Electrophysiological actions of the rubrospinal tract in the anaesthetised rat

The rubrospinal tract (RST) of the rat is widely used in studies of regeneration and plasticity, but the electrophysiology of its spinal actions has not been described. In anaesthetised rats with neuromuscular blockade, a tungsten microelectrode was located in the region of the red nucleus (RN) by combining stereotaxis with recording of antidromic potentials evoked from the contralateral spinal cord. Single stimuli through this electrode typically elicited two descending volleys in the contralateral dorsolateral funiculus (DLF) separated by about 1 ms, and one volley recorded from the ipsilateral DLF. Latencies of the ipsilateral and the early contralateral volley were similar. The activation of these volleys depended on the location of the stimulation site in or near the RN. Evidence is adduced to show that: (a) the late contralateral volley is carried by fibres of RST neurones, synaptically activated; (b) the early contralateral volley is mostly carried by RST fibres stimulated directly; (c) the ipsilateral volley is sometimes carried by RST fibres from the RN on the side contralateral to the stimulus; (d) otherwise, either early volley may derive from fibres in other tracts. Synaptic potentials related to the volleys were recorded within the cervical enlargement and their distribution plotted on cross-sections of the spinal cord. These measurements suggest that the great majority of RST terminations are on interneurones in the intermediate region contralateral to the RN. Direct synaptic actions on motoneurones are likely to be weak. Stimulation parameters appropriate for specific activation of the RST in future studies are suggested.