7^e Colloque de la Société des neurosciences

May 18-20, 2005, Lille (France)

For lectures and symposia, time indicates the beginning of the session.
For posters, authors are expected to be present at the time indicated.

Poster J-41 - 20/05/05, 15:00 - Lille Métropole I
Session J - Neurophysiology II
Ref.: 7^e Colloque de la Société des neurosciences, Lille 2005, J.41.

Author(s)	Schmied-Doussain A. (1), Vedel J. P. (1), Türker K. (2), Mazzocchio R. (3) & Rossi A. (3)
Addresse(s)	(1) UMR P3M 6196, Marseille, France ; (2) Dept. Physiol. Univ. Adelaide, Australie ; (3) Neurol. , CHU Sienne, Italie
Title	Pharmacologically induced enhancement of recurrent inhibition in humans: effects on motoneurone discharge patterns.
Text	The aim of the present study was to explore the effects of spinal recurrent inhibition on human motoneurone discharge patterns. The investigation was performed on the Extensor Carpi Radialis (ECR), the Abductor Digiti Minimi (ADM), the Tibialis Anterior (TA) and the Masseter (MAS) muscles. The existence of recurrent inhibition is well established in the ECR and TA motoneurone pools whereas it is thought to be lacking in the ADM and strongly questioned in the MA. The tonic discharge of motor unit (MU) pairs was recorded during voluntary isometric contraction. While undergoing continuous intravenous saline (NaCl 0.9 %) perfusion, the subjects were given a 2 min- injection of L-acetylcarnitine (L-Ac), known to potentiate recurrent inhibition in humans. The variability and synchronization of the MU discharges were analysed before, during and after the injection. In both the ECR and TA muscles, the L-Ac injection led to 1) a significant decrease in the variability of the inter-spike interval (ISI) with no consistent changes in the mean duration of the ISIs and, 2) a significant increase in the synchronous activity. In contrast, L-Ac injection did not alter in any way the pattern of MU discharge in the ADM and the MAS muscles. The contrasting effects observed with ECR and TA motoneurones known to undergo recurrent inhibition and with ADM motoneurones known to lack recurrent inhibition strongly suggest that Renshaw cell activity enhanced by L-Ac injection had promoted the changes in variability and in synchronization in the ECR and TA muscles. These data support the contribution of recurrent inhibition in limiting the variability of steadily discharging motoneurones and in synchronizing motoneurone activity. In contrary, the absence of effect of L-Ac injection in the MAS muscle can be taken as an argument favouring the lack of recurrent inhibition which is presently the matter of a debate in this muscle.