Acta physiol. scand. 1972. 86. 59-81 From the Department of Physiology, University of Goteborg, Sweden Facilitation from Ipsilateral Primary Afferents of Interneuronal Transmission in the Ia Inhibitory Pathway to Motoneurones BY L. FEDINA~ and H. HULTBORN Received 4 February 1972 Abstract FEDINA, L. and H. HULTBORN. Facilitation from ipsilateral primary afferents of interneuronal transmission in the l a inhibitory pathway to motoneurones. Acta physiol. scand. 1972. 86. 59-81. The effects on transmission in the Ia inhibitory pathway to motoneurones by volleys in ipsi- lateral primary afferents were investigated with intracellular recording from motoneurones. In spinal cats under chloralose anesthesia I a 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 affeients (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. Antidromic impulses in motor axons depress transmission in the Ia inhibitory path- way to motoneuroner by a postsynaptic inhibition of the Ia inhibitory interneurones, evoked via a-motor axon collaterals and Renshaw cells (Hultborn, Jankowska and Lindstrom 1971 a, b)-henceforth referred to as recurrent depression of Ia IPSPs. Although the most striking result in the investigation by Hultborn et al. (1971 a) was the efficient recurrent depression of Ia IPSPs in all species of motoneurones they reported that also some IPSPs evoked from the ipsilateral flexor reflex afferents (FRA) were slightly depressed by volleys in ventral roots. IPSPs from Ib afferente were on the other hand never affected. The recurrent depression of the FRA IPSPs Present address: Kiskrleti Kutatd Laboratbrium, U116i ut 78/a, Budapest VIII, Hungary. 59