PHYSICAL EXAMINATION

LABORATORY STUDIES

DIAGNOSIS

Duchenne muscular dystrophy

PATHOPHYSIOLOGY OF KEY SYMPTOMS

Muscle tension development depends on the contraction of the muscle filaments as described by the sliding filament model. Depolarization of the muscle cell causes Ca⁺⁺ to be released from the sarcoplasmic reticulum (Fig. 6-1). The Ca⁺⁺ binds to troponin C, pulling the tropomyosin away from the G-actin site. Myosin binds to the exposed actin site, and the myosin head pivots using adenosine triphosphate (ATP) as an energy source. The pivoting of the myosin head causes the actin filaments to slide past the myosin filaments, shortening the muscle (Fig. 6-2). This shortening is transmitted to the muscle cell cytoskeleton by a number of proteins, including the elastic protein titin.

FIGURE 6–1 Excitation-contraction coupling in the muscle, showing (1) an action potential that causes release of calcium ions from the sarcoplasmic reticulum and then (2) reuptake of the calcium ions by a calcium pump.