An electrotonically coupled pathway in the central nervous system of some teleost fish, Gymnotidae and Mormyridae.

Morphological and physiological results indicate that the magnocellular mesencephalic nucleus (MMN) is characteristic of all species of weakly electric fish, so far investigated. The potentials recorded from this nucleus are due to the fish's own electric organ discharge (EOD). After abolition of the EOD by curare the mesencephalic potential is suppressed, but it can still be obtained by artificial electric stimulation. After injection of curare the medullary pacemaker activity responsible for the EOD remains undisturbed. The results, therefore, let us exclude the hypothesis that the mesencephalic response is evoked by recurrent pacemaker activity. The very short delay (1 msec) of the mesencephalic potential is explained by the existence of a rapid conduction electrosensory pathway where the peripheral, rhombencephalic and mesencephalic elements are electrotonically coupled. This kind of transmission provides a constant synaptic delay and a one-to-one input-output ratio. The fish can receive, by means of this electrosensory pathway, a binary signal for each electrical event whether it comes from its own electric organ or from neighbouring fish.