SS-4-2 Disorder of the reciprocal Ia inhibitory pathway in spasticity

Abstract

This chapter deals with the identification of the reciprocal Ia inhibition in humans and modulation of its activity in relation to voluntary movements. The aim is to answer the question whether the Ia inhibitory system contributes to the antagonist inhibition, actually occurring during voluntary movements. Voluntary ankle dorsiflexion—that is, contraction of the tibialis anterior muscle, causes an inhibition of its antagonistic triceps surae motoneurons as exemplified by depression of the H-reflex. If the excitability of Ia inhibitory interneurons to the triceps surae motoneurones increases together with this antagonist inhibition, it strongly suggests that this system takes part in revelation of the inhibition. Most prominent findings are that, in normal human subjects, interneurones mediating Ia inhibition to the ankle extensor motoneurones appear to be depressed in a resting state but are active in parallel with depression of the extensor motoneurones during a steady voluntary contraction of ankle flexors. Subliminal facilitation of these interneurons already appears nearly 100 msec prior to the onset of voluntary electromyogram in the flexors in parallel to that of agonist α-motoneurones.