Fig. 20.
Curves of muscle contraction obtained by stimulation of 3 and 4 points situated at equal distances from each other on the sciatic nerve of the frog. The increase of length of the nerve stretch corresponds with an equal increase of the latent period of contraction. From this follows, that the rapidity of the wave of excitation is the same at all points of the entire length of the nerve. (After Engelmann.)
There is, nevertheless, a third point of considerable difference between the types of conduction of excitation in the rhizopods and in the nerve. Whereas in the rhizopods the rapidity of conduction is dependent upon the intensity of the stimulus, it has been long known as the result of investigation by Rosenthal, Brücke and Lautenbach and at a more recent date by Gotch[89] and Piper,[90] that in the nerve of the frog, as well as in man, the velocity is not dependent upon the intensity of stimulation. (Figure [21].) Contrary results have been obtained by a few early observers wherein the latent period was shorter when the stimulation was strong. Nicolai[91] explains this shortening of the latent period, resulting from the application of strong electrical stimuli, to a spreading out of the “Stromschleifen” from the point of application and consequently there is a shortening of the stretch of nerve between the point of stimulation and the indicator.
Fig. 21.
Course of the action current of the nerve. The thin line indicates the action current produced by a weak, the thick line the action current produced by a strong stimulus. The duration of the action current is the same in both cases. (After Gotch.)
This conspicuous difference in the conduction of the two extreme types of living substance, which we have already observed, arouses the question as to what properties of living substance bring about these differences. In order to answer this question, it is necessary, first of all, to make some general statements concerning the processes of conductivity.
As already emphasized, all living substance possesses the capability of conducting excitations to a definite degree. We may, therefore, assume that the same fundamental property of conductivity exists in all substances. A fact to be considered in the conduction of excitation, is that the primary breaking down of the complex molecules at the position of stimulation act in turn as exciting stimuli upon the neighboring portion of the living substance, which in turn undergoes a similar decomposition. And so this process continues. This fact is evident from the observations on the process of excitation. But the nature of the stimulus which produces the breaking down of the complex molecules upon the surrounding molecules is a problem which can only be studied later. Here only one point will be mentioned in advance concerning the intensity of the stimulus. It is apparent from the experiments on the rhizopods, that the greater the intensity of the stimulus the more extensive must be the breaking down of the living substance. A stronger primary stimulation must also secondarily produce a stronger stimulus in the neighborhood. In other words: the conduction of excitation is a function of irritability. The greater the irritability, that is, the greater the number of molecules broken down in a unit of time and space by a stimulus of a certain intensity, the greater also is the conductivity of the living system, that is, the stronger, the more rapidly and the further excitation is extended. Conductivity of excitation is, therefore, unthinkable without irritability. Both are inseparably connected. The conclusion forced upon us by this chain of reasoning admits of no argument. Nevertheless the endeavor has been made, because of certain evidence at hand, to show that the property of conductivity could exist without irritability. A number of authors, such as Schiff,[92] Erb,[93] Grünhagen,[94] Effron,[95] Hirschberg[96] and G. Weiss,[97] have observed the fact that in spite of a more or less strong decrease of excitability of a stretch of nerve, stimuli applied above this stretch can still produce a conduction of excitation through the affected part. They have concluded from this that it is possible to separate the conductivity from irritability. Erb and G. Weiss have even gone so far as to directly express the opinion that capability of conduction and irritability involve two different histological elements. In contrast to this, other investigators, such as Hermann,[98] Szpilmann and Luchsinger,[99] Gad,[100] Piotrowski[101] and Wedenski,[102] have more or less decidedly taken the stand that an actual separation of irritability and of conductivity does not exist. The apparently contradictory evidence as well as the conflicting theoretical views have been cleared up by Werigo,[103] Dendrinos,[104] Noll[105] and Fröhlich.[106] These investigators have shown that the length of the narcotized stretch of the nerve plays an important rôle in the obliteration of conductivity. It has been found by the application of a stimulus above the narcotized stretch of nerve, that the longer this stretch is, the less is the reduction of irritability which obliterates the excitation wave reaching this area. Further: The shorter the stretch, the greater must be the reduction in irritability before this result is brought about. (Figure [22].) In other words, the conductivity in the narcotized nerve is dependent upon the length and the irritability of the narcotized stretch. From this observation the important fact is evolved, that the wave of excitation meets with a decrement of its intensity in the narcotized area. This decrement becomes larger as the wave progresses through the involved stretch. Further it is progressively increased as the amount of the irritability is reduced. Finally, when the stretch is long enough, the wave of excitation is obliterated. This important fact has been further established by the experiments of Boruttau and Fröhlich,[107] in which they studied the intensity of the current of action, produced by a wave of excitation, from two points in the narcotized stretch. The wave of negative variation, brought about by the excitation, gradually decreases in the narcotized stretch as the electrode is further removed from the point of entrance. Beside a decrement of intensity, as the investigations of Fröhlich[108] prove, the wave of excitation shows a decrement of the velocity in the narcotized stretch. And it is probable that the wave of excitation extends with progressive reduction in the velocity, corresponding to the decrement of intensity. The work of Koike[109] under the direction of Garten, in which the conclusion arrived at is that the reduction in the velocity is the same throughout the narcotized area, should not be accepted as conclusive in spite of the delicate method employed. These investigations are extremely difficult, being in the field of the most delicate of present-day methods. The decrement, which the wave of excitation meets with in its progress in the narcotized stretch, makes the conflicting testimony concerning the apparent separation of irritability and conductivity intelligible. It depends entirely upon the length of the narcotized area, and the amount of reduction in irritability on the one hand, and the strength of the stimulus used for testing the irritability on the other, whether the conductivity will disappear before the irritability or vice versa. If I test the irritability in the narcotized stretch with a weak stimulus, just slightly above the threshold, then by slight reduction in the irritability complete absence of response occurs, when the same stimulus is applied. This occurs at a time when excitation reaches the narcotized area from above and meets with a decrement so slight that it can pass through the whole narcotized stretch, that is, when the narcotized stretch is short enough. If I test the irritability of the narcotized area with a strong stimulus, far above that of the threshold, irritability will be found to be present at a time when the conductivity for the excitation, coming from above, is already obliterated. This is due to the fact that the decrement in the narcotized area is already great enough to bring about the complete disappearance of the wave of excitation coming from above. This, of course, only occurs provided the length of the narcotized stretch is great enough. The separation of conductivity and irritability is, therefore, only an apparent one. In reality, the facts obtained from experimentation indicate that with the reduction of irritability the decrement of the wave of excitation increases, whilst the shorter the stretch, the smaller is the decrement. This shows that conductivity is a manifestation of irritability.
