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In the artificial stimulation, by means of electric shocks applied to the nerve of a cold blooded animal, there are many minor conditions which have considerable influence on the irritability, as evidenced by the response given by the attached muscle to weak stimuli. The more important of these are :
1. Temperature changes. In the case of a frog's nerve, a rise of temperature to 32° C. causes an increase in its excitability. Also a fall of temperature below zero tends to make the nerve more easily excited. Both these conditions have, however, a very fleeting effect, for the nerve soon dies at the temperatures named, and, probably, the increased irritability is only to be taken as a sign of approaching death. It thus appears that a medium temperature is the optimum for nerve work.
Diagram illustrating the variations of irritability of different parts of a nerve during the
passage of polarizing currents of varying strength through a portion of it. A Anode: B Cathode ; AB - Intra.polar district; gl. Effect of weak current ; y2 =
Effect of medium current; 33 = Effect of strong current.
of the curves from the straight line. The part of curve below the line corresponds to
2. The part of the nerve stimulated is also said to have some effect on the result of a given strength of stimulus. The further from the muscle, the more powerful the contraction produced, other things being equal. So that the impulse is supposed to gather force as it goes, as in the case of a falling body, and hence has been spoken of as the avalanche action of nerve impulse.
3. A new section of the nerve is said to increase its irritability, as does, indeed, any slightly stimulating influence, such as drying, and chemical or mechanical meddling of any kind. This increase in irritability probably depends upon injurious changes going on in the nerve, as the influences just alluded to lead to complete loss of excitability, if carried too far.
4. The electrotonic state. The most remarkable changes in the excitability of a nerve are those brought about by the action of a constant current passing through the nerve, so as to set up the conditions just described as anelectrotonus and catelectrotonus.
Diagram to show the meaning of the terms ascending and descending currents, used in
speaking of the law of contraction. The end of the vertebral column, sciatic nerves and
call muscles of a frog are shown. The arrows indicate the direction of the ascending current, A, on the left, and the descend
ing current, D, on the right, according as the positive pole of the battery, C, is below or above.
The irritability of the nerve is increased in the region near the cathode, and is diminished in the neighborhood of the anode.
The increase of irritability is in proportion to the intensity of the catelectrotonic and the decrease in proportion to the intensity of the anelectrotonic state. Thus, the increase is most marked in the immediate neighborhood of the cathode, and fades with the distance from the negative pole ; and similarly, the decrease
is strongest at the anode, and becomes less and less as it passes away from the positive pole. In the same way, in the part of the nerve between the two poles—the intra-polar region—the decrease and increase of irritability become less marked toward the middle point between the cathode and the anode, so that here we find an unaffected part, which has been called the indifferent point.
It is a remarkable fact that this indifferent point is not always midway between the two poles, but decreases its distance from the cathode in proportion as the polarizing current is made stronger. That is to say, with strong polarizing currents the indifferent point is near the cathode (B); with weak currents it lies near the anode (A) (Fig. 203).
Besides becoming less irritable in proportion as the polarizing current becomes more powerful, the anelectrotonic region of the nerve loses its ability to conduct impulses, and may finally, with a very strong current, even when applied for a short time, become quite incapable of conducting an impulse.
If the polarizing current be now opened, so as to stop its passage through the nerve, and remove the anelectrotonic and the catelectrotonic states, a kind of rebound occurs in the condition of both the altered regions, and the part which has just ceased to be catelectrotonic, and was, therefore, over-irritable, becomes, by a kind of negative modification, very much lowered in its irritability; while, on the other hand, the anelectrotonic part, by a positive rebound, becomes more excitable than in its normal state. The rebound over the line of normal irritability lasts a very short time; but as we shall see presently, it is of greater duration than the passage of the negative variation along the
THE LAW OF CONTRACTION. Upon the foregoing facts, and others already mentioned-viz., that the impulse starts in the nerve from different poles and with different force, with a making and a breaking shock-depends the law of contraction, which would be difficult to understand without bearing in mind all these interesting points.
It was found that, with the same strength of stimulation, not only were different degrees of contraction produced with making
and breaking shocks, but also that, other things being similar, a different result followed when the current was sent through the nerve in an upward direction (i. e., from the muscle), and when it was sent in a downward direction (i. e., toward the muscle). The stimulating current is spoken of, in the former case, as an ascending current, and in the latter as a descending current.
The following is a tabular view of the law of contraction :
To explain this law, the following points must be kept in view :1. In a breaking shock, it is the disappearance of anelectrotonus
which causes the stimulation to start from the anode. 2. In a making shock, it is the appearance of catelectrotonus
which causes the stimulation to start from the cathode. 3. With the same current the make is more powerful than the
break. 4. Anelectrotonus causes reduction of irritability and conduc
tivity of the nerve. 5. Catelectrotonus causes increase of irritability and conduc
tivity of the nerve. 6. With ascending currents the part of the nerve next the
muscle is in a state of reduced functional activity (anelec
trotonus). 7. With descending currents the part of the nerve next the
muscle is in a state of exalted activity (catelectrotonus). 8. The reduction or exaltation of activity is much greater with
strong currents. That only making shocks cause contraction with very weak currents, simply depends on the greater efficacy of the entrance of catelectrotonus into the nerve, which causes the making stimulation. That contraction follows in all four cases, with medium stimulation, is explained by assuming that the depression of the functional activity of the nerve is not sufficient to affect its conductivity.
The want of response to a making shock, in the case of the strong descending current, depends upon the fact that the part of the nerve near the muscle, around the anode, is in a state of lowered activity, and is, therefore, unable to conduct the impulse which has to pass through this region from the cathode, where the stimulation takes place, in order to reach the muscle.
The absence of contraction at the breaking of a strong descending current, is caused by the same lowering of the conductivity of the nerve between the point of stimulation and the muscle, because at the cessation of strong catelectrotonus, the region near the cathode rebounds from exalted to depressed activity, and at the moment of stimulation the greater part of the intra-polar region is anelectrotonic.
The special function of nerve fibres may be briefly stated to be their power of rapidly intercommunicating between distant parts. The axis cylinder has undergone a special development, by which it is enabled to conduct impulses much more quickly than ordinary protoplasm. Each muscle tissue transmits impulses about thirty times more slowly than a nerve fibre. A highly-organized animal body, without nerve fibres, would be in a worse condition than a highly-organized state without a telegraphic or even a postal system.
NERVE CORPUSCLES OR TERMINALS. These are the real actors in the nerve operations, while the fibres are merely their means of communicating with one another. One set of terminals is placed on the surface of the body and is adapted to the reception of the various external influences which are brought to bear on it from without by its surroundings. These receivers of extrinsic stimuli are necessarily much varied, so as to be capable of appreciating all the different kinds of stimulation presented to them. They are either distributed over the entire surface so as to meet with general mechanical and