The classical homunculus is a useful didactic device but a poor model of how M1 actually represents movement. Penfield himself noted overlap and reciprocity between body part maps in his original 1937 paper, and subsequent microstimulation and population-coding work in primates has shown that M1 neurons code for direction, force, and end-effector trajectory rather than individual muscles. The hand area in particular displays a characteristic knob in axial MRI (Yousry’s “hand knob”), which serves as a reliable anatomical landmark for the localisation of the primary motor representation of the contralateral hand.

The corticospinal tract descends from layer V through the corona radiata into the posterior limb of the internal capsule, crosses in the medullary pyramidal decussation, and continues as the lateral corticospinal tract in the spinal cord. About 85 percent of fibres cross at the pyramids, the remainder descending uncrossed as the anterior corticospinal tract. Lesions of M1 produce contralateral weakness with a classical pyramidal pattern: extensor weakness in the arm, flexor weakness in the leg, and a strong Babinski sign on plantar stimulation.

The premotor and supplementary motor cortices in BA 6 contribute to movement preparation via projections to M1, to direct cortico-cortical connections with parietal cortex (for sensory-guided action), and to the basal ganglia and cerebellum via separate cortico-subcortical loops. Damage to SMA alone, sparing M1, can produce alien-hand phenomena and transient akinesia.