Upper versus lower motor neuron lesions

What it is

The upper motor neuron carries an inhibitory effect on the muscle stretch reflex. The lower motor neuron is the motor component of that same reflex. Lose the first and the reflex is released; lose the second and the reflex is gone.

How it presents

An upper motor neuron lesion gives spastic paralysis, hyperreflexia, hypertonic muscle, disuse atrophy, no fasciculations, a positive Babinski sign, loss of the abdominal and cremasteric reflexes, voluntary movement that is decreased in speed rather than lost, and a large area of the body involved. Tone is read further: decorticate rigidity marks a lesion above the midbrain, decerebrate rigidity a lesion below the midbrain. A lower motor neuron lesion gives weak muscle with decreased tone, decreased deep tendon reflexes, wasting atrophy, fasciculations present, no Babinski sign, voluntary movement lost, and a small area of the body involved. Amyotrophic lateral sclerosis shows both at once, so it is the best case for holding the two side by side: lower motor neuron symptoms are muscle atrophy and fasciculation, from loss of neurons in the anterior horn, and upper motor neuron symptoms are spasticity, hyperreflexia and pathologic reflexes, from degeneration and atrophy of the lateral corticospinal tracts.