ELSEVIER Electroencephalography and clinical Neurophysiology 97 (1995) 63-68 Inhibition of hand muscle motoneurones by peripheral nerve stimulation in the relaxed human subject. Antidromic versus orthodromic input M. Inghilleri, A. Berardelli, G. Cruccu *, M. Manfredi, A. Priori, J.C. Rothwell a Department of Neurosciences, University of Rome "La Sapienza," Viale Universit~ 30, 1-00185 Rome, Italy, and MRC Human Mo~,ement and Balance Unit, Institute of Neurology, Queen Square, London, UK Accepted for publication: 8 August 1994 Abstract In active muscle, a supramaximal conditioning stimulus to peripheral nerve produces a classic silent period in the EMG. The present experiments examined the effect of this type of conditioning stimulus on motoneurone excitability in relaxed muscle. EMG responses evoked by transcranial magnetic stimulation of the brain were recorded from the first dorsal interosseus muscle (FDI) in 10 healthy subjects and 5 patients with sensory neuropathy. These responses (motor evoked potentials) were conditioned by supramaximal peripheral nerve stimuli given 0-150 msec beforehand. In the normal subjects, the classic silent period in the FDI lasted about 100 msec. The same conditioning stimulus only abolished motor evoked potentials when the conditioning-test interval was so short that the antidromic peripheral nerve volley collided with the orthodromic volley set up by magnetic brain stimulation. At longer conditioning-test intervals, although remarkably inhibited (65% mean suppression between 10 and 40 msec), the test motor potential was never completely abolished and gradually recovered by 100 msec. Inhibition of cortically evoked motor potentials did not depend upon activity set up by the conditioning stimulus in peripheral nerve sensory fibres. The patients with complete peripheral sensory neuropathy had the same extent and time-course of inhibition as the normal subjects. We conclude that in relaxed subjects the inhibitory effect of peripheral conditioning results almost exclusively from the motoneuronal inhibitory mechanisms consequent to antidromic invasion. Keywords: Electromyography; Magnetic brain stimulation; Excitability of alpha-motoneurones; Conditioning by peripheral nerve stimulation; (Human) 1. Introduction Supramaximal electrical stimulation of a mixed nerve produces the classic "silent period," which interrupts steady voluntary activity (Merton, 1951) for some 100 msec. Most of the information available on the excitability changes of hand muscle motoneurones after a peripheral conditioning has been obtained with this technique, be- cause of the difficulty in recording H reflexes in these muscles. Many authors believe that the silent period results from several mechanisms. Antidromic impulses from the condi- tioning volley and orthodromic impulses from voluntary activity collide along the motor nerve fibres; uncollided antidromic impulses invade the motoneuronal cell body, * Corresponding author. produce an after-hyperpolarisation potential (AHP) and activate the recurrent inhibition system via motor axon collaterals (Renshaw, 1946; Kimura, 1983a). The periph- eral nerve shock directly excites sensory fibres, which can inhibit motoneurQnes reflexly (McLeUan, 1973; Shahani and Young, 1973). Finally, the muscle twitch induces changes in the proprioceptive input, e.g. spindle unloading and activation of Golgi tendon organs (Granit, 1950; Mer- ton, 1951). In the present experiments we have attempted to ana- lyse changes in excitability of hand muscle motoneurones when the supramaximal peripheral conditioning shock is given in the relaxed state. Instead of using voluntary contraction to activate the spinal motoneurones we used magnetic transcranial stimulation of the motor cortex. To evaluate the relative importance of orthodromic and an- tidromic input, we conditioned the motor evoked potentials with stimulation of mixed and cutaneous nerves, in healthy subjects and in virtually "deafferented" patients. 0924-980X/95/$09.50 © 1995 Elsevier Science Ireland Ltd. All rights reserved SSDI 0013-4694(94)00225 -8 EEM 93708