Chemical changes in the adductor muscle of the cheliped of the crayfish in relation to the double motor innervation

Work from this laboratory has shown that a number of the striated muscles of the legs of the crayfish show a typical motor innervation (1). Such muscles are innervated by two motor fibers only, each causing a different type of contraction. One of these, resulting from stimulation of the thicker fiber, is termed the "fast" contraction, while the other is called the "slow." Both anatomical evidence and physiological experiments indicate that each muscle fiber is innervated by these two axons and that both contractions occur in the same muscle fibers (2, 3). The most striking difference between the two contractions is found in the adductor muscle of the cheliped of the crayfish. In this case, a single impulse in the thicker motor fiber causes a twitch-like contraction, and the system behaves in almost all respects as does a single motor unit of a vertebrate muscle: single shocks of different strengths give twitches of a considerable strength and a constant height, and during tetanic contraction the action currents are all of the same magnitude. In contrast to this, single shocks given to the thinner fiber produce no visible response in the muscle, but faradic stimulation causes action currents which grow in height and a contraction with a long latent period. The action currents in this case are always much smaller than those of the fast contraction. The chemical changes occurring in the adductor muscle of the cheliped of the crayfish Cambarus clarkii during these two types of contraction were investigated in order to obtain evidence on two possible mechanisms by which the two contractions might occur in the same muscle fiber. In the first place, two contractile substances might be present, a "phasic" and a "tonic" one, analogous to those supposed by Botazzi (4) in the vertebrate striated muscle, or the same substance might contract in both cases, the difference between the contractions being due solely to differences in the transmission mechanism between the nerve impulse and the contractile substance. Methods were devised for the rapid removal of the stimulated cheliped to liquid air and for the analysis of the frozen muscle tissue. The changes in phosphate and in lactic acid content were used as indices of the chemical changes occurring