Neuronal control of locomotion in hydrozoan medusae

Summary

• 1. The swimming control systems of 13 hydromedusan species were examined electrophysiologically and morphologically. Despite obvious differences in the structure, behavior and life style of these medusae, the basic organization of swimming system components is similar.

• 2. Motor neurons that activate swimming muscles are located in the inner nerve-ring comprising electrically-coupled condensed networks (Figs. 2, 3). Individual neurons of these networks are of far larger diameter than other neurons of the nerve-rings (Figs. 4–6).

• 3. Spontaneous activity persists in the swim motor neuron networks in seawater containing excess Mg⁺⁺ suggesting that the network may perform a pacemaker function.

• 4. The swimming muscle sheet includes circular, striated epitheliomuscular cells of the subumbrella and velum, and an interposed non-muscular epithelial region which overlies the inner nerve-ring (Fig. 1). Gap junctions are common throughout this tissue sheet (Figs. 13, 14, 18). Electrical(Fig. 10) and dye-coupling (Fig. 3) of cells in this sheet suggests that direct current spread between myocytes is important in transmission of excitation throughout the subumbrella.

• 5. Recordings from epithelial cells immediately adjacent to swim motor neurons reveal graded potentials presumably of synaptic origin (Figs. 2, 10). The synaptic potentials, and muscle action potentials, are blocked in seawater containing excess Mg⁺⁺. Synaptic contacts were observed between swim motor neurons and overlying epithelial cells throughout the inner nerve-rings of all medusae examined (Figs. 19–21).