In the CNS, the motor cortex, basal ganglia, cerebellum, and parts of the brainstem, are directly involved in the planning and initiation of movement. In contrast, the precise timing and pattern of movement is generated by MNs located in the spinal cord (Figure 1; Kiehn, 2016). The corticospinal (anterior and lateral) tract is the largest descending tract in humans. The lateral corticospinal tract originates in the primary motor cortex, which lies in the precentral gyrus and sends fibers to muscles in the extremity. This is via contralateral cortical innervation, so that the left motor cortex controls the right extremities and vice versa, to control the voluntary movement of contralateral limbs (Javed and Lui, 2018). MNs outputs are not confined to the peripheral muscles however, but also include excitatory terminals to a group of interneurons, Renshaw cells, and also to other MNs.