Sensory Function II

Sensory Function II - Lecture 33

Vision:
anatomy
cells
neurons

Vision - very complicated neural processing - this will not be covered in detail here

Receptors sensitive to small portion of light spectrum (electromagnetic energy - photons)

radio waves	microwaves	infrared	visible	ultraviolet	X-rays	gamma rays
wave 10⁴ meters	10^-4	10^-5	10^-6	10^-7	10^-10	10^-11

visible 400-700 nm (blue (short), green, red (long))

Audio - Lecture -

lens and cornea - optical system - focus image on retina

retina - neural tissue containing light sensitive cells

bending of light -- refraction -- physics

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focusing of light from object - gives an inverted image on the retina

shape of cornea and lens determines where object is focused - primarily cornea

ciliary muscles - adjust lens shape

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Pupil - light passes through pupil

-amount of light is controlled by iris but also there are changes in the receptors

receptor adaptation to dark rooms -- more photochemicals are produced -- thus in a dark room - pupil dilates and more photochemicals

Defects -nearsighted or myopic (can not see distant objects) - farsighted or hyperoptic (can not see close)

Control of iris

radial muscle - sympathetics cause contraction which dilates the pupil

circular muscle - parasympathetics cause contraction which constricts pupil

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Audio - Lecture -

Properties of receptor cells

RODS	CONES
100 million/retina	3 million/retina
shades of gray (achromatic)	color vision (chromatic) red (560 nm), green (530), blue (420) there is some overlap thus different colors
high sensitivity (low light levels)	low sensitivity
night vision	day vision
low acuity	high acuity
much convergence in retinal pathways	little convergence in retinal pathways
more numerous in periphery	concentrated in fovea
Rhodopsin	3 type of photochemicals opsin + chromophore retinal (or retinene) retinal is a variation of Vit A

the retinal is the same in all photopigments -- only the opsin varies

No neurons are found directly in front of the fovea centralis (about the size of a pin head)

Retina is pigmented epithelium - no light scattering - cats have white shinning eyes in the dark because the retina is white - reflects light - see 40 x better

4 types of neurons

-bipolar - B - receptor to ganglion
-ganglion - G - out optic nerve (to occipital lobe)
-horizontal - H - connect receptors
-amacrine - A - connect B to G

Audio - Lecture -

Response to light -

1) receptor response: photopigment is bound to membrane layered in outer segment of rods and cones

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Summary - Receptors

A) depolarized in dark, therefore, increase transmitter release - Ach, GABA, etc.

B) hyperpolarized in light, therefore, decrease transmitter release

2) Bipolar cell response - Neurotransmitter from receptor cells can cause either depolarize or hyperpolarize, depending upon bipolar cell

** In most bipolar cells - NT inhibits BP

The bipolar cells:

only produces G.P. (because of very high threshold potential)

- but it has a very short length

in the dark -- increased NT from receptor cells -- inhibits bipolar, therefore, less stimulation of ganglion cells

with rods, bipolar cells may connect several rods to a single ganglion and each rod to several ganglions, therefore, amplified response - more sensitive to low light levels -

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this does not promote vision acuity i.e. ability to determine between 2 points

Cones on the other hand - have a direct line - little cross over, good acuity, bad for low light

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In fovea centralis - only one receptor -- one bipolar cell -- one ganglion cell

3) Ganglion cells - spontaneous activity, therefore, they can either increase or decrease firing, finer control

-AP frequency determines intensity of light

if dark:

rods --- depolar -- increase NT -- inhibits NT from BP --- less firing of ganglion cells

4) Horizontal cells and Amacrine are important in color coding and changes in illumination read text for additional information

the ganglion cells receive input from area called a receptive field - circle -

systems of response : On and Off - ganglion cells (these are different cells)

On : Spot of light--- increase frequency AP on ganglion

Off : light --- decrease freq. AP on ganglion cells

dark --- increase freq. AP on ganglion cells

difference is due to NT released from receptor cell and bipolar response to it

1) On system : NT from receptor cell produces hyperpolarization of bipolar -- the NT from receptor is glutamate

spot of light

light--- decrease NT ---- depolarization of bipolar --- increase AP ganglion

these cells respond to white spot on black background

Color Vision was thought to be determined by the wave length of light reflected by the object and sensed by the cones in the eye. However, if the light illuminating the object changed then the color reflected would change. This does not happen -- the phenomenon is called "color constancy". Actually the cones (RGB) sense different degrees of lightness. This does not change as the illumination changes. The brain then interprets the different degrees of lightness sensed by the 3 cones and then produces a color. Thus, the color actually comes from the brain and not the object. INTERESTING????

This was illustrated by Land who showed that a black and white photo taken through a red lens will produce color when projected through a green lens.

REVIEW QUESTIONS

Which of the following is true? Rod cells of the eye
1. are sensitive to high intensity light
2. are responsible for color vision
3. contain the protein hemoglobin
4. are used for sharp daytime vision
5. none of the above are true
The layer of the eye where the photoreceptors are located is the
1. lens
2. cornea
3. pupil
4. iris
5. retina
Which of the following is not true? Cone photoreceptor cells of the eye
1. are sensitive to high intensity light
2. are responsible for black and white vision
3. contain a photopigment opsin
4. are used for sharp daytime vision
Which of the following is correct?
1. Receptors in the eye respond to light by increasing the production of neurotransmitter.
2. Bipolar cells never form an action potential.
3. Receptors in the eye use cGMP as the neurotransmitter.
4. Rods produce less photopigment when exposed to dark.
5. Receptors in the eye depolarize when stimulated by light.
Contraction of the the circular muscle of the iris:
1. Two of the above are correct.

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