[Testing the excitability of a spiking human motor neuron during the interspike interval]

Excitability of voluntary activated human triceps surae motor units was tested by monosynaptic excitatory Ia afferent volley evoking weak H-reflex. Firing index was used as a quantitative measure of motoneuron excitability. During transition of a low firing rate of a motoneuron (up to 6 imp/s) to moderate one (6-8 imp/s) the firing index fell; with a further increase in the rate (up to 12 imp/s) it practically did not change. Dependence of the firing index on the rate is based on changes in the motoneuron excitability within the interspike interval. The results indicate that at low firing rates of a motoneuron which are characteristic of its natural activation, the firing rate is one of the factors influencing the excitability of the firing motoneuron and hence its input-output relations.