Differences in muscle fibre properties as a factor in “fast” and “slow” contraction in Carcinus

Abstract

1. 1. An investigation was made of the electrical and mechanical responses to direct and indirect stimulation in Carcinus closer muscles. 2. 2. Three main types of muscle fibre could be distinguished on the basis of electrical responses to indirect stimulation. They were called Types A, B and C. 3. 3. Type A muscle fibres showed large “fast” p.s.p.'s and electrically excitable membrane responses, but usually no response to “slow” axon stimulation. Type B muscle fibres showed large, slowly decaying “slow” p.s.p.'s and small or no “fast” p.s.p.'s. In Type C muscle fibres “fast” and “slow” p.s.p.'s were both present. Both were small at low frequencies of stimulation of the motor axons but increased in size with increasing frequency of stimulation. 4. 4. The electrical properties of the membranes of the three muscle fibre types were found to differ. Type A muscle fibres had low membrane resistances, small time constants and electrically excitable membranes. Type B muscle fibres had high membrane resistances, large time constants and electrically inexcitable membranes. Type C muscle fibres had intermediate properties. The electrical responses to indirect stimulation could be largely explained in terms of the membrane properties of the fibres from which they were recorded. 5. 5. The average membrane potential at which detectable tension was developed by the muscle in response to potassium depolarization was 55 mV. 6. 6. Tension was developed by single directly stimulated Type C muscle fibres when the depolarization at the current-passing microelectrode was 42 mV to 58 mV. 7. 7. During both fast and slow contractions, electrical responses can be observed which exceed the estimated membrane potential threshold for tension development.