Facilitation from ipsilateral primary afferents of interneuronal transmission in the Ia inhibitory pathway to motoneurones.

Abstract

AbstractThe effects on transmission in the Ia inhibitory pathway to motoneurones by volleys in ipsilateral primary afferents were investigated with intracellular recording from motoneurones. In spinal cats under chloralose anesthesia Ia IPSPs in flexor as well as extensor motoneurones were regularly facilitated by volleys in cutaneous, high threshold muscle and joint afferents. In decerebrate cats with a low pontine lesion IPSPs were not facilitated from high threshold muscle and joint afferents, although volleys in low threshold cutaneous afferents evoked a facilitation also in this preparation. It is postulated that the Ia inhibitory interneurones receive excitatory actions from the ipsilateral flexor reflex afferents (transmission depressed in the decerebrate state) and through a separate pathway from low threshold cutaneous afferents. The recurrent effects from motor axon collaterals were investigated on inhibitory transmission to motoneurones from different afferents. A strong positive correlation was revealed between recurrent depression of IPSPs evoked from different afferents and facilitation of Ia IPSPs by the same afferent volleys. This suggests that the recurrent depression of IPSPs from different primary afferents depends on an excitatory convergence from them onto the Ia inhibitory inter‐neurones, which mediate at least part of the IPSP evoked in the motoneurone from these afferents.