Electroreception

Many groups of fish have receptors that are specialized for the detection of electric fields. The existence of these electroreceptors was first clearly indicated in weakly electric fish, which continually set up low voltage electric fields around themselves by means of their electric organs. If the water over a receptor opening is replaced by air, a local electrode need pass much less current to produce a given voltage change outside the receptor; the shunting by the water is reduced. Although the degree of residual shunting by the skin and remaining water has not been satisfactorily evaluated, the current voltage relation measured under these conditions is likely to have considerable contribution from the receptor cells because the input resistances become large, as high as several megohms. Receptor cells of phasic receptors contribute importantly to the external potentials under these conditions. Another important characteristic of the input–output relationship of the receptor synapses is sensitivity. The input–output relationship of synaptic membrane of phasic receptor cells differs from that of tonic receptor cells in that there is little resting release of transmitter. It is difficult to be confident of the shape of the potential-secretion relationship because of active processes in the receptor cells. It is likely but uncertain that the secretory membrane of phasic receptors does have an input–output relationship that has a much greater slope than that at known interneuronal and neuromuscular synapses.