On the time required for diffusion and its relation to processes in muscle

Abstract

Two problems of diffusion are considered in relation to muscle: (A) How rapidly can chemical changes occurring inside a muscle be detected and measured outside by some device (e. g. an electrode) specifically sensitive to one of the products capable of diffusing out? (B) If, as seems probable, the process of excitation is limited to the surface of a fibre, could diffusion be rapid enough for contraction to be propagated inwards by the arrival at each point of some chemical substance liberated at the surface? As regards (A), it is shown that an indication of chemical change can be given rapidly if the layers of tissue and fluid between muscle fibres and recording surface are kept thin, but that the attainment of final equilibrium is too slow to allow accurate following of changes occurring at all quickly, unless the layers of tissue and fluid are reduced to a few microns in thickness. As regards (B), it is shown that if the twitch of a muscle fibre is assumed to involve the contraction of the whole of its contents, diffusion from the outer surface could not be fast enough to account for observed speeds of contraction. If, however, in a twitch the outer region only, say one-half of the fibre, were involved, which agrees with the known fact that the ratio of twitch tension to tetanus tension is usually considerably less than unity, then diffusion over the shorter distances involved would be amply quick enough. The considerable difference between the temperature coefficients of the rates of diffusion and contraction shows that the time course of contraction cannot be determined by diffusion. The times calculated for diffusion, however, are short enough to allow us to suppose, if we wish, that the contractile process begins at any point when the concentration there of some hypothetical substance liberated at the surface during excitation, and diffusing inwards, reaches a critical value. The connexion between excitation and contraction is discussed.