Neuromuscular systems in neurogenic arthropod hearts

Abstract

Neuromuscular transmission has been studied in detail by various authors in neurogenic hearts of decapod and stomatopod crustaceans, horseshoe crabs, and spiders. In these hearts, bursts of impulses generated in the cardiac ganglion at regular intervals produce depolarizations of the muscle fibers. Each depolarization is associated with a heart contraction. The depolarization is composed of many excitatory junction potentials (ejp's), each produced by a single nerve impulse. There is no evidence in Homarus, Squilla, or Limulus hearts that single ejp's or composites of ejp's give rise to regenerative membrane responses; in these hearts, spontaneous depolarizations never overshoot the zero reference level. Overshooting occurs in certain crab and crayfish hearts, and it is possible that muscle fibers of these hearts produce regenerative membrane events.The muscle fibers of Limulus, Tachypleus and Homarus hearts are polyneuronally innervated.Pulse stimuli applied to nerve branches evoke ejp's that facilitate in hearts of Squilla and Homarus. In addition to facilitation in Homarus, there is also depression; at certain frequencies of stimulation both facilitation and depression can be observed. Experiments in tarantula, Limulus, and Homarus hearts show that L-glutamic acid mimics the natural transmitter substance. © 1973 by the American Society of Zoologists.