Irritability: A Physiological Analysis of the General Effect of Stimuli in Living Substance

by Max Verworn

Fig. 18.

Cyphoderia margaritacea. Result of localized mechanical stimulation in the middle of a long extended pseudopod.

Fig. 19.

A pseudopod of Orbitolites complanatus (cf. Fig. [7]). a—In normal condition. b—Severed by a cross section near the end. b-f—Five successive stages of the effect. b-d—The pseudopod retracts by centripetal flowing of the protoplasm contracted in the shape of microscopic balls and spindles. e and f—The pseudopod begins to extend again. The centripetal flowing balls and spindles begin to disappear.

A sharp contrast to this type is formed by the other extreme as represented by that of the medullated nerve. As an indicator of the course of excitation we will take the action current in an isolated nerve of the frog. If this is stimulated at one end, we can test the intensity of the conducted excitation by leading off the action current from two points at varying distances from the one influenced by the stimulus. Since the classical discovery of Du Bois-Reymond of the action current of the nerve, we know that in the fresh medullated nerve, if observed under favorable experimental conditions, no decrement of intensity of excitation during its course from the point of stimulation along the length of the nerve can be demonstrated.[82] If unpolarizable electrodes are applied to a nerve in such a position that they are equidistant from the cross section and are connected with apparatus for testing the current, it will be found that there exists an “unwirksame Ableitung” in the sense of Du Bois-Reymond, that is, in which there is no demarcation current. When a tetanizing current is applied to one end of the nerve, no difference of potential between the two nonpolarizable electrodes is observed, which indeed would be the case if excitation with its current of action would have a decrement on its way from one to the other point of leading off the current. This fact, which has been repeatedly confirmed, shows us that the medullated nerve, under normal conditions, conducts excitation without a perceptible decrement of the intensity.

This specific property of a medullated nerve is in conformity with the conditions in connection with the rapidity of conductivity. Since Helmholtz[83] has devised the method for measuring the rapidity of conduction in the nerve, this investigator himself and numerous others have studied the rate in different nerves.[84] Helmholtz found the rate for motor nerves of the frog to be 27 meters per second, for the sensory nerves of man 60 meters, and the motor nerves of man 34 meters. Other investigators have obtained quite different results; Hirsch, for the sensory nerves of man, 34 meters; Schelske, for the same, 25–33 meters; De Jaager, 26 meters; v. Wittich, 34–44 meters, and Kohlrausch, 56–225 meters; v. Wittich for the motor nerves of man, 30 meters; Piper[85] finally in the most recent investigations about 120 meters in the second.

These differences may be explained in a large measure by the variety of the methods used, in part also by the difference in the structures. The methods employed for the study of the velocity have also been used to solve the question, whether the velocity of the excitation wave in its course over the nerve meets with a decrement as it moves further and further away from the point of stimulation. Here the endeavor was made to study the difference in time of the latent period, which is observed by the indicator, when the nerve is stimulated at two points at different distances from the muscle, used as an indicator, or from the wires leading the current to the indicator. The more recent investigators, as René Du Bois-Reymond,[86] Engelmann,[87] G. Weiss,[88] have arrived at the same conclusion, that the rate of conductivity in the medullated nerve under normal conditions is the same at all distances from the point of stimulation. (Figure [20].)

The medullated nerve shows, therefore, under normal conditions neither a decrement of its conductivity, nor of its irritability, as the distance of the wave of excitation increases from that of the position of stimulation; this means, in other words, that excitation is conducted with the same intensity with which it is started, and with a constant rate throughout the entire course of the nerve.