Action potential and mechanical response of isolated cross striated frog muscle fibres at different degrees of stretch.

Abstract

Summary. 1) Simultaneous recording of the action potential and the mechanical response showed that the rising phase of the intracellular action potential had traversed the entire length of the fibre before the first sign of twitch tension appeared. Correcting for the time necessary for propagation of the action potential to the leading-off electrode the extracellular action potential terminated before the middle of the mechanical latency (Fig. 3). 2) As a function of stretch the conduction velocity of the action potential remained unaltered up to a stretch of about fifty per cent. With further elongation it increased, the increase being the greater the larger the fibre diameter. The maximum increase in velocity was fifty per cent with 100 per cent stretch in fibres of 135 μ diameter (Fig. 4). The increase in conduction velocity could not be accounted for by assuming a “folded membrane”. It is explained in terms of a change in the specific resistance and capacitance of the muscle fibre membrane with stretch. 3) The decrease in amplitude of the action potential with stretch amounting to 40 per cent at 100 per cent stretch is attributable to the decrease in the cross sectional area of the muscle fibre (Fig. 6). 4) There was no correlation between the decrease in twitch tension and the changes in the electrical response with stretch. At 100 per cent stretch where twitch tension was practically zero the amplitude of the extracellular action potential was reduced by only forty per cent (Fig. 5). 5) The 50 per cent decrease in conduction velocity when transferring the muscle fibre to humid air was associated with a decrease in twitch tension of 20 per cent, with an increase in the mechanical latency of about 50 per cent and a decrease in the maximum rate of rise of the twitch tension of 30 per cent. The shorter duration and wave length of the action potential with the fibre in air is suggested to be the cause of the changes in the mechanical response (Table 1).