Studies on the flexor reflex.—II. The reflex response evoked by two centripetal volleys

Abstract

When an afferent nerve is stimulated by two similar break-shocks at a sufficiently short interval, it has been found that the ensuing reflex response is no greater than the response to a single break-shock. The shortest interval at which the second stimulus produces an additional response has been called “the least interval for muscular summation” (Adrian and Olmsted, 1922). Different observes have found fairly comparable values for this interval in the mammalian preparation: 0·75 σ to 1·31 σ (Sherrington and Sowton, 1915); 1·2 σ to 2·4 σ (Adrian and olmsted, 1922); 1·0 σ to 2·4 σ (Forbes, Querido, Whitaker and Hurxthal, 1928). It was at first thought that the ineffectiveness of the second stimulus at intervals less that “the least interval” was due to a refractory period of the central part of the reflex arc; consequently the “least interval” was used as a measure of the duration of the absolutely refractory period of the reflex arc. Forbes (1922), however, pointed out that the duration of the “least interval” was similar to the duration of the absolutely refractory period of the afferent nerve ( cf . Sherrington, 1921, p. 257), and concluded that the ineffectiveness of the second stimulus was probably due to its inability to set up centripetal impulses in the afferent nerve, and not to a blocking of such impulses by a refractory period in some part of the central reflex pathway. Later (1928) Forbes and his co-workers suggested that the reflex response added by the second centripetal volley, when it followed the first at very short intervals, might not be due to a second discharge of those motoneourones which had already responded to the first volley. Other motoneurones might be excited by summation of the central effects of the two centripetal volleys. If that were so, “the least interval for muscular summation” would have no relation to the refractory period of the reflex pathway.