VISUAL SYSTEMKey pointsRelations of neural retina to other structures in the eyeKnowledge of types of photoreceptor and their distribution across the retinaFoveal versus peripheral retinaKnowledge of the different cell types that make up the retinaThere are different types of retinal ganglion cell – thus parallel outputs from the retina and beyond….Central visual pathways – projections to LGB and other subcortical visual structuresOrganization of visual fields – partial decussation at optic chiasm, binocularity, topographyProjection to primary visual cortex – optic radiationsEffects of lesions on visual function

ReadingNolte (4th Ed), pgs 397-431 Crossman and Neary, pgs 129-133Fitzgerald, pgs 189-196, 201-202Kiernan (7th Ed), pgs 364-381Pritchard and Alloway, pgs 283-315

The human eye

Mouse retinal wholemount

NB Retina and optic nerve are derived from the neural tube – are of CNS origin

Photoreceptors

rods cones

LIGHT

light

light

130 million rods – monochromatic – high sensitivity – convergence

7 million cones – colour – central retina – high acuity – photopic vision

1 million retinal ganglion cells per eye – efferent cell

Cone photopigments

Retinal circuitry

Photoreceptors

Horizontal cells

Bipolar cells

Amacrine cells

Ganglion cells

ONL

OPL

INL

IPL

GCLoutput signals

light

conerod

There are different classes of retinal ganglion cell:

On/off cells, large and small receptive fields, sustained versus transient responses, monochromatic versus colour……(etc!!)

Visual Pathways

• Retina (rods, cones, fovea)– bipolars– ganglion cells - optic nerve

• Chiasm (nasal fibres cross)• Optic tract

– suprachiasmatic nuclei (diurnal rhythms)– pretectum (for eye movements)– superior colliculus (new objects, co-ordination of eye and

head/neck movements)– lateral geniculate body of the thalamus (to cortex)

Visual fields and central visual pathways in the human

Lateral geniculate nucleushuman monkey

Visual topography – maps

Control of Eye Movements• Medial longitudinal fasciculus connects all the cranial

nerve nuclei (III, IV, VI) together• Descending cortical input• Input from superior colliculus• Input from ascending cervical proprioception

Optic ReflexesPupillary constriction (Edinger-Westphal)due to light (consensual…both eyes at once)On accommodation - convergence

Classes of RGC

• CAT– On/off cells– sustained X/transient Y– not much colour

• MONKEY– On/off– Parvo (P) colour

(red/green) and form– Magno (M) motion

Monkey area 17 (V1)

Ocular dominance stripes in layer IV

Last Slide