Introduction: Due to its large gyrencephalic brain, the pig is increasingly used as an animal model for the preclinical testing of novel neuroprostheses. However, our understanding of the pig’s motor system remains limited compared to the common species used for neuroscience research. We aimed to characterize the forelimb and hindlimb representation of the pig motor cortex using intracortical microstimulation and chronic implanted electrodes for awake stimulation during treadmill walking.
Methods: For the intracortical microstimulation study, three domestic pigs (Sus scrofa) were placed in a modified stereotactic frame and kept under sedation with intravenous Propofol. We mapped the motor cortex with intracortical stimulations, delivered at different coordinates and depths, using a microelectrode and micromanipulator. Responses were measured visually (video recording) and with electromyography of hindlimb muscles. For the awake stimulation experiment, we implanted chronic electrodes in the hindlimb representation of the motor cortex of a fourth domestic pig, with an epidural grid on one side, and an intracortical grid in the other side. We delivered stimulations at rest and during locomotion on a modified treadmill.
Results: Microstimulations uncovered a large forelimb representation, with stereotypical contralateral responses; whereas the hindlimb representation was small, buried in the interhemispheric fissure, and bilateral responses were evoked after a summation of high-amplitude stimulations. A summation of stimulation could evoke a pattern of alternating background activity in hindlimb muscles, and bilateral alternating response to stimulations. Stimulations in the awake pig, either at rest or on the treadmill, could not evoke hindlimb responses.
Conclusion : We found a small representation of the hindlimb on the medial surface of the motor cortex, which when stimulated repeatedly causes bilateral alternating movements of the hindlimbs, presumably through the cortico-reticulo-spinal pathway. The representation of the forelimb was larger, activated at lower amplitude, and triggered consistent contralateral forelimb movements. Altogether, our results add to the neuroanatomical literature in suggesting that the porcine cerebral cortex can directly command forelimb movements through the corticospinal pathway but relies mostly on brainstem nuclei to control hindlimb motor activity.
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