Muscular Dystrophy and Myasthenia Gravis

Muscular dystrophy14 is a collective term for several hereditary diseases in which the skeletal muscles degenerate, lose strength, and are gradually replaced by adipose and fibrous tissue. This new connective tissue impedes blood circulation, which in turn accelerates muscle degeneration in a fatal spiral of positive feedback. The most common form of the disease is Duchenne15 muscular dystrophy (DMD), caused by a sex-linked recessive allele. Like other sex-linked traits (see chapter 4), DMD is mainly a disease of males. It occurs in about 1 in 3,500 male live births, but is not usually diagnosed until the age of 2 to 10 years. Difficulties begin to appear early on, as a child begins to walk. The child falls frequently and has difficulty standing up again. The disease affects the hips first, then the legs, and progresses to the abdominal and spinal muscles. The muscles shorten as they atrophy, causing postural abnormalities such as scoliosis. DMD is incurable but is treated with exercise to slow the atrophy and with braces to reinforce the weakened hips and correct the posture. Patients are usually confined to a wheelchair by early adolescence and rarely live beyond the age of 20.

The DMD gene was identified in 1987, and genetic screening is now available to inform prospective parents of whether or not they are carriers. The normal allele of this gene makes dystrophin, a large protein that links to actin filaments at one end and to membrane glycoproteins on the other. In DMD, dystrophin is absent, the plasma membranes of the muscle fibers become torn, and the muscle fibers die.

A less severe form of muscular dystrophy is facioscapulohumeral (Landouzy-Dejerinem) muscular dystrophy, an autosomal dominant trait that begins in adolescence and affects both sexes. It involves the facial and shoulder muscles more than the pelvic muscles and disables some individuals while it barely affects others. A third form, limb-girdle dystrophy, is a combination of several diseases of intermediate severity that affect the shoulder, arm, and pelvic muscles.

Myasthenia gravis17 (MY-ass-THEE-nee-uh GRAV-is) (MG) usually occurs in women between the ages of 20 and 40. It is an autoimmune disease in which antibodies attack the neuromuscular junctions and bind ACh receptors together in clusters. The muscle fiber then removes the clusters from the sarcolemma by endocytosis. As a result, the muscle fibers become less and less sensitive to ACh. The effects often appear first in the facial muscle (fig. 11.25) and commonly include drooping eyelids and double vision (due to weakness of the eye muscles). The initial symptoms are often followed by difficulty in swallowing, weakness of the limbs, and poor physical endurance. Some people with MG die

Famous People With Myasthenia Gravis
Figure 11.25 Myasthenia Gravis. This disorder especially affects the muscles of the head. It is characterized by drooping of the eyelids, weakness of the muscles of eye movement, and double vision resulting from the divergence (strabismus) of the eyes.

quickly as a result of respiratory failure, but others have normal life spans. One method of assessing the progress of the disease is to use bungarotoxin, a protein from cobra venom that binds to ACh receptors. The amount that binds is proportional to the number of receptors that are still functional. The muscle of an MG patient sometimes binds less than one-third as much bungarotoxin as normal muscle does.

Myasthenia gravis is often treated with cholinesterase inhibitors. These drugs retard the breakdown of ACh in the neuromuscular junction and enable it to stimulate the muscle longer. Immunosuppressive agents such as Prednisone and Imuram may be used to suppress the production of the antibodies that destroy ACh receptors. Since certain immune cells are stimulated by hormones from the thymus, removal of the thymus (thymectomy) helps to dampen the overactive immune response that causes myasthenia gravis. Also, a technique called plasmapheresis may be used to remove harmful antibodies from the blood plasma.

14dys = bad, abnormal + trophy = growth

15Guillaume B. A. Duchenne (1806-75), French physician

16Louis T. J. Landouzy (1845-1917) and Joseph J. Dejerine (1849-1917), French neurologists

Saladin: Anatomy & I 11. Muscular Tissue I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

438 Part Two Support and Movement

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