Interhemispheric inhibition in human wrist muscles

Abstract

The interhemispheric interactions between homologous wrist extensor and flexor muscles representations in the right and left primary cortex (M1) were studied using a paired-pulse transcranial magnetic stimulation in healthy subjects. The magnitude of interhemispheric inhibition (IHI) was studied in 9 right-handed subjects at short (10 ms, SIHI) and long (40 ms, LIHI) interstimulus intervals between the magnetic conditioning (CS) and test stimulus in the motor dominant to non-dominant cortex and vice versa, while the right or left hand was at rest or performing a unimanual sustained tonic contraction (holding a pen with the hand contralateral to the CS). A bidirectional powerful interhemispheric inhibition could be elicited at the short and long IHI phases (SIHI and LIHI) in wrist extensor and flexor muscles in most of the subjects at rest. SIHI but not LIHI was significantly bidirectionally reduced during unimanual contraction of the hand contralateral to the CS stimulation in comparison with rest. The amount of IHI after the stimulation of the "non-dominant" right hemisphere was not reduced in comparison with IHI after stimulation of the "dominant" left hemisphere whatever the active or resting condition. IHI directed to the wrist muscles had a similar level than IHI directed to digit muscles (FDI) at rest. Our data indicate that contralateral wrist muscles activity evokes a global, bidirectional reduction in IHI which was more pronounced for SIHI. These results provide additional evidence that changes in interhemispheric interactions between the M1s are involved in the control of unimanual movements including suppression of unwanted motor activity in the opposite limb during unilateral movements.

Does abnormal interhemispheric inhibition play a role in mirror dystonia?

“…In normal subjects we had previously shown that SIHI was more bidirectionally reduced than LIHI in wrist muscles in comparison with rest during a postural task (holding a pen as in the present study) involving the hand contralateral to the CS stimulation. This was more pronounced for the ECR than for the FCR muscle . These results fit those of Nelson et al, who described a bidirectional decrease of IHI for flexor digitorum indicis (FDI) muscles when the subjects performed isometric contraction of the FDI muscle in the context of holding a pen.…”

“…A possible explanation is the predominant involvement of flexor muscles in the dystonic hand of most of our patients and also a possible decrease in reciprocal inhibition . The physiologically weaker FCR contraction compared with ECR contraction observed in normal subjects when holding a pen may have also amplified these differences . However, although IHI disturbances were observed bilaterally in both muscles and only in patients with mirror dystonia, background FCR EMG differences were strictly unilateral and observed in both patient groups, and so their impact was likely minor.…”

“…The intensities of both the conditioning and test stimuli were adjusted to elicit an MEP of ≈1 mV in the contralateral ECR and FCR muscles, irrespective of whether the muscle was at rest or active. This was done to normalize IHI to the increase in corticospinal excitability caused by voluntary contraction …”

“…Using dual-site TMS protocol, we have recently reported that a bidirectional powerful IHI can be elicited at the short and long IHI phases in wrist exten-sor and flexor muscles in healthy right-handed subjects at rest. 11 The purpose of the current study was to compare the level of bidirectional SIHI and LIHI in wrist flexor and extensor muscles at rest and during the task of holding a pen performed by the hand contralateral to the conditioning stimulation in a group of writer's cramp patients with a mirror dystonia (PATMIR1), a group of writer's cramp patients without mirror dystonia (PATMIR2), and a group of healthy subjects.…”

Generic Inhibition of the Selected Movement and Constrained Inhibition of Nonselected Movements during Response Preparation

“…Alternatively, if CR operates in a more specific manner, then we expected to observe greater inhibition in the condition involving homologous response alternatives. Competitive interhemispheric interactions reflecting the mirror organization of the motor system have been observed in many behavioral and physiological studies (Diedrichsen, Wiestler, & Krakauer, 2013; Sattler, Dickler, Michaud, & Simonetta-Moreau, 2012; Hinder, Schmidt, Garry, & Summers, 2010). As such, competition is likely to be related to the degree of similarity between the response alternatives.…”