Distribution of recurrent inhibition in the human upper limb

Abstract

The distribution of homonymous and heteronymous recurrent inhibition among the motor nuclei innervating the main muscles of the human upper limb has been investigated in 25 healthy subjects. Homonymous recurrent inhibition was studied with a specially designed electrophysiological method with paired H-reflexes, previously described by Bussel & Pierrot-Deseilligny (1977), combined with a pharmacological study using a cholinergic agonist, the L-acetylcarnitine (Rossi & Mazzocchio 1991). These methods were used to investigate the Flexor Carpi Radialis (FCR), Extensor Carpi Radialis (ECR), Opponens Pollicis (OP) and Abductor Digiti Minimi (ADM) motor nuclei. In the Deltoid, Triceps, Flexor Carpi Ulnaris (FCU) and Extensor Carpi Ulnaris (ECU) motor nuclei in which it was impossible to evoke clearly distinguishable H-reflexes, homonymous recurrent inhibition was studied with the PSTH technique: homonymous recurrent inhibition was found in Deltoid, Triceps, FCR, FCU, ECR, ECU motor nuclei but not in OP and ADM motor nuclei. Heteronymous recurrent inhibition was studied with the PSTH technique in the Deltoid, Biceps, Triceps, FCR, ECR, FCU, ECU, Flexor Digitorum Communis (FDC), Extensor Digitorum Communis (EDC) motor nuclei as well as those innervating the intrinsic muscles of the hand. The following results were obtained: (1) motor neurones innervating muscles acting at digits do not receive any heteronymous recurrent inhibition; (2) motor neurones innervating muscles acting at the wrist give recurrent inhibition to motor nuclei of proximal muscles but do not receive any recurrent projections from the latter; and (3) motor neurones innervating proximal muscles (acting at shoulder or elbow) are interconnected by recurrent inhibition and receive heteronymous recurrent projections from some wrist muscles but not from intrinsic hand muscles.

Depression of Renshaw recurrent inhibition by activation of corticospinal fibres in human upper and lower limb.

“…This pattern has already been described in detail in previous work (Rossi & Mazzocchio, 1992;Katz et al 1993). Cortical stimulation effects were restricted to that part of the curve where an increase in Hi reflex led to a fall in H' reflex.…”

“…Thus the test H' reflex can be at the very most equal to the HI reflex. In the soleus motor nucleus (Bussel & PierrotDeseilligny, 1977), quadriceps and pretibial motor nuclei (Rossi & Mazzocchio, 1991) and flexor carpi radialis and extensor carpi radialis motor nuclei (Rossi & Mazzocchio, 1992;Katz et al 1993), when the HI reflex increases beyond a certain value, the absolute value of the test H' reflex decreases indicating that an increasingly smaller fraction of the motoneurones which have given rise to HI are involved in the H' response. Provided that the conditioning-test interval is more than 9 ms, the effect of Ib fibre stimulation by the conditioning stimulus can be eliminated since firstly, it has been demonstrated that in humans the duration of lb inhibition from triceps to soleus motoneurones is less than 10 ms , and secondly, the amount of inhibition of the test H' reflex is independent of the conditioning stimulus strength and is only related to the size of the conditioning HI reflex (Pierrot-Deseilligny & Bussel, 1975;.…”

“…1 Bussel, Held & Katz, 1976). In the upper limb, the reflex arc being much shorter, interstimulus intervals of 10, 12 and 15 ms were used (see Katz et al 1993). At these intervals, possible Ib effects from the conditioning S1 stimulus should be over, if the duration of Ib inhibition of soleus motoneurones is taken as a parameter (see Pierrot-Deseilligny, Katz & Morin, 1979).…”

“…However, the descending pathways mediating these effects can only be hypothesized. In addition, although the presence and distribution of recurrent inhibition in the human upper limb has been recently reported (Rossi & Mazzocchio, 1992;Katz, Mazzocchio, Penicaud & Rossi, 1993), it is not yet known whether Renshaw cell activity in the upper limb is subjected to the same control as in the lower limb. We therefore tested whether the activity of Renshaw cells could be modified by cortical stimulation in humans at rest.…”

A central back-coupling hypothesis on the organization of motor synergies: a physical metaphor and a neural model

“…There is substantial variability in the organization and the strength of inhibitory projections mediated by Renshaw cells in different muscles (Katz et al 1993). These projections can be modulated pharmacologically and by descending projections (Mattei et al 2003;Hultborn et al 2004).…”