Cortical and brainstem control of locomotion.

While a basic locomotor rhythm is centrally generated by spinal circuits, descending pathways are critical for ensuring appropriate anticipatory modifications of gait to accommodate uneven terrain. Neurons in the motor cortex command the changes in muscle activity required to modify limb trajectory when stepping over obstacles. Simultaneously, neurons in the brainstem reticular formation ensure that these modifications are superimposed on an appropriate base of postural support. Recent experiments suggest that the same neurons in the same structures also provide similar information during reaching movements. It is suggested that, during both locomotion and reaching movements, the final expression of descending signals is influenced by the state and excitability of the spinal circuits upon which they impinge.