Abstract
Intracellular recordings from the muscle fibers in contracting hearts of the crayfish and the lobster show a sequence of potential changes superficially resembling the vertebrate cardiac action potential. In the Crustaceans, however, the plateau part of the depolarization is produced by fused junctional potentials, elicited by the firing of motor neurons whose cell bodies lie in the cardiac ganglion. The first or second JP elicits an active membrane response that probably is produced by Ca^2+^ influx, since it is blocked by Mn^2+^. Local currents can flow between the individual muscle fibers, presumably by way of gap junctions. Simultaneous records were made extracellularly from one of the efferent nerves coming from the lobster cardiac ganglion and intracellularly from a muscle fiber. Four motor axons were identified in the efferent nerve and the firing of each of these sets up a JP at each point on the muscle membrane. A fifth motor neuron may also run to each muscle fiber. In the isolated heart the motor outflow from the cardiac ganglion is quite variable. Nevertheless, the system is such that the potential changes on the muscle membrane are almost indistinguishable from beat to beat. The hermit crab cardiac muscle usually has only two JPs per beat; the first JP sets up an overshooting spike.