In response to an instantaneous stimulus, such as occurs in the secondary coil on breaking the primary current, a muscle gives a momentary twitch or spasm, commonly spoken of as a single contraction, which is of so short duration, that without the graphic method of recording the motion we could not appreciate the phases which are seen in the curve.

The curve drawn on the recording surface of a pendulum myograph, by such a single contraction, is represented in Fig. 185. The short vertical stroke on the abscissa, or base line, is drawn by touching the lever when the muscle is in the uncontracted state, and indicates the time of stimulation. The upper curved line is drawn by the lever during the contraction of the muscle.

In such a curve the following stages are to be distinguished: -

1. A short period between the moment of stimulation and that at which the lever begins to rise, during which the muscle does not move. This is known as the latent period. In the skeletal muscles of the frog this period lasts nearly.01 sec.

2. A period during which the lever rises, at first slowly, then more quickly, then again slowly, until it ceases to rise. This stage has been called the period of rising energy. It lasts about.04 sec.

3. When the highest point is attained the lever commences to fall, at first slowly, then more quickly, and at last slowly.

Curve drawn by a frog's gastrocnemius on the Pendulum Myograph; below is seen the tuning fork record of the time occupied by the contraction.

Fig. 185. Curve drawn by a frog's gastrocnemius on the Pendulum Myograph; below is seen the tuning-fork record of the time occupied by the contraction. Parallel to the latter is the abscissa. The little vertical mark at the left shows the moment of stimulation, and the distance from this to the beginning of the rise of the curve gives the latent period, which is followed by the ascent and descent of the lever.

There is then no pause at the height of contraction. The stage of relaxing has been called the period of falling energy. It occupies, when quite fresh, about the same time as the second period, viz., about.04 sec.

Thus, a stimulus occupying an almost immeasurably short time sets up a change in the molecular condition, which, taking nearly 1/10 sec. to run its course, and requiring 1/100 sec. before it exhibits any change of form, then in 4/100 sec. attains the maximum height of contraction, and, without waiting in the contracted condition, spends 4/100 sec. in relaxing.

The latent period which appears in a single contraction curve drawn by a muscle stimulated in the usual way, through the medium of a nerve, is not entirely occupied by preparatory changes going on in the substance of the muscle, but a certain part of the time recorded as latent period corresponds to the time required for the transmission of the impulse along the nerve. This may be shown by stimulating first the far end of the nerve, and then the muscle itself. In this case two curves will be drawn, having different latent periods, that obtained by direct stimulation of the muscle being shorter, and representing the real latent period, while the longer one includes the time taken by the impulse to travel along the piece of nerve between the electrodes and the muscle.

Wave Of Contraction

If one extremity of the muscle be stimulated without the aid of the nerve (it is best to employ a muscle from a curarized animal), the contraction passes along the muscle from the point of stimulation in a wave which travels at a definite rate of 3-4 metres per sec. in a frog, and 4-5 metres per sec. in a mammal. Reduction of temperature and fading of vital activity cause the velocity of the wave to be lessened, until finally the tissue ceases to conduct; then only a local contraction occurs, severe stimulus causing simply an elevation at the point of contact. This seems analogous to the idio-muscular contraction, which marks the seat of severe mechanical stimulation after the general contraction has ended.

Variations In The Phases Of A Single Contraction

The latent period varies much in different kinds of muscle, in the same kind of muscle of different animals, and in the same individual muscle under different conditions. As a rule, the slow-contracting muscles have a longer latent period. Thus the non-striated slow-contracting muscles found in the hollow viscera have a latent period of some seconds. The striated muscles of cold-blooded animals have a longer latency than the same kind of muscle in birds and mammalia. The same gastrocnemius of a frog has a shorter latent period when strongly stimulated, or when its temperature is raised, and vice versa.

The latent period is considerably lengthened by fatigue. If the weight be so applied that it does not extend the muscle before contraction, but only bears on it the instant it commences to shorten, the duration of the latent period increases in proportion to the weight the muscle has to lift.

The duration of the single contraction of striated muscle varies in different cases and under varying circumstances. With submaximal stimulation the length of the curve increases with the strength of the stimulation. When the maximal strength of stimulus (i. e., that exciting a maximal contractiori) is reached, no further lengthening of the curve takes place.

Curves drawn by the same muscle in different stages of fatigue.

Fig.186. Curves drawn by the same muscle in different stages of fatigue - A, when fresh; B, C, D, E, each immediately after the muscle had contracted 200 times. Showing that fatigue causes a low, long contraction.