Spatial Facilitation in the ‘Direct’ Inhibitory Pathway

Abstract

OF the various inhibitory actions converging on to spinal motoneurones the ‘direct’ inhibition is the one that has been the most extensively investigated^1–6. It is provoked by impulses in large muscle afferents from annulo-spinal endings (Ia fibres)^7–9. Monosynaptic excitatory action to motoneurones is also supplied by Ia fibres, and, with any particular group of motoneurones, the inhibitory action is caused by Ia fibres originating from the muscles antagonistic to those supplying excitatory action. The ‘direct’ inhibitory action is caused by a brief hyperpolarization of the motoneuronal membrane, the inhibitory post-synaptic potential¹⁰. The inhibitory post-synaptic potentials have a minimal latency of about 1.2–1.4 msec. relative to the time when the Ia volley reaches the spinal cord or approximately 0.8 msec. longer than for the monosynaptic excitatory post-synaptic potential. It has been postulated that this longer latency is due to the existence of an interneurone in the ‘direct’ inhibitory pathway, and experimental evidence has been presented suggesting that these interneurones are located in the intermediate nucleus¹¹.