Fine structure of the medullary command nucleus of the electric organ of the skate.

Many axodentritic and axosomatic synapses of chemical type exist in the medullary command nucleus which controls the electric organ discharge in the skate (Raia clavata and Raia basis). The pre- and postsynaptic membranes are separated from each other by a space of about 250 A and they lack regions of tight junction, which is thought to form the morphological basis of electrical synapses elsewhere. Direct neuronal soma-soma contacts, occasionally encountered in the command nucleus, are desmosome-like with no region of tight junction. In contrast to conditions observed in many other electric fish, the apparent absence of electrical synapses in the skate can be correlated with the asynchronous nature of its electric organ discharge.