Fibers originate in motor cortex and descend via posterior limb of internal capsule to basis pedunculi of midbrain
Longitudinal bundles branch upon entering basis pontis and rejoin to enter pyramids of medulla
At lower medulla, bulk of fibers cross median plane to form lateral corticospinal tract; some fibers continue downward in ipsilateral lateral corticospinal tract; others descending ipsilateral anterior corticospinal tract
Synapse occurs at spinal level: Lateral corticospinal fibers synapse on ipsilateral anterior horn cells; anterior corticospinal fibers synapse on contralateral anterior horn cells
Anterior corticospinal tract Lateral corticospinal tract
Figure 2.20 Corticospinal Tracts
The corticospinal, or pyramidal, tract is the major motor tract that controls voluntary movement of the skeletal muscles, especially skilled movements of distal muscles of the limbs. All structures from the cerebral cortex to the anterior horn cells in the spinal cord constitute the upper portion of the system (upper motor neuron). The anterior horn cells and their associated axons constitute the lower portion of the system (lower motor neuron).
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