Neuromuscular Properties of Mesenteries from the Sea-Anemone Metridium

1. A method is described for a simultaneous electrical and mechanical recording from isolated mesenteries of the sea-anemone Metridium senile (L.). 2. The retractor muscle gives quick and slow contractions. Shock intervals of 0.15-2 sec. produce facilitated twitches on all stimuli after the first, usually followed by a slow contraction. Trains of stimuli at lower frequencies are followed by slow contractions only. Slow contractions may also arise spontaneously. 3. Two types of potential were found using external suction electrodes, provisionally interpreted as nerve-net impulses and muscle action potentials. The first is an all-or-none compound pulse, whose brief components summate (1-3 msec, height up to 0.4 mV.). Conduction speed in an expanded anemone would be 70-120 cm./sec. at 14-16° C., corresponding to that of the through-conduction system. The second always precedes a twitch and is a smooth, graded potential lasting 50-100 msec. Its size, up to 1.2 mV., is inversely related to stimulus interval, but unlike the quick mechanical rseponse it shows but slight initial facilitation. A muscle potential follows a nerve-net impulse and is propagated at the same speed. The refractory period of the nerve-net probably exceeds that of the muscle. Potentials showing similar features have been recorded from the oral disk and tentacles. 4. These results support the suggestion that the sites of facilitation and initiation of contractions are neuromuscular junctions throughout the retractor muscle. Quick contractions are co-ordinated by the nerve-net, whereas it is possible that slow contractions, however initiated, may at least in part be propagated in the muscle itself.