Input-output organization of reticulospinal neurones, with special reference to connexions with dorsal neck motoneurones in the cat

Summary

Dorsal neck motoneurones receive disynaptic tectal and pyramidal EPSPs via common reticulospinal neurones (RSNs). This study was aimed at identification of the RSNs projecting directly to neck motoneurones and mediating these EPSPs. 1. Stimulation of the tectum and the cerebral peduncle evoked monosynaptic descending volleys in the spinal cord, which were chiefly mediated by reticulospinal neurones in the pons and the medulla. Systematic tracking of the C3 and C7 segments was made to locate descending volleys in the spinal funiculi. The tectal monosynaptic volley was largest in the medial part of the ventral funiculus and decreased gradually as the recording electrode was moved to the lateral part of the ventral funiculus and the lateral funiculus. In contrast, the peduncle-evoked monosynaptic volley was distributed rather evenly in the ventral funiculus and the ventral half of the lateral funiculus. 2. Differences in funicular distribution of the two descending volleys suggest the existence of subgroups of RSNs which differed in strength of inputs from the two descending fibre systems and in the funicular location of descending axons. 3. The RSNs were classified into the following four groups; (1) mRSNs which descended in the medial part of the ventral funiculus, (2) in RSNs which descended in the ventrolateral funiculus, (3) 1RSNs which descended in the dorsal 2/3 of the lateral funiculus and (4) coRSNs which descended in the contralateral funiculi. The mRSNs were located in a fairly localized region corresponding to the nucleus reticularis pontis caudalis (N.r.p.c.), while inRSNs, 1RSNs and coRSNs were mainly in the nucleus reticularis gigantocellularis (N.r.g.), in the nucleus reticularis magnocellularis (N.r.m.) and in the nucleus reticularis ventralis (N.r.v.). RSNs were further divided into three types depending on the levels of projection. L-RSNs projected to the lumbar spinal segments. C-RSNs descended to the C6–C7 spinal segment but not to the lumbar segments. N-RSNs projected to the C3 but not to the C6–C7 segments. 4. Stimulation of the tectum and the cerebral peduncle produced monosynaptic negative field potentials in the medial two thirds of the reticular formation in the pons and medulla. Tectal field potentials were largest in the N.r.p.c. and the rostral part of the N.r.g., while pyramidal field potentials were largest in the N.r.g. Correspondingly, RSNs in the N.r.p.c. (mRSNs) received larger monosynaptic EPSPs from tectal than from pyramidal volleys, while RSNs in the N.r.g. (in-, 1- and coRSNs) received stronger input from the peduncle than from the tectum. 5. Stimulation of the C7 ventral but not the lateral funiculus evoked monosynaptic EPSPs on all the dorsal neck motoneurones tested. Stimulation of the L1 segment only produced monosynaptic EPSPs in 35% of the motoneurones. The L1 evoked EPSPs were much smaller than C7 evoked EPSPs. 6. The C7 evoked EPSPs (C7 EPSP) showed complete occlusion (collision) with the tectal or pyramidal disynaptic EPSPs. Similar results were obtained with L1 EPSPs. These results indicate that tectal and pyramidal disynaptic EPSPs in dorsal neck motoneurones were mediated chiefly by C-mRSNs and C-inRSNs and partly by L-RSNs.