DOES A C3-C4 PROPRIOSPINAL SYSTEM TRANSMIT CORTICOSPINAL EXCITATION IN THE PRIMATE - AN INVESTIGATION IN THE MACAQUE MONKEY

1. Synaptic responses to electrical stimulation of the contralateral p yramidal tract were measured in intracellular recordings from 206 uppe r limb motoneurones in ten chloralose-anaesthetized macaque monkeys. T he objective was to search for evidence of a disynaptic excitatory pat hway via C3-C4 propriospinal interneurones similar to that in the cat. 2. In monkeys with intact spinal cords, only a small proportion of mo toneurones (19%) responded with late EPSPs to repetitive stimulation o f the pyramid; only 3% had segmental latencies that were appropriate f or a disynaptic pathway. 3. From previous studies in the cat, it was e xpected that a lesion to the dorsolateral funiculus (DLF) at C5 would interrupt the corticospinal input to the spinal segments supplying upp er limb muscles, whilst leaving intact excitation transmitted via a C3 -C4 propriospinal system, the descending axons of which travel in the ventral part of the funiculus. In five of the monkeys a lesion was mad e to the DLF at C5 which spared the ventrolateral columns. It severely reduced the monosynaptic EPSPs and disynaptic IPSPs evoked from the p yramidal tract that were present in the intact monkey spinal cord, and which might have masked the presence of disynaptic EPSPs. However, ev en after the lesion the proportion of motoneurones with such late EPSP s was still low (18%); 14% of motoneurones had EPSPs within the disyna ptic range. 4. In addition, some EPSPs with relatively long segmental latencies (>1.1 ms) were recorded before and after the C5 lesions, but since these effects could be evoked by single stimuli, had stable lat encies and did not facilitate with repetitive shocks, it is likely tha t they represent monosynaptic EPSPs evoked by: slowly conducting corti cospinal fibres which survived the lesions. 5. In seven of the monkeys motoneurone responses to stimulation of the ipsilateral lateral retic ular nucleus (LRN) were also tested. Most motoneurones showed EPSPs wi th short latencies (1.2-2.5 ms) and other properties characteristic of monosynaptic activation. This is consistent with the presence of coll aterals of C3-C4 propriospinal neurones to the LRN, as demonstrated in the cat. 6. These short-latency EPSPs evoked from the LRN were just a s common before (77 %) as after (75%) the C5 lesion. They had small am plitudes both before (mean +/- S.D. 1.1 +/- 0.59 mV) and after (1.2 +/ - 0.72 mV) the lesion. Unlike the situation in the cat, only a small p roportion (16%) of motoneurones activated from the LRN showed late EPS Ps after repetitive stimulation of the pyramid. 7. The results provide little evidence for significant corticospinal excitation of motoneuro nes via a system of C3-C4 propriospinal neurones in the monkey. The ge neral absence of responses mediated by such a system in the macaque, u nder experimental conditions similar to those in which they are seen i n the cat, show that extrapolation of results from the cat to the prim ate should be made with considerable caution.