Final answer:
UMN lesions are characterized by symptoms like hypertonicity, which leads to spasticity and the clasp-knife response, and abnormal reflexes such as hyperreflexia and a positive Babinski sign, as opposed to the flaccidity and muscle atrophy often seen in LMN lesions.
Step-by-step explanation:
Two symptoms of an Upper Motor Neuron (UMN) lesion are hypertonicity and abnormal reflexes. Hypertonicity refers to an increased muscle tone, which leads to spasticity or excessive contraction in response to stretch. This condition can cause the muscles to exhibit the clasp-knife response, which is when a limb initially resists movement and then suddenly relaxes, akin to a pocket knife closing. Abnormal reflexes in the case of UMN lesion include hyperreflexia, which is an exaggerated reflex response, and may also include the presence of pathologic reflexes like a positive Babinski sign.
Contrastingly, Lower Motor Neuron (LMN) lesions are characterized by symptoms such as flaccidity or decreased muscle tone, and can show signs including fasciculations and muscle atrophy, where muscle fibers waste away due to lack of innervation.