Non‐monosynaptic transmission of the cortical command for voluntary movement in man.

Abstract

1. The possibility was investigated that, in man, some of the descending command for tonic voluntary wrist extension is transmitted to extensor motoneurones over a nonmonosynaptic pathway. 2. Stimulation of the cutaneous superficial radial nerve at 3 times perceptual threshold depressed the electromyogram (EMG) of extensor carpi radialis (ECR) and the discharge of single ECR motor units, both with a mean central delay of 4-2 ms. Such stimuli depressed the response to transcranial magnetic stimulation of the motor cortex, but had little effect on the H reflex. 3. The possibility that the relative sparing of the H reflex was due to an alteration in transmission of the afferent volley for the H reflex was excluded. 4. The central latency of the cutaneous-induced depression of the discharge of single motor units in biceps brachii (C5-C6) was shorter by about 1 ms than that of the more caudal wrist and finger extensor motor units. This suggests that the locus for the cutaneousinduced effects was spinal but above the cervical enlargement. 5. The pattern of EMG depression (evoked by superficial radial but not palmar stimuli, in wrist extensors but not wrist flexors) is that previously described for the presumed propriospinal system of human subjects. 6. It is concluded that a significant component of the voluntary command for tonic wrist extension reaches the relevant motoneurone pool via a non-monosynaptic pathway. It is suggested that the interposed neurones could be C3-C4 propriospinal neurones.

Mechanisms underlying spinal motor neuron excitability during the cutaneous silent period in humans

“…Low threshold cutaneous afferents converge on a number of known interneuron classes that affect motoneurons, such as Ib interneurons, 27 propriospinal interneurons, 4,5 and the first-order interneurons in circuits mediating presynaptic inhibition of antagonist Ia muscle afferents. 39 The combined action of these interneurons is likely to account for the mixture of excitatory and inhibitory reflexes that can be observed in hand muscle.…”

Selective facilitation of responses to cortical stimulation of proximal and distal arm muscles by precision tasks in man.

“…1993), and the increase in MEP size might depend on changes in excitability of these subcortical structures, which are tonically facilitated more during the precision than the power tasks. On the other hand, the premotoneuronal C3-C4 'propriospinal' system ) and the spinal inhibitory interneurones (Iles & Pisini, 1992) can be activated monosynaptically by functional populations of cortical cells, and their discharge can reach the motoneurones sufficiently early to contribute significantly to the motor potential (Nielsen et al 1993;Burke, Gracies, Mazevet, Meunier & Pierrot-Deseilligny, 1994). Therefore, either tonic or phasic excitability modification in subcortical structures might be envisaged as the cause of MEP changes.…”

Multijoint Reflex Responses to Constant-Velocity Volitional Movements of the Stroke Elbow

“…Descending facilitation of the propriospinal neurons has been demonstrated during voluntary contractions (Burke et al 1994;Pierrot-Deseilligny 2002) and thus increased excitability of propriospinal interneurons could explain the coupling of joint actions following stroke (Pierrot-Deseilligny 1996, 2002. The central delays of the spatial facilitation of weak peripheral and corticospinal volleys indicate that the interneurons that transmit corticospinal excitation to motoneurons are probably located rostral to the motoneurons (Pierrot-Deseilligny 1996).…”