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electric circuit of which the muscle forms a part, so as to make or break the current; and thus a variation of intensity equal to the entire strength of the current takes place in the muscle, and acts as a stimulus.
The direct current from a battery (continuous current) is used to stimulate a muscle in certain cases, but a current induced in a secondary coil by the entrance or cessation of a current in a primary coil of wire induced current) is more commonly employed on account of the greater efficacy of its action. The instrument used for this purpose in physiological laboratories is Du Bois-Reymond's inductorium, in which the strength of the stimulus can be reduced by removal of the secondary coil from the primary. It is supplied with a magnetic interrupter, by means of which repeated stimuli may be given by rapidly making and breaking the primary current (interrupted current) (Fig. 18
The irritability of muscle substance is not so great as that of the motor nerves; that is to say, a less stimulus applied to the nerve of a nerve-muscle preparation* will cause contraction than if applied to the muscle directly. In experimenting on the contraction of muscle, as already stated, the nerve is commonly used to convey the stimulus, because, when an electric current is applied to the nerve, the stimulus is the more safely and completely distributed throughout the muscle fibres than when it is applied directly.
CHANGES OCCURRING IN MUSCLE ON ITS ENTERING THE
ACTIVE STATE. Changes in Structure. The examination of muscle with the microscope during its contraction is attended with considerable difficulty, and in the higher animals has not led to satisfactory results. In the muscles of insects, where the differentiation of the contractile substance is more marked, certain changes can be observed. The fibres, and even the fibrillæ within them, can easily enough be seen to undergo changes in form corresponding to those
* By a nerve-muscle preparation is meant a muscle of a frog (commonly the gastroc. nemius and the half of the femur to which it is attached) and its nerve, which have been carefully separated from other parts and reinoved from the body.
of the entire muscle, namely, increase in thickness and diminution in length. A change in the position and relative size of the singly and doubly refracting portions of the muscle element has been described, and some authors state that the latter increases at the expense of the former after an intermediate period in which the two substances seem fused together.
Chemical Changes.—During the contracted condition, the chemical changes which go on in passive muscle are intensified, and certain new chemical decompositions arise of which not much is known.
Active muscle takes up more oxygen than muscle at rest, as is shown by the facts that, during active muscular exercise, more oxygen enters the body by respiration, and the blood leaving active muscles is poorer in oxygen than when the same muscles are passive. This absorption of oxygen may be detected in a muscle cut out of the body, but a supply of oxygen is not necessary for its contraction, since an excised frog's muscle will contract in an atmosphere containing no oxygen. From this it would appear that a certain ready store of oxygen must exist in some chemical constituent of the muscle substance. It is possible that some chemical compound, constantly renewed by the blood, is the normal source of oxygen, and not the oxyhæmoglobin.
The amount of CO, given off by a muscle increases in its state of activity. This may be seen (a) by the greater elimination from the lungs during active muscular exercise, and (3) by the fact that the venous blood of a limb, when the muscles are contracted, contains more CO, than when they are relaxed. (v) The increase of CO, can also be detected in a muscle removed from the body and kept in a state of contraction. increase in the formation of CO, takes place whether there is a supply of oxygen or not, (f) and the quantity of CO, given off exceeds the quantity of oxygen that is used up. So that it is not exclusively from the newly-supplied oxygen that the CO, is produced.
Muscle tissue, when passive, is neutral or faintly alkaline; during contraction, however, it becomes distinctly acid. The
This litmus which it changes from blue to red is permanently altered, and the conclusion follows that CO, is not the only acid that makes its appearance. The other acid is sarcolactic acid, which is constantly present in muscle after prolonged contraction, and varies in amount in proportion to the degree of activity the muscle has undergone. If artificial circulation be kept up in the muscle, the quantity of sarcolactic acid found in the blood is very great. It varies directly with the CO2, which would seem to suggest a relationship between the origin of the two acids.
The amount of glycogen and grape sugar is said to diminish in muscle during its activity, and it is stated that sarcolactic acid can be produced from these carbohydrates by the action of certain ferments.
Active muscle contains more of those substances than can be extracted by alcohol, and less that are soluble in water than passive muscle.
The chemical changes which take place during muscle contraction are probably the result of a decomposition of some carbohydrates, in which the albuminous substances do not take any part that requires their own destruction. This seems supported by the fact that the increased gas exchange in muscle during active exercise can be recognized in a corresponding alteration in the gas exchange in pulmonary respiration ; and there seems no relation between muscular labor and the amount of nitrogenous waste, as estimated by the urea elimination, which we should expect if muscular activities were the outcome of a decomposition of nitrogenous (albuminous) parts of the muscle substance.
The chemical changes, then, said to take place in muscle during its contraction are :
1. The contractile substance, which is normally neutral or faintly alkaline, becomes acid in reaction, owing to the formation of sarcolactic acid.
2. More oxygen is taken up from the blood than when the muscle is at rest. This using up of oxygen occurs also in the isolated muscle, and its amount appears to be independent of the blood supply:
3. The extractives soluble in water decrease; those soluble in alcohol increase.
4. A greater amount of CO, is given off, both in the isolated muscle and in the muscles in the body, and the change in the quantity of CO, has no exact relation to that of the oxygen used.
5. A diminution is said to occur in the contained glycogen, and certainly prolonged inactivity causes an increase in the amount of glycogen.
6. A peculiar muscle sugar makes its appearance.
I. Change in Elasticity.--The elasticity of a muscle during its state of contraction is less than in the passive state. That is to say, a given weight will extend the same muscle more if attached to it while contracted (as in tetanus) than when it is relaxed. The contracted muscle is then more extensible. If a weight which is just over the maximum load the muscle can lift be hung from it and the muscle stimulated, it should become extended, because the change to the active state lessens its elastic power, while it cannot contract, being over-weighted.
II. Electrical Changes.-If a muscle, in connection with a galvanometer, so as to show the natural current, be stimulated by means of the nerves, a marked change occurs in the current. The galvanometric needle swings toward zero, showing that the current is weakened or destroyed. This is called the negative variation of the muscle current, which initiates the change to the active condition. When the muscle receives but a momentary stimulus sufficient to give a single contraction, this negative variation takes place in the current, but, owing to its extremely short duration, the galvanometric needle is prevented by its inertia from following the change. Only the most sensitive and well-regulated instruments show the electric change of a single contraction, but when the muscle is kept contracted by a series of rapidly repeated stimulations the inertia of the needle is readily overcome.
Rheoscopic Frog.-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
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. upon the negative variation of the natural current between + and — stimulates the nerve (N'), and excites the muscles of B to action.
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.
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