What is the primary outcome of upper motor neuron lesions?

The primary outcome of upper motor neuron lesions is characterized by spasticity and weakness. Upper motor neurons originate in the brain and descend to influence lower motor neurons, which directly innervate muscles. When there is damage to these upper motor neurons, the regulation and inhibition they provide are lost. This results in uncontrolled excitatory signals reaching the muscles, leading to increased muscle tone or spasticity.

Additionally, weakness occurs because the descending pathways that facilitate voluntary movement are impaired. Individuals typically exhibit a combination of increased muscle tone (spasticity), exaggerated deep tendon reflexes, and weakness or paralysis of the affected muscles. This contrasts with lower motor neuron lesions, which lead to flaccid paralysis and muscle atrophy due to the direct involvement of the lower motor neurons.

Increased reflex action is indeed a result of upper motor neuron lesions, but it is a secondary phenomenon related to the loss of inhibitory control rather than the primary outcome. Normal muscle tone does not occur with upper motor neuron lesions; instead, the tone is pathologically altered. Therefore, spasticity and weakness are the most defining characteristics of the consequences of upper motor neuron lesions.