Bilateral responses of upper limb muscles to transcranial magnetic stimulation in human subjects

• Published in: Experimental brain research Vol. 158; no. 3; pp. 385 - 390
• Main Authors: BAWA, P, HAMM, J. D, DHILLON, P, GROSS, P. A
• Format: Journal Article
• Language: English
• Published: Berlin Springer 01-10-2004; Springer Nature B.V
• Subjects: Adult; Arm - innervation; Arm - physiology; Biological and medical sciences; Efferent Pathways - physiology; Electric Stimulation; Electrodes; Electromyography; Female; Forearm - innervation; Forearm - physiology; Functional Laterality - physiology; Fundamental and applied biological sciences. Psychology; Humans; Male; Middle Aged; Motor control and motor pathways. Reflexes. Control centers of vegetative functions. Vestibular system and equilibration; Motor Cortex - physiology; Movement - physiology; Muscle Contraction - physiology; Muscle, Skeletal - innervation; Muscle, Skeletal - physiology; Transcranial Magnetic Stimulation; Vertebrates: nervous system and sense organs; Human; Transcranial magnetic stimulation; Motor pathway; Central nervous system; Electrophysiology; Ipsilateral responses; Motor evoked potentials; Muscle contraction; Motor control; Hand; Encephalon; Phasic movements; Motor cortex; Biceps brachii muscle; Bilateral responses; Electromyography; Upper limb; Motor evoked potential
• ISSN: 0014-4819; 1432-1106
• DOI: 10.1007/s00221-004-2031-x

Anatomical and behavioural work on primates has shown bilateral innervation of axial and proximal limb muscles, and contralateral control of distal limb muscles. The following study examined if a clear boundary exists between the distal and proximal upper limb muscles that are controlled contralaterally or bilaterally. The right motor cortical area representing the upper limb was stimulated, while surface EMG was recorded bilaterally from various upper limb muscles during rest and phasic voluntary contractions. Peak-to-peak amplitude of motor evoked potential (MEP) was measured for each muscle on both sides. The ratio R = (ipsilateral MEP: contralateral MEP) was calculated for seven pairs of muscles. For each of the seven pairs, R was less than 1.0, implying that for each muscle and subject, the contralateral control is stronger. The boundary where R changed from almost zero to a clearly measurable magnitude depended on the subject. Ipsilateral MEPs from trapezius and pectoralis could be recorded with a small background contraction from almost all subjects; on the other hand, in deltoid and biceps brachii, ipsilateral MEPs were observed only with bimanual phasic contractions. The forearm and hand muscles, in general, did not show any ipsilateral MEPs. Major differences between subjects lay in the presence or the absence of ipsilateral MEPs in biceps brachii and deltoid, without defining a sharp boundary between proximal and distal muscles.