Individual motoneuron responses to a variety of afferent inputs have been examined. At a given input some motoneurons respond to every trial, some to no trial, and some respond to a certain percentage of trials that is characteristic for the motoneuron at that input. The performance of a motoneuron is expressed by means of a firing index that relates the number of responses to the number of trials.
In a representative assemblage of individual motoneurons some 20 to 30 per cent display intermediate firing indices. This number, comprising an "intermediate zone" remains fairly constant at different levels of input although the individuals within it may be entirely different at two 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 first approximation, is normal.
The individual motoneurons are approximately equally frequently distributed with respect to transmitter potentiality of their monosynaptic reflex afferent connections.
The 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.