On the Relation Between Discharge Zone and Subliminal Fringe in a Motoneuron Pool Supplied by a Homogeneous Presynaptic Pathway *

This note is concerned with a description of the growth, in a motoneuron pool, of a discharge zone within and from the subliminal fringe, as excitation by a homogeneous afferent system is progressively increased. On being subjected to an afferent influx, a motoneuron pool, previously considered to be in the "resting" state, may be divided into two zones, one resting, the other excited (Sherrington8). Once reflex threshold is reached, the excited zone, in turn, contains a discharge zone and a subliminal fringe (Denny-Brown and Sherrington'). It seems certain that discharge from a neuron pool in the central nervous system demands summation of the action of a number of synaptic knobs (cf. Eccles and Sherrington2) and the conditions, for such summation are: (1) virtually synchronous arrival of impulses at the motoneuron (Lorente de No5'6); (2) sufficiency in number of impulses impinging upon the motoneuron (Ecdes and Sherrington2); and (3) immediate proximity of the active synapses (Lorente de No5 8). Of most immediate concern to the present account is the last of these conditions. Lorente de No has shown that activation of a very large number of synapses belonging to a given presynaptic path may remain ineffective in the sense of securing a postsynaptic discharge, yet a few of these same synapses active in concert with others of different origin, themselves of subliminal effect in isolation, may evoke a large postsynaptic discharge.5 6, Obviously the spatial decrement of subliminal stimulation must be great and the location of active synapses of prime importance in securing transmission. Anatomical studies on the distribution of synaptic knobs on motoneuroris of the spinal cord (Lorente de No6) shows (1) that each presynap~tic fiber has on anv neuron but a small number of knobs