The descending fibers of the CST normally keep the ascending sensory stimulation from spreading to other nerve roots. This response does not rule out pathology. Sometimes there is no response to stimulation. The motor response which leads to the plantar flexion is mediated through the S1 root and tibial nerve. Nociceptive input travels up the tibial and sciatic nerve to the S1 region of the spine and synapse with anterior horn cells. The response results from nociceptive fibers in the S1 dermatome detecting the stimulation. Stimulation of the lateral plantar aspect of the foot (S1 dermatome) normally leads to plantar flexion of the toes (due to stimulation of the S1 myotome). Damage anywhere along the CST can result in the presence of a Babinski sign. The remainder originates from primary sensory areas, the parietal cortex, and the operculum. Sixty percent of the CST fibers originate from the primary motor cortex, premotor areas, and supplementary motor areas. The CST is considered the upper motor neuron (UMN) and the alpha motor neuron is considered the lower motor neuron (LMN). Fibers from the CST synapse with the alpha motor neuron in the spinal cord and help direct motor function. The CST is a descending fiber tract that originates from the cerebral cortex through the brainstem and spinal cord. The Babinski reflex tests the integrity of the corticospinal tract (CST).
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