Carmona, L. M. | Search bioRxiv

bioRxiv2023

Topographical and cell type-specific connectivity of rostral and caudal forelimb corticospinal neuron populations

Corticospinal neurons (CSNs) synapse directly on spinal neurons, a diverse group of neurons with unique structural and functional properties necessary for body movements. CSNs modulating forelimb behavior fractionate into caudal forelimb area (CFA) and rostral forelimb area (RFA) motor cortical populations. Despite their prominence, no studies have mapped the diversity of spinal cell types targeted by CSNs, let alone compare CFA and RFA populations. Here we use anatomical and RNA-sequencing methods to show that CSNs synapse onto a remarkably selective group of spinal cell types, favoring inhibitory populations that regulate motoneuron activity and gate sensory feedback. CFA and RFA CSNs target similar spinal cell types, with notable exceptions that suggest these populations differ in how they influence behavior. Finally, axon collaterals of CFA and RFA CSNs target similar brain regions yet receive surprisingly divergent inputs. These results detail the rules of CSN connectivity throughout the brain and spinal cord for two regions critical for forelimb behavior.

neuroscience↗

bioRxiv2022

Corticothalamic Neurons in Motor Cortex Have a Permissive Role in Motor Execution

The primary motor cortex (M1) is a central hub for motor learning and execution. M1 is composed of heterogeneous cell types, many exhibiting varying relationships to movement. Here, we employed an unbiased screen to tag active neurons at different stages of performance of a motor task. We characterized the relative cell type composition of active neurons across training and identified one cell type consistently enriched as training progressed: corticothalamic neurons (M1CT). Using two-photon calcium imaging, we found that M1CT activity is largely suppressed during movement, and this negative correlation with movement scales with movement vigor and augments with training. Closed-loop optogenetic manipulation of this population revealed that increasing M1CT activity during forelimb movement significantly hinders execution, an effect that became stronger with training. Similar optogenetic manipulations, however, had little effect on locomotion. In contrast to M1CT neurons, we observed that M1 corticospinal neurons positively correlate with movement, and that this positive correlation increases with learning. Finally, by examining the connectivity between M1CT and corticospinal neurons, we uncovered that M1CT neurons can suppress M1 corticospinal activity via feedforward inhibition, and that this inhibition scales with training. These results identify a novel permissive role of corticothalamic neurons in movement execution through suppression of inhibition of corticospinal neurons.

neuroscience↗