The irritability of nerves varies according to certain conditions and circumstances. While uninjured in the body, the irritability of a nerve depends upon -
1. A supply of blood sufficient to supply nutriment, and to carry off any injurious effete matters that may be produced by its molecular changes.
2. A suitable amount of rest. Prolonged activity causes fatigue and loss of irritability, no doubt from the same causes mentioned as bringing about fatigue in muscles. The chemical changes taking place in nerves have not yet, however, been made out with any degree of accuracy.
3. Uninjured connection with the nerve centres. When a spinal nerve is cut, the part connected with the periphery rapidly undergoes degenerative changes which seem to depend upon faulty nutrition, since they are accompanied by structural changes - fatty degeneration. This appears to commence in a very short time after the section - often in about three to five days. The part of the nerve remaining in direct connection with the cord retains its irritability for a very much longer time.
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 320 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.
Fig. 203. 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; yl = Effect of weak current; y2 = Effect of medium current; y3 = Effect of strong current.
The degree of change effected in the irritability of the part is estimated by the distance of the curves from the straight line. The part of curve below the line corresponds to decrease, that above to increase of irritability. Where the curves cross the line is called the indifferent point. With strong currents this approaches the cathode. (From Foster, after Pfliiger).
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.
Fig. 204. 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 calf muscles of a frog are shown.
The arrows indicate the direction of the ascending current, A, on the left, and the descending 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 nerve.