Monosynaptic and disynaptic reticulospinal actions on lumbar motoneurons of the rat

Abstract

The postsynaptic effects evoked in lumbar motoneurons by stimulation of the ipsilateral medullary reticular formation were studied by means of intracellular recordings in rats with intact CNS and with chronically destroyed vestibular and red nuclei. Reticulospinal volleys mediated by fast-conducting fibers descending in the ventral and lateral funiculi elicited, in flexor and extensor motoneurons, short-latency simple-shaped EPSPs with monosynaptic segmental delay. The mean amplitude of the monosynaptic EPSPs evoked by maximal reticulospinal volleys ranged from 0.6 to 3.8 mV with an average of1.96 ± 0.19mV. The operating characteristics of the monosynaptic reticulospinal projections were similar to those reported in the cat. In some cells large unitary monosynaptic EPSPs were evoked by threshold stimuli applied to the reticular structures. The growth of monosynaptic EPSPs as a function of reticular pulse strength suggests the existence of sharply localized groups of reticulomotoneuronal cells. In addition to their monosynaptic excitatory actions the fast reticulospinal units gave rise to a disynaptically mediated excitation and inhibition. The characteristic property of disynaptic reticulospinal actions was the prominent frequency potentiation. The reticulospinal inhibition was associated with an increase in conductance. The results are discussed in relation to the properties of reticulomotoneuronal and corticomotoneuronal projections in different mammals.