Differential participation of the corticospinal and corticorubral neurons during motor execution in the rat

The sensorimotor cortex is crucial for learning and executing new movements with precision (Nudo & Frost, 2007). It selectively modulates sensory information flow and represents motor information in a spatially organized manner (Canedo, 1997; Chen et al., 2017). The pyramidal system is made up layer 5 pyramidal tract neurons (PTNs), which are organized into populations with distinct morphological, genetic and functional properties. These subpopulations project to different subcortical structures in a segregated manner (Nudo & Frost, 2007).

To understand whether PTNs projecting to different structures play distinct functional roles in motor control, we characterized two types of layer 5 neurons in the motor cortex: corticorubral (CR) neurons, which project to the red nucleus, and corticospinal (CS) neurons, which project to the spinal cord.

To analyze movement performance in rats, we compared the selective optogenetic inhibition of motor cortex CS or CR neurons during lever movement execution in response to a light stimulus. As the animals progressed through the training sessions, the variability of lever trajectories decreased, and the movements became more stereotyped. Photoinhibition of CS or CR neurons increased the performance variability of learned movements but differentially affected kinematic parameters. CR neuron inhibition affected amplitude, duration, reaction times, speed, and acceleration of the movement. In contrast, inhibition of CS neurons mainly altered the duration, speed, and acceleration of the movement. We conclude that CS and CR are complementary pathways for transmitting information rather than copies of the same motor command.