LO5: anatomy of eye/physiology of vision

cranial nerves for vision

CNIII, CNIV, CNVI

cornea

transparent layer that covers anterior structures of the eye

iris

coloured portion of the eye containing muscles that can constrict/dilate to change pupil diameter. controlled by sympathetic + parasympathetic NS.

pupil

round central opening that allows light to enter eye

lens

transparent and flexible. changes shape to focus light on retina. held in place by suspensory ligaments called ciliary zoniles attached to ciliary muscles

lens shape

round and thickened unless external force is applied by suspensory ligaments.

ciliary muscles

circular muscles surrounding the lens

suspensory ligaments

extend from muscle to lens to hold in place

ciliary muscle contraction

suspensory ligaments loosen > lens become round and thicken

ciliary muscle relax

suspensory ligaments tighten which pull at circumference of lens > lens flatten.

distant vision

ciliary muscle relaxes, controlled by sympathetic system. light rays are almost parallel.

close vision

ciliary muscle contracts, controlled by parasympathetic system. light rays are on divergent paths.

anterior chamber

between cornea and lens; contains aqueous humour.

aqueous humour

supplies nutrients and oxygen to the lens and cornea, removes waste.

posterior chamber

posterior to lens; contains vitreous humour.

vitreous humour

gelatinous substance that supports posterior surface of eye and holds retina in place.

3 layers of eyeball

fibrous, vascular, nervous

fibrous layer

contains sclera and cornea

sclera

white, tough, opaque connective tissue, covers most of eye

cornea

transparent region at front of eye.

vascular layer

contains choroid, ciliary body and iris

choroid

highly vascularised areac

ciliary body

forms ciliary muscle rings around lens

iris

adjustable muscular layer that controls diameter of pupil.

retina: nervous layer

contains photoreceptors, fovea centralis and otpic disk

photoreceptors

rods and cones

fovea centralis

visual activity

optic disk

blind spot

rods

receptors for dim light and peripheral vision

cones

receptors for bright light, colour and visual activity

photoreceptors

light detecting receptor cells. light reflects off objects and travel through eyes to photoreceptors of the retina.

light

travels in rays and reflects (aka bounces off objects). it travels in a straight line until it reaches a different medium

refraction

bending of light rays as it passes through different mediums (i.e. humours, cornea and lens)

accomodation

changing the shape of the lens, increasing/decreasing refractory power of the lens. (increasing or decreasing amount of bending the light rays).

pathway of light

1. cornea

2. aqueous humour

3. lens

4. vitreous humour

refraction power

constant amount of refraction for all structures except lends because lens is highly elastic and changes shape (accomodation) for focussing. the more convex (fatter) the lens, the greater the refraction.

converting light into action potentials

light stimulates rhodopsin breakdown, causing rod cell membrane to depolarise, producing action potential of optic nerve. cones functions similarly using different type of rhodopsin.

rhodopsin

contained in rods, breakdown promoted by light

optic disk

region of the eyeball where the optic nerve exits the eye, area lacks photoreceptors so light focussed on blind spot cannot be seen.

fovea centralis

region next to blind spot where all retinal structures other than cones are displaced. allows light to pass directly to receptors. enhanced visual acuity and detailed colour vision.

optic chiasma

where both optic nerves meetla

lateral fibre

stay on the same side of the brain

binocular vision

whereby human eyes are located anteriorly so the field of vision from both eyes overlap.

optic pathway

optic nerve > chiasma > tract > radiation