Limb movements are initiated and controlled by nerve cells in several brain regions. Of these, neurones in the red nucleus supplement actions of corticospinal neurones and may even substitute them after injuries of the motor cortex. We have investigated how commands sent from the red nucleus to spinal neurones, and via them to limb muscles, are monitored by a population of spinal neurones providing the cerebellum with feedback information on operation of spinal neuronal networks. The results show that actions expected to be evoked by neurones in the red nucleus on motoneurones may be monitored by the cerebellum, as actions of corticospinal neurones are, although they are monitored by spinal rather than reticulospinal neurones. These results may provide us with a better understanding of mechanisms contributing to the recovery of motor functions following brain injuries.
We recently demonstrated that feline ventral spinocerebellar tract (VSCT) neurones monitor descending commands for voluntary movements initiated by pyramidal tract (PT) neurones as well as locomotor movements relayed by reticulospinal (RS) neurones. The aim of the present study was to examine whether VSCT neurones likewise monitor descending commands from the red nucleus (RN). Extracellular records from the spinal border (SB) subpopulation of VSCT neurons revealed that a third (31%) of SB neurones may be discharged by trains of stimuli applied in the RN. Moreover, when RN stimuli failed to discharge SB neurones they facilitated activation of some of these neurones by RS and/or PT neurones, while activation of other SB neurones was depressed. We propose that the facilitation and depression of actions of RS neurones by RN neurones might serve to reflect a higher or lower excitability of motoneurones and therefore a likely higher or lower efficacy of the RS descending commands, prompting the cerebellum to adjust the activation of reticulospinal neurones. Activation of SB neurones by RN stimuli alone would also allow monitoring and adjusting the RN descending commands. Intracellular records from SB neurones revealed both monosynaptic and disynaptic EPSPs and disynaptic IPSPs evoked by RN stimuli. The disynaptic actions remained following transection of axons of reticulospinal neurones within the medullary longitudinal fascicle (MLF) and were therefore taken to be relayed primarily by spinal neurones, in contrast to EPSPs and IPSPs evoked by PT stimuli found to be relayed by reticulospinal rather than spinal neurones.