A: Upper and lower motor neurons are components of motor pathways-neural pathways that originate in the brain or brainstem and descend down the spinal cord. These pathways control posture, reflexes, muscle tone, and voluntary movements.
Upper motor neurons originate in the cerebral cortex and descend through the spinal cord, where they interact with lower motor neurons. A patient with an upper motor neuron disease will exhibit:
[black small square] weakness with minimal associated atrophy (atrophy may be absent)
[black small square] hyperactive reflexes
[black small square] increased muscle tone
[black small square] spasticity
[black small square] rigidity
[black small square] minimal paralysis of voluntary movement
[black small square] tremor
[black small square] chorea (random involuntary contractions of the extremities)
[black small square] athetosis (slow, irregular movements in the distal extremities)
[black small square] dystonia (sustained, involuntary twisting movements).
The patient will also have positive Babinski's and Hoffman's reflexes. To test for Babinski's reflex, run a semisharp object up the patient's foot, starting at the heel and curving toward the great toe. If you observe plantar flexion of the great toe in response, Babinski's reflex is present. To test for Hoffman's reflex, tap on the third finger of the patient's hand. If the reflex is present, the patient will bring his thumb and forefinger together.
Lower motor neurons are located in the central and peripheral nervous systems and are responsible for innervating skeletal muscle. When the patient has a lower motor neuron disease, you'll note:
[black small square] loss of muscle tone
[black small square] ipsilateral (same side) weakness of individual muscles
[black small square] flaccidity
[black small square] atrophy
[black small square] weak or absent deep tendon plantar reflexes and abdominal reflexes
[black small square] fasciculations (muscle twitching).
Bell's palsy is caused by lower motor neuron disease associated with the seventh cranial nerve, which controls facial expression. You'll see a sagging of the face on the side of paralysis and constant tearing of the eyes. The patient can't close his eyes, wrinkle his forehead, smile, whistle, or grimace.
Amyotrophic lateral sclerosis (ALS) is an example of a mixed upper and lower motor neuron disease. It's characterized by muscle wasting caused by the destruction of lower motor neurons in the brainstem and degeneration of the upper motor neuron pyramidal tracts. A patient with ALS will experience muscle weakness and atrophy followed by spasticity and hyperactive reflexes. Other signs and symptoms include dysarthria (uncoordinated speech), dysphagia (difficulty swallowing), and dyspnea.
When differentiating upper and lower motor neuron disease, remember that upper motor neurons are responsible for motor movement, whereas lower motor neurons prevent excessive muscle movement. Upper motor disorders usually cause spasticity; lower motor disorders usually cause flaccidity.
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