Electrical and dye coupling in an identified group of neurons in a coelenterate.

A compressed network of "giant" neurons, lying within the inner nerve-ring of the hydrozoan jellyfish Polyorchis, functions as the overall pattern generator and the motor neuron system for the subumbrellar swimming musculature. The neurons that form the network are all electrically coupled. The coupling is tight, so that action potentials and slow membrane-potential oscillations are synchronous throughout the network. The fluorescent dye Lucifer Yellow CH passes throughout the network following iontophoretic injection into a single neuron. The sites of both current and dye passage are presumably the numerous gap junctions which are found where the giants run together. Based on the morphological identification of the giant network from the dye injections and ultrastructural studies, the electrophysiological data on the firing pattern and input--output relations of the network, and its position relative to other neurons in the inner nerve-ring, the giant network can be considered an identified neuronal group.