Anatomy of Vision

Question 1

Describe the anatomy of the eyeball

Answer 1

Ant chamber between cornea and lens
-filled with aqueous humour

Post chamber between lens and retina
-filled with vitreous humour

Pupil surrounded by iris (sphincter)

Lens suspended by zonular ligaments
-originate from body of cilliary muscles

Optic disc - entry point for vessels supplying retina

Floaters - bubbles, particles in vitreous humour

Question 2

Describe the eye light reflex
-pathway
Describe the pathway of light onto the retina
-Why do we have depth perception

Answer 2

Consensual pupillary light reflex
-CNII => pretectal midbrain nucleus => EWN => CNIII

LIGHT INVERTED ALONG BOTH AXES
Lateral light => nasal retina
Nasal light => lateral retina
Sup light => inf retina
Inf light => sup retina

Binocular zone => depth perception (stereopsis)

Question 3

Describe corneal refraction and lens accommodation with light at different distances

Answer 3

Most refraction at cornea
Accommodated at lens
=> Focus light onto retina

Distant object

• light rays are almost parallel
• ciliary muscles relax => flat lens

Close object

• light rays need to be bent
• ciliary muscles contract => thick and round lens

Question 4

Cross section of retina

Answer 4

Inner => Outer

Blood vessels
Nerve fibre layer
Ganglion cell layer

Retinal ganglion cells (RGC)
Inner plexiform layer (IPL)
Inner nuclear layer
Outer plexiform layer (OPL)
Outer nuclear layer

Layer of photoreceptor outer segments
Pigment epithelium

Question 5

Photoreceptor structure, distribution and types

Answer 5

Rods
-rhodopsin (GPCR) + retinal in stacks of membranous discs in outer segments

Cones

• photopsin + retinal in stacks of membranous discs in outer segments
• 3 types of opsin that work at different wavelengths = RGB

More rods than cones

• rods work better in dark (scotopic vision)
• cones work better in light (photopic vision)

Most cones concentrated in fovea
Peripheral retina contains mainly rods

Dark
-GC produces cGMP => depolarisation
Light
-rhodopsin bleaching => PD breaks down cGMP =>hyperpolarization

Question 6

Visual processing in retina

-describe the pathway of getting visual info from the photoreceptors to the optic nerve

Answer 6

Hyperpolarisation of photoreceptor => bipolar cell => ganglion cell => axons converge into optic nerve

Horizontal cells - integrate photoreceptor receptors to adjust contrast

Amacrine cells - shape retinal output of ganglia

Question 7

Describe the visual pathway from the optic nerve to the occipital lobe

Describe the visual pathway from the occipital lobe to the dorsal and ventral stream

Answer 7

RGC axons from nasal retina in CN2 decussate at optic chiasm

Optic tract => LGN in thalamus
Optic radiations => primary visual cortex in occipital

Fibres from inf part of retina => Meyer’s loop
(loop around inf horn of lateral ventricle in temporal lobe)
Fibres from sup part of retina => Baum’s loop
(straight post projection to primary visual cortex)

Dorsal stream - mv perception (akinetopsia)
Ventral stream - object recognition (prosopagnosia)

Question 8

Describe the 3 non image forming pathways from the retina

Answer 8

Pupillary accommodation reflex
-fibres to pretectal midbrain nucleus and sup colliculi

Circadian rhythm maintenance
-Intrinsic photosensitive ganglion cells => hypothalamus

Cerebrum, cerebellum integration
-saccadic, vergence, optokinetic mvs