13-6. Source(s) of Central Instruction Signals

Voluntary control systems differ from reflex control systems in that the former operate with instruction signals generated centrally within the cerebral cortex (Figure 9C), whereas the latter are driven primarily by peripheral stimuli. The motor cortex receives instruction signals that designate the content of the movement to be performed. For example, this can be a desired trajectory for an arm movement or even a complex program of movements. The primary motor cortex forms motor command signals and sends them to the brainstem and spinal segmental levels via the corticospinal descending tract. In higher nonhuman primates and humans, part of the primary motor cortex projects corticospinal fibers to directly innervate shoulder, elbow, hand, and finger motoneurons (Rathelot and Strick, 2009). On the other hand, the rostral region of the primary motor cortex represents the “old” primary motor cortex, which sends its descending commands to motoneurons indirectly via segmental circuits (see Lemon, 2008).