The electrical properties of the muscle fibre membrane

Abstract

An analysis is made of the electric properties of frog muscle using the 'rectangular pulse' technique of Hodgkin & Rushton (1946). The experiments were made on isolated fibres and small bundles of the M. adductor magnus and on the M. extensor longus dig. IV. Sub-threshold currents of about 0.1 sec. duration were applied and wave form and attenuation of the extrapolar potential changes determined. In the vicinity of the cathode the relation between current and voltage across the fibre membrane is non-linear, and there is evidence of a local electric response with currents of more than 30% threshold strength. At the anode, however, the membrane behaves as a conductor of approximately constant resistance (section A). At an average temperature of 22 degrees C, the following mean values were obtained: (a) Fibre diameter: $75\mu $ in the bundles of M. adductor magnus, and $45\mu $ in M. extensor longus dig. IV. (b) Characteristic length of the muscle fibres: 0.65 and 1.1 mm. respectively. (c) Membrane time constant: 9 and 18.5 msec. (d) Specific resistance of the myoplasm: about 230 $\Omega \text{cm}$. (e) Transverse resistance of the fibre membrane: 1500 and 4300 $\Omega \text{cm.}^{2}$. (f) Membrane capacity: about 5 $\mu $F/cm.$^{2}$. The numerical differences between isolated fibres and whole muscle arose chiefly from a different value of the membrane resistance, the significance of which is discussed (section section B1 and D). The value of the membrane capacity of muscle is about five times higher than that reported for various other cell membranes and confirmed here for isolated crustacean nerve fibres. The large membrane capacity must be an important factor in determining the slow electrical time scale of muscle. The relations between the electric constants of the resting muscle fibre and some of its physiological properties (time factor of excitation, propagation velocity, rate of decline of the end-plate potential) are discussed (section E). The location of the membrane, or interface, at which electromotive changes occur is discussed, and a number of reasons are given which indicate that the site of the electrotonic potential changes must be at the surface of the muscle fibres.