Slow and Fast Responses in Cockroach Leg Muscle

Abstract

1. Intracellular recording from individual muscle fibres of the flexor tibiae of the cockroach shows two types of action potentials. One of these represents fast fibre activity, while the other represents slow fibre activity in this muscle. 2. The fast response consists of an all-or-none potential varying from 40 to 85 mV., with 6o% of the responses between 55 and 70 mV. This potential has an overshoot over the resting potential, and is followed by an absolute and a relative refractory period. For these reasons it is considered likely that the potential is a propagated one. The slow response is a small, facilitating potential, varying from 8 to 20 mV., and is quite likely a local phenomenon. 3. All the evidence indicates that the two types of activity do not take place in the same muscle fibres, but rather represent the responses of two different types of fibres, as is the case in some amphibian muscles. 4. Gradation of activity in the fast system seems to depend solely on the activation of a very small number of motor units and on mechanical summation. The response of this system shows little, if any, facilitation as the frequency of stimulation is increased. The facilitating slow system may be concerned with slower, more finely graded movements. 5. The resting potential in roach muscle has an amplitude of 45 ±9 mV., which falls within the range expected on the basis of the intracellular and extracellular potassium concentrations. 6. Slow and fast fibre activity in insects and Crustacea is discussed, and a revision of the earlier work on Crustacea with the use of intracellular recording is suggested.