Nerve fibres in the corticospinal tract originate from pyramidal cells in layer V of the cerebral cortex. Fibres arise from the primary motor cortex (about 30%),supplementary motor area and the premotor cortex (together also about 30%), and the somatosensory cortex, parietal lobe, and cingulate gyrus supplies the rest.The cells have their bodies in the cerebral cortex, and the axons form the bulk of the pyramidal tracts. The nerve axons travel from the cortex through theposterior limb of internal capsule, through the cerebral peduncle and into the brainstem and anterior medulla. Here they form two prominences called the pyramids of the medulla. Below the prominences, the majority of axons cross over to the opposite side from which they originated. The axons that cross over move to the outer part of the medulla and form the lateral corticospinal tract, whereas the fibres that remain form the anterior corticospinal tract About 80% of axons cross over and form the lateral corticospinal tract; 10% do not cross over and join the tract, and 10% of fibres travel in the anterior corticospinal tract.
The nerve axons traveling down the tract are referred to as upper motor neurons. These axons travel down the tracts in the white matter of the spinal cord until they reach the vertebral level of the muscle that they will innervate. At this point, the axons synapse with lower motor neurons. The majority of axons do not directly synapse with lower motor neurons, but instead synapse with an interneuron that then synapses with a lower motor neuron. This generally occurs in the anterior horn of the spinal cord. Nerve axons of the lateral corticospinal tract that did not cross over in the medulla do so at the level of the spinal cord they terminate in.
These tracts contain more than 1 million axons and the majority of the axons are myelinated. The corticospinal tracts myelinate largely during the first and second years after birth. The majority of nerve axons are small (<4μm) in diameter. About 3% of nerve axons have a much larger diameter (16μm) and arise from Betz cells, mostly in the leg area of the primary motor cortex. These cells are notable because of their rapid conduction rate, over 70m/sec, the fastest conduction of any signals from the brain to the spinal cord.