Essentials of Anatomy and Physiology

BOX7–3 MUSCULAR DYSTROPHY

Muscular dystrophyis really a group of genetic

diseases in which muscle tissue is replaced by

fibrous connective tissue or by fat. Neither of these

tissues is capable of contraction, and the result is

progressive loss of muscle function. The most com-

mon form is Duchenne’s muscular dystrophy, in

which the loss of muscle function affects not only

skeletal muscle but also cardiac muscle. Death usu-

ally occurs before the age of 20 due to heart failure,

and at present there is no cure.

Duchenne’s muscular dystrophyis a sex-

linked(or X-linked) trait, which means that the

gene for it is on the X chromosome and is recessive.

The female sex chromosomes are XX. If one X chro-

mosome has a gene for muscular dystrophy, and

the other X chromosome has a dominant gene for

normal muscle function, the woman will not have

muscular dystrophy but will be a carrier who may

pass the muscular dystrophy gene to her children.

The male sex chromosomes are XY, and the Y has

no gene at all for muscle function, that is, no gene

to prevent the expression of the gene on the X

chromosome. If the X chromosome has a gene for

muscular dystrophy, the male will have the disease.

This is why Duchenne’s muscular dystrophy is more

common in males; the presence of only one gene

means the disease will be present.

The muscular dystrophy gene on the X chromo-

some has been located, and the protein the gene

codes for has been named dystrophin. Dystrophin is

necessary for the stability of the sarcolemma and

the proper movement of ions. Treatments for mus-

cular dystrophy that are being investigated include

the injection of normal muscle cells or stem cells

into affected muscles, and the insertion (using

viruses) of normal genes for dystrophin into

affected muscle cells.

Increased muscle

contraction

Increased cell

respiration

Increased ATP

production

Increased need

for O 2

Increased CO 2

production

Increased heat

production

Increased

sweating

Increased respiration

Increased heart rate

Vasodilation in muscles

Figure 7–6. Responses of the

body during exercise.

QUESTION:Name all the organ

systems depicted here.

BOX7–4 MYASTHENIA GRAVIS

which acetylcholine bonds and stimulates the entry

of Naions. Without these receptors, the acetyl-

choline released by the axon terminal cannot cause

depolarization of a muscle fiber.

Treatment of myasthenia gravis may involve

anticholinesterase medications. Recall that cholin-

esterase is present in the sarcolemma to inactivate

acetylcholine and prevent continuous, unwanted

impulses. If this action of cholinesterase is inhibited,

acetylcholine remains on the sarcolemma for a

longer time and may bond to any remaining recep-

tors to stimulate depolarization and contraction.

Myasthenia gravisis an autoimmune disorder

characterized by extreme muscle fatigue even after

minimal exertion. Women are affected more often

than are men, and symptoms usually begin in middle

age. Weakness may first be noticed in the facial or

swallowing muscles and may progress to other mus-

cles. Without treatment, the respiratory muscles will

eventually be affected, and respiratory failure is the

cause of death.

In myasthenia gravis, the autoantibodies (self-

antibodies) destroy the acetylcholine receptors

on the sarcolemma. These receptors are the sites to

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