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Schematic of the motor system. Upper motor neurons (UMNs) project to lower motor neurons (LMNs) in the brainstem and spinal cord. LMNs innervate skeletal muscle fibers. Different classes of α-MNs innervate diverse skeletal muscle fibers. FF-MNs, fast-fatigable MNs. SFR-MNs, slow fatigue resistant MNs. FFR-MNs, fast fatigue resistant MNs.
