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Publisher Summary This chapter discusses the simplest of several possible corticospinal control systems—the localization in the cortical arm area of the baboon of colonies of pyramidal neurones, which project monosynaptically to sample motoneurones belonging to different muscle groups of the upper limb. Such colonies form significant functional groupings as the motor units to which they project are the ultimate elements in the organization of muscular contraction. The monosynaptic actions of the corticospinal discharges are recorded from the sampled motoneurones with intracellular microelectrodes. The cortico-motoneuronal colonies for motor units of different muscles overlap and they differ in their spatial extent and in the quantity of monosynaptic excitatory action they command on the test motoneurones. The colonies projecting to motoneurones controlling distal muscles tend to occupy smaller cortical areas and command larger monosynaptic actions; the colonies projecting to the motoneurones of proximal muscles tend to occupy larger areas, but command smaller monosynaptic actions.