Implication of the ipsilateral motor network in unilateral voluntary muscle contraction: the cross-activation phenomenon

Abstract

Voluntary force production requires that the brain produces and transmits a motor command to the muscles. It is widely acknowledged that motor commands are executed from the primary motor cortex (M1) located in the contralateral hemisphere. However, involvement of M1 located in the ipsilateral hemisphere during moderate to high levels of unilateral muscle contractions (>30% of the maximum) has been disclosed in recent years. This phenomenon has been termed cross-activation. The activation of the ipsilateral M1 relies on complex inhibitory and excitatory interhemispheric interactions mediated via the corpus callosum and modulated according to the contraction level. The regulatory mechanisms underlying these interhemispheric interactions, especially excitatory ones, remain vague, and contradictions exist in the literature. In addition, very little is known regarding the possibility that other pathways could also mediate the cross-activation. In the present review, we will therefore summarize the concept of cross-activation during unilateral voluntary muscle contraction and explore the associated mechanisms and other nervous system pathways underpinning this response. A broader knowledge of these mechanisms would consequently allow a better comprehension of the motor system as a whole, as distant brain networks working together to produce the motor command.

Neural Correlates of Motor Skill Learning Are Dependent on Both Age and Task Difficulty

“…A similar finding was found in a previous study using a visuomotor tracking task, where smaller pre-training beta activity over the ipsilateral cortex was related to better performance after practice (Espenhahn et al, 2019). Although the functional role of the ipsilateral cortex in motor control and learning is still under debate (Uehara and Funase, 2014;Barany et al, 2020;Cabibel et al, 2020), these results are consistent with the view that a more lateralized activity pattern is beneficial for motor performance (Takeuchi et al, 2012).…”

Handedness Does Not Impact Inhibitory Control, but Movement Execution and Reactive Inhibition Are More under a Left-Hemisphere Control

“…Finally, it has also been demonstrated that cooperation between the two motor cortices during skill learning harnesses plasticity in the ipsilateral hemisphere improving motor learning [30]. Taken together, this evidence supports the idea that the ipsilateral M1 activation plays a crucial role in the control of unilateral hand movements [18,19]. It should be underlined that the activation of the two motor cortices during unilateral hand movements is not the same.…”

“…It has been well established that unimanual arm movements elicit bilateral activity in the sensorimotor areas [18,19]. However, the meaning of ipsilateral hemisphere signals is considerably debated.…”

“…It is now very well known that unilateral movements are not controlled only by the motor cortices of the hemisphere contralateral to the moving limb. Several studies have shown that the sensorimotor areas of the ipsilateral hemisphere are also activated [18,19]. The role of the ipsilateral hemisphere in unilateral limb movements is not yet clear.…”

Evidence for a Window of Enhanced Plasticity in the Human Motor Cortex Following Ischemic Stroke

“…10-19 These changes within the contralesional cortex could contribute directly to recovery. For example, in humans, both hemispheres are recruited during execution or learning of unilateral motor tasks, 44-46 suggesting both motor cortices may work cooperatively during motor learning. 47 In people with stroke, increased activation of the contralesional cortex on imaging is reported during paretic upper limb movement, 48,49 while suppression of contralesional activity has been shown to impair motor performance.…”