Phys Exam 2 "Quiz" 3

eye structure

a globe made up of 3 concentric layers

outer, middle, and inner

outer layer of the eye

tunica fibrosa and sclera of cornea

middle layer of eye

pigmented, vascular layer

tunica vasculosa or uvea

choroid, iris, and ciliary body

ciliary body

controls the shape of the lens for focus and secretes the aqueous humor

inner layer of the eye

neural layer

tunica interna

comprising the retina, retinal vessels, and start of the optic nerve

iris job

controls light entering the eye

radial dilatory pupillae muscle

sympathetic nerves supply this to dilate the pupil by the iris

concentric sphincter pupillae

parasympthatetic nerve supply

contrics pupil by iris

anisocoria

unequal pupils

horner’s syndrome

results in ipsilateral disruption of the sympathetic nerves

causes ipsilateral pupil constriction, ptosis, unilateral facial erythemma, and anhidrosis

CN III palsy

affects the parasympathetic supply tot he eye causes pupil dialtion

results from compression of the nerve → uncal herniation

where are images that we see from points of light recreated

on the retina

optical pathway

cornea, aqueous humor, lens, and the vitreous humor

most refraction is done by what

cornea

how is focus achieved

through variable refraction by the lens

refractive index and the eye

structures of the eye have different RI and cause light rays to bend in order to focus them on the retina

focal length of the lens

the distance from the lens at which parallel light rays passing through it will be focused

the greater the lens bends light leads to what

the greater refractive power and smaller focal length

expressed as diopters = 1 m/focal length

concave lens

diverge light

negative focal length

convex lens

converge light

focal length is positive

becomes negative if light converges before the retina

how does the eye focus on near objects

close objects have diverging rays so the lens must change its shape to increase the refractive power

Explain what refraction is and why light must be refracted for clear vision

Refraction is the bending of light when it passes from one medium to another with a different refractive index.

The eye needs to bend (refract) those rays so they converge and focus precisely on the retina.

identify the optical components of the eye

cornea

aqueous humor

lens

virtuous humor

pupil

iris

cornea job

provides most of the eye's refractive power

bends incoming light.

aqueous humor job

fluid between the cornea and lens

light passes through it.

Lens job

fine-tunes the focus of light

especially for near vs. far objects.

vitreous humor

gel behind the lens that light passes through on its way to the retina.

pupil job

opening that controls how much light enters the eye

iris job

controls pupil size and therefore the amount of light entering.

what parts are responsible for refraction

cornea and lens

which is responsible for most of the refractive power

cornea

which has variable refractive power

lens

accommodation

the adjustment of lens shape to change its focal length in order to focus light on the retina

how is accommodation accomplished

through the contraction of the ciliary muscles which acts on the lens

relaxed ciliary muscle

pulls outward on lens to cause it to flatten so its focal length is longer and reduces its power

contracted ciliary muscle

slacks on the lens causing it to become more rounded so its focal length is shortened and its power increases

ciliary muscle is supplied by what nerves

parasympathetic nerves

what is used to dilate the pupil

parasympathetic blockers which relaxes the ciliary muscles

this blocks accommodation → ability to focus

why must the refractive power of the lens must increase to focus on near objects

must increase its refractive power to bend light more strongly so the diverging rays from a near object focus on the retina.

Explain the effect of parasympathetic antagonists on the pupil and on accommodation and why that makes them useful agents for evaluation of the eye

they dilate the pupil and relax the ciliary muscle, which decreases accommodation.

They are useful for eye evaluation because dilating the pupil allows more of the retina to be viewed, while preventing accommodation helps the examiner measure refractive error accurately.

emmetropia

normal eye

parallel rays come to focus on the retina when the ciliary muscle is relaxed

hyperopia

far sightedness

the eye is too shallow so the light becomes focused behind the retina

The eye's refractive power is too weak.

Near objects are especially blurry because the eye has difficulty providing enough focusing power.

myopia

near sightedness

eye is too deep so light becomes focused in front of the retina

The eye has too much refractive power.

Distant objects are blurry, while nearby objects can be clearer.

correction of myopia

glasses that are concave are used to diverge light rays

moves focal point back on retina

correction of hyperopia

use a convex lens to converge light

adds refractive power to bring the focal point forward onto the retina.

20/20

ability to see at 20 feet that a normal person can see at 20 feet

20/40

must be at 20 feet to see what a normal person sees at 40 feet

15/20

can see at 20 feet what a normal person can see at 15 feet

intraocular fluid job

keeps eyeball round and distended

2 fluid chambers

aqueous humor

vitreous humor

aqueous humor produced by what

fluid produced by ciliary body

aqueous humor flows through what

through pupil into anterior chamber through canal of Schlemm

virtuous humor is where

behind the lens

vitreous humor job

holds retina against back of eye

intraocular pressure

is determined by resistance to outflow of aqueous humor in canal of Schlemm

increased intraocular pressure is caused by what

increased resistance of outflow

glaucoma

damage to optic nerve caused by increased intraocular pressure

increased IOP compresses blood vessels and axons

tonometry

the measurement of IOP

two kinds of glaucoma

open angle

closed angle

closed angle glaucoma

10% of cases

sudden closure of iridocorneal angle blocks fluid flow and acutely increased IOP and causes ocular pain

medical emergency

open angle glaucoma

90% of cases

reduced flow through trabecular meshwork due to obstruction

insidious- no pain initially

meds used to increase fluid flow or decrease fluid production

Explain how glaucoma causes damage that leads to blindness

causes increased intraocular pressure that damages the optic nerve, leading to progressive loss of vision and potentially blindness.

presbyopia

with age the lens stiffens and is no longer able to change shape so the ability to change focus is lost

astigmatism

the shape of the cornea or lens is irregular

light entering in different planes will be bent to different degrees disrupting focus

how is astigmatism corrected

with glasses, contacts, or surgery

cataracts

clouding of the lens

how is light sensed

it must be absorbed by the visual pigment within the retina

two kinds of photoreceptor cells in retina

rods and cones

rods contain what visual pigment

rhodospin

rods are sensitive to what

light and dark

rods location and concentration

most numerous type of photoreceptor

concentrated around the periphery

how many different types of cones

3 types

3 different types of cones for what

each with different types of visual pigment

each is maximally sensitive to a different color

cones distribution and concentration

concentrated in the area of central vision → macula lutea

what are cones needed for

vision in daylight and color discrimination

macular degneration

leading cause of vision loss

degeneration of cells in the macula lutea

what does vitreous humor press the retina against

the choroid

1st order neurons for vision

bipolar cells the process input from photoreceptors

2nd order neurons for vision

ganglion cells whose axons form the optic nerve

pigment epithelium job

absorbs light

retina layers

vitreous humor

nerve fiber layer

ganglion cell layer

bipolar cells

cones and rods

pigment epithelium

choroid

sclera

choroid

vessels of choroid supply out half of retina

central retinal artery

supply inner layers of retina

fovea centrtalis

is a pit where there are no rods

the cones are highly concentrated and the nerves and vessels are displaced to the side in order to maximize resolution in the very center of vision `

Explain the importance of the fovea centralis

highest concentration of cones, providing the sharpest, most detailed central vision.

blind spot

optic nerve exits at the optic disc medial to macula

no receptor cells there so a blind spot is created

lateral to center of vision in each visual field

macula

The central region of the retina responsible for central, detailed vision; it contains the fovea centralis.

detached retina

shrinkage of vitreous with age can allow retina to pull away from the choroid

causes loss of vision

generating the optic nerve signal

rods and cones continuously release a NT in the dark that inhibits the basal cells they synapse with

when they absorb light changes induced in their visual pigments cause the cell to hyper polarize

bipolar cells are disinhibited and in turn stimulate ganglion cells

ganglion cells produce action potentials

rods and cones in the dark

Rods/cones are depolarized → continuously release neurotransmitter → inhibit bipolar cells

Photoreceptor releases NT → bipolar cell is inhibited → ganglion cell is less active

rods and cones in the light

• Light activates the visual pigment in the rod/cone.

• This causes the photoreceptor to hyperpolarize.

• Hyperpolarization means it releases less neurotransmitter.

• Because less neurotransmitter is being released, the bipolar cell is no longer inhibited = disinhibited.

• The bipolar cell can now stimulate the ganglion cell.

• The ganglion cell produces action potentials that travel through the optic nerve to the brain.

visual pigments of rods and cones are composed of what

chromophore

retinal

opsin

retinal and chromophore

absorb light

opsin

protein that is slightly different in each of the 4 types of photoreceptors

vitamin A deficiency can cause what

night blindness

is retinol

rods are sensitive to what

very sensitive to low levels of light but can only distinguish light and dark

cones sensitivity

less sensitive to light but can distinguish colors

each type of cone is maximally sensitive to either blue, green, or red light

rods in the dark

increase cGMP which opens Na+ channels that depolarize the cell inducing the release of NT at its synapse with bipolar cells

rods in the light

when cis-retinal of rhodospin absorbs light is isomerizes to trans-retinal, dissociates from its opsin, and activates phosphodiesterase that hydrolyzes the cGMP

this then closes Na+ channels and the membrane hyperpolarizes

rods back to dark

an enzyme converts trans back to cis retinal which rejoins the scotospin and the cell again depolarizes