VISUAL SYSTEM Key points Relations of neural retina to other structures in the eye Knowledge of types of photoreceptor and their distribution across the retina Foveal versus peripheral retina Knowledge of the different cell types that make up the retina There 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 structures Organization of visual fields – partial decussation at optic chiasm, binocularity, topography Projection to primary visual cortex – optic radiations Effects of lesions on visual function Reading Nolte (4th Ed), pgs 397-431 Crossman and Neary, pgs 129-133 Fitzgerald, pgs 189-196, 201-202 Kiernan (7th Ed), pgs 364-381 Pritchard 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

Cone photopigments 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

Retinal circuitry light ONL OPL INL IPL GCL output signals Photoreceptors Horizontal cells Bipolar cells Amacrine cells Ganglion cells rod cone light output signals

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 nucleus human 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 Reflexes Pupillary constriction (Edinger-Westphal) due to light (consensual…both eyes at once) On accommodation - convergence

Classes of RGC CAT MONKEY On/off cells sustained X/transient Y not much colour MONKEY On/off Parvo (P) colour (red/green) and form Magno (M) motion

Ocular dominance stripes in layer IV Monkey area 17 (V1) Ocular dominance stripes in layer IV Last Slide