Common mechanisms of weakness thus include dysfunction of Upper motor neurons (corticospinal and corticobulbar tract lesions) Lower motor neurons (eg, due to peripheral polyneuropathies or anterior horn cell lesions) Neuromuscular junction Muscle (eg, due to myopathies) The location of certain lesions correlates with physical findings: Upper motor neuron dysfunction disinhibits lower motor neurons, resulting in increased muscle tone (spasticity) and increased muscle stretch reflexes (hyperreflexia).