On the Excitation of Crustacean Muscle

Abstract

1. The nature of inhibition in the muscles of the limbs of Carcinus maenas has been studied. In all the muscles there is a definite threshold for inhibition which is normally higher than the threshold for excitation. Under the influence of inhibition the rate of contraction of the muscle is depressed. This depression is greatest at low frequencies of excitation. It is scarcely appreciable at higher frequencies. This absolutely precludes the possibility that inhibition is connected with the Wedensky phenomenon. 2. It is shown that inhibition is essentially due to a depression of neuromuscular facilitation, so that compared with normal excitation a higher frequency of stimulation is required to cause a response by summation. 3. Certain evidence suggests that inhibition is of a humoral nature. The effects of inhibition last longer than the duration of excitation of the inhibitory nerve, and there is slight evidence that an inhibitory substance may be transmitted by artificial perfusion so as to cause depression of facilitation in a second muscle. 4. The relation of excitation and inhibition in the reciprocal action of the extensor and flexor muscles of the dactylopodite of the limb is examined. In both muscles excitation occurs at a lower intensity than inhibition. The reciprocal responses obtainable in the amputated limb are due to the respective thresholds of the excitor and inhibitor nerves of each muscle. A weak stimulus excites the extensor, but has no action on the flexor. A strong stimulus inhibits the extensor and excites the flexor, these thresholds being close together. A still stronger stimulus inhibits the flexor. This interpretation differs from that originally given by Biedermann, and the reasons for this are discussed. 5. The functional significance of inhibition is discussed. It is pointed out that as a means of bringing contraction to an end inhibition is less efficient than mere absence of excitation. But it is also pointed out that in the resting limb there is probably a background of low-frequency excitation continually present. By itself this suffices only to maintain the muscle in tone. Under these conditions, however, the muscle can be made to contract by a brief series of rapid stimuli (Pantin, 1936). Such a contraction once produced is thereafter maintained and can only be brought to an end by inhibition. This mechanism is contrasted with neuromuscular action in the skeletal muscle of the vertebrates.