Muscles and Muscle Tissue
“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
How is the multinucleate condition achieved during
development of skeletal muscle ﬁbers?
What does it mean when we say “muscles get stringier
How can we defer (or reverse) some of the effects of age on
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
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
, 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
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
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
For more chapter study tools, go to the Study Area of
There you will ﬁnd:
Practice Anatomy Lab
Videos, Practice Quizzes and Tests, MP3 Tutor Sessions, Case
Studies, and much more!
Overview of Muscle Tissues
Types of Muscle Tissue
Skeletal muscle is attached to the skeleton, is striated, and can be
Cardiac muscle forms the heart, is striated, and is controlled
Smooth muscle, located chieﬂy in the walls of hollow organs, is
controlled involuntarily. Its ﬁbers are not striated.
Special Characteristics of Muscle Tissue
Special functional characteristics of muscle include excitability,
contractility, extensibility, and elasticity.
Muscles move internal and external body parts, maintain posture,
stabilize joints, generate heat, and protect some visceral organs.
Gross Anatomy of a Skeletal Muscle
Connective tissue coverings protect and strengthen skeletal
muscle ﬁbers (cells). Superﬁcial to deep, these are epimysium,
perimysium, and endomysium.
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
Skeletal muscle ﬁbers are long, striated, and multinucleate.
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.
Te sarcoplasmic reticulum (SR) is a system of membranous
tubules surrounding each myoﬁbril. Its function is to release and
then sequester calcium ions.