Studies on the flexor reflex.-IV. After-discharge

In many preparations the flexor reflex elicited by the application of a moderately strong break-shock to an ipselateral afferent nerve has an after-discharge following the initial reflex discharge, even when the strength of the break-shock it such that it sets up no more than a single centripetal volley (Sherrington, 1921, a; 1921, b ; Adrian and Forbes, 1922). The prolonged excitatory condition which must occur at some part of the central reflex pathway clearly has some affinity to the persistence of the c. e. s. which forms the basis of facilitation (Eccles and Sherrington, 1930). It is of interest therefore to investigate the effect of an antidromic volley on after-discharge. It must, however, be remembered that an antidromic volley set up during an after-discharge will be prevented from reaching some motoneurones by meeting centrifugal (reflex) impulses. Denny-Brown (1929, p. 273) observed that an antidromic volley set up during the after-discharge of either a flexor or extensor reflex (in response to a tetanic stimulation) was followed by a period of quiescence owing to a temporary lapse of the after-discharge. The duration of this period seemed to be too long for a central refractory period set up by the antidromic volley, so he suggested that there might be a temporary exhaustion of the central exciting agent (c. e. s.). II. Method. The general technique is as described previously (Eccles and Sherrington, 1931, a). In all cases the muscle (tibialis anticus) has been completely deafferented. Tetanic stimuli have been provided by a neon-tube oscillator.