Mapping the Anatomical basis for motor control of a single vibrissa muscle across Rat Brain

Abstract

How does the cerebral cortex control a single muscle to orchestrate movement? Rabies virus was injected into the intrinsic muscle that protracts the rat C3 vibrissa. The retrograde transneuronal transport of rabies virus was used to identify the most direct output pathways from the cerebral cortex to a single muscle that moves facial vibrissae. <br/> The results show that the most direct cortical output pathways responsible for control of this muscle originate from almost entire primary motor cortex (M1), vibrissae-related primary somatosensory cortex – barrel cortex(S1), secondary somatosensory (S2)and motor cortex(M2), and insular cortex. Apart from the barrel cortex, all these regions exhibit bilateral organization. Interestingly, M1 contains distinct subregions in both hemispheres that can differentially contribute to the control of contralateral and ipsilateral vibrissa muscles.<br/> For comparison, the rabies virus was also injected into a single forepaw muscle, extensor digitorium communis (EDC). The direct cortical output pathways for forelimb motor control originate majority from contralateral cortical regions that were also represented by the vibrissae, particularly in M1, M2, and S2 and minor overlap in S1. However, output pathways from S1 are topographically organized and remain segregated for forelimb and vibrissae movements. This topological distinction suggests fundamental differences in how cortex integrate information for different muscle groups. <br/> Surprisingly, while infected neurons were observed in the insular cortex for vibrissa control, no neurons were detected in this region for forelimb control. This suggests that its role is specific to facial motor functions. This also highlights that the cortical output system for controlling a single vibrissa muscle is more complex than that for a single forelimb muscle. <br/> In the final part of this study, I also examined the distribution of rabies-infected motoneurons (vMNs) in the facial nucleus (FN) in higher-order brains. I observed a higher number of infected vMNs along with labeling in the contralateral FN, indicating the presence of indirect interhemispheric projections. Additionally, mean size of these vMNs was lower compared to second and third-order brains, indicating potential differences in functional roles or connectivity patterns of these vMNs. <br/> In conclusion, the findings reveal that multiple functionally distinct cortical areas, both within and beyond the primary motor cortex (M1), contribute in parallel to the control of individual muscles. The evidence shows that robustness rabies virus tracing is a powerful tool for studying the anatomical basis of motor control in rats. These insights offer a foundation for further research into the hierarchical and integrative mechanisms underlying motor control in mammals.