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George E. Carvell, PhD, PT   University of Pittsburgh
Ben.  fig 13-31  p 421
Neuronopathies are diseases of the motor neuron. Lower Motor Neuron (LMN) Disorders have a classic constellation of signs and symptoms, see LMN/UMN REVIEW. A LMN Disorder is characterized by a flaccid paresis or paralysis with no sensory system involvement. Severe atrophy of the muscle is due to denervation of the involved motor unit's muscle fibers. Deep Tendon Reflexes are hypoactive or absent for the involved muscles. Clinical Electromyographic Needle Exam reveals electrical signs of denervation including abnormal potentials at rest (fibrillation potentials, positive sharp waves, and fasciculation potentials). Requests for volitional contraction shows that Motor Unit Recruitment is abnormal (reduced number of motor units that fire at a
higher rate than normal). Motor unit potentials that survive in incomplete lesions are abnormal (polyphasic and giant potentials). The surviving motor unit axons sprout new axon collaterals within the muscle to "rescue" denervated muscle fibers; this results in an expanded motor unit territory for surviving motor units. Missing in a true Anterior Horn Cell (LMN) Disease are the pathological reflexes typical of an Upper Motor Neuron Disorder. Abnormal synergies are absent, though the individual may substitute remaining muscles to create actions to accomplish the task at hand. Weakness can be quantified with a manual muscle test that isolates specific muscles/muscle groups. The individual has no difficulty in isolating specific muscle actions but those affected muscles will be weak (reduced force production). Diseases of the spinal cord may show a combined Upper Motor Neuron and Lower Motor Neuron Syndrome due to involvement of the descending tracts and ventral horn gray, respectively. Amyotrophic Lateral Sclerosis (ALS) shows such a combined UMN/LMN disease process. ALS typically involves motor nuclei within the brainstem that may affect speech, swallowing, tongue movements, and facial expressions. The extraocular muscles are often sparred at least until late in the course of this fatal disease.
GMOMM  2001
Recruit motor units more often/
Unit of force