ABSTRACT
The vertebrate optic nerve has proved a popular model for studies of central nerve degeneration and regeneration. One valuable aspect is that the interpretation of the results of axotomy are simplified by the fact that the optic nerve represents a nerve tract composed entirely or almost entirely of axons of one origin, namely optic axons arising from retinal ganglion cells. Another advantage is that the parent cell somata form a well demarcated population, within the innermost cellular layer of the retina. Optic axons converge to form the optic nerve and on reaching the brain, traverse the optic tracts to terminate in contralateral, and usually to a lesser extent, in ipsilateral visual brain centres. The main visual centre in non-mammals is the optic tectum. In mammals there are two prominent primary visual centres, namely the superior colliculus, the homologue of the optic tectum, and the lateral geniculate nucleus. In the major visual centres, axons terminate in a topographically ordered fashion, related to the distribution of their somata within the retina. Many anatomical, physiological, pharmacological and behavioural studies describe the normal visual projection, providing a basis for studies of axonal degeneration and regeneration (Jacobson, 1991).
