the visual system: eye and retina

Question 1

in what ways does light interact with the environment (optics)?

Answer 1

• reflection
• absorption
• refraction - bending due to difference in speed of light through different media

Question 2

function of the pupil

Answer 2

lets light inside the eye
- black so every wavelength is absorbed

Question 3

function of the iris

Answer 3

contains muscles which control the amount of light entering the eye

Question 4

function of the cornea

Answer 4

glassy, transparent covering of the pupil and iris
- refracts light and focuses it to back of retina

Question 5

function of sclera

Answer 5

tough, protective wall of eyeball to give it its shape

Question 6

function of the extraocular muscles

Answer 6

move the eyeball , controlled by oculomotor nerve (cranial nerve III)

Question 7

function of optic nerve (cranial nerve II)

Answer 7

carries axons (sensory info) from retina to brain

Question 8

what is the optic disk?

Answer 8

origin of blood vessels and optic nerve, cannot sense light (blind spot but filled in by cortex)

Question 9

what is the macula?

Answer 9

• region of retina for central vision
• devoid of large blood vessels to improve visual quality

Question 10

what is the fovea?

Answer 10

area of highest visual acuity

Question 11

what is the retina?

Answer 11

contains the sensory receptor cells and afferent neurons
divided into nasal and temporal retina (split by fovea)

Question 12

what controls the lens?

Answer 12

suspended by zonal fibres which are attached to ciliary muscle enabling stretching of the lens

Question 13

how are images formed?

Answer 13

light rays focused onto the retina (ideally fovea)
refraction occurs at the cornea (80%) and lens (20%)

Question 14

what determines the degree of refraction?

Answer 14

• difference in refractive indices between the two media
• angle at which light hits the interface between these two media

Question 15

how does refraction occur at the cornea?

Answer 15

• light arrives at cornea through air but cornea is mainly water
• light travels slowly through water due to density = refraction occurs
• distance from refractive surface to convergence of parallel light rays = focal distance

Question 16

how does the lens accommodate for objects at different lengths?

Answer 16

distant objects - stretched thin as light rays almost parallel, relaxed ciliary muscle

closer objects - fattening of lens for additional refraction as rays are not parallel, contracted ciliary muscle

Question 17

what is hyperopia?

Answer 17

see things at a distance well but blurry close up (shorter eyeball so less distance for refraction)

convex lenses makes rays more parallel before entering eye

Question 18

what is myopia?

Answer 18

see things close up but blurry at a distance (longer eyeball so image too much refraction and image formed before retina)

concave lenses diverge rays before entering eye

Question 19

what is the laminar organisation of the retina?

Answer 19

• light must pass through ganglion and bipolar cells before it reaches the photoreceptors
• no ganglion and bipolar cells at fovea so highest visual acuity
• light that passes all the way through the retina is absorbed by pigmented epithelium

Question 20

cells of the retina

Answer 20

• ganglion cells - output from retina (only cells that produce AP)
• bipolar cells - connect photoreceptors to ganglion cells
• photoreceptors - sensory transducers (rods and cones)

Question 21

how is light absorbed by photoreceptors?

Answer 21

membranous disks which contain light-sensitive photopigments that absorb light

Question 22

what is duplicity theory?

Answer 22

can’t have high sensitivity and high resolution in a single receptor
(therefore there are rods and cones)

Question 23

features of rod photoreceptors

Answer 23

• greater number of disks
• higher photopigment concentration
• 1000 times more sensitive to light than cones
• enable vision in low light conditions
• low visual acuity

Question 24

features of cone photoreceptors

Answer 24

• fewer disks
• used during daylight conditions
• colour vision
• high visual acuity
• lower sensitivity

Question 25

how does retinal structure vary with region?

Answer 25

fovea contains only cones
central retina = low convergence of cells, low sensitivity, high resolution
peripheral retina = high convergence of cells, high sensitivity, low resolution