An analysis of connexions between 2nd and 3rd order of relay neurones in the spinal reflex.

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Dorsal neck motoneurones innervationg splenius (SPL) and biventer cervicis and complexus (BCC) muscles receive disynaptic excitatory effects after stimulation of the tectum and the pyramid in the cats. These effects were mediated by reticulospinal neurones (RSN) in the nucleus reticluaris pontis caudalis (NRPC) and the nucleus reticularis gigantocellularis (NRG). The majority of RSNs in NRPC received larger inputs from the tectum than from the pyramid, and descended in the ipsilateral ventral funiculus in the C3 segment. RSNs in the NRG, in contrast, had a stronger inputs in general from the pyramid than from the tectum, and descended in the ventrolateral or lateral funiculus of the ipsilateral side or in the contralateral funiculi.Studies of systematic threshold mapping for evoking antidromic spikes of RSNs in C2-C3 segments and intra-axonal staining of their descending axons in C1-C3 segments with horseradish peroxidase(HRP) revealed the following. Among RSNs, those descending to the brachial segments (C6-Th1) but not down to lumbar segments (C-RSNs) issued collaterals to the ventral horn of C1-C3 segments, while RSNs descended to the lumbar segments gave off collaterals rarely in C1-C3 segments. Direct monosynaptic excitatory connexions of C-RSNs in NRPC and NRG with SPL and BCC motoneurones were demonstrated by spike triggered averaging methods. Their connexions were muscle specific.The origin of corticoreticular projections was examined. Retrogradely labelled neurones were located, following injection of HRP in NRPC, in the prefrontal cortex(PF), areas 6 and medial half of area 4. In contrast, they were distributed in the lateral 2/3 of area 4 and area 6 following injection in the NRG. Intracortical stimulation evoked dominant EPSPs in RSNs in NRPC from area 6 and the medial half of area 4 and small EPSPs from PF, while it evoked large EPSPs in RSNs in NRG from the lateral 2/3 of area 4 and small EPSPs from area 6 but not from PF.