EVIDENCE FOR CORTICOSPINAL EXCITATION OF PRESUMED PROPRIOSPINAL NEURONS IN MAN

1. The possibility that stimulation of the motor cortex facilitates tr ansmission in the pathway mediating non-monosynaptic ('propriospinal') excitation from low-threshold afferents to upper limb motoneurones wa s investigated. 2. Convergence between peripheral afferent volleys (fr om the ulnar or musculo-cutaneous nerve) and corticospinal volleys (ev oked by magnetic stimulation of the motor cortex) was investigated usi ng the spatial facilitation technique. Thus the effects of these volle ys on the flexor carpi radialis H reflex were compared when applied se parately and together. When cortical stimulation was optimal for the m uscle from which the conditioning volley originated the facilitation o f the reflex on combined stimulation was significantly larger than the algebraic sum of the effects of separate stimuli. 3. The extra facili tation on combined stimulation had all the characteristics of 'proprio spinal' excitation (low threshold, long central delay, brief duration and depression when the afferent input was increased), and it is sugge sted that this reflects corticospinal excitation of 'propriospinal' ne urones. 4. When varying the time interval between cortical and test st imulations, it was shown that extra facilitation on combined stimulati on began 1 ms later than the onset of the control reflex facilitation. Assuming that the latter onset reflects the arrival of the monosynapt ic corticospinal volley at the motoneurone pool, this 1 ms delay sugge sts a disynaptic pathway for the cortical excitation of motoneurones t hrough 'propriospinal' neurones. 5. As at the onset of voluntary movem ent, the pattern of the cortical excitation of 'propriospinal' neurone s was quite specific: extra facilitation of the reflex on combined sti mulation only occurred when the cortical volley was preferentially dir ected to motoneurones supplying the muscle from which the afferents us ed for the peripheral volley originated. 6. It is concluded that corti cospinal axons activate human 'propriospinal' neurones and thereby pro duce disynaptic excitation of the motoneurone pool. Given temporal sum mation with the monosynaptic excitation, this 'propriospinally mediate d' disynaptic excitation might make a significant contribution to the evoked EMG potential.