Abstract
BackgroundPrevious studies with Transcranial Magnetic Stimulation (TMS) have focused on the cortical representation of limited group of muscles. No attempts have been carried out so far to get simultaneous recordings from hand, forearm and arm with TMS in order to disentangle a ‘functional’ map providing information on the rules orchestrating muscle coupling and overlap. The aim of the present study is to disentangle functional associations between 12 upper limb muscles using two measures: cortical overlapping and cortical covariation of each pair of muscles. Interhemispheric differences and the influence of posture were evaluated as well.Methodology/Principal FindingsTMS mapping studies of 12 muscles belonging to hand, forearm and arm were performed. Findings demonstrate significant differences between the 66 pairs of muscles in terms of cortical overlapping: extremely high for hand-forearm muscles and very low for arm vs hand/forearm muscles. When right and left hemispheres were compared, overlapping between all possible pairs of muscles in the left hemisphere (62.5%) was significantly higher than in the right one (53.5% ).The arm/hand posture influenced both measures of cortical association, the effect of Position being significant [p = .021] on overlapping, resulting in 59.5% with prone vs 53.2% with supine hand, but only for pairs of muscles belonging to hand and forearm, while no changes occurred in the overlapping of proximal muscles with those of more distal districts.Conclusions/SignificanceLarger overlapping in the left hemisphere could be related to its lifetime higher training of all twelve muscles studied with respect to the right hemisphere, resulting in larger intra-cortical connectivity within primary motor cortex. Altogether, findings with prone hand might be ascribed to mechanisms facilitating coupling of muscles for object grasping and lifting -with more proximal involvement for joint stabilization- compared to supine hand facilitating actions like catching. TMS multiple-muscle mapping studies permit a better understanding of motor control and ‘plastic’ reorganization of motor system.
Highlights
About 30 years ago a revolutionary technique was introduced which allowed stimulation of the human brain through the skull with non-invasive, high-intensity, extremely brief electric pulses [1–5], followed –few years later- by a new device that employs strong and brief, time-varying magnetic fields able to induce electric currents flowing within the brain without any discomfort [6]; for a review see [7]
Despite their potential ability to properly follow-up such a Modern concepts on the functional organization of cortical primary motor areas maintain a somatotopic view only for major body districts, each being sustained by a network serving broadly distributed functions which involve large populations of neurons in spatially separate clusters
The main goal of the present study was to investigate whether the use of a non-invasive technique like Transcranial Magnetic Stimulation (TMS)
Summary
About 30 years ago a revolutionary technique was introduced which allowed stimulation of the human brain through the skull with non-invasive, high-intensity, extremely brief electric pulses [1–5], followed –few years later- by a new device that employs strong and brief, time-varying magnetic fields able to induce electric currents flowing within the brain without any discomfort [6]; for a review see [7]. Focal TMS to different cells of a grid overlying the primary motor cortex (M1) while recording electromyographic (EMG) responses from the contralateral ‘target’ muscles, made it possible to obtain reliable maps of cortical motor output in awake and cooperative subjects [8–21]. The aim of the present study was to investigate overlap and covariation of cortical motor output during TMS mapping, and simultaneous recordings from 12 individual upper limb muscles in order to measure cortical overlapping and covariation of pairs of muscles, interhemispheric differences and influence of arm/hand posture. No attempts have been carried out so far to get simultaneous recordings from hand, forearm and arm with TMS in order to disentangle a ‘functional’ map providing information on the rules orchestrating muscle coupling and overlap. The aim of the present study is to disentangle functional associations between 12 upper limb muscles using two measures: cortical overlapping and cortical covariation of each pair of muscles. Interhemispheric differences and the influence of posture were evaluated as well
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