The negative variation of a single contraction can be easily shown on the sensitive animal tissues. For this purpose the sciatic nerve of a frog's leg is placed upon the surface of the gastrocnemius of another leg, so as to pass over the middle and the extremity of the muscle. When the second (stimulating) muscle is made to contract, its negative variation acts as a stimulus to the nerve lying on it, and so the first (stimulated) muscle contracts. Not only does this show the negative variation of a single contraction, but it also demonstrates that the continued (tetanic) contraction, produced by interrupted electric stimulation, is associated with repeated negative variations. We shall see that the continued contraction is brought about by a rapidly repeated series of stimulations, so that the electric condition of the stimulating muscle undergoes a series of variations. The contraction of the stimulated muscle, whose nerve lies on the stimulating muscle, responds to the electric variations of the stimulator, and contracts synchronously with it.
Fig. 184. Diagram illustrating the arrangement in the Rheoscopic Frog.
A = stimulating limb. B = stimulated limb. The current from the electrodes passes into nerve (N) of stimulating limb (A), causing its gastrocnemius to contract. Whereupon the negative variation of the natural current between + and - stimulates the nerve (N'), and excites the muscles of 13 to action.
If an isolated part of a muscle be stimulated, the contraction passes from that point as a wave to the remainder of the muscle. This contraction wave is preceded by a wave of negative variation which passes along the muscle at the rate of three metres per second (the same rate as the contraction wave), lasting at any one point.003 of a second, so that the negative variation is over before the contraction begins, for the muscle requires a certain time, called the latent period, before it commences to contract.
The origin of the electric currents of muscle will be discussed with nerve currents, to which the reader is referred.
Long since it was observed in the human subject that the temperature of muscles rose during their activity. In frog's muscle a contraction lasting three minutes caused an elevation of.18° C. A single contraction is said to produce a rise varying from.001° to.005° C, according to circumstances.
The production of heat is in proportion to the tension of the muscle. When the muscles are prevented from shortening, a greater amount of heat is said to be produced.
The amount of heat has also a definite relation to the work performed. Up to a certain point the greater the load a muscle has to move, the greater the heat produced; when this maximum is reached any further increase of the weight causes a falling off in the heat production. Repeated single contractions are said to produce more heat than tetanus kept up for a corresponding time.
The fatigue which follows prolonged activity is accompanied by a diminution in the production of heat.
The most obvious change a muscle undergoes in passing into the active state is its alteration in shape. It becomes shorter and thicker. The actual amount of shortening varies according to circumstances. (a) A muscle on the stretch when stimulated will shorten more in proportion than one whose elasticity is not called into play before contraction, so that a slightly weighted muscle shortens more than an unweighted one with the same stimulus. (b) The fresher and more irritable a muscle is, the shorter it will become in response to a given stimulus; and, conversely, a muscle which has been some time removed from the body, or is fatigued by prolonged activity, will contract proportionately less. (V) Within a certain limit, the stronger the stimulus applied the shorter a muscle will become. (d) A warm temperature augments the amount of shortening, the amount of contraction of frogs' muscles increasing up to 330 C. A perfectly active frog's muscle shortens to about half its normal length. If much stretched and stimulated with a strong current it may contract nearly to one-fourth of its length when extended. Muscles are seldom made up of perfectly parallel fibres, the direction and arrangement varying much in different muscles. The more parallel to the long axis of the muscle the fibres run, the more will the given muscle be able to shorten in proportion to its length.
The thickness of a muscle increases in proportion to its shortening during contraction, so that there is but little change in bulk. It is said, however, to diminish slightly in volume, becoming less than 1/1000 smaller. This can be shown by making a muscle contract in a bottle filled with weak salt solution so as to exclude all air, and to communicate with the atmosphere only by a capillary tube into which the salt solution rises. The slightest decrease in bulk is shown by the fall of the thin column of fluid in the tube.
Since a muscle loses in elastic force and gains but little in density during contraction, the hardness which is communicated to the touch depends on the difference of tension of the semifluid contractile substance within the muscle sheath.