PATTERN OF DESCENDING EXCITATION OF PRESUMED PROPRIOSPINAL NEURONS ATTHE ONSET OF VOLUNTARY MOVEMENT IN HUMANS

The pattern of activation of presumed 'propriospinal' neurones was inv estigated in human subjects during phasic voluntary contractions of on e of the following muscles: biceps, triceps, flexor carpi radialis (FC R), flexor carpi ulnaris (FCU) and extensor carpi radialis (ECR). Chan ges in the amplitude of the H reflex (FCR, ECR), or the tendon jerk (b iceps, triceps) were used to assess the excitability of the correspond ing motorneurone pools after conditioning stimulation. Conditioning st imuli were applied to the musculo-cutaneous, triceps and ulnar nerves. In most cases reflex facilitation was not observed at rest and was on ly disclosed at the onset of contraction. The characteristics of this facilitation (3-4 ms central delay, short duration, low threshold, dep ression when the afferent input was increased) are consistent with tho se previously attributed to 'propriospinal' excitation. It is argued t hat the contraction-associated facilitation was descending in origin. The descending facilitation of the 'propriospinal' system had a charac teristic pattern in that the pathways selected by higher centres were those receiving the afferent feedback from the contracting muscle. The se results provide further insight into the organization of human 'pro priospinal' pathways: (1) it is confirmed that afferents from each mus cle activate a specific subset of neurones; and (2) it is suggested th at the projections of each subset are divergent, implying that individ ual neurones project onto diverse motor nuclei, an organization that w ould favour the coordination of multi-joint movements. Such an organiz ation is discussed in relation to the possible role of the propriospin al system in the control of normal human upper limb movements.