Propriospinal pathways in the dorsolateral funiculus and their effects on lumbosacral motoneuronal pools

Abstract

Propriospinal pathways descending in the dorsolateral funiculus (DLF), which link middle lumbar with lower lumbar and sacral segments, were studied in cats in terms of effects evoked by activation of these pathways in lumbosacral motoneuronal pools. Selective activation of propriospinal fibers was achieved by preceding (12–16 days before) ipsilateral hemisection of the spinal cord in upper lumbar segments and subsequent degeneration of long descending fibers. Under such conditions stimulation of DLF evoked a mass discharge descending with a conduction velocity of about 40 m/sec in the DLF. This volley exerted a considerable conditioning influence upon monosynaptic test reflex discharges of motoneurons. Moderate DLF stimulation facilitated discharges evoked from flexor muscle afferents and inhibited discharges from extensor afferents. The duration of conditioning effects was about 15–20 msec. With increased strength of DLF stimulation secondary prolonged inhibition of both flexor and extensor reflexes was added to initial reciprocal effects. Segmental delay of propriospinal action upon motoneurons did not exceed 0.8–1.0 msec, and was identical for facilitation and inhibition. This indicates the possibility of a monosynaptic nature of both effects. DLF stimulation evoked postsynaptic potentials in motoneurons: predominantly EPSPs in flexor units and IPSPs or mixed PSPs in extensor units. Latency values of both EPSPs and IPSPs agree with the suggestion concerning the monosynaptic nature of at least part of these actions. Strong frequency potentiation is a special feature of ‘propriospinal’ PSPs. The functional significance of propriospinal pathways in DLF and their probable participation in transmission of activity from descending pathways to motoneurons are discussed.