Chapter 9
Muscles and Muscle Tissue
315
9
“explosive” movements needed to rise from a chair. Even mod-
erate activity, like taking a walk daily, improves neuromuscular
function and enhances independent living.
Smooth muscle is remarkably trouble free. Most problems that
impair gastrointestinal function, for instance, stem from irritants
such as excess alcohol, spicy foods, or bacterial infection. Under
such conditions, smooth muscle motility increases in an attempt
to rid the body of irritating agents, and diarrhea or vomiting occur.
Check Your Understanding
20.
How is the multinucleate condition achieved during
development of skeletal muscle fibers?
21.
What does it mean when we say “muscles get stringier
with age”?
22.
How can we defer (or reverse) some of the effects of age on
skeletal muscles?
For answers, see Appendix H.
Te capacity for movement is a property of all cells but, with
the exception of muscle, these movements are largely restricted
to intracellular events. Skeletal muscles, the major focus of this
chapter, permit us to interact with our external environment in an
amazing number of ways, and they also contribute to our internal
homeostasis as summarized in
System Connections
.
In this chapter we have covered muscle anatomy from gross
to molecular levels and have considered muscle physiology in
some detail. Chapter 10 continues from this point to explain
how skeletal muscles interact with bones and with each other,
and then describes the individual skeletal muscles that make up
the muscular system of the body.
being tested is coaxing dystrophic muscles to produce more
utrophin
, a similar protein present in low amounts in adults but
at much higher levels in fetal muscles. In mice at least, utrophin
can compensate for dystrophin deficiency.
As we age, the amount of connective tissue in our skeletal
muscles increases, the number of muscle fibers decreases, and
the muscles become stringier, or more sinewy. By age 30, even
in healthy people, a gradual loss of muscle mass, called
sarco-
penia
(sar-co-pe
9
ne-ah), begins. Apparently the same regulatory
molecules (transcription factors, enzymes, hormones, and others)
that promote muscle growth also oversee this type of muscle at-
rophy. Because skeletal muscles form so much of the body mass,
body weight and muscle strength decline in tandem. By age 80,
muscle strength usually decreases by about 50%. Tis “flesh wast-
ing” condition has serious health implications for the elderly, par-
ticularly because falling becomes a common event.
Muscles can also suffer indirectly. Aging of the cardiovascu-
lar system affects nearly every organ in the body, and muscles
are no exception. As atherosclerosis takes its toll and begins
to block distal arteries, a circulatory condition called
intermit-
tent claudication
(klaw
0
dĭ-ka
9
shun; “limping”) occurs in some
individuals. Tis condition restricts blood delivery to the legs,
leading to excruciating pains in the leg muscles during walking,
which forces the person to stop and rest.
But we don’t have to slow up during old age. Regular ex-
ercise helps reverse sarcopenia, and frail elders who begin to
“pump iron” (liF leg and hand weights) can rebuild muscle mass
and dramatically increase their strength. Performing those liF-
ing exercises rapidly can improve our ability to carry out the
Chapter Summary
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.
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Overview of Muscle Tissues
(pp. 276–278)
Types of Muscle Tissue
(p. 277)
1.
Skeletal muscle is attached to the skeleton, is striated, and can be
controlled voluntarily.
2.
Cardiac muscle forms the heart, is striated, and is controlled
involuntarily.
3.
Smooth muscle, located chiefly in the walls of hollow organs, is
controlled involuntarily. Its fibers are not striated.
Special Characteristics of Muscle Tissue
(p. 277)
4.
Special functional characteristics of muscle include excitability,
contractility, extensibility, and elasticity.
Muscle Functions
(pp. 277–278)
5.
Muscles move internal and external body parts, maintain posture,
stabilize joints, generate heat, and protect some visceral organs.
Skeletal Muscle
(pp. 278–305)
Gross Anatomy of a Skeletal Muscle
(pp. 278–279)
1.
Connective tissue coverings protect and strengthen skeletal
muscle fibers (cells). Superficial to deep, these are epimysium,
perimysium, and endomysium.
2.
Skeletal muscle attachments (origins/insertions) may be direct
or indirect via tendons or aponeuroses. Indirect attachments
withstand friction better.
Microscopic Anatomy of a Skeletal Muscle Fiber
(pp. 279–285)
3.
Skeletal muscle fibers are long, striated, and multinucleate.
4.
Myofibrils are contractile elements that occupy most of the cell
volume. Teir banded appearance results from a regular alternation
of dark (A) and light (I) bands. Myofibrils are chains of sarcomeres;
each sarcomere contains thick (myosin) and thin (actin)
myofilaments arranged in a regular array. Te heads of myosin
molecules form cross bridges that interact with the thin filaments.
5.
Te sarcoplasmic reticulum (SR) is a system of membranous
tubules surrounding each myofibril. Its function is to release and
then sequester calcium ions.
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