
Ion exchange occurs at Nodes of Ranvier in myelinated sheaths
Causes action potential to travel faster = saltatory conduction
Impulse jumps from node to node
F. Transmission across a synapse:
Axon – axon branch – axon bulb (axon terminal) on end
Axon bulb lies close to dendrite of cell body of next neuron
Space=synaptic cleft
Region of close proximity = synapse
action potential
Transmission of action potential across synapse accomplished by neurotransmitters
which are molecules stored in synaptic vesicles
Steps in transmission
1) Nerve impulse travels along axon to axon terminal
2) As nervous impulse reaches bulbs, gated channels open and Ca2+ enters bulb – as
concentration of Ca2+ increases, causes synaptic vesicles to merge with pre-synaptic
membrane (NB diagram)
3) Neurotransmitters released into cleft, diffuse across and bind with receptor proteins on
post synaptic membrane -
4) Cause post synaptic neuron excitation (causes Na+ to diffuse into post synaptic neuron)
or inhibition (causes K+ to diffuse out of post synaptic neuron) depending on
neurotransmitter – once response is initiated, neurotransmitter removed from cleft by
a) Post synaptic membrane enzyme that inactivates the NT or/
b) Pre synaptic membrane reabsorbs the NT possibly for recycling
This is required so constant stimulation or inhibition of post synaptic membrane doesn’t
occur
F. Synaptic integration
1000 – 10,000 synapses/neuron is common therefore must have a method to integrate signal =
summing up all the signals received
If neuron receives more excitory signals has depolarizing effect – axon will transmit a nerve
impulse after reaching threshold signal – either by 1 axon sending a rapid # of signals or many
signals from different neurons
Or if receives more inhibitory signals – has a hyperpolarizing effect – can stop axon from firing
takes it further from an action potential
G. Divisions of the Nervous System
1. Central Nervous System:
Definition: lies in midline of body =spinal cord and brain - Sensory info received,
voluntary motor impulses initiated here
Both protected by bone – vertebrae (spinal cord), skull (brain)
Both wrapped in protective membrane known as meninges
Space filled with cerebrospinal fluid to cushion and protect CNS parts
0
+2
2
4
7
8
Time (milli seconds)
threshold
resting
potential
excitatory signal
inhibitory signal
integration
a) Spinal Cord:
1) Structure of spinal cord – fig 13.7
Located at base of brain and into vertebral canal
Vertebra joined so spinal cord passes thru middle
Spinal nerves pass thru lateral openings between vertebra
Spinal cord contains:
Central canal -contains cerebrospinal fluid
Grey matter – central H shaped, contains cell bodies and short non-myelinated
fibers – contains parts of sensory and motor neurons and interneurons
White matter – around grey matter, contains myelinated axons in bundles called
tracts
Tracts
- Ascending – take info to brain, located dorsally
- Descending – take info from brain, located ventrally
- Both cross just after they enter and exit brain therefore left side –
controls rt side and vice versa
2) Function of spinal cord
Provides a means of communication between brain and peripheral nerves that
leave the cord.
Reflex arc centre (talked about later)
Integration of incoming info from many sensory neurons before motor impulse
sent out.
b) Brain Structure and Function
Handout– be able to id main parts of brain
Ventricles: brain has 4 ventricles = interconnecting cavities that produce and act as a
reservoir for cerebrospinal fluid.(2x lateral, third, fourth,)
Main parts of brain:
1) Cerebrum
2) diencephalon
3) Cerebellum
4) Brain stem
1) Cerebrum
Largest part of the brain in humans
Left and rt cerebral hemispheres
Highest centre to receive info- commands voluntary responses
Higher thought processes – learning, memory, speech
Cerebral cortex (grey matter of cerebrum)
Thin convoluted outer layer of grey matter
Sensation, voluntary movement, consciousness
Cerebral hemispheres (handout – be able to id main parts)
Cerebrum is divided into left and right hemispheres by longitudinal fissure
Each hemisphere divided into lobes by sulci (grooves)
i. Parietal lobe – back, top
ii. Temporal lobe – lies below frontal and parietal lobe – (sides by ears)
iii. Occipital lobe – very back
iv. Frontal lobe – front
i. Parietal lobe – back
a) Primary somatosensory area
Sensory info from skin and skeletal muscles
b) Primary taste area – taste sensations
c) Somatosensory association area – integrates sensory info from skin and muscles
ii. Temporal lobe – lies below frontal and parietal lobe
Primary auditory area – receives info from ears
Auditory association area – integrates sensory info
Speech area – called Wernicke’s area – helps us understand written and spoken
word and send to Broca’s area
iii. Occipital lobe – dorsal to parietal lobe
Primary visual area - receives info from eyes
And association area –associates new and previously received visual info
iv. frontal lobe: front of cerebral cortex
a) pre motor area – organizes motor functions for skilled motor activities/ sends to
primary motor area
b) primary motor area - voluntary muscle commands begin here, sends to cerebellum
c) Broca’s area – motor speech area – sends to 1o motor area
d) Prefrontal area – reasoning and plan actions, receives info from other association
areas
(Association area = any part of the cerebral cortex involved in the integration of
information – and where memory is stored)
White matter of cerebrum
Rest of cerebrum composed of white matter
Consists of long myelinated fibers organized into tracts
Ascending from lower brain centres , sends info to primary somatosensory area
Descending communicates with lower brain centres
Tracts inside cerebrum take info between different sensory, motor, and other
association areas
Corpus callosum – contains tracts that join 2 cerebral hemispheres – like a bridge
between 2 hemispheres
2) Diencephalon
Region that encircles 3rd ventricle
Contains thalamus and hypothalamus and pineal gland
Thalamus = integrates sensory info (except smell) and relays to cerebrum – involved in memory
and emotion
Hypothalamus = homeostasis - integration centre for autonomic system – regulates hunger,
sleep, thirst, temperature, water balance and controls pituitary gland (serves as link btwn
nervous system and endocrine system)
Pineal gland = secretes hormone – (not sure of its role) – gland pokes out from under
hypothalamus
3) Cerebellum
Posterior to brainstem, separated by 4th ventricle
Has 2 portions surface = grey matter inside = white matter
Integrates sensory and motor information
i.e. posture, balance, skeletal muscles and coordination
receives info from sensory (eyes, ears, joints etc) as to where body is presently
positioned then receives motor output from cerebral cortex about where it should be
then sends message to skeletal muscles to correct
learning new motor skills
4) Brain Stem Contains:
a) Midbrain – also has reflex centre for visual, auditory and tactile senses
-Relay stn for tracts going to and from spinal cord and cerebrum or spinal cord and
cerebellum
b) Pons (bridge)
-This contains bundles of axons travelling between cerebellum and CNS
-Helps with breathing rate and head movement
c) Medulla oblongata – lies between spinal cord and pons
-Contains vital centres which regulate heart beat, breathing, blood pressure
-Contains reflex centres for vomiting, coughing, sneezing hiccupping and swallowing etc.
Reticular formation
Network of nuclei which are masses of grey matter and fibers which extend length of
brainstem
Receives sensory signals – pass up to higher centres
Receives motor signals – passes down to spinal cord
2. Peripheral Nervous System
a) Location and Structure:
PNS lies outside the CNS (brain and spinal cord)
Contains nerves (bundles of axons)
Nerve=bundle of nerve fibers (axons)/Single nerve fiber=axon
2 types of nerves: cranial - arise from the brain/ spinal arise from spinal cord
Sensory fibers/nerves– send info to CNS
Motor nerves – info from CNS
Cell bodies are in CNS or in ganglia (ganglia=collection of cell bodies in PNS
b) Types of Nerves:
1) Cranial nerves – 12 pairs, attached to brain
Some sensory,
Some motor (don’t need to know function of each one)
some mixed – contain both sensory and motor fibers
Most cranial nerves inervate head, neck, and face - but vagus nerve (which
begins in medulla oblongata) goes to most organs as well as pharynx and larynx
2) Spinal nerves
31 paired nerves
Emerge from spinal cord by 2 roots or branches
Both roots join to form spinal nerve that leaves CNS – becomes a mixed nerve = both
sensory and motor bundled together in one nerve.
Conduct impulse away from cord to effectors
c) Divisions of the PNS:
The peripheral nervous system has 2 divisions:
1) Somatic nervous system
2) Autonomic nervous system
1) Somatic Nervous system
Nerves that take sensory impulses to CNS from sensory receptors and motor commands
away.
Serve the skeletal muscles, skin, and tendons
Voluntary response = brain
Involuntary response = reflex = spinal cord or brain
Reflex Arc – short circuit response thru sensoryinterneuronsmotor nerves in spinal
cord – integrate info from sensory neurons and relay signals to motor neurons – allows
response rapidly to sensory stimulus
Some info also reaches brain (say ‘Ow” later)
2) Autonomic nervous system
Fig 13.17? good review of sympathetic and parasympathetic structure and function – see
contrary effects – give example
Regulates cardiac and smooth muscle, organs, and glands
Has 2 divisions: Sympathetic and parasympathetic
Both:
Act automatically and are usually involuntary
Innervate all internal organ
They use 2 neurons and 1 ganglion for each impulse
1st neuron – cell body in CNS and preganglionic fiber that enters the ganglion
2nd neuron – cell body in ganglion and postganglionic fiber that leaves the ganglion
Reflex actions of the autonomic system regulate things such as blood pressure and
breathing rate and are important to maintain homeostasis
Autonomic motor nerves vs somatic motor nerves
1. Sympathetic division – fight or flight
Used in flight or fight situations
Preganglionic fibers from middle of spinal cord (thoracic-lumbar region)
Pre-ganglionic fiber is short, terminate in ganglion close to spinal cord
Postganglionic fiber is long (contacts organ)
Causes increased heartbeat, increased ventilation, decreased digestion
Usually neurotransmitter is Norepinephrine (acts like adrenaline)
2. Parasympathetic division – rest and digest
Includes some cranial nerves (i.e., vagus nerve etc.) and sacral nerves (arise from the
sacral {bottom} part of the spinal cord)
therefore this division may be referred to as craniosacral portion of the autonomic
nervous system
preganglionic fiber is long
ganglia lies near or in organ
postganglionic fiber is short
this division promotes normal steady state – relaxed
promotes digestion, decreased heartbeat
uses neurotransmitter called acetylcholine
Summary of somatic and autonomic motor nerves:
Autonomic motor pathways
Somatic motor pathway
Sympathetic motor Parasympathetic motor
Control Vol/invol invol involuntary # of neurons/message
1 2 (preganglionic shorter than post)
2 (preganglionic longer than post)
Loctn of motor fiber Most cranial nerves and all spinal nerves
Thoracic-lumbar spinal nerves
Cranial and sacral spinal nerves
Neurotransmitter acetylcholine Norepinephrine (NE) Acetylcholine (Ach) effectors Skeletal muscle,
skin, tendons Smooth & cardiac muscle, glands, & organs
Smooth & cardiac muscle, glands, & organs
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