A 75-year-old male patient who has recent hemiparesis came to the hospital for followup. On physical examination, stroking the plantar surface of foot produced a reflex extension of large toe and fanning of lateral toes. This is due to damage to one of the following structures

Upper motor neurons typically refer to coicospinal tract neurons that innervate spinal motor neurons, but they can also include brainstem neurons that control spinal motor neurons. Damage to these neurons initially causes muscles to become weak and flaccid but eventually leads to spasticity, hypeonia (increased resistance to passive movement), hyperactive stretch reflexes, and abnormal plantar extensor reflex (positive Babinski sign). The Babinski sign is dorsiflexion of the great toe and fanning of the other toes when the lateral aspect of the sole of the foot is scratched. In adults, the normal response to this stimulation is plantar flexion in all the toes.