Paradoxical e¡ect of digital anaesthesia on force and corticospinal excitability Julie Duque, 1,CA Yves Vandermeeren, 1 Thierry M. Lejeune, 2,3 Jean-Louis Thonnard, 2 Allan M. Smith 4 and Etienne Olivier 1 1 Laboratoire de Neurophysiologie; 2 Unite¤ de Re¤ adaptation et Me¤ decine Physique; 3 Cliniques Universitaires Saint-Luc, Universite¤ Catholique de Louvain, Bruxelles, Belgium; 4 De¤ partement de Physiologie, Centre de Recherche en Sciences Neurologiques, Universite¤ de Montre¤ al, Montre¤ al, Canada CA Corresponding Author: julie.duque@nefy.ucl.ac.be Received10 November 2004; accepted 29 December 2004 The role played by sensory information in maintaining motor cor- tical representations is still incompletely understood. We investi- gated the e¡ect of digital anaesthesia of the index ¢nger and thumb on the amplitude of motor evoked potentials to transcranial magnetic stimulation (TMS) recorded from the ¢rst dorsal interos- seus, F-wave response probability and maximal key pinch force. Whereas digital anaesthesia led to a 29% decrease in maximal force, both motor evoked potential amplitudes and F-wave prob- ability remained unchanged.This dramatic decrease in maximal vol- untary contraction following digital anaesthesia may result from a lack of proper sensory feedback during the task. NeuroReport 16:259^262  c 2005 Lippincott Williams & Wilkins. Key words: Dea¡erentation; Maximal voluntary contraction; Motor maps; Plasticity; Transcranial magnetic stimulation INTRODUCTION In the past decade, there have been considerable advances in understanding the neuronal bases of sensory and motor representations in the cortex [1]. It is now widely accepted that in the primary motor cortex, muscle representations are not invariant but are susceptible to rapid adaptation following a central [2] or a peripheral lesion [3] or after an amputation [4]. In particular, within minutes following anaesthesia, it has been shown that the cortical representation of muscles immediately proximal to the paralysed body part increases [5]. However, in most studies, both afferent and efferent signals were interrupted indistinguishably by lesion or anaesthesia and therefore their specific contribution to the reorganization of the motor cortex remains unclear. In monkeys, the hand area in the primary motor cortex receives abundant projections from the corresponding region of the somatosensory cortex [1] and these projections may play a critical role in controlling motor excitability [6]. In humans, several studies have tried to investigate the specific impact of sensory information on muscle represen- tations in the motor cortex. To do so, they examined the effects of a radial and median nerve block at the wrist level on both the excitability and extent of muscle representations in the primary motor cortex [7,8]. The main finding was a decrease in size of the cortical representation of muscles spared by anaesthesia, but enveloped in the deafferented region, whereas the representation of muscles outside this region remained unaltered [7]. However, because in these studies the anaesthesia involved a large cutaneous territory and affected motor nerves, these results could have been biased by the thenar muscle paralysis or the pain resulting from the anaesthesia. The purpose of the present study was to investigate the specific influence of sensory information from a restricted area encompassing the index finger and thumb on intrinsic hand muscle representations. We studied the effect of local ‘ring’ anaesthesia of the thumb and index finger on both the maximal key pinch force and the amplitude of motor evoked potentials (MEPs) recorded from the first dorsal interosseus in response to transcranial magnetic stimulation (TMS) of the primary motor cortex. We hypothesized that removing sensory inputs from the first two digits would yield a decrease in the corticospinal excitability of the first dorsal interosseus, a muscle controlling the index finger abduction and flexion. We expected this decrease to be correlated with a reduction in the maximal voluntary pinch force. METHODS The present study was performed in 12 right-handed healthy adults (age: 2771.1 years). The experimental procedure was approved by the Ethics Committee of the Universite ´ Catholique de Louvain and all participants gave their written informed consent. Anaesthesia procedure: The conduction of the digital nerves of the index finger and thumb of the right hand was blocked at the level of the proximal phalanx of each digit by injection of B1 ml bupivacain (0.5%) in order to achieve ring-block anaesthesia. By using this protocol, we ensured that only sensory inputs, including cutaneous and some joint information, were interrupted. This anaesthesia protocol, in contrast to previous studies, spared all intrinsic SOMATOSENSORY SYSTEMS, PAIN NEUROREPORT 0959-4965  c Lippincott Williams & Wilkins Vol 16 No 3 28 February 2005 259 Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited. Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.