keletal muscles respond to use and dis-
use. Forcefully exercised muscles enlarge,
. Unused muscles
decreasing in size and strength.
The way a muscle responds to use also
depends on the type of exercise. Weak contrac-
tions, such as in swimming and running, activate
slow, fatigue-resistant red fibers. In response,
these fibers develop more mitochondria and
more extensive capillary networks, which increase
fatigue-resistance during prolonged exercise,
although sizes and strengths of the muscle ±
may not change.
In forceful exercise, such as weightlifting,
a muscle exerts more than 75% of its maximum
tension, using predominantly the muscle’s fast,
fatigable white fibers. In response, existing
muscle fibers synthesize new filaments of actin
and myosin, and as their diameters increase, the
entire muscle enlarges. However, no new muscle
bers are produced during hypertrophy.
The strength of a contraction is directly pro-
portional to the diameter of the muscle ±
an enlarged muscle can contract more strongly
than before. However, such a change does not
increase the muscle’s ability to resist fatigue dur-
ing activities such as running or swimming.
Microscopic muscle damage can occur with
too-frequent weight lifting (strength training).
This is why trainers advise lifting weights every
other day, rather than daily.
If regular exercise stops, capillary networks
shrink, and muscle fibers lose some mitochon-
dria. Actin and myosin filaments diminish, and
the entire muscle atrophies. Injured limbs immo-
bilized in casts, or accidents or diseases that inter-
fere with motor nerve impulses, cause muscle
atrophy. An unexercised muscle may shrink to
less than one-half its usual size in a few months.
bers whose motor neurons are severed
not only shrink but also may fragment, and in
time fat or ±
brous tissue replaces them. However,
reinnervation of such a muscle within the ±
months following an injury can restore function.
New technologies can compensate for some
muscle loss. “Targeted muscle reinnervation,” for
example, can tap into the neuromuscular system
to assist a person who has lost an upper limb. A
surgeon reattaches muscles from a severed arm
to the patient’s chest wall, then uses electromyo-
graphy to detect the electrical activity that still
reaches those muscles. The information is sent
to a microprocessor built into an attached pros-
thetic arm, where a “neural-machine interface”
enables the patient to move the replacement arm
at will, just as he or she would consciously direct
the movement of the missing part.
Use and Disuse of Skeletal Muscles
smooth muscle respond as a single unit. When one f
stimulated, the impulse moving over its surFace may excite
adjacent f bers that, in turn, stimulate others. Some visceral
smooth muscle cells also display
—a pattern oF
spontaneous repeated contractions.
These two Features oF visceral smooth muscle—
transmission oF impulses From cell to cell and rhythmicity—are
largely responsible For the wavelike motion called
oF certain tubular organs (see chapter 17, pp. 654 and 656).
Peristalsis consists oF alternate contractions and relaxations
oF the longitudinal and circular muscles. These movements
help Force the contents oF a tube along its length. In the
intestines, For example, peristaltic waves move masses oF
partially digested Food and help to mix them with digestive
uids. Peristalsis in the ureters moves urine From the kid-
neys to the urinary bladder.
Visceral smooth muscle is the more common type oF
smooth muscle and is Found in the walls oF hollow organs,
such as the stomach, intestines, urinary bladder, and uterus.
Usually smooth muscle in the walls oF these organs has two
thicknesses. The f
bers oF the outer coats are longitudinal,
whereas those oF the inner coats are circular. The muscular
layers change the sizes and shapes oF the organs as they con-
tract and relax.
Smooth Muscle Contraction
Smooth muscle contraction resembles skeletal muscle
contraction in a number oF ways. Both mechanisms reﬂ
The contractile mechanisms oF smooth and cardiac mus-
cles are essentially the same as those oF skeletal muscles.
However, the cells oF these tissues have important structural
and Functional distinctions.
Smooth Muscle Fibers
Recall From chapter 5 (p. 163) that smooth muscle cells are
shorter than the f bers oF skeletal muscle, and they have sin-
gle, centrally located nuclei. Smooth muscle cells are elon-
gated with tapering ends and contain f laments oF actin and
myosin in myof
brils that extend throughout their lengths.
However, the f
laments are thin and more randomly distrib-
uted than those in skeletal muscle f
bers. Smooth muscle
cells lack striations and transverse tubules, and their sarco-
plasmic reticula are not well developed.
The two major types oF smooth muscles are multiunit
and visceral. In
multiunit smooth muscle,
the muscle f
are less well organized and Function as separate units, inde-
pendent oF neighboring cells. Smooth muscle oF this type is
Found in the irises oF the eyes and in the walls oF blood ves-
sels. Typically, multiunit smooth muscle contracts only aFter
stimulation by motor nerve impulses or certain hormones.
Visceral smooth muscle
(single-unit smooth muscle)
is composed oF sheets oF spindle-shaped cells held in close
contact by gap junctions. The thick portion oF each cell lies
next to the thin parts oF adjacent cells. ±ibers oF visceral