Absence of a linear relationship between the reflex flexor shortenings evoked by a graded series of flexion-producing stimuli and the “inhibitory" lengthenings of a constant extension reflex evoked by the same stimuli

When each of a graded series of flexion-producing stimuli is compounded with a constant extension-producing stimulus, a common reflex effect is that both of two antagonistic muscles—a flexor and an extensor—exhibit a smaller magnitude of shortening in the compound reflex than in their respective simple reflexes. Such a series of compound reflexes affords the opportunity of analys­ing various possible relationships. One relationship—that between the “simple” and the “compound” flexor shortening—was examined in a pre­ceding paper. The reverse aspect of this particular relationship is obtained by contrasting each different magnitude of “simple” flexor shortening with the corresponding magnitude of concurrent lengthening (“relaxation”) in the extensor muscle. The present paper is devoted to an examination of this particular relationship. The relationship of flexor shortening and concurrent extensor lengthening is that which has chiefly been used by Sherrington for the establishment of the principle of the “reciprocal innervation of antagonistic muscles.” Not­ withstanding the interest and importance of this relationship, the reflex magni­tudes involved have not hitherto been examined. All examinations have been more or less qualitative, and any theoretical illustrations—by the giving of hypothetical numerical values to flexion and extension (including, in each case, “excitation” and “inhibition”)—have been made on the assumption of equality between “excitation” and “inhibition.” The writer has, however, shown that one particular reflex phenomenon—the concurrence in compound reflexes of a remainder of shortening in both of the antagonistic muscles—is inexplicable on the assumption that there is equality between “excitation” and “inhibition.”