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Monday, September 26th 2011

Optic Nerve

It’s fair to say that vision is the most important sensation, the one from which we draw the most information.

The first order neurons of the visual pathway are the rods and cones lying deep in the retina.

On the diagram light would enter from the left, pass through the cornea, lens, vitreus humor and multiple layers of the retina to reach the first order neurons in the pathway. Rod cells are peripherally located cells with a low activation threshold, some responding to single photons. They are primarily associated with “night vision” and peripheral vision. Many rod cells may converse on a single second order neuron. This convergance of data means that the acuity of rod cells is not nearly as great as the more centrally located cone cells which converge nearly one to one on second order bipolar and amacrine cells.

The physiology of the activation of rod and cone cells is complex but here is a good very basic summary.

The short bipolar and amacrine cells in the retina relay the signal to the third order ganglion cells. The long axons of the ganglion cells converge to form the actual optic nerve.

The axons converge at the disc and form the optic nerve which exits posteriorly where it becomes myelinated and takes up the dural covering of the brain. The nerve exits the orbit at the optic canal. The tracts within the optic nerve are grossly somatically organized.

Generally the temporal fields of both retina travel laterally in the nerve and the nasal fields from both retina travel medially. Even this gross simplification takes some thinking about. Essentially objects of focus in the right visual field project onto the nasal fields of the right eye and the temporal fields of the left eye.

This is something to think about as the optic nerves come to the optic chiasm. At the chiasm approximately half of the ganglion cell axons will decussate. Primarily the axons carrying information from the nasal fields decussate. Functionally what you end up with is information from the right visual fields(temporal fields of the left eye and nasal fields of the left eye) in the left optic tract and information from vision to the right (temporal fields of the right eye and nasal fields of the left eye) in the optic tract on the right.

However, because the lens inverts the image prior to it reaching the retina, things you see to your left actually end up as information traveling through the left optic tract and vice/versa.

The first branches from the optic tracts travel to the superior colliculi and the pretectal area. These represent the afferent limb of the pupillary light reflex. They will synapse on the pretectal nuclei which will send axons to the accessory nucleus of the third nerve, then on to the cillary ganglion, then on to the sphinter pupillae muscle causing narrowing of the pupil.

The majority of the axons in the optic tracts however continue on the lateral geniculate bodies in the thalami.

The above image sourced here.

The neurons in the lateral geniculate body send axons which make up the optic radiations which run posteriorly around the temporal and occipital horns of the lateral ventricles to the occipital lobe, particularly the medial surface, Brodmann Area 17.

The primary clinical considerations are understanding how lesions along the pathway might manifest in terms of visual field deficits.