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Updated: Jun 23, 2026
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Measuring and Manipulating Functionally Specific Neural Pathways in the Human Motor System with Transcranial Magnetic Stimulation
Published on: February 23, 2020
Motor cortex bilateral motor representation depends on subcortical and interhemispheric interactions.
Marcel Brus-Ramer1, Jason B Carmel, John H Martin
1Department of Neuroscience, Columbia University, New York, New York 10032, USA.
Summary
The uncrossed corticospinal system is crucial for limb control recovery after injury. Ipsilateral motor cortex responses depend on the opposite hemisphere, revealing critical interhemispheric interactions.
Area of Science:
- Neuroscience
- Motor Control
- Corticospinal Tract
Background:
- The corticospinal tract is primarily crossed, but the primary motor cortex (M1) shows bilateral activation during unilateral movements.
- Understanding the uncrossed corticospinal system is key for limb control recovery after unilateral brain injury.
Purpose of the Study:
- To investigate the bilateral motor representation in rat M1.
- To determine the dependence of ipsilateral forelimb responses on the contralateral M1 and interhemispheric connections.
Main Methods:
- Intracortical microstimulation (ICMS) in rats to map forelimb joint representation in M1.
- Reversible inactivation of M1 and pyramidal tract lesions to assess functional dependencies.
- Corpus callosum sectioning to evaluate interhemispheric transfer.
Main Results:
- Most M1 sites representing ipsilateral forelimb joints also represented the same joint contralaterally.
- Ipsilateral responses were abolished by contralateral M1 inactivation or pyramidal tract lesion.
- Corpus callosum sectioning increased ipsilateral ICMS thresholds, indicating a role for interhemispheric transfer.
Conclusions:
- Ipsilateral corticospinal responses are dependent on the contralateral M1 and interhemispheric communication.
- Interactions between corticospinal systems from both hemispheres are significant, particularly at the spinal level.
- Findings highlight the potential for recruiting the ipsilateral corticospinal system for motor recovery post-injury.
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