The onset of shortening in striated muscle

Abstract

When a muscle is stimulated a change occurs such that, at the end of the latent period, the contractile mechanism is already fully active. This has been demonstrated by Hill (1949cHill ( , 1950, using rapid stretches. The present experiments show that at the end of the latent period the part of the muscle at the point of stimulation begins to shorten with its maximum speed, i.e. transition from rest to full activity is abrupt.When the stimulus is applied to one end of a muscle the part of the muscle distant from the electrodes becomes active only after an extra delay representing the time taken for activity to spread along the muscle to that part. The shortening of the whole muscle therefore begins gradually; the abrupt change in each element is masked by a time dispersion between the elements.The velocity of propagation of the contraction wave, as distinct from the electrical excitation wave, along the muscle has been studied. Hill (1949b) made allowance for the time of spread of activity along the muscle in order to obtain more precisely the heat production in a twitch. This propagation time was deduced from the first derivative of the heat curve. Similar arguments apply to the effect of propagation time on the form of the shortening curve during a twitch. Comparison of the onset of shortening under a small load when a muscle is directly stimulated at one end, with that when the same muscle is simultaneously stimulated at many points along its length, has enabled the propagation velocity of activity to be calculated and related to that of the action potential. Parallel fibred muscles were required for these experiments, in which the fibres ran the length of the muscle, or, at least, in which no two independent fibres were joined end to end. Sartorius muscles from English frog (Rana temporaria) and toad (Bufo bufo) were used in the first group, and coracohyoid muscles from medium-sized small-spotted dogfish (Scyllium canicula) in a later group of experiments. METHOD An electrode assembly was constructed by Mr A. C. Downing which enabled a muscle to be stimulated either at one end or at many points along its length simultaneously. Twelve pieces of 32 s.w.g.(0.27 mm.) silver wire were embedded as electrodes 2-5 mm. apart in a perspex block, and the

Inward spread of activation in vertebrate muscle fibres

“…in muscle; Eccles, Katz & Kuffler, 1941;Abbott & Ritchie, 1951), and it seems at first sight unlikely that such a high Q10 would be given by a passive electrotonic system. A Q10 of only about 1-3 is to be expected from the change of resistivity of the contents of the tubules.…”

The toxin fromGymnodinium veneficumBallantine

“…Stimulation could be applied to either nerve or muscle. When the anterior byssus retractor muscle of Mytilus was used the muscle was mounted on a multi-electrode assembly and again connected to the same recording apparatus (Abbott & Ritchie, 1951). Stimulation was by square wave pulses from an electronic stimulator.…”

Contraction in molluscan smooth muscle

“…ACTIVE SMOOTH MUSCLE In the case of the PR muscle, measurements were made of the speed at which the mechanical events are propagated along the length of the muscle. The method used for this experiment has been described by Abbott & Ritchie (1951). Unloaded isotonic twitches obtained with stimulation at one end only were compared with those resulting from all-over multi-point stimulation.…”