13-2: Physiology of Vision

Question 1

Forming an image on the retina involves __ processes: ____.

Answer 1

4; refraction, accommodation, constriction of the pupil, and convergence

Question 2

refraction

Answer 2

bending of light as it passes from one medium to another of a different density

Question 3

What are the 4 steps of refraction?

Answer 3

1) from the air into the denser cornea
2) from the cornea to the less dense aqueous humor
3) from the aqueous humor into the denser lens
4) from the lens into the less dense vitreous humor

Question 4

During refraction, the rays must bend so that they fall on the ___ for sharpest vision.

Answer 4

macula lutea (fovea centralis)

Question 5

accommodation

Answer 5

a change in the curvature of the lens to adjust for focusing at various distances

Question 6

In near vision, the ciliary muscle ___, causing the lens to become ___ curved.

Answer 6

contracts; more

Contraction releases tension on the lens, which causes it to shorten, thicken and become more curved (PANS)

Question 7

In far vision, the ciliary muscle ___, causing the lens to ____.

Answer 7

relaxes; flatten

Relaxation pulls on ligaments, flattening the lens for focusing on objects at a distance (SANS)

Question 8

presbyopia

Answer 8

lit “elderly eyes”; as we age, lens loses its elasticity and becomes less able to accommodate for near vision

Question 9

myopia

Answer 9

nearsighted; see clearly at short distances only (concave lens)

Question 10

hyperopia

Answer 10

farsighted; see distant things more clearly than up close (convex lens)

Question 11

astigmatism

Answer 11

uneven curve in cornea or lens, always slightly blurry (custom lenses)

Question 12

constriction of the pupil

Answer 12

narrowing the diameter of the hole through which light enters the eye prevents diffuse rays from passing through the edge of the lens, which would not focus properly

Question 13

How does the pupil constrict?

Answer 13

Contraction of the circular smooth muscle of the iris: increases clarity and depth of focus (PANS)

Question 14

convergence

Answer 14

the medial movement of the eyeballs so that they are BOTH directed at the viewed object. The goal is to keep the object focused on the fovea centralis of both eyes for shapr vision

Question 15

The nearer the object, the ___ degree of convergence

Answer 15

greater

Question 16

Convergance is accomplished using the ___ muscles.

Answer 16

medial rectus

Question 17

After the image is formed on the retina, the __ energy must be converted into a ___.

Answer 17

light; nerve impulse

Question 18

photopigments

Answer 18

substances that absorb light and undergo a structural change that leads to a nerve impulse; embedded in areas of membrane that form discs

Question 19

Photopigments are made of ___ (derived from ___), plus ___ (a type of ___). There are __ different pigments, each with a different type of ___.

Answer 19

retinal; vitamin A; opsin; protein; 4; opsin

Question 20

The photopigment in rods is ___.

Answer 20

rhodopsin

Question 21

Rhodopsin forms and accumulates very slowly in the ___; it is sensitive to ___ levels of light, which cause it to decompose rapidly.

Answer 21

dark; low

Question 22

What generates a nerve impulse in the rods?

Answer 22

the breakdown of rhodoposin into retinal and opsin generates a nerve impulse

Question 23

Rods function best ____. Why?

Answer 23

at night or in a dark room; due to the rapid breakdown of rhodoposin in bright light

Question 24

There are ___ types of cones, each made of ___ and ___. Each type of cone responds best to light of a given ___.

Answer 24

3; retinal; 1 of 3 different opsins; color (red, blue, and green cones)

Question 25

Photopigments in the cones reform ____, and are decomposed by ___.

Answer 25

very quickly; bright light

Question 26

dark adaptation

Answer 26

after a long period of time in bright light, most photopigments are decomposed. As a result, the eye is less sensitive to light.

If you move into a dark room, you see nothing at first because it takes 20-30 minutes for enough rhodoposin to reform allowing you to see clearly in dim light. Cones do not function in the dark.

Question 27

light adaptation

Answer 27

after a long period in the dark, the concentraiton of photopigments in the rods and cones is very high. As a result, the eye is very sensitive to light.

If you turn on a bright light, you are “blinded” as large amounts of photopigment decompose instantly, overgenerating the nerve impulse. It takes ~60 seconds for the eye to adjust.

Question 28

Why does the eye adjust faster to bright light than darkness?

Answer 28

Photopigments in the cones decompose instantly in bright light, but rhodoposin in the rods takes longer to reform

Question 29

What are the steps in the visual pathway?

Answer 29

light > cornea > aqueous humor > pupil > lens > virtreous humor > retina > macula lutea

Light image is on the back of the eye, but still cannot “see” - need a nerve impulse

light passes through the ganglionic and bipolar neurons to hit the rods and cones > photopigments decompose > generates a nerve impulse

nerve impulse travels to the bipolar neurons > ganglion cells > axons leave eye as part of the optic nerve

optic nerve > optic chiasma > optic tract > thalamus (1 branch to superior colliculi for reflex mvts)

visual cortex in occipital lobe

now you see!

Question 30

The ___ of the ganglion cells leave the eyeball as part of the ___ at the ___.

Answer 30

axons; optic nerve; optic disc

Question 31

optic chiasma

Answer 31

spot at the base of the brain where axons cross over to the other side

Question 32

optic tract

Answer 32

axons from the optic nerve now entering the brain, terminating at the thalamus (1 branch to superior colliculi)

Question 33

color blindness

Answer 33

congenital lack of 1 or more cone types

Question 34

night blindness

Answer 34

vitamin A deficiency leading to decreased rod function