Also known as the pyramidal tract or system, it consists in fact of two descending tracts from layer 5 of the cortex containing the axons from neurons in the primary and secondary motor cortex and somatosensory cortex to nuclei in the brain stem, and to motoneurons and interneurons in the spinal cord. The ventromedial (or indirect) tract descends through the thalamus, midbrain, and medulla in the brain stem (see Figure A below). The dorsolateral tract passes through the internal capsule, decussates in the pyramids (75-80% cross), with about 80% of axons becoming the lateral corticospinal tract and 20% the ventral corticospinal tract (see Figure B below). The axons terminate onto internuncial neurons or alpha motoneurons of the ventral horn. These tracts control fine movements, and their destruction leads to reduced dexterity of the hands and fingers, and some loss of muscle tone. There is no effect, however, on posture or on the use of limbs for reaching. The functional development of the CST in humans is still not completely understood, with some claiming functionality already in the newborn based on transcranial magnetic stimulation.
