Anatomy Motor Exam: Upper Motor Neuron Lesion
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Upper Motor Neuron Lesion
The corticospinal tract has its main influence on the motor neurons that innervate the muscles of the distal extremities- the hand and the foot (motor neurons in the lateral part of the ventral horn). The corticospinal tract also (and this a key point) has collaterals that modulate and control the indirect brainstem motor centers so that we are not a stiff statue opposing gravity but rather we can move at will and have just the right amount of supporting tone. So when there is a lesion of the upper motor neuron (the UMN is the corticospinal tract and it's collaterals to the brainstem motor nuclei) the clinical findings are a combination of the loss of direct effect of the corticospinal tract on the LMN plus the loss of control and modulation of the indirect brainstem motor control centers.