Dysfunction of either the upper or the lower motor neurons may cause weakness, but there are distinguishing signs that may also occur, or which may occur even without weakness. Volitional muscle strength is quantified on a scale from zero to five compared to a population normal: zero is no muscle contraction; one is contraction without movement; two is movement but inability to overcome gravity; three is antigravity strength but no resistance to the examiner; four is some resistance to the examiner but not full strength; and five is full strength. Normal strength varies by age, sex, and muscle mass. A heavily-muscled young man and a frail elderly woman will have very different amounts of strength that will be normal (5/5) for them.
The lower motor neuron signs include hyporeflexia, hypotonia, atrophy, and fasciculations. Hyporeflexia refers to diminished muscle stretch reflexes. Hypotonia refers to diminished muscle tone, which is the resistance of muscles to stretch in the absence of volitional contraction. Tone may involve a similar mechanism as that of the muscle stretch reflex, so that in a similar way dysfunction of lower motor neurons may cause hypotonia. Atrophy refers to diminished muscle bulk. The mechanism of muscle atrophy with lower motor neuron dysfunction is unclear, but apparently skeletal myocytes need neuronal stimulation to maintain bulk. Fasciculations refer to involuntary twitching contractions of muscle that are visible through the skin. The mechanism of fasciculations is unclear, but may involve increased sensitivity of denervated myocytes to stimuli, leading to involuntary contractions.
The upper motor neuron signs include hyperreflexia, clonus, spasticity, and the extensor plantar response. Hyperreflexia refers to increased muscle stretch reflexes. The mechanism of hyperreflexia with upper motor neuron dysfunction is unclear, but may involve increased sensitivity of denervated lower motor neurons to stimuli, leading to increased responses to muscle spindle activation. Clonus refers to involuntary alternating contraction of antagonist muscles triggered by muscles stretch. Clonus probably represents hyperreflexia, where antagonist muscle get in a cycle of activating each other’s stretch reflex repeatedly. Spasticity is a type of hypertonia that is velocity dependent, such that the faster the muscle is stretched, the more resistance occurs. The mechanism of spasticity is unclear, but may be similar to that of hyperreflexia. The extensor plantar response refers to toe extension with tactile stimulation of the foot. Tactile stimulation of the foot, usually by scraping the hard end of a reflex hammer along the inferior foot, usually produces toe flexion, which is called the flexor plantar response. With upper motor neuron dysfunction toe extension may occur, which is called the extensor plantar response, or Babinski’s sign; the mechanism of this is unclear.
Reflexive limb withdrawal is a more complex motor reflex that occurs when noxious stimuli to a limb activates lower motor neurons at multiple spinal levels, causing movement at multiple joints, leading to withdrawal of the entire limb. Upper motor neuron dysfunction may cause increased reflexive limb withdrawal to noxious stimuli, or the reflex may occur with nonnoxious stimuli.