The greatest difference is observed in the muscles found in different kinds of animals. The contraction of some kinds of muscle tissue (non-striated muscle of mollusca, for example) occupies several minutes, and reminds one of the slow movement of protoplasm; while the rapid action of the muscle of the wing of a horsefly occurs 330 times a second. Various gradations between these extremes in the rapidity of muscle contraction may be found in the contractile tissues of different animals. The following table gives the rate of contraction of some insects' muscles, which may help to show the extent of these variations: -
. 330 contractions per second.
.... 190 " "
9 " "
Among the vertebrata the duration of the contraction of the skeletal muscles varies considerably, according to the habits of the animal. The limb muscles of the tortoise and the toad take a very long time to finish their contraction; other muscles of the same animals act more quickly, but do not attain the rapidity of contraction of the skeletal muscles of warm-blooded animals.
The duration of a single contraction of the same muscle is also capable of considerable variation. It seems to be lengthened by anything that leads to an accumulation of the chemical products which arise from muscle activity. Hence fatigue or overstimulation causes a slow contraction (Fig. 186).
Fig. 187. Six curves drawn by the same muscle when stretched by different weights. Showing that as the weight is increased the latency becomes longer and the contraction less in height and duration.
Fig. 188. Curves drawn by the same muscle at different temperatures. Showing that with elevation of temperature the latency and the contraction become shorter. (The muscle had been previously cooled).
Moderate increase of temperature greatly shortens the time occupied by the single contraction of any given muscle. Excessive heat causes a state of continued contraction.
The reduction of temperature causes a muscle to contract more slowly, and when extreme, the muscle remains contracted long after the stimulus is removed.
The altitude of the curve which represents the extent of the contraction varies in the same way as the latent period and the duration.
Fig. 189. Curves drawn by the same muscle while being cooled. Showing that the latency and the contraction become longer as the temperature is reduced.
The extent to which a muscle will contract depends upon the conditions in which it is placed, and varies with the load, its irritability, the temperature, and the force of the stimulus. A fresh muscle at the ordinary temperature, with a medium load, will contract more and more as the intensity of the current employed increases. There is a limit to this increase, and with comparatively weak stimulation an effect is produced which cannot be surpassed by the same muscle with further increment of stimulus. The height of the contraction is the same for all medium stimuli while the muscle is fresh. This is called the maximum contraction, being the greatest shortening which can be produced by a single stimulus.
Fig. 190. Pendulum Myograph tracings showing summation.
1. Curve of maximum contraction drawn by first stimulus, the exact time of application o which is shown by the small upstroke of the left hand of the base line.
2. Maximum contraction resulting from second simple stimulation given at the moment indicated by the other small upstroke.
3. Curve drawn as the result of double stimulation sent in at an interval indicated by the distance between the upstrokes, showing summation of stimulus and consequent increase in contraction over the " maximum contraction".
Each time a muscle receives an induction shock of medium strength, it responds with a "maximal contraction," but this is not the maximum amount the muscle can contract with repeated stimulation. If a second stimulus be given while the muscle is in the contracted state, a new maximum contraction is added to the contraction already arrived at by the muscle at the moment of the second stimulation. If stimulated when the lever is at the apex of the curve, the sum of'the effect produced will be equal to two maximum contractions.
If applied in the middle of the period of the ascent or descent of the lever, a second stimulation gives rise to i}4 maximum contractions, and so on, in various parts of the curve, a new maximum curve is produced, arising from the point at which the lever is when the second stimulus is applied (Fig. 190).
During the latent period a second stimulation produces the same effect, but the summation only begins at the end of the latent period of the second contraction, when the effect of the first stimulus is as yet small. It is difficult to demonstrate the summation when the stimuli are very close, but if the second stimulus comes after an interval of more than 1/600 sec, summation can easily be appreciated.
This summation of effect also takes place when the stimulus is insufficient to produce a maximum contraction. The first few weak stimuli give rise to the same extent of contraction as if the muscle were at its normal length at the time of each successive stimulation. The following tracings (Figs. 191-193) show the effects of repeated stimulations applied at the various periods indicated by the numbers on the abscissa line.