The motor cortex is the region of the cerebral cortex responsible for mediating voluntary movements. In rodents, the primary cortex (M1) is large and comprises almost all of the frontal cortex (Gioanni and Lamarche, 1985; Neafsey et al., 1986; Brecht et al., 2004; Yu et al., 2008; Hira et al., 2013; Paxinos, 2014), whereas in primates, the frontal cortex is compartmentalized into specialized premotor subfields and M1 is relatively small in comparison (Ferrier, 1875; Leyton and Sherrington, 1917; Asanuma and Rosén, 1972; Dickey et al., 2013; Riehle et al., 2013; Young et al., 2013; Ebbesen and Brecht, 2017). M1 plays a central role in controlling movement. This involves specialized UMNs located in layer V of this region (Broadman area 4), the giant Betz cells or corticospinal MNs. These MNs are the cortical components of the MN circuit that initiates and modulates precise voluntary movement, through long-range projections to the spinal cord. Approximately ∼30–50% of corticospinal projections originate from M1 MNs and they begin modulating their firing rate several hundred ms before movement of the limb is initiated (Georgopoulos et al., 1982; Porter and Lemon, 1993). In most mammals, the axons of cortical MNs terminate at spinal interneurons, but they also make direct connections to MNs (Lemon, 2008; Rathelot and Strick, 2009). This constitutes the final efferent pathway to the muscle to generate or suppress movement (Ramírez-Jarquín and Tapia, 2018).