The Nervous SystemChapter 14.1-14.2

Pages 446-454

your external senses such as sight, hearing, taste, smell and touch send signals about your outside world to your brain.your internal senses monitor:

blood pH, blood pressure blood volume, osmolarity

these sensors send signals about your internal environment to your brain to help you maintain homeostasis.

Category and type of Category and type of ReceptorReceptor

Examples of ReceptorExamples of Receptor StimulusStimulus

PhotoreceptorsPhotoreceptors

Vision rods and cones in the eye visible light

Chemoreceptors

taste taste buds on the tongue food particles in saliva

smell olfactory receptors in the nose odour molecules

internal sense receptors in the carotid artery and aorta

blood pH

MechanoreceptorsMechanoreceptors

touch/pressure/pain receptors in the skin mechanical pressure

hearing hair cells in the inner ear sound waves

balance hair cells in the inner ear fluid movement

body position proprioceptors in the muscles and tendons, and the joints

muscle contraction, stretching, and movement

ThermoreceptorsThermoreceptors

Temperature heat and cold receptors in the skin

change radiant energy

sensation occurs when the neural impulses arrive at the cerebral cortex (brain).

different stimuli will be picked up and trigger a neural impulse that is sent to the brain for interpretation.

perception is the interpretation of sensory information by the cerebral cortex.

we are constantly over stimulated by the environment we live in

our receptors become accustomed to the stimulus and adapt. sensory adaptation occurs once you have

adjusted to a change in the environment

sensory receptors become less sensitive when stimulated repeatedly.

sensory neurons supply the central nervous system with information about the external and internal environment.there are many different types of sensors found in the body.

sometimes many different types of receptors work at the same time in one place. your skin has pressure, and

temperature receptors.

THE EYE

Main parts of the vertebrate eye: The sclera: white outer layer, including cornea, protective

layer The cornea: the window to the eye, the front of the sclera

that bulges out The iris: regulates the size of the pupil, the coloured part of

the external eye The lens: focuses light on the retina, found right behind the

iris, the ciliary muscle change the shape. The ciliary muscle: attched to ligaments and

alter the shape of the lens

Main parts of the vertebrate eye: The choroid: pigmented layer, the middle layer The retina: the innermost layer, contains 4 different

layers Pigmented epithelium (prevents light scattering) light-sensitive cells (rods and cones) Bipolar cells (relay message) Cells of the optic nerve (send message to brain)

The optic disk: a blind spot in the retina where the optic nerve attaches to the eye- no rods or cones!

The fovea centralis: most light sensitive area, contains only cones!

the eye is divided into two cavities separated by the lens and ciliary body:

The anterior (front) cavity is filled with watery aqueous humor the ciliary body produces the aqueous

humorThe posterior (back) cavity is filled with

jellylike vitreous humor

Fig. 50-18

Pupil

Aqueoushumor

Lens

Vitreous humorOptic disk(blind spot)

Central artery andvein of the retina

Opticnerve

Fovea centralis (center of visual field)

ChoroidSclera RetinaCiliary body

Suspensoryligament

Cornea

Iris

the innermost layer of the eye is the retina which comprises of four different layers of cells:

- pigmented epithelium

- light-sensitive cells

- bipolar cells

- cell of the optic nerve

• is positioned between the choroid layer and the light sensitive cells

pigmented granules in this layer prevent light that has entered the eye from scattering.

there are two types of light sensitive cells called the rods and the cones.

The rods respond to low-intensity light

The cones that require high intensity light, identify colour.

both rods and cones act as sensory receptors

Evolutionarily however, the dog and the human each developed the visual system that worked best for them. Humans have depended on their diurnal ability and a sense of color throughout time to help them find food. Dogs on the other hand, were not originally diurnal animals, until humans domesticated them. Consequently, the ability to see at night was originally more important to the dog than color. After all, their prey is often camouflaged with the surroundings, so they are unable to rely on color vision cues as heavily as humans do to find food.

The retina of the eye is lined with both rods and cones in humans and dogs. The rods are much more prevalent in both species, but even more so in the dog than the human. The rods are adapted to work best in low light and are used for motion detection. The central retina of the canine eye contains about 20% cones, while humans have an area of 100% cones called the fovea. The cones work best in mid to high levels of light and have the ability to detect color.

Color and Acuity Differencesbetween Dogs and Humans

by Jennifer Davishttp://www.uwsp.edu/psych/dog/LA/davis2.htm

once excited, the nerve message is passed from rods and cones to the bipolar cells. the bipolar cells then relay the message to the cells of the optic nerve.the optic nerve then carries the nerve impulse to the central nervous system.

rods and cones are unevenly distributed on the retina. in the centre of the retina there is a tiny

depression referred to as the fovea centralis. the fovea centralis

is the most sensitive part of the eye. has many cones packed very close together. when you look at an object most of light falls here. is surrounded by rods

(often you can see things in your peripheral vision without being able to identify its colour.)

There are no rods and cones in the area which the optic nerve comes in contact with the retina.

because there is no photosensitive cells we call this area the “blind spot”

http://serendip.brynmawr.edu/bb/blindspot/

there are three types of cones each which absorb different wavelengths of light.

the combination of cones that can detect red, blue and green wavelengths of light allows us to see range of colours. colour blindness is an inherited condition

(occurs more in males) is a deficiency in particular cones, usually red or green

there are about 160 million rods surrounding the colour-sensitive cones in the centre of the eye.

The rods contain a light-sensitive pigment called rhodopsin.

The cones contain a similar pigment but they are less sensitive to light.

Rhodopsin is composed of a form of vitamin A and large protein called opsin

when a single photon of light strikes a rhodopsin molecule it divides into two components Retinene, the pigment portion Opsin, the protein portion.

this division alters the cell membrane of the rods and produces an action potential. neurotransmitters are released from the

end plates of the rods and the nerve message is conducted across the synapse to the bipolar cells and to a neuron of the optic nerve.

a terminal vitamin A deficiency can damage the rods.

Food, Standard Amount Vitamin A(μg RAE)

Calories

Organ meats (liver, giblets), various, cooked, 3 oza 1490-9126 134-235

Carrot juice, ¾ cup 1692   71

Sweetpotato with peel, baked, 1 medium 1096 103

Pumpkin, canned, ½ cup 953 42

Carrots, cooked from fresh, ½ cup 671 27

Spinach, cooked from frozen, ½ cup 573 30

Collards, cooked from frozen, ½ cup 489 31

Kale, cooked from frozen, ½ cup 478 20

Mixed vegetables, canned, ½ cup 474   40

Turnip greens, cooked from frozen, ½ cup 441 24

rhodopsin is extremely sensitive to light. in bright light rhodopsin is broken down faster

than it is restored the opsins used for colour vision are much less

sensitive and operate best with greater light intensity.

only rods are active during periods of limited light intensity, this is why images appear in shades of grey.

(rods are most effective at dusk and dawn)

an example of an after image is the blue or green lines that stay in your vision after a camera flash has gone off.

there are two types of after images, positive and negative ones. a positive afterimage occurs after you look

into a bright light and close your eyes. a negative afterimage occurs when the

eyes are exposed to bright coloured light for long periods of time. cone cells adapt from the over stimulation and lose

sensitivity

As light enters the eye it is bent towards the pupil by the cornea.

as light enters the more dense medium it is refracted (bent).light is bent to a focal point and an inverted image is projected on the light sensitive retina.

Ciliary muscles control the shape of the lens.Suspensory ligaments maintain a constant tension.

when close objects are viewed the ciliary muscles contract and the lens becomes thicker.

the thicker lens provides additional bending of the light for near vision.

When far away objects are viewed, the ciliary muscles relax causing the lens to be thinner.

The adjustment of the lens is known as the accommodation reflex, objects 6 meters away from the viewer need no accommodation.

The importance of the accommodation reflex becomes more pronounced with age. as the years add up so does layers of transparent

protein covering the lens making it harder. by the age of 40 near point accommodation has

reduced so much people usually have problems reading.

a secondary adjustment occurs during the accommodation reflex.

when objects are viewed from a distance, the pupil dilates letting in as much light as possible.

when objects are viewed close up the pupil constricts in an attempt to bring the object into focus.

a constricted pupil makes it so light passes through a small opening and falls on the most sensitive part of the retina, the fovea centralis.

Ex. The Inuit’s Snowblindness Glasses

Glaucoma caused by a buildup of aqueous humour in the

anterior chamber of the eye. tiny ducts usually drain out any excess liquid

that is produced every day. if the ducts get blocked, the fluid builds up

and pressure inside the eye increases. the retinal ganglion cells slowly die from the

increased pressure, which leads to vision loss.

Cataractthe lens becomes opaque and prevents some of the light from passing through.the traditional solution is to remove the lens and fit the patient with strong eye glasses.

Astigmatismfor most people the lens and cornea are symmetrical.

incoming light is refracted along identical angles for both the dorsal (back) and ventral (front) surfaces, this forms a sharp focal point.

in some people, the lens or cornea are irregularly shaped leading to astigmatism.

Test your vision here http://www.perret-optic.ch/optometrie/test_vision_astigmatisme/opto_test_astigmatisme_gb.htm

NearsightednessAlso known as myopia

occurs when the eyeball is too long. The lens cannot flatten enough to project the

image on the retina The distant image is brought into focus in front of the retina.

Someone who is nearsighted is able to focus close objects but has difficulty seeing objects at a distance. Glasses with concave lens can correct

nearsightedness.

Farsightedness Also known as hyperopia

is caused by an eyeball that is too short. distant images are brought into focus behind the

retina, instead of on it. A farsighted person can focus on distant objects, but

has trouble seeing objects that are close up. Can be corrected by glasses that have a convex

lens.