Synapses made by myelinated fibers running to teleost and elasmobranch retinas

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Abstract
Myelinated fibers are described in teleost (carp, goldfish) and elasmobranch (dogfish) retinas which make synapses onto amacrine cell bodies and bipolar cell terminals. The fibers run in parallel with bundles of optic nerve fibers radiating from the optic disk until at some point they traverse the inner plexiform layer, turning again horizontally to course under the layer of amacrine cell bodies for 0.5–0.75 mm. En route each fiber makes several en passant synapses characterized by a cessation of the myelin sheath, a local increase in axonal diameter, an accumulation of agranular vesicles 300–400 Å in diameter and increased densification of pre‐ and post‐synaptic membranes. These fibers are not post‐synaptic to any retinal element. The evidence that the myelinated fibers described form an efferent system is based on their resemblance in many particulars to the centrifugal fiber system innervating the pigeon retina. Degeneration experiments, however, could not distinguish conclusively between a true efferent system, originating in some central nucleus, and a recurrent collateral system branching from afferent optic nerve axons.