Special cutaneous receptor organs of fish: The tuberous organs of Eigenmannia

Abstract

AbstractThe special cutaneous receptor organs of the fresh water weakly electric fish have previously been proposed to be electroreceptors. In the gymnotid, E. virescens, two types of special cutaneous receptor organs, ampullary and tuberous, are distinguished from each other, as well as from the ordinary lateral line receptor organs, by their characteristic distribution and size. The tuberous organs usually contain 25 to 35 elongate nonciliated receptor cells within a cellular capsule. A single layer of supporting cells is present between the base of the receptor cells and the base of the capsule. A single thin myelinated nerve fiber innervates each group of organs and branches so that the base of each receptor cell is supplied with a single nerve ending. Synaptic contact is made at many points on each nerve ending. The synapses are characterized by fingers of receptor cell cytoplasm which contain dense presynaptic rods. The organ capsule is open toward the surface of the fish. A cellular plug partly obscures the opening, but continuity is maintained between the intracapsular fluid and the external water. Microvilli, projecting from the surfaces of the receptor cells, maintain an open gap between adjacent receptor cells. About 95% of the surface area of these cells is therefore in contact with the fluid. The functional implications of some of the ultrastructural observations are discussed.