1. The response of certain limb muscles in Carcinus maenas to stimuli of different frequencies and intensities has been analysed. The precautions necessary to obtain reproducible results in crustacean muscle are recorded. The material must be fresh; the duration of stimulation short; and each individual shock must be less than the true chronaxie, to prevent multiple excitation of the nerve.
2. A single stimulus produces a microscopic response or none at all. A succession of shocks, however, causes a contraction, the rate of which increases with the frequency, till this reaches the high values of 300-400 shocks per sec. The rate of contraction varies absolutely continuously with the frequency from 300 per sec. down to the microscopic response observed at less than 10 per sec. The rate of contraction increases very rapidly indeed between frequencies of 50 and 200 per sec, so that this range includes almost all rates of contraction.
3. The limiting frequency of 300-400 per sec. is close to the refractory period. For pairs of stimuli, the absolute refractory period is about 1σ at 18° C. This is followed by a relative refractory phase and sometimes by a supernormal phase. The excitability has returned to normal after about 4σ. In repetitive stimulation the absolute refractory period lengthens.
4. With stimuli of increasing intensity, the responses of both flexor and extensor muscles show first a threshold for excitation of the motor nerve, and, at a higher intensity, a threshold for inhibition. At very high intensities (10-20 times the true threshold) large contractions may be obtained owing to repetitive excitation.
5. With suitable precautions it can be shown that between the threshold of excitation and the threshold of inhibition there is great independence between the response and the intensity of the stimulus. The system behaves as a single excitable system and possibly in some cases a single axon supplies the entire muscle.
6. The chronaxie of the nerve to single shocks and to repetitive stimulation is of the order of 0.2-0.4σ. Single shocks of high intensity give multiple excitation, and the thresholds for this simulate a chronaxie curve. False chronaxies up to 30σ can be obtained in this way.
7. There is no evidence of a double excitable system in the muscles of the walking leg of Carcinus such as has sometimes been recorded in crustacean claws. There is no doubling of intensity-duration or refractory period curves.
8. All the effects observed are explicable in terms of neuromuscular facilitation. The response is governed entirely by the frequency and number of stimuli. Each shock in a series brings more and more muscle fibres into action. With increasing frequency of stimulation, not only are there more contraction increments in a given time, but the increment following each shock is larger.
9. At low and moderate frequencies the rate of development of tension is governed by the rate at which impulses reach the muscle. At the highest frequencies a limit is set to the rate of contraction by the physical properties of the muscle.
10. There is a close analogy between the neuromuscular mechanism disclosed here and the neuromuscular mechanism of the Coelenterata. In both there is a tendency for an entire effector to behave as a single system in which the response is governed by the number and frequency of impulses received by the muscle. This system is distinguished sharply from that of vertebrate skeletal muscle in which gradation of response is brought about through the multiplicity of motor units.
On The Excitation of Crustacean Muscle
