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 fibrillar within them, can easily enough be seen to undergo changes in form corresponding to those 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.
* By a nerve-muscle preparation is meant a muscle of a frog (commonly the gastrocnemius and the half of the femur to which it is attached) and its nerve, which have been carefully separated from other parts and removed from the body.
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 oxyhaemoglobin.
The amount of C02 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 (/B) by the fact that the venous blood of a limb, when the muscles are contracted, contains more C02 than when they are relaxed. (r) The increase of C02 can also be detected in a muscle removed from the body and kept in a state of contraction. (d) This increase in the formation of C02 takes place whether there is a supply of oxygen or not, (e) and the quantity of C02 given off exceeds the quantity of oxygen that is used up. So that it is not exclusively from the newly-supplied oxygen that the C02 is produced.
Muscle tissue, when passive, is neutral or faintly alkaline; during contraction, however, it becomes distinctly acid. The litmus which it changes from blue to red is permanently altered, and the conclusion follows that C02 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 C02, 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 C02 is given off, both in the isolated muscle and in the muscles in the body, and the change in the quantity of CO2 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.
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.
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.