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Hemiparesis is the partial paralysis of one side of the body. It is generally caused by the lesions of the corticospinal tract, which runs down from the cortical neurons of the frontal lobe to the motor neurons of the spinal cord (see the second paragraph of Amyotrophic lateral sclerosis) and is responsible for the movements of the muscles of the body and its limbs.
On its way the tract passes through several parts of the brain stem; namely the midbrain, pons and medulla, respectively. The tract crosses to the opposite side (or decussates) on the lowest portion of the medulla (forming an anatomical structure named as the pyramids) and goes down along the opposite side of the spinal cord to meet the contralateral motor neurons. For this reason, one side of the brain controls the muscle movements of the opposite side of the body, and thus the disruption of the right corticospinal tract on brain stem or upper brain structures causes a hemiparesis on the left side of the body and vice versa. On the other hand, the lesions of the tract on the spinal cord lead to a hemiparesis on the same side of the body. The facial muscles are also controlled by the same tract.
The tract activates the facial nuclei (see ganglion) and the facial nerve emerging from these nuclei activate the facial muscles during voluntary facial muscle contraction. Since the facial nuclei are located in the pons above the decussation, the lesions of the tract on the pons or upper structures give rise to a hemiparesis on the opposite side of the body and a paresis on the same side of the face and that is called a crossed hemiparesis. If the patient's face is not involved, this is highly suggestive of a lesion of the tract on lower parts of the brain stem or spinal cord. Since the spinal cord is a very small structure, it is very unusual for only one side to be affected by a lesion and usually both tracts are affected. Therefore, the spinal cord lesions usually present with the paralysis of both arms and legs (quadriparesis) or both legs (paraparesis).
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