Identifying Disorders of the Motor Unit

Motor unit: anterior horn cell, peripheral motor nerve, muscle. Manifestations: flaccid weakness, wasting, depression of tendon reflexes (Table 125.1).

Tests to distinguish neurogenic from myopathic disorders: EMG, nerve conduction studies (see Chapter 15); muscle biopsy, serum levels sarcoplasmic enzymes (especially creatine kinase).

Definitions

Atrophy: word with three meanings: (a) visible wasting of muscle from any cause; (b) small muscle fibers under light microscope; (c) name of neurogenic diseases.
Myopathy: symptoms due to dysfunction of muscle with no clinical, EMG, or muscle biopsy signs of denervation.
Dystrophies: myopathies with five characteristics: (a) inherited; (b) all symptoms due to limb or cranial muscle weakness; (c) weakness progressively more severe; (d) symptoms result from dysfunction in voluntary muscles (heart, viscera may also be involved); (e) no histologic abnormalities in muscle other than degeneration, regeneration, or inflammatory-fibrous reaction to muscle changes.