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The corticospinal tract (also known as the pyramidal tract) is a descending white matter tract primarily concerned with motor function that extends caudally from the motor cortex to synapse with motor neurons of the spinal cord in the anterior horns.
Corticospinal fibers are axons from upper motor neurons in the cerebral cortex. Over half of these arise in the primary motor cortex with additional contributions from the premotor cortex, supplementary motor cortex and even the sensory cortex. In the primary motor cortex, fibers arise specifically from Betz cells 1,2,4.
Organized somatotopically, these fibers descend through subcortical white matter and form the anterior two-thirds of the posterior limb of the internal capsule. They pass through the ventral midbrain, specifically the central and lateral portions of the inner crus, and continue through the pons. In the medulla oblongata, corticospinal fibers collect into a discrete bundle forming the pyramid 1,2.
The pyramid is a discrete triangular column on the ventral medulla oblongata next to the midline. This is why the corticospinal tract is also called the pyramidal tract. Within the pyramids approximately 90% of the corticospinal fibers decussate, forming the lateral corticospinal tract. The remaining 10% fibers remain ipsilateral (i.e. do not decussate) and form the anterior corticospinal tract 1-4.
The anterior and lateral tracts are discussed separately.