Special Cutaneous Receptor Organs of Fish: Synaptic Structures of Eigenmannia

Abstract

Special cutaneous receptor organs of fresh-water weakly electric fish are grossly similar to other receptor organs of the acoustico-lateralis system, but differ in numerous cytological details. They are presumed to function as detectors of weak electric currents. Those of Eigenmannia are of two types that have been designated tuberous and ampullary receptor organs. Parts of the lateral and basal surfaces of the receptor cells are in contact with accessory cells. A single relatively large, almost spherical, nerve ending is inserted between the basal end of each receptor cell and the subjacent accessory cell, lying within a hemispherical concavity in the base of the receptor cell.The presumed synaptic areas between each receptor cell and the nerve ending associated with it are not contiguous, but are morphologically discrete structures, separated by nonsynaptic areas of membrane (Fig. 1). Approximately thirty synaptic areas are found on each receptor cell in both kinds of organs. The surface contours of the apposed cell membranes are irregular in the nonsynaptic areas, and hence the extracellular space is variable in width. In the synaptic areas the membranes of both cells appear slightly thickened (Fig. 2), and are separated by a constant gap of about 200 Å. Fine fibrillar material spans the synaptic gap.