Electroreceptive single units in the mesencephalic magnocellular nucleus of the weakly electric fish gymnotus carapo

Abstract

1. Extra- and intracellular recordings from single units in the magnocellular mesencephalic nucleus (MMN) of the torus semicircularis, related to the fast electrosensory system are reported for the weakly electric fish Gymnotus carapo (Gymnotidae). 2. The non-spontaneously active units responded with single action potentials to the electric organ discharge (EOD) and to artificial electrical pulses with a very short latency of 0.8–1.5 ms. This strongly suggests, in agreement with morphological data, that transmission takes place through electrical synapses. 3. The dynamic range (probability and latency of the single action potential) of the response is extremely narrow and about the same as found in the relevant electrosensory fibres. Intracellular stimulation gives the same response characteristics and dynamic range. 4. The recovery of the response was studied in detail using different stimulus combinations of double pulses at varying delays. Under all conditions, the recovery period to evoke a test response after a conditioning stimulus and response increased in length with the strength of the conditioning stimulus. Inversely, the conditioning stimulus to prevent the unit from firing again had to be stronger as the delay between the two stimuli was increased. 5. Since there is no evidence of neural inhibition causing the long lasting and graded recovery characteristics for MMN units, an attempt was made to explain the findings by classical neurophysiological considerations adapted for electrical synaptic transmission (“current sink” theory). 6. This neural mechanism means that, if at all, the relatively weaker stimulus is not responded to, which protects the fish from being jammed by external pulses of physiological amplitude. In contrast, very strong foreign pulses can completely abolish responses to own EODs especially when timed appropriately. Both effects are discussed in view of their significance for the fish's electrosensory system and communication.