SYSTEM.] PHYSIOLOGY 25 positive pole it is diminished, that is to say, a stimulus from the induction-coil, not sufficient to excite the nerve so much as to cause a muscular contraction if applied near the positive pole, will at once do so if applied near the negative pole ; or, a stimulus so strong as to cause tetanus in the muscle when applied near the negative pole may produce no effect when applied near the positive pole. In other words, the nerve near the negative pole is more excitable than in the normal state, whilst near the positive pole it is less so, indicating that at least one of the physiological properties of the nerve has been changed by the action of the continuous current. But a nerve-fibre has also the property of conducting the effects of an impression, or the nerve-force travels with a certain velocity along a nerve, as will be shown lower down. It lias been ascer tained that near the negative pole the rate of conductivity is increased, whilst near the positive pole it is diminished. Finally, a piece of living nerve, when connected with the terminals of a galvanometer, so that the one terminal touches the surface whilst the other touches the transverse section of the nerve, shows the existence of a current of electricity travelling from the surface of the nerve through the galvanometer to the transverse section, that is, the surface is positive to the transverse section. This condition is also modified by the transmission through the nerve of a continuous current, so that the difference of potential is increased near the positive pole and diminished near the negative. These results are thus summed up. State of Nerve. Functions of Nerve. Near positive pole Near negative pole Electromotive force. Increased Diminished Conductivity. Diminished Increased Excitability. Diminished Increased ec- cal mula- nof nsory rves. nipolar
- cita-
on. The properties of the nerve, therefore, are altered by the passage through it of a continuous current, and the altered condition is termed the "electrotonic state," the condition in the neighbourhood of the positive pole, or anode, being termed "anelectro tonic," whilst that near the negative pole or katode is called " katelectrotonic." A certain portion of nerve near each pole is thrown during the passage of a continuous current into these conditions of anelectrotonus and of katelectrotonus, whilst the amount of nerve thrown into the one condition or the other depends on the strength of the current. Further, there is always be tween the two poles a point of indifference, in which the properties of the nerve seem to be unaltered, and the position of this point depends on the strength of the current. Thus, with a current of medium strength the point is midway between the poles ; with a weak current the point is near the positive pole, that is, a large portion of the nerve near the negative pole is in the katelectro- tonic state in which the excitability is increased ; and with a strong current the point is near the negative pole, that is, a large portion of the nerve near the positive pole is in the anelectrotonic state in which the excitability is diminished. Now, according to Pfliiger, the stimulating effect of closing the current occurs at the katode only, whilst the stimulating effect of opening the current occurs at the anode only, or a nerve is stimulated by a current on the appearance or increase of katelectrotonus, on closing the circuit, or by the disappearance or diminution of anelectrotonus on opening the circuit. If we suppose that this depends on the modification of excitability near the negative pole, by the molecules of the nerve becoming more mobile, the matter is in telligible. Thus the passage of the molecules from the normal stable con dition to the katelectrotonic less stable condition acts as a stimulus, whilst the passage backwards has no effect. On the other hand, the passage from the more stable condition in anelectrotonus to the normal stable condition acts as a stimulus, whilst, again, the reverse action has no effect. This explains why it is that a weak current gives contraction on closing, because on closing a large portion of the nerve near the negative pole passes from the normal into the katelectrotonic state, and this acts as a stimulus. On the other hand, a strong current causes contraction on opening, because on opening a large por tion of nerve near the positive pole passes back from the anelectrotonic state into the normal state, and this acts as a stimulus. Again, with currents of medium strength, as both states are equally produced, there is contraction both on opening ami on closing. Thus Pfliiger s theory accounts for most of the facts ; but its weak point is that no reason can be given why a nerve is stimulated only by the appearance of katelectrotonus and by the disappearance of anelectrotonus. It remains only to add that currents passing transversely through nerves produce no stimulating effect. In ascending currents the shorter the piece of nerve between the electrodes the greater the stimulating effect, whereas in descending currents the reverse holds good (Hermann). (b. ) Electrical Stimulation of Seiisomj Nerves. The effect of stim ulating sensory nerves as distinguished from the direct stimulation of sensory or terminal organs has not been sufficiently studied, but, so far as is known, the laws seem to be the same as those relating to motor nerves. When a sensory nerve is stimulated the test must be the resulting sensation. As stimulation of the motor nerve in the condition of anelectrotonus or of katelectrotonus may or may not be followed by a contraction, so stimulation of the sensory nerve may or may not be followed by a sensation, or the character of the sensa tion may vary just as the muscular contraction ma} 7 be weak or strong. Further, Bonders has shown that electrical stimulation of the vagi or-pneumogastric nerves is attended by analogous pheno mena, so far as the movements of the heart are concerned. In this case, however, as will be shown lower down in discussing the pheno mena of nervous inhibition, the result is not movement but arrest of movement. (c.) Chauveau s Researches mi Unipolar Excitation. Chauveau has studied the comparative influence of the two poles of any arrange ment supplying a continuous current, that is, he has tried the stimulating effect, supposing either the positive or the negative pole be applied to the nerve whilst the other is in contact with another part of the body. He has found, amongst other more abstruse and less practical results, that there is in each case a certain intensity of current corresponding to the physiological con dition of the nerve by which the influence of one pole is the same as that of the other. If the intensity of the current be below this medium strength the effect of the negative pole on motor nerves is greater than that of the positive ; but, if the intensity be above, the reverse is the case, that is, the positive pole is the stronger excitant. In the case of sensory nerves Chauveau found that application of the negative pole with a moderately strong current was more painful than application of the positive pole. Thus the influence of unipolar excitation with a strong current on motor nerves is the reverse of that on sensory nerves, that is, the positive pole is the more powerful on motor nerves, the negative pole on sensory nerves. (d.) Production of Tetanus. Tetanus or cramp of a muscle is Produc- produced when its nerve is stimulated by successive irritations tion of at intervals so short that the muscle has no time to relax between tetaiius. them, and consequently it passes into a state of more or less firm contraction. A single muscular contraction may be called a twitch of the muscle, but in tetanus or cramp the individual contractions are fused together so as to maintain a rigid state of the muscle for some time. A rapid series of induction shocks, each of short duration, always produces tetanus, even if they are sent to the muscle at the rate of 15 per second. A continuous current, on the other hand, usually causes contraction only at the moment of open ing and closing the circuit, but occasionally tetanus may be seen during the passage of the current. Tetanus during the passage of a constant current has been attributed to electrolytic changes in the nerve. Pfliiger holds that this is a normal production of tetanus and may be seen even with feeble currents ; but certainly it is very difficult to demonstrate. Long ago Ritter showed that, if a constant current of sufficient intensity be sent up a nerve for a considerable time, say half an hour, and then be suddenly interrupted, tetanus lasting for eight or ten seconds may be seen, which disappears on again closing the current. Hitter s tetanus, according to Pfliiger, is really due to the stimulation caused by the disappearance of an electrotonus, which occurs, as we have seen, when the current is opened, and the proof he offers is that the tetanus disappears when the muscle is cut off from the anelectrotonic portion. Tetanus may also be caused by the mechanical irritation of the nerve, or by heat, or by chemical substances. Nervous Conductivity. When a nerve is irritated at any point Nervous in its course a change is produced which is propagated along the conduct- nerve, that is, the nerve conducts, and the phenomenon is called ivity. the "nerve-current." The velocity of transmission can be measured only by the use of delicate apparatus, as the time occupied is too short to directly affect consciousness. For example, when the tip of the finger is touched the mind apparently perceives the contact without any loss of time. But it can be shown that an appreciable interval of time elapses between the instant the finger is touched and the instant the mind perceives the impression. During this time a change passes along the nerve from the point touched to the brain. The method usually employed for determining the velocity of the nerve-current consists in preparing the gastrocnemius muscle of a frog with the sciatic nerve attached, and connecting it with a recording apparatus, so that if the muscle be caused to contract by irritating the nerve the record of the contraction may be made on a rapidly-moving surface. If, then, the nerve be irritated in two consecutive experiments, first close to the muscle, and secondly at a distance from it, and the muscle be caused to contract in each case, it will be found that it does not contract so soon when the nerve is irritated at a distance from the muscle as when it is irritated close to it ; in other words, if the nerve be irritated at a distance from the muscle the transmission of the nervous impression from the point irritated to the muscle occupies an appreciable time. If, then, we know the length of nerve between the two points irritated, we can determine the length of time the nerve-current took in passing along that distance of nerve. (1.) Measurement of Velocity in Motor Nerves. Many ingenious methods have been devised for this purpose, but the simplest is the use of the "spring myo- graphion " of Du Bois-Reymond (see fig.3). The apparatus consists of a smoked-glass plate, which is driven in front of the re cording stylet of the myograph by the recoil of a steel spring C. Underneath the frame carrying the glass plate are two binding screws 1 and 2, to one of which is attached a rectangular arm of brass 1, which can so move horizontally as to establish metallic connexion between the two binding screws (marked Break, F). By means of these binding screws the myograph is interposed in the circuit of a galvanic element and the primary coil I of an induction-machine, and the brass arm is so placed as to connect both bind ing screws, thus completing the circuit. From underneath the frame carrying the NERVE R 200VIB.PER SEC. smoked -glass plate there descends a small mntnt^v .>- ii^i-iC"/!- nnir>i-Vii- flange, which (when the glass plate, by RAPIDITY OF NERVE CURRENT releasing a catch not seen in the figure, F IG . 3._Diagram showing arrange- but close to C, is driven across by the men t of apparatus in measuring spiral spring from left to right) pushes rapidity of nerve-current, the brass arm aside and thus interrupts the circuit of the primary coil. When this occurs an opening shock is trans mitted from the secondary coil II to a commutator E, an instrument by which electric currents may be transmitted to the nerve either at a point close to the muscle at A, or at a distance from it at B. Suppose the ap paratus all arranged so as to send the shock to the nerve at a point close
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