MONOSYNAPTIC REFLEX RESPONSES OF INDIVIDUAL MOTONEURONS

Abstract

Individual motoneuron responses to a variety of afferent inputs were examined. At a given input some motoneurons respond to every trial, some to no trial, and some to a certain percentage of trials characteristic for the motoneuron at that input. Performance of a motoneuron is expressed by means of a firing index that relates number of responses to number of trials. In a representative assemblage of individual motoneurons some 20-30% display intermediate firing indices. This number, comprising an "intermediate zone" remains fairly constant at different levels of input although individuals within it may be entirely different at 2 different levels of input. Frequency distribution of individuals with respect to firing indices is U-shaped. Intermediacy of firing indices depends upon temporal fluctuation of excitability which, in the 1st approximation, is normal. Individual motoneurons are approximately equally frequently distributed with respect to transmitter potentiality of their monosynaptic reflex afferent connections. Distribution of motoneurons with respect to transmitter potentiality of their monosynaptic reflex connections is considered representative of a natural pool in that the sum of their individual post-tetanic response behaviors accurately reproduces the course of post-tetanic potentiation in a natural pool.