The Centrifugal Visual System of Vertebrates: A Century-Old Search Reviewed

Abstract

Publisher Summary This chapter reviews a comparative perspective from both anatomical and functional points of view, group by group, from the most primitive vertebrates to the most evolved vertebrates. The centrifugal system of birds is considered because of the large volume and diversity of work done in this vertebrate class. Extrinsic axons to the retina are found in most of the large taxonomic groups of vertebrates, from the lamprey to the human. These fibers always terminate upon amacrine cells, more rarely upon ganglion cells, and upon interplexiform cells (IPC). However, upon entering the retina, they show a considerable degree of collateralization and thus may be considered to be divergent. These fibers are generally of small diameter and unmyelinated (except in the human). These axons are usually of large diameter and myelinated. It is also demonstrated that the retinopetal fibers are predominantly convergent in birds. In teleosts, several neuropeptides are colocalized within retinopetal fibers, including luteinizing hormone-releasing hormones (LHRH)-like, molluscan cardioexcitatory tetrapeptide (FMRF) amide-like, and substance P-like peptides.