Interference of two series of stimuli. A—Effect of the one series alone. Development of tonus by summation. The dots below the curve indicate the points of time at which the stimuli of the second series will operate. B—Effect resulting from the interference of both series. By the addition of the second series the frequency has been doubled. The result consists in an inhibition.
The greatest number of instances of the interference of two series of stimuli have been given to us by investigation of the physiology of the nervous system. In the functionation of the nervous system the fact that two series of stimuli from different tracks affect the same ganglia plays a very important rôle. It is this to which Sherrington[191] has alluded as “the principle of the common path.” Where two nervous excitations involve the same paths, there arises an interference of the effect of the two series of stimuli, for the impulses in the nervous system, as already stated, possess a rhythmic character. This principle has a broad application in the phenomena of association in the cerebral cortex. The simpler and, therefore, the most easily understood cases are, however, in the spinal cord. The motor neurons of the anterior horns of the spinal cord are the junction of a great number of tracks, for example, the sensory neurons of the spinal cord at different levels, the neurons of the cerebellum, the pyramidal tracks from the motor areas of the cerebral cortex, etc. On the contrary, for example, the sensory neurons of the spinal cord are strictly “private paths” in the sense of Sherrington, for excitation can enter by this means only from the special paths of the spinal ganglia and, therefore, from the periphery. The motor neurons of the anterior horns offer, therefore, excellent opportunities for the experimental investigation of the interference of two series of excitations which enter by different paths. The spinal cord consequently has become a much-used object of investigation for this purpose. In fact, we can observe and produce all types of interference in the spinal cord. These conditions have been quite thoroughly investigated by Sherrington[192] and his coworkers on the dog, and Fröhlich,[193] Vészi,[194] Tiedemann[195] and Satake[196] on the frog.
A summation of two excitations was observed already by Exner. This investigator connected the abductor pollicis of the rabbit with an apparatus for making graphic records. He then stimulated first the paw and then the motor areas of the cerebral cortex with faradic shocks, the intensity of which was just sufficient to bring about perceptible effect. If both stimuli were simultaneously operative, an increase in the response was observed. Even when the stimuli were sub-threshold in type, as a result of summation there was a perceptible muscle contraction. (Figure [56].) Exner had at that time referred to this increase of the response as “Bahnung” (reinforcement). However, the word “Bahnung” has more than one meaning, for processes of various types are involved in this term. Thus writers have differentiated real and apparent “Bahnungen.” On account of this lack of clearness in the meaning of the term “Bahnung,” I wish to discard its use as it is not at all essential. We will speak simply of a summation of excitation, for here it is simply a question of summation of two excitations of the motor cells of the spinal cord.
Fig. 56.
Summation of two excitations in the rabbit. The one proceeds from the paw, the other from the motor sphere of the cerebral cortex. S—Time in seconds. Pf—Stimulation of the paw. H—Stimulation of the motor sphere. M—Contractions of the abductor pollicis. (After Exner.)
Fröhlich has shown that summation of two excitations upon a motor cell of the anterior horn coming by way of different paths is more readily obtained when the stimuli are somewhat strong, or when the duration of the excitation processes in the ganglion cells are somewhat prolonged by fatigue.
Fig. 57.
Summation of two excitations in the spinal cord produced by stimulation of the ninth and tenth posterior root. Lower line indicates faradic stimulation of the tenth, upper line of the ninth root.