Descending Monosynaptic and Reflex Control of γ‐Motoneurones

Abstract

The supraspinal and reflex control of γ‐motoneurones has been studied with intra‐ and extracellular recording from lumbosacral γ‐motoneurones in the cat. Monosynaptic EPSPs were recorded in some γ‐motoneurones on stimulation of the brain stem. These effects were evoked from the Deiters' nucleus and from fibres descending in the medial longitudinal fascicle probably originating in the pontine reticular formation. Previous investigations have revealed monosynaptic connections to α‐motoneurones from these regions and our results suggest parallel effects to α‐ and part of the γ‐motoneurone population supplying one muscle. Indirect evidence suggests that this monosynaptic effect is exerted only on static γ‐motoneurones, thus implying a linkage between the descending monosynaptic control of α‐ and static γ‐motoneurones via these pathways. The reflex effects to γ‐motoneurones have been studied with graded electrical stimulation of ipsi‐ and contralateral hindlimb nerves. Five γ‐motoneurones, presumably all belonging to extensor motor nuclei, were found to receive IPSPs from group I afferents and it is suggested that only static γ‐motoneurones are influenced. It has not been possible to decide if these IPSPs are evoked from Ib or Ia afferents. The reflex effects from group II and III muscular afferents, joint and cutaneous afferents seem to conform to the effects evoked in α‐motoneurones from these afferents.