13-2: Physiology of Vision Flashcards
Forming an image on the retina involves __ processes: ____.
4; refraction, accommodation, constriction of the pupil, and convergence
refraction
bending of light as it passes from one medium to another of a different density
What are the 4 steps of refraction?
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
During refraction, the rays must bend so that they fall on the ___ for sharpest vision.
macula lutea (fovea centralis)
accommodation
a change in the curvature of the lens to adjust for focusing at various distances
In near vision, the ciliary muscle ___, causing the lens to become ___ curved.
contracts; more
Contraction releases tension on the lens, which causes it to shorten, thicken and become more curved (PANS)
In far vision, the ciliary muscle ___, causing the lens to ____.
relaxes; flatten
Relaxation pulls on ligaments, flattening the lens for focusing on objects at a distance (SANS)
presbyopia
lit “elderly eyes”; as we age, lens loses its elasticity and becomes less able to accommodate for near vision
myopia
nearsighted; see clearly at short distances only (concave lens)
hyperopia
farsighted; see distant things more clearly than up close (convex lens)
astigmatism
uneven curve in cornea or lens, always slightly blurry (custom lenses)
constriction of the pupil
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
How does the pupil constrict?
Contraction of the circular smooth muscle of the iris: increases clarity and depth of focus (PANS)
convergence
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
The nearer the object, the ___ degree of convergence
greater
Convergance is accomplished using the ___ muscles.
medial rectus
After the image is formed on the retina, the __ energy must be converted into a ___.
light; nerve impulse
photopigments
substances that absorb light and undergo a structural change that leads to a nerve impulse; embedded in areas of membrane that form discs
Photopigments are made of ___ (derived from ___), plus ___ (a type of ___). There are __ different pigments, each with a different type of ___.
retinal; vitamin A; opsin; protein; 4; opsin
The photopigment in rods is ___.
rhodopsin
Rhodopsin forms and accumulates very slowly in the ___; it is sensitive to ___ levels of light, which cause it to decompose rapidly.
dark; low
What generates a nerve impulse in the rods?
the breakdown of rhodoposin into retinal and opsin generates a nerve impulse
Rods function best ____. Why?
at night or in a dark room; due to the rapid breakdown of rhodoposin in bright light
There are ___ types of cones, each made of ___ and ___. Each type of cone responds best to light of a given ___.
3; retinal; 1 of 3 different opsins; color (red, blue, and green cones)
Photopigments in the cones reform ____, and are decomposed by ___.
very quickly; bright light
dark adaptation
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.
light adaptation
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.
Why does the eye adjust faster to bright light than darkness?
Photopigments in the cones decompose instantly in bright light, but rhodoposin in the rods takes longer to reform
What are the steps in the visual pathway?
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!
The ___ of the ganglion cells leave the eyeball as part of the ___ at the ___.
axons; optic nerve; optic disc
optic chiasma
spot at the base of the brain where axons cross over to the other side
optic tract
axons from the optic nerve now entering the brain, terminating at the thalamus (1 branch to superior colliculi)
color blindness
congenital lack of 1 or more cone types
night blindness
vitamin A deficiency leading to decreased rod function
