Propriospinal control of last order interneurones of spinal reflex pathways in the cat

Abstract

In studies from our laboratory on the supraspinal control of segmental spinal mechanisms, the problem has occasionally arisen as to whether some segmental effects evoked by stimulation of a supraspinal tract were due to activation of long propriospinal neurones. For this and other reasons it became necessary to obtain more information about long propriospinal neurones, not only with respect to their segmental origin, the trajectory of their axons and their synaptic activation, but also regarding their effects on segmental mechanisms. To analyze segmental synaptic effects evoked by propriospinal fibres we chose to stimulate the axons directly at the low thoracic level, and limited ourselves to effects on motoneurones and last order interneurones in spinal reflex pathways. In this report it will be shown that propriospinal fibres terminate on some interneurones which project directly to motoneurones, and that these iriterneurones are interposed in two well defined segmental reflex pathways. Both conclusions are based on intracellular records of postsynaptic potentials (PSPs) evoked in motoneurones (in L6-SI spinal segments) by stimulation ofpropriospinal and peripheral nerve fibres, and on an analysis of interactions between effects evoked from these two sources. The experiments were performed on 8 cats anaesthetized with chloralose, in which the spinal cord had been hemisected at a high cervical (C3) level, 2-13 weeks before, in order to produce degeneration of fibres of supraspinal origin. Propriospinal fibres originating between high cervical and low thoracic segments were stimulated at Thl 1. Since our attention was focussed on the long propriospinal pathways which descend in the middle of the lateral funicle 9, we transected the following fibre systems caudal to the level of stimulation: the dorsal column, the contralateral spinal half, the ipsilateral superficial dorsomedial part of the lateral funicle (including the spinocervical tract), the ipsilateral ventral funicle and the very ventral part of the lateral funicle. Despite these lesions it cannot be excluded that some segmental effects evoked by stimulation at Th 11 may be collateral to antidromic activation of ascending axons. To investigate this possibility, control experiments were made in animals after a chronic hemisection of the spinal cord at a low thoracic (Thl0) level. This lesion