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material moistened in saline solution (.65 per cent.) is brought into direct contact with the muscle, and, by means of saturated solution of zinc sulphate, into electrical connection with amalgamated zinc terminals from the galvanometer. Thus the muscle is not injured, and the zinc solution prevents the metal terminals from producing adventitious currents.
Small glass tubes drawn to a point, the opening of which is plugged with china clay moistened with salt solution, make a suitable receptacle for the zinc solution. If a pair of such electrodes be applied to the middle of the longitudinal surface at () (Fig. 182), and of the transverse surface at (), respectively, and then be brought into connection with a delicate galvanometer, it is found that a current passes through the galvanometer from the longitudinal to the transverse surface. A current in this direction can be detected in any piece of muscle, no matter how much it be divided longitudinally, and probably would be found in a single fibre, had we the means of examining it. The nearer to the centre of the longitudinal and transverse sections the electrodes are placed, the stronger will be the current received by them. If both the electrodes be placed on the longitudinal section or on the transverse surfaces, a current will pass through the galvanometer from that electrode nearer the middle of the longitudinal section (called the equator of the muscle cylinder) to the electrode nearer the centre of the transverse section (pole of muscle cylinder). If the electrodes be placed equidistant from the poles or from the equator no current can be detected.
The central part of the longitudinal surface of a piece of muscle is then positive, compared with the central part of the extremities or transverse sections. And between these partsthe equator and poles of the muscle cylinder, where the difference is most marked-are various gradations, so that any point near the equator is positive when compared with one near the poles.
There is, then, a current passing through the substance of the piece of muscle from both the transverse sections or extremities of the muscle block to the middle of the longitudinal surface,
whether it be a cut surface (longitudinal section) or the natural surface of the muscle. This is called the muscle current, or sometimes natural muscle current.
If the cylinder in the accompanying figure be taken to represent a block of muscle, e would correspond to the equator, and p to the poles, and the arrow heads show the direction of the currents passing through the galvanometer, the thickness of the lines indicating their force. The dotted lines o are connected
Diagram to illustrate the currents in muscle.
) Polar regions of cylinder representing the extremities of the muscle. The arrow heads show the direction of the surface currents, and the thickness of lines indicates the strength of the currents. (After Fick.)
with points where the electro-motive force is equal, and, therefore, no current exists.
The electro-motive force of the muscle current in a frog's gracilis has been estimated to be about .05-08 of a Daniell cell. It gradually diminishes as the muscle loses its vital properties, and is also reduced by fatigue. The electro-motive force rises with the temperature from 5° C. until a maximum is reached at about the body temperature of mammals.
These muscle currents are very weak if the uninjured muscle be examined in situ, the tendon being used as the transverse section ; they soon become more marked after the exposure of the muscle, and if the tendon be injured they appear at once in almost full force. In animals quite inactive from cold the muscles naturally are but slowly altered by exposure, etc., and the muscle currents do not appear for a considerable time, which is shortened on elevating the temperature. It has, therefore, been supposed that in the perfectly normal state of a living animal there are no muscle currents so long as the muscle remains in the passive state.
ACTIVE STATE OF MUSCLE. A muscle is capable of changing from the passive elongated condition, the properties of which have just been described, into a state of contraction or activity. Besides the change in form, obvious in the contracted state of the muscle, its chemical, elastic, electric, and thermic properties are altered. The capability of passing into this active condition is spoken of as the irritability of muscle. This is directly dependent upon its chemical condition, and therefore related to its nutrition and to the amount of activity recently exerted, which, as will hereafter appear, changes its chemical state.
Under ordinary circumstances, during life, the muscles change from the passive state into that of contraction in response to certain impulses communicated to them by nerves, which carry impressions from the brain or spinal cord to the skeletal muscles. The influence of the will generally excites most skeletal muscles to action. Nearly all muscular contraction depends on nervous impulses of one kind or another. But there are many other influences which, when applied to a muscle, can bring about the same change. These influences are called stimuli.
We utilize the nerve belonging to a muscle in order to throw it into the contracted state, but the great majority of stimuli can bring about the change when applied to the muscle directly. Since the nerves branch in the substance of the muscle, and are distributed to the individual fibres, it might, as has been argued, be the stimulation of the terminal nerve ramifications that brings about the contraction, even when the stimulus is applied to the muscle directly, for the terminal branches of the nerves are affected by the stimulus applied to the muscle.
That muscles can be stimulated without the intervention of nerves is satisfactorily proved by the following facts :
1. Some parts of muscles, such as the lower end of the sartorius, and many muscular structures which have no nerve terminals in them, respond energetically to all kinds of muscle stimuli.
· 2. There are some substances which act as stimuli when applied directly to the muscle, but have no such effect upon nerves, viz., ammonia.
3. For some time after the nerve has ceased to react, on account of its dying after removal from the body, the attached muscle will be found quite irritable if directly stimulated.
4. The arrow poison, Curara, has the extraordinary effect of paralyzing the nerve terminals, so that the strongest stimulation of the nerve calls forth no muscle contraction. If the muscles in an animal under the influence of this poison be directly stimulated, they respond with a contraction.
MUSCLE STIMULI. The circumstances which call forth muscle contraction may be enumerated thus :
1. Mechanical Stimulation.—Any sudden blow, pinch, etc., of a living muscle causes a momentary contraction, which rapidly passes off when the irritation is removed.
2. Thermic Slimulation.—If a frog's muscle be warmed to over 30° C. it will begin to contract, and before it reaches 40 C. it will pass into a condition known as heat rigor, which will be mentioned presently. If the temperature of a muscle be reduced below o° C. it shortens before it becomes frozen.
3. Chemical Stimulation. A number of chemical compounds act as stimuli when they are applied to the transverse section of a divided muscle. Among these may be named : (1) the mineral acids (HCI, . I per cent.) and many organic acids ; (2) salts of iron, zinc, silver, copper and lead ; (3) the neutral salts of the alkalies of a certain strength; (4) weak glycerine and weak lactic acid; these substances only excite nerves when concentrated ; (5) bile is also said to stimulate muscle in much weaker solutions than it will nerve fibres.
4. Electric Stimulation.—Electricity is the most convenient form of stimulation, because we can accurately regulate the force of the stimulus. The occurrence of variation in the
Du Bois-Reymond's Inductorium with Magnetic Interrupter. c. Primary coil through which the primary, inducing, current passes, on its way to the
electro-magnet (b). i. Secondary coil, which can be moved nearer to or further from the primary coil (c),
thereby allowing a stronger or weaker current to be induced in it. This induced
current is the stimulus. b. Electro-magnet, which on receiving the current breaks the contact in the circuit of
the primary coil by pulling down the iron hammer (k), and separating the spring from the screw of e. When using Helmholtz's modification (g'), é is screwed up, and the current brings the spring in contact with the point of the pillar (a), and so
demagnetizes (6) by “short circuiting" the battery. When tetanus is to be produced, the wires from the battery are to be connected with g and a.
When a single contraction is required, the magnetic interrupter is cut out by shifting the wire from a to the binding screw to the right of f.
intensity of an electric current passing through a muscle causes it to contract. The sudden increase or decrease in the strength of a current acts as a stimulus, but a current of exactly even intensity may pass through a muscle without further exciting it, after the initial contraction has ceased. A common method of producing such a variation is that of opening or closing an