Modulation of spontaneous locomotor and respiratory drives to hindlimb motoneurons temporally related to sympathetic drives as revealed by Mayer waves.

In this study we investigated how the networks mediating respiratory and locomotor drives to lumbar motoneurons interact and how this interaction is modulated in relation to periodic variations in blood pressure (Mayer waves). Seven decerebrate cats, under neuromuscular blockade, were used to study central respiratory drive potentials (CRDPs, usually enhanced by added CO2) and spontaneously occurring locomotor drive potentials (LDPs) in hindlimb motoneurons, together with hindlimb and phrenic nerve discharges. In four of the cats both drives and their voltage-dependent amplification were absent or modest, but in the other three, one or other of these drives was common and the voltage-dependent amplification was frequently strong. Moreover, in these three cats the blood pressure showed marked periodic variation (Mayer waves), with a slow rate (periods 9-104 s, mean 39 ± 17 SD). Profound modulation, synchronized with the Mayer waves was seen in the occurrence and/or in the amplification of the CRDPs or LDPs. In one animal, where CRDPs were present in most cells and the amplification was strong, the CRDP consistently triggered sustained plateaux at one phase of the Mayer wave cycle. In the other two animals, LDPs were common, and the occurrence of the locomotor drive was gated by the Mayer wave cycle, sometimes in alternation with the respiratory drive. Other interactions between the two drives involved respiration providing leading events, including co-activation of flexors and extensors during post-inspiration or a locomotor drive gated or sometimes entrained by respiration. We conclude that the respiratory drive in hindlimb motoneurons is transmitted via elements of the locomotor central pattern generator. The rapid modulation related to Mayer waves suggests the existence of a more direct and specific descending modulatory control than has previously been demonstrated.

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.

Olfactory ensheathing cell grafts have minimal influence on regeneration at the dorsal root entry zone following rhizotomy.

The effectiveness of grafts of olfactory ensheathing cells (OECs) as a means of promoting functional reconnection of regenerating primary afferent fibers was investigated following dorsal root injury. Adult rats were subjected to dorsal root section and reanastomosis and at the same operation a suspension of purified OECs was injected at the dorsal root entry zone and/or into the sectioned dorsal root. Regeneration of dorsal root fibers was then assessed after a survival period ranging from 1 to 6 months. In 11 animals, electrophysiology was used to look for evidence of functional reconnection of regenerating dorsal root fibers. However, electrical stimulation of lesioned dorsal roots failed to evoke detectable cord dorsum or field potentials within the spinal cord of any of the animals examined, indicating that reconnection of regenerating fibers with spinal cord neurones had not occurred. In a further 11 rats, immunocytochemical labeling and biotin dextran tracing of afferent fibers in the lesioned roots was used to determine whether regenerating fibers were able to grow into the spinal cord in the presence of an OEC graft. Although a few afferent fibers could be seen to extend for a limited distance into the spinal cord, similar minimal in-growth was seen in control animals that had not been injected with OECs. We therefore conclude that OEC grafts are of little or no advantage in promoting the in-growth of regenerating afferent fibers at the dorsal root entry zone following rhizotomy.