Characterization of limb representation in the pig’s motor cortex

Due to its large gyrencephalic brain, the pig is increasingly used for neuroscience research, especially 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. Here, we aimed to characterize the forelimb and hindlimb representation of the pig motor cortex using intracortical microstimulation (ICMS). Three domestic pigs ( Sus scrofa) were placed in a modified stereotactic frame and maintained under intravenous propofol sedation. We mapped the motor cortex using ICMS, applied at varying cortical coordinates and depths. For each site, we recorded the electrode depth eliciting the maximal limb response and determined the motor threshold. Responses were assessed visually and via electromyographic recordings. ICMS uncovered a large forelimb representation, with stereotypical contralateral responses. Conversely, the hindlimb representation was smaller and located within the interhemispheric fissure. The mean threshold of the five most responsive forelimb sites was 75 ± 25 μA, compared to 280 ± 45 μA for hindlimb sites (p<0.01). A summation of stimulations in the hindlimb representation of the motor cortex unilaterally triggered bilateral alternating hindlimb movements. These results suggest that while the porcine cortex can directly command forelimb movements via the corticospinal pathway, cortical control of hindlimb likely relies on polysynaptic pathways through the brainstem, such as the cortico-reticulospinal pathway.