A presynaptic mechanism evoked from brain stem reticular formation in the lumbar cord and its temporal significance

Abstract

Summary (1) In the decerebrate cat, stimulation of a large spectrum of sites in the brain stem reticular formation (RF) evoked negative and postive dorsal root potentials (DRPs) in the lumbar spinal cord. (2) Both negative and postive DRPs are developed as a function of the stimulus intensity delivered to the reticular sites, although the relationship is not linear. (3) There is a well correlated temporal course of the negativity of DRP, the inhibitory phase of monosynaptic reflexes (MSRs), as well as the increased excitability of intraspinal Ia terminals revealing the presence of primary afferent depolarization (PAD), under the influence of the same RF stimulation. These evidences thus point to the existence of a presynaptic inhibitory mechanism exerted from the RF onto the lumbar cord. (4) Concomitant positive DRP and facilitation of MSR on a parallel time basis, the latter of which overrides a preceding inhibition which corresponds in time to the negative wave, has been shown. This is interpreted as the reflection of a hyperpolarization of the primary afferent terminals (PAH) underlying a presynaptic facilitation. (5) It is concluded that at least two separate groups of reticular neurons are exerting their effects, one initiating PAD and the other PAH on the primary afferent terminals in the lumbar cord. These two would be modulating sensory inputs at the presynaptic level through a competitive process.