Muscles and Muscle Tissue
soon as the muscles are immobilized. Under such conditions,
muscle strength can decline at the rate of 5% per day!
As noted earlier, even at rest, muscles receive weak in-
termittent stimuli from the nervous system. When totally
deprived of neural stimulation, a paralyzed muscle may at-
rophy to one-quarter of its initial size. Fibrous connective
tissue replaces the lost muscle tissue, making muscle reha-
Check Your Understanding
How do aerobic and resistance exercise differ in their effects
on muscle size and function?
For answers, see Appendix H.
Compare the gross and microscopic anatomy of smooth
muscle cells to that of skeletal muscle cells.
Compare and contrast the contractile mechanisms and the
means of activation of skeletal and smooth muscles.
Distinguish between unitary and multi unit smooth muscle
structurally and functionally.
Except for the heart, which is made of cardiac muscle, the mus-
cle in the walls of all the body’s hollow organs is almost en-
tirely smooth muscle. Te chemical and mechanical events of
contraction are essentially the same in all muscle tissues, but
smooth muscle is distinctive in several ways, as summarized in
on p. 310.
Microscopic Structure of Smooth Muscle
Smooth muscle ﬁbers are spindle-shaped cells of variable size,
each with one centrally located nucleus
cally, they have a diameter of 5–10 μm and are 30–200 μm long.
Skeletal muscle ﬁbers are up to 10 times wider and thousands
of times longer.
Smooth muscle lacks the coarse connective tissue sheaths
seen in skeletal muscle. However, a small amount of ﬁne con-
nective tissue (endomysium), secreted by the smooth muscles
themselves and containing blood vessels and nerves, is found
between smooth muscle ﬁbers.
Most smooth muscle is organized into sheets of closely
apposed ﬁbers. Tese sheets occur in the walls of all but the
smallest blood vessels and in the walls of hollow organs of
the respiratory, digestive, urinary, and reproductive tracts. In
most cases, there are two sheets of smooth muscle with their
ﬁbers oriented at right angles to each other, as in the intestine
, the muscle ﬁbers run parallel to
the long axis of the organ. Consequently, when these ﬁbers
contract, the organ dilates and shortens.
Te moderately weak but sustained muscle activity required for
endurance exercise does not promote signiﬁcant skeletal muscle
hypertrophy, even though the exercise may go on for hours. Mus-
cle hypertrophy—think of the bulging biceps and chest muscles
of a professional weight li²er—results mainly from high-intensity
(typically under anaerobic conditions) such
as weight li²ing or isometric exercise, which pits muscles against
high-resistance or immovable forces. Here strength, not stamina, is
important, and a few minutes every other day is suﬃcient to allow
a proverbial weakling to put on 50% more muscle within a year.
Te additional muscle bulk largely reﬂects the increased size
of individual muscle ﬁbers (particularly the fast glycolytic variety)
rather than an increased number of muscle ﬁbers. [However, some
of the bulk may result from longitudinal splitting of the ﬁbers and
subsequent growth of these “split” cells, or from the proliferation
and fusion of satellite cells (see p. 312). Te controversy is still rag-
ing.] Vigorously stressed muscle ﬁbers also contain more mitochon-
dria, form more myoﬁlaments and myoﬁbrils, store more glycogen,
and develop more connective tissue between muscle cells.
Collectively these changes promote signiﬁcant increases in
muscle strength and size. Resistance activities can also convert
fast oxidative ﬁbers to fast glycolytic ﬁbers. However, if the spe-
ciﬁc exercise routine is discontinued, the converted ﬁbers revert
to their original metabolic properties.
Resistance training can produce incredibly bulging muscles,
but if done unwisely, some muscles may develop more than
others. Because muscles work in antagonistic pairs or groups,
opposing muscles must be equally strong to work together
smoothly. When muscle training is not balanced, individuals
, which means they lack ﬂexibility,
have a generally awkward stance, and are unable to make full
use of their muscles.
A Balanced Exercise Program
Whatever the activity, exercise gains adhere to the
. Forcing a muscle to work hard increases muscle strength
and endurance. As muscles adapt to these greater demands,
they must be overloaded even more to produce further gains.
However, always follow a heavy-workout day with a day of
rest or an easy workout to let your muscles recover and repair
themselves. Doing too much too soon, or ignoring the warning
signs of muscle or joint pain, increases your risk of
that may lead to lifetime disability.
Endurance and resistance exercises produce diﬀerent pat-
terns of muscular response, so it is important to know what
your exercise goals are. Li²ing weights will not improve your
endurance for a triathlon. By the same token, jogging will do lit-
tle to improve your muscle deﬁnition or enhance your strength
for moving furniture. A program that alternates aerobic and
anaerobic activities provides the best balance for optimal health.
±o remain healthy, muscles must be active. Immobilization due
to enforced bed rest or loss of neural stimulation results in
(degeneration and loss of mass), which begins almost as