Optic nerve

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The photic energy of light, its amount regulated by the pupillary aperture is converted to nerve action potentials by retinal rod, cone and ganglion cells. The lens, under control of the ciliary muscle causes the image on the retina to be inverted (1). An object in the lower part of the visual field is projected to the upper retina and one in the temporal field to the nasal retina. Each optic nerve (2), sheathed in pia-arachnoid meninges, carries axons from retinal ganglion cells to the lateral geniculate bodies.

At the optic chiasm (3), fibres travelling in the nasal portions of the optic nerves cross, join uncrossed temporal fibres of each optic nerve and form each optic tract. The fibres synapse at the lateral geniculate body (4). One optic tract thus carries fibres from the temporal ipsilateral retina and the nasal contralateral retina. Some

1. Paracentral scotoma-retinal lesion.
2. Mononuclear field loss-optic nerve lesion.
3. Bitemporal hemianopia-chiasmal lesion.
4. Homonymous hemianopia-optic tract lesion.
5. Homonymous quadrantanopia-temporal lesion.
6. Homonymous quadrantanopia-parietal lesion.
7. Homonymous hemianopia-occipital cortex or optic radiation.
8. Homonymous hemianopia-occipital pole lesion.

optic tract fibres reaching the lateral geniculate bodies pass to the brainstem to control refraction (lens) and pupillary aperture.

From the lateral geniculate body, fibres pass in the optic radiation through the parietal and temporal lobes (5 and 6) to reach the visual, or calcarine, cortex of the occipital lobe (7 and 8). The upper retinae (lower visual fields) project in the optic radiation through the parietal lobes to the upper part of the visual cortex, and the lower retinae (upper fields) through the temporal lobes, beneath the parietal, to the lower visual cortex. Impulses reach the cortex in strictly maintained vertical topographical order (i.e. upper field to lower retina, tract, radiation and cortex, and vice versa, lower to upper). Within the visual cortex itself there are synaptic connections between groups of cells which detect lines, orientation, shapes, movement, colour, and depth. These are processed by the neighbouring visual association areas.

Visual field defects caused by lesions of each optic tract, radiation and cortex are called 'homonymous' to indicate the different (i.e. bilateral) origins of each unilateral pathway. (A homonym is the same word used to denote different things.)

Field defects are hemianopic when half the field is affected by a lesion of the optic tract, radiation or cortex (e.g. left homonymous hemianopia), and quadrant anopic when a quadrant is affected. Congruous denotes symmetry and incongruous lack of it. Bitemporal defects (damage to crossing nasal fibres) are caused by lesions of the optic chiasm (e.g. pituitary tumour).

• Papilloedema
• Pupil
• Optic neuritis
• Optic atrophy and chiasm
• Horner's syndrome