A review of axon collateralization in the mammalian visual system

Axon collateralization appears to represent a prominent feature of the mammalian visual system. Both anatomical and electrophysiological evidence reveal that axon branching occurs in the retinofugal, geniculocortical and visual corticifugal projections. Most of this evidence is provided by studies on the cat, but enough data are available from investigations on the rat and monkey to permit certain interspecies differences to be recognized and evaluated. Axon branching allows individual axons to provide innervation to two or more targets and generally to transmit the same type of visual information to these targets. There is abundant evidence to suggest that two of the three functional classes of retinal ganglion cells and geniculate relay cells (namely Y and W ganglion and relay cells) utilize axon branching; however, few details regarding this subject are currently available. The third functional class of ganglion and relay cells (X ganglion and relay cells) essentially lacks axon branches. This review has three primary goals: (1) to review the pertinent anatomical and electrophysiological literature dealing with axon branching and to discuss areas in which information is meager and further investigation necessary; (2) to emphasize the need for applying recently developed techniques, such as double-labeling of neurons and electrical collision, to the study of axon collateralization, and (3) to formulate some hypotheses concerning the functional significance of axon branching.