Fig. 4.

Ampullary electroreceptors of fish. a This example from a sturgeon shows a receptor cell terminal (R) containing synaptic ribbons with their ends (with active zones) invaginating/protruding into the large postsynaptic process (called a nerve terminal by the authors; T; modified from figure 3B in Teeter et al. 1980; reprinted with permission from Springer). Scale bar is 500 nm. b, c Drawings of electroreceptor synapses from a transparent catfish (b; Szamier and Bennett 1973) and a weakly electric gymnotid (c; Szamier and Wachtel 1969). Both have irregular presynaptic rods surrounded by vesicles. In c, the rod branches into the two lobes of the ending of the invaginating terminal protrusion. Note also the postsynaptic cluster of vesicles adjacent to the lobes; most are larger than the presynaptic vesicles. Compare to vesicles of some of the horizontal processes in photoreceptor invaginations in figure 7; are they both examples of presynaptic vesicles in a postsynaptic process?