Sub/supraependymal axonal net in the brains of sharks and probable targets in parasynaptic relationship.

A sub/supraependymal system of varicose axons in the tectal midline ridge formation of shark brains is described. Lengths of axons and their terminals are seen freestanding in the cerebrospinal fluid (CSF) as well as in close contiguity with the large CSF-contacting and supraependymal mesencephalic trigeminal neurons of the midline ridge formation. The relationship of axonal varicosities and terminals to both CSF and supraependymal somata is such as to suggest a nonsynaptic association. This finding is discussed in the context of its utility in testing the concept of parasynaptic modulation in vertebrate brains. The topographical arrangement described is one in which the essential operational elements of parasynaptic activity can be identified in a neuronal assemblage whose functional association is known.