Pattern of descending excitation of presumed propriospinal neurones at the onset of voluntary movement in humans.

The pattern of activation of presumed 'propriospinal' neurones was investigated in human subjects during phasic voluntary contractions of one of the following muscles: biceps, triceps, flexor carpi radialis (FCR), flexor carpi ulnaris (FCU) and extensor carpi radialis (ECR). Changes in the amplitude of the H reflex (FCR, ECR), or the tendon jerk (biceps, triceps) were used to assess the excitability of the corresponding motoneurone pools after conditioning stimulation. Conditioning stimuli were applied to the musculo-cutaneous, triceps and ulnar nerves. In most cases reflex facilitation was not observed at rest and was only disclosed at the onset of contraction. The characteristics of this facilitation (3-4 ms central delay, short duration, low threshold, depression when the afferent input was increased) are consistent with those previously attributed to 'propriospinal' excitation. It is argued that the contraction-associated facilitation was descending in origin. The descending facilitation of the 'propriospinal' system had a characteristic pattern in that the pathways selected by higher centres were those receiving the afferent feedback from the contracting muscle. These results provide further insight into the organization of human 'propriospinal' pathways: (1) it is confirmed that afferents from each muscle activate a specific subset of neurones; and (2) it is suggested that the projections of each subset are divergent, implying that individual neurones project onto diverse motor nuclei, an organization that would favour the co-ordination of multi-joint movements. Such an organization is discussed in relation to the possible role of the propriospinal system in the control of normal human upper limb movements.