Muscles and Muscle Tissue
uterine smooth muscle, causing the uterus to grow to adult size.
During pregnancy, high blood levels of estrogen stimulate uter-
ine hyperplasia to accommodate the growing fetus.
Types of Smooth Muscle
Te smooth muscle in diﬀerent body organs varies substantially
in its (1) ﬁber arrangement and organization, (2) innervation,
and (3) responsiveness to various stimuli. For simplicity, how-
ever, smooth muscle is usually categorized into two major types:
Unitary Smooth Muscle
Unitary smooth muscle
, commonly called
cause it is in the walls of all hollow organs except the heart, is far
more common. All the smooth muscle characteristics described
so far pertain to unitary smooth muscle.
For example, the cells of unitary smooth muscle
Are arranged in opposing (longitudinal and circular) sheets
Are innervated by varicosities of autonomic nerve ﬁbers and
o±en exhibit rhythmic spontaneous action potentials
Are electrically coupled by gap junctions and so contract as a
unit (for this reason recruitment is not an option in unitary
Respond to various chemical stimuli
Multi Unit Smooth Muscle
Te smooth muscles in the large airways to the lungs and in large
arteries, the arrector pili muscles attached to hair follicles, and the
internal eye muscles that adjust pupil size and allow the eye to fo-
cus visually are all examples of
multi unit smooth muscle
In contrast to unitary muscle, gap junctions and spontaneous
depolarizations are rare. Like skeletal muscle, multi unit smooth
Consists of muscle ﬁbers that are structurally independent of
Is richly supplied with nerve endings, each of which forms a
motor unit with a number of muscle ﬁbers
Responds to neural stimulation with graded contractions
that involve recruitment
However, skeletal muscle is served by the somatic (volun-
tary) division of the nervous system. Multi unit smooth muscle,
like unitary smooth muscle, is innervated by the autonomic (in-
voluntary) division and also responds to hormones.
Check Your Understanding
Compare the structures of skeletal and smooth muscle ﬁbers.
Calcium is the trigger for contraction of all muscle types.
How does its binding site differ in skeletal and smooth
How does the stress-relaxation response suit the role of
smooth muscle in hollow organs?
For answers, see Appendix H.
entry into the sarcoplasm. Tey include certain hormones,
histamine, excess carbon dioxide, low pH, and lack of oxygen.
Te direct response to these chemical stimuli alters smooth
muscle activity according to local tissue needs and probably is
most responsible for smooth muscle tone. For example, the hor-
mone gastrin stimulates stomach smooth muscle to contract so
it can churn foodstuﬀs more eﬃciently. We will consider acti-
vation of smooth muscle in speciﬁc organs as we discuss each
organ in subsequent chapters.
Special Features of Smooth Muscle Contraction
Smooth muscle is intimately involved in the functioning of most
hollow organs and has a number of unique characteristics. We
have already considered some of these—smooth muscle tone,
slow prolonged contractions, and low energy requirements.
But smooth muscle also responds diﬀerently to stretch and can
lengthen and shorten more than other muscle types. Let’s take
Response to Stretch
Up to a point, when skeletal muscle is
stretched, it responds with more vigorous contractions. Stretch-
ing of smooth muscle also provokes contraction, which auto-
matically moves substances along an internal tract. However,
the increased tension persists only brieﬂy, and soon the muscle
adapts to its new length and relaxes, while still retaining the
ability to contract on demand.
allows a hollow organ to ﬁll
or expand slowly to accommodate a greater volume without
causing strong contractions that would expel its contents. Tis is
an important attribute, because organs such as the stomach and
intestines must store their contents long enough to digest and
absorb the nutrients. Likewise, your urinary bladder must be
able to store the continuously made urine until it is convenient
to empty your bladder, or you would spend all your time in the
Length and Tension Changes
Smooth muscle stretches
much more and generates more tension than skeletal muscles
stretched to a comparable extent. As we saw in Figure 9.22, pre-
cise, highly organized sarcomeres limit how far a skeletal muscle
can stretch before it is unable to generate force.
In contrast, the irregular, overlapping arrangement of
smooth muscle ﬁlaments and the lack of sarcomeres allow them
to generate considerable force, even when they are substantially
stretched. Te total length change that skeletal muscles can
undergo and still function eﬃciently is about 60% (from 30%
shorter to 30% longer than resting length), but smooth muscle
can contract when it is anywhere from half to twice its resting
length—a total range of 150%. Tis capability allows hollow or-
gans to tolerate tremendous changes in volume without becom-
ing ﬂabby when they empty.
All muscle cells can hypertrophy (increase in cell
size), but certain smooth muscle ﬁbers can also undergo
, that is, they divide to increase their numbers. Consider
the response of the uterus to the hormone estrogen. At puberty,
girls’ blood estrogen levels rise. As estrogen binds to uterine
smooth muscle receptors, it stimulates the synthesis of more
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