Single medullary reticulospinal neurons exert postsynaptic inhibitory effects via inhibitory interneurons upon alpha-motoneurons innervating cat hindlimb muscles

This study was aimed at elucidating the brainstem-spinal mechanisms of postural suppression evoked by stimulating the dorsal portion of the caudal tegmental field (DTF) in the pons. For this purpose, we first sampled a group of reticulospinal neurons located in the medial part of medullary reticular formation, which were activated orthodromically and antidromically by stimulating the DTF area and the first lumbar spinal segment, respectively (DTF-RS neurons; N = 26). These DTF-RS neurons were located within the nucleus reticularis gigantocellularis (NRGc) and projected their descending axons to the lumbar spinal cord through the ventrolateral funiculus. The postsynaptic inhibitory effects of single DTF-RS neurons upon hindlimb alpha-MNs intracellularly recorded (N = 78) were then studied with spike-triggered averaging. Twelve DTF-RS neurons evoked IPSPs in 21 hindlimb alpha-MNs. Five DTF-RS neurons exerted postsynaptic inhibitory effects upon more than one alpha-MNs. These alpha-MNs were located from L5 to S1 segments of the spinal cord. A mean latency of IPSPs which was measured from the onset of the trigger spike was 5.1 ms with time to peak of 1.8 ms, and the mean segmental delay of the IPSPs was 1.5 ms, which was measured from the onset of the descending axonal volley recorded extracellularly adjacent to alpha-MNs. Amplitudes of the IPSPs were augmented with an increase in the firing frequencies of the DTF-RS neurons, the increase being produced by iontophoretic application of glutamate. These characteristics of the IPSPs suggest that reticular effects are mediated at least by a single spinal inhibitory interneuron. These results suggest that the DTF-NRGc system participates in generalized motor inhibition.