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The corticospinal tract is a group of axons that extend from the brain to the spinal cord. It originates at the cortex of the brain, and it ends in the ventral horn of the spinal cord. This tract is also called the motor system because its primary function is the transmission of signals for voluntary or willed and skilled movements.
The corticospinal tract is also called the pyramidal tract. The descriptive term “pyramidal” does not refer to the fact that it originates from pyramidal neurons in the cortex. Instead, the term refers to the arrangement of the tract through the medulla. Anatomically, the dense nerve fiber bundle looks like a pyramid.
About 50 percent of the corticospinal tract fibers originate from the primary motor cortex, particularly in layer V cells called Betz cells. Additional fibers come from the premotor cortex, supplementary motor area, somatosensory cortex, cingulate gyrus and parietal lobe. The neuronal cell bodies from the cortex, and their axons are known as upper motor neurons (UMNs). The motor neurons located in the brain stem and in the spinal cord's ventral horns are referred to as lower motor neurons (LMNs). The corticospinal tract consists only of UMNs.
The distinction between UMNs and LMNs is important for neurologists because it helps them in the localization of pathologies. For instance, with UMN pathology of the corticospinal tract, a person’s muscle tone would be spastic, which means that there would be a catch and a yield when a muscle group is moved. UMN pathology is also characterized by increased reflexes, an inability to perform skilled and fine movements such as writing and the presence of the extensor plantar response, or Babinski sign. These signs are found in addition to symptoms of weakness on one side of the body or paralysis.
About 80 percent of the corticospinal fibers decussate or cross the midline at the level of the medulla oblongata. This is called pyramidal decussation. After this crossing, these fibers are collectively called the lateral corticospinal tract. Ten percent stay on the same side and another 10 percent decussate as they exit the spinal cord, thus the label "anterior corticospinal tract." All of these fibers eventually form synapses with the neurons of the spinal cord.
Knowledge of decussation helps in the leveling of a neurological pathology. For instance, when a lesion is above the level of the medulla, the symptoms would appear on the same side of the lesion. Alternatively, when the lesion is below the medulla, the symptoms would appear on the opposite side of the lesion.