Pt9 Vision

Question 1

Describe the visual system of a dog.

Answer 1

-binocular vision & wider periphery
-image of object goes to area of centralis = retinal area that provides the greatest acuity

Question 2

Describe the choroid.

Answer 2

-loose CT
-pigmented cells
-vasculature
-nutritive
-diurnal animals = melanocytes that absorb light passed by photoreceptors without stimulating them
-nocturnal & domestics = patch of reflective material “tapetum lucidum”
>enhances dark vision under dim light

Question 3

Describe the iris.

Answer 3

-pigmented structure
-dilator & sphincter muscles

behind iris = lens

Question 4

Describe the pupillary dilator muscle.

Answer 4

> radially arranged
opposes action of sphincter
part of pigmented anterior epithelial
contraction = dilation “mydriasis”
dilation = sympathetic (pain, fear, anger)

Question 5

Describe the pupillary sphincter muscle.

Answer 5

-circular arrangement near pupil
-contraction = decrease in pupil size “miosis”
-innervated by parasympathetic fibers
size of pupil regulates amount of light entering eye

Question 6

Describe the lens of the eye.

Answer 6

-suspended by suspensory ligaments
-fibers attached to ciliary body
-muscle near base of iris
>increase/decrease tension on the lens
>make lens curve more/less convex to focus near/far

Question 7

Describe the vitreous humor.

Answer 7

-behind lens
-gelatinous fluid = gives spherical shape of eye
-phagocytic cells
-behind vitreous humor is the retina

Question 8

Describe the retina.

Answer 8

-where light is transformed into electrical activity of neurons
-interrupted where axons of retina ganglion cell layer leave eye on their way to brain = optic disc/blind spot = optic nerve (CN II)
> arteries & veins enter retina at optic disc
> provide nutrition of retina & vessels at choroid
-more axons in both optic nerves than in dorsal root of spinal cord
-specialized sensory epithelium = photoreceptors & other cells arranged in layers

Question 9

What are the 2 types of photoreceptor cells?

Answer 9

cone & rod

OUTER SEGMENT:
-photosensitive region
-membranous invaginations in cones
-flattened membranous sacs in rods (stack of coins)
-membrane of both contain photopigments
>convert light stimulus to receptor potential

INNER SEGMENT:
-metabolic region

Question 10

Describe rod cells.

Answer 10

-rhodopsin
>photochemical neurotransmitter for perception of shades of grey
>low threshold of excitability (high sensitivity)
~stimulated by low intensity light
>night vision
>95% of photoreceptor in retina

Vit A imp for formation of rhodopsin
>deficiency = night blindness

Question 11

Describe cone cells.

Answer 11

color/cone pigments
>photochemical neurotransmitter = perception of color
>requires high intensity light
-3 diff types of cones in primates
each one a diff color pigment & absorptions/wavelengths
>blue, green, red = polychromatic vision

(the visual system mix & contrast effect of each cone cell)

Question 12

Describe the photoreceptor cells of most mammals.

Answer 12

-2 types of cones = dichromatic
>detect blue & yellow (no reds/oranges)
-birds, lizards, turtles, fish = 4 types
>3 types + UV sensitive cone
>richer color perception

(Color is brains interpretation of differences in wavelengths of light)

Question 13

What are the 5 layers of the retina?

Answer 13

1. Photoreceptor
2. Horizontal
3. Bipolar
4. Amacrine
5. Ganglion
   all the cells cause distortion of light rays = fovea (area centralis) minimizes distortion

Question 14

Describe the fovea of the retina.

Answer 14

-center of retina “area centralis”
-acute/detailed vision
-composed of mostly cones
>other cells displaced to allow light to pass unimpeded to cones

Question 15

Compare rods & cones.

Answer 15

Question 16

Describe the activation of photoreceptors by light.

Answer 16

-photoreceptors & ganglion = receptor potentials
>retinal neurons conduct visual signals by electric current = electronic conduction
>degree of hyperpolarization in rod & cone flow to cytoplasm to the synaptic body
~neurotransmitter released & transmitted to next cell
-allow graded conduction of signal strength
>strength of hyperpolarizing output signal related to intensity of illumination
>not all or none

Question 17

Describe the pathway of the photoreceptors in the dark.

Answer 17

not sending signals = depol
1. Dark = cGMP phosphodieserase is inactive (PDE6) enzyme
2. CGMP bind to ligand gated Na/Ca ion channels
3. Leakage of cations into rod = depol
4. Ca transported out by anti porter
5. Energy from Na/K down gradient
6. Force Ca against gradient

Question 18

Describe visual acuity.

Answer 18

Acuity of visual image reflects many factors
>pop of retinal cells
>ratio of rods to cone photoreceptor cells
>ratio of photoreceptor cells to ganglion cells

Cone cells have better acuity than rods
>100s of rod signals via bipolar cells to a single ganglion
>few cone signals to single ganglion

*more ganglion in area centralis than peripheral portion of retina
>area centralis = highest acuity in retina
(More ganglion = high acuity)

Question 19

How are images formed?

Answer 19

*eye catches light reflected by objects & guide passage until image is formed
*sequence of transparent media causes light to be refracted (direction change)
1. Air & anterior surface of cornea
2. Posterior surface of cornea & aqueous humor
3. Aqueous humor & anterior surface of lens
4. Posterior surface of lens & vitreous humor

Question 20

What controls the amount of light that passes through the pupil?

Answer 20

Iris!
-dark = dilated pupil = mydriasis
-light = contracted pupil = miosis

Question 21

How do the lens work when forming an image?

Answer 21

-converges light rays to a focal point on retina
-light rays = unobstructed access to photoreceptors
-central retina = area of highest visual acuity
-no distinct fovea

Question 22

How are images on the retina formed?

Answer 22

-real, inverted, & smaller than the object
-distance between the light rays emanating from the top & bottom of tree get closer together as they approach the eye - at some point they cross & reverse
-by reversing & decreasing the image, the eye can visualize objects much larger than it

Question 23

How are the images formed on the retina transformed?

Answer 23

-by cones & rods into nerve impulses
-visual signals carried by optic nerve fibers that form optic nerve, optic chiasm, & optic tract
-fibers terminate in the thalamus for visual perception (lateral geniculate nucleus) or mid brain for visual reflexes
EX: body & ocular reflex, pupillary constriction

brain processes & reorients image