Pattern of propriospinal‐like excitation to different species of human upper limb motoneurones.

Abstract

SUMMARY1. The pattern of distribution of non-monosynaptic (propriospinal-like) excitation to various motor nuclei (deltoid, extensors and flexors of the elbow, the wrist and the fingers) was investigated.2. Changes in the firing probability of individual voluntarily activated motor units were studied following conditioning stimuli. Conditioning volleys were evoked by weak electrical stimuli applied to various mixed nerves (circumflex, musculocutaneous, median, radial, ulnar) and to the skin.3. In all investigated nuclei stimulation of the 'homonymous' nerve evoked a peak of increased firing probability with a latency which was 2-7 ms longer than the monosynaptic I a latency. The average central delay of the late excitation, measured from monosynaptic latency, seems to depend only on the segmental level of the motor nucleus: the more caudal the nucleus the longer the latency. This strongly suggests a transmission through neurones located above the cervical enlargement, as are C3-C4 propriospinal neurones in the cat.4. Both group I muscle and cutaneous afferents were shown to contribute to propriospinal-like excitation. It is argued that a spatial facilitation of the effects evoked by these two inputs might explain why the threshold of late excitation is always below that of the monosynaptic Ia excitation in motoneurones.5. The pattern of distribution of propriospinal-like excitation was diffuse: stimulation of each mixed nerve was able to evoke excitation in all investigated motor nuclei. Similarly, stimulation of a given skin field could produce excitation of biceps and wrist flexor and extensor units.6. Each motor nucleus therefore receives excitation from a multimodal and wide range peripheral input. However, it is argued that what appears as a diffuse pattern might simply reflect connections which are not used in each movement but appropriately selected by higher centres.

Pattern of cutaneous inhibition of the propriospinal‐like excitation to human upper limb motoneurones.

“…A non-monosynaptic excitation of a given motoneurone pool from different nerves was necessary for the purpose of this study, which excluded the use of triceps brachii (see Gracies et al 1991). Finger muscles were not used either, since cutaneous stimulation may produce an inhibition exerted directly onto their motoneurones (Garnett & Stephens, 1980;Malmgren & PierrotDeseilligny, 1988b) and this would have made the interpretation of the results difficult.…”

“…The methods for recording individual voluntarily activated motor units, stimulating the median, radial (below the brachio-radialis muscle branch) and musculo-cutaneous nerves, constructing a post-stimulus time histogram (PSTH) and assessing or estimating the monosynaptic Ia latencies were as described in the companion paper (Gracies et al 1991). The monosynaptic latency is the zero of all the histograms.…”

“…In most of the experiments the effects of stimulating the palmar and the dorsal side of both the second and fourth fingers were compared: stimuli were weak (2 x perception threshold, 2 x PT), and the site of stimulation was chosen so that the same sensation was experienced on both sides. Conduction velocities in I a fibres (see Gracies et al 1991) and in cutaneous fibres (see below) were calculated, and the interval between the cutaneous stimulation and that eliciting the propriospinallike excitation was adjusted so that the cutaneous volley preceded the other volley by about 1 ms at the spinal level.…”

“…flexion or extension) mediated by propriospinal neurones should be inhibited from a specific cutaneous field which can come into contact with the target at the end of the movement. We have therefore investigated whether the pattern of the cutaneous inhibition of the propriospinal-like excitation varies with muscle function, taking advantage of the existence of such an excitation in motoneurones supplying many different human upper limb muscles (Gracies, Meunier, Pierrot-Deseilligny & Simonetta, 1991).…”

Spinal-Like Regulator Facilitates Control of a Two-Degree-of-Freedom Wrist

“…The proprioceptive sensory outputs (muscle spindle primary Ia and Golgi tendon organs Ib) excited the propriospinal pathways (PN in Figs. 2, 3) (Malmgren and Pierrot-Deseilligny, 1988a;Gracies et al, 1991;Burke et al, 1992). The propriospinal circuit models directly excited the homonymous motoneurons and also formed the following connections with other motoneurons: "selective" synapses whose gain could be excitatory or inhibitory for the partial synergists and inhibitory synapses for the true-antagonist muscles.…”