Fig. 58.
Fig. 59.
On the other hand, the conditions for the production of inhibition are favored when the intensity of the series of stimuli is weak. Here it is a question of the development of a relative refractory period for the weak stimuli by increase in their frequency. A relative fatigue of the motor ganglion cells for weak stimuli rapidly occurs, and there develops a state of equilibrium beneath that of the threshold of perceptible effect throughout the continuation of stimulation. Vészi succeeded in isolating these types of summation and inhibition in the spinal cord. His method consisted in cutting the posterior roots of the spinal cord of the frog and stimulating faradically the central ends, and at the same time graphically recording the response of the gastrocnemius muscle. Upon faradic stimulation of the ninth posterior root, one obtains tetanic reflex contraction of this muscle. When the tenth posterior root is then stimulated, tetanus is also produced but of somewhat shorter duration. If, while obtaining tetanus reflexly by stimulation of the ninth root, a faradic current of short duration and not too weak is applied to the tenth root, then a summation of excitation occurs, an increase in the reflex contraction. (Figure [57], A and B.) When, on the other hand, the tenth root is stimulated with weak shocks, one can obtain an increase of the tetanus of short duration followed by inhibition. Here, as the result of interference, we have an instance of inhibition with primary tetanus. (Figure [58].) When the tenth root is stimulated with very weak shocks, inhibition of the tetanus produced simultaneously from the ninth root occurs without primary summation. (Figure [59].) The fact that two series of stimuli, both of which produce dissimilative excitation, bring about an inhibition by their combined action, is sufficient to show the untenability of the Gaskell-Hering hypothesis, that inhibitory processes result from assimilatory excitation. It would be impossible to understand how two dissimilatory exciting stimuli, by their simultaneous action, could bring about assimilatory excitation. When the eighth or the seventh root is stimulated with stronger faradic shocks during the time when tetanus is produced reflexly by faradic stimulation of the ninth, an inhibition is practically always obtained. Indeed, faradic currents that are so weak as to be far below the threshold of perceptible response bring about when applied to the seventh or eighth root a decided inhibition of the tetanus, brought about by simultaneous stimulation of the ninth root. The inhibitory effect of weak sub-threshold excitations are here particularly apparent. This inhibition resulting from excitation far below that of the threshold of perceptible response is a common occurrence in the functional activities of the central nervous system. In various parts of the nervous system, the excitation in its conduction is weakened when passing through intervening ganglion stations so that it has undergone a strong decrement before reaching the responding structure, where an inhibitory effect may be manifested. In this connection it is of interest that the reciprocal “antagonistic reflexes” discovered by Sherrington,[197] who recognized their importance in the functional processes of the nervous system, can be explained, as Fröhlich showed, upon this principle of inhibition resulting from weakened excitation. On the basis of numerous investigations in the Göttingen laboratory as well as that of Bonn[198] we have come to look upon the reflex arc in the spinal cord as consisting of the following elements: a neurone in the spinal ganglion, a neurone in the posterior horn and a motor neurone in the anterior horn. This is the most direct route between the point of stimulation and that of the responding organ of a unilateral reflex. (Figure [60].) It is known that the excitation becomes weaker in passing from the entrance of the excitation into the spinal cord to the motor elements of a lower level on the same side or to those on the opposite side. In order to obtain a response a stronger stimulus is necessary. Here the weakening of the excitation as well as the prolongation of the reaction time is brought about by the introduction of intercalated neurones. The reflex arc contains more stations. (Figure [61].) If we accept the most plausible assumption that the central connection of antagonistic muscles possesses like relations, then the effects discovered by Sherrington are self-explanatory. In this case stimulation of the sensory path, which brings about a strong reflex excitation of the motor neurons of the anterior horns controlling a muscle, at the same time stimulates the antagonistic muscle with sub-threshold stimuli. The result of this as shown by the experiments of Vészi is not a motor response of the antagonists, but an inhibition if the motor neurons of the antagonists are at the time in a state of excitation. It is, therefore, understandable that reflex excitation of a muscle under normal conditions of irritability has an inhibitory effect on its antagonist.
Fig. 60.
Scheme of the simplest unilateral reflex arc of the spinal cord.
Fig. 61.