Autogenetic inhibition of gamma-motoneurons in the spinal cat uncovered by Dopa injection.

Abstract

The response of identified gastrocnemius γ-efferents to stretch of the triceps surae muscle was recorded in the acute spinal cat before and after the injection of Dopa. A strong tonic inhibition during static muscle extension was detectable only in those γ-efferents which had no resting and reflex activity in the spinal cat, but which responded to Dopa injection with an initially high discharge rate. In contrast γ-motoneurons with background and reflex activity before Dopa, and some previously quiescent units acquiring a low discharge rate after Dopa did not exhibit any signs of a stretch-dependent inhibition. It is concluded that the observed autogenetic inhibition is limited to the static γ-motoneurons. The responsiveness to stretch-evoked inhibition was found to be correlated with that to antidromic inhibition induced by repetitive stimulation of the ventral root. This could imply that the activation of α-motoneurons by muscle stretch leads to a Renshaw inhibition of the γ-motoneurons. However, this pathway may be only partly responsible for the autogenetic inhibition of γ-motoneurons in the spinal cat after Dopa, since the inhibitory effects of muscle stretch were more pronounced than those obtained by antidromic activation of α motor axons in the ventral root.