Impairment Of Vision

ANATOMY AND PHYSIOLOGY

Anatomically the visual system is contained in the supratentorial compartment. It is composed of peripheral receptors in the retina, central pathways and cortical centres. The control of ocular movement and pupillary responses are closely integrated.

The retina: three distinct layers of the retina are identified:

Pigmented choroid ^

Light source

Pigmented choroid ^

Light source

. Rods and cones

Bipolar cells , Ganglion cells

. Rods and cones

Bipolar cells , Ganglion cells

Rods - responsible for night/twilight vision and for detection of peripheral movement.

Cones - responsible for day vision/ colour vision.

Rods and cones synapse with bipolar cells.

The bipolar cells synapse with ganglion cells from which unmyelinated fibres run to the optic disc, where they become myelinated and leave the eye as the optic nerve.

The macular region of the retina is its most important area for visual acuity. Here, cones lie in the greatest concentration whereas rods are more numerous in the surrounding retina.

The optic nerve leaves the orbit through the optic foramen and passes posteriorly to unite with the opposite optic nerve at the optic chiasma. Here, partial decussation occurs (axons from ganglion cells on the nasal side of the retina cross over to the opposite side).

The optic tract consisting of ipsilateral temporal and contralateral nasal fibres passes to the lateral geniculate body. A few fibres leave the tract before the lateral geniculate body and pass to the superior colliculus (fibres concerned with pupillary light reflex).

Axons of cell bodies in the lateral geniculate body make up the optic radiation. This enters the hemisphere in the most posterior part of the internal capsule, courses deep in parietal and temporal lobes and terminates in the calcarine cortex of the occipital lobe.

Visual cortex '

Lateral geniculate body

Visual cortex '

Lateral geniculate body

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