Control from the brainstem of synchrony of discharge between gamma motoneurones in the cat.

An assessment has been made of the effect of partial and complete section of the spinal cord on the discharges of gamma motoneurones to hind limb muscles in the decerebrated cat. The degree to which the discharges of pairs of individual gamma motoneurones exhibit short-term synchrony and the variability in interspike intervals of the discharge in individual neurones was measured. Variability of discharge was assessed as coefficient of variation of interspike intervals and degree of synchronization assessed from cross correlation analysis. The discharges of gamma motoneurones in the decerebrated cat with intact spinal cord are regular (low coefficient of variation) and show no tendency to synchrony for up to 24 h following decerebration. Section of the more medial part of the dorsolateral funiculus, on either side of the thoracic spinal cord, resulted in a less regular discharge and the development of short-term synchrony between gamma motoneurones. A dependence of synchrony on variability of discharge was established. The synchrony and increased variability in the spinal cat persisted for at least 24 h. Both were markedly reduced following administration of the precursors of monoamines (either L-Dopa or 5-HTP). We conclude that a bilateral, monoaminergic pathway descending in the dorsolateral funiculus from the brainstem controls synchrony of gamma motoneurone discharge in the decerebrated cat. The possibility is discussed that synchrony of discharge between alpha motoneurones may be controlled by a similar pathway.