Muscle
289
Although most changes in smooth muscle contractile
activity induced by chemical messengers are accompanied by
a change in membrane potential, this is not always the case.
Second messengers—for example, inositol trisphosphate—can
cause the release of calcium from the sarcoplasmic reticu-
lum, producing a contraction without a change in membrane
potential (review Figure 5–10).
Local Factors
Local factors, including paracrine agents, acidity, oxygen con-
centration, osmolarity, and the ion composition of the extra-
cellular fl uid, can also alter smooth muscle tension. Responses
to local factors provide a means for altering smooth muscle
contraction in response to changes in the muscle’s immediate
internal environment, which can lead to regulation that is inde-
pendent of long-distance signals from nerves and hormones.
Many of these local factors induce smooth muscle relax-
ation. Nitric oxide (NO) is one of the most commonly encoun-
tered paracrine agents that produces smooth muscle relaxation.
NO is released from some nerve terminals as well as from a vari-
ety of epithelial and endothelial cells. Because of the short life
span of this reactive molecule, it acts as a paracrine agent, infl u-
encing only those cells that are very near its release site.
Some smooth muscles can also respond by contracting
when they are stretched. Stretching opens mechanosensi-
tive ion channels, leading to membrane depolarization. The
resulting contraction opposes the forces acting to stretch the
muscle.
It is important to remember that seldom is a single
agent acting on a smooth muscle. Instead, the state of con-
tractile activity at any moment depends on the net magnitude
of the signals promoting contraction versus those promoting
relaxation.
Types of Smooth Muscle
The great diversity of the factors that can infl uence the contrac-
tile activity of smooth muscles from various organs has made it
diffi cult to classify smooth muscle fi bers. Many smooth muscles
can be placed, however, into one of two groups, based on the
electrical characteristics of their plasma membrane:
single-
unit smooth muscles
and
multiunit smooth muscles.
Single-Unit Smooth Muscle
The muscle cells in a single-unit smooth muscle undergo syn-
chronous activity, both electrical and mechanical; that is, the
whole muscle responds to stimulation as a single unit. This
occurs because each muscle cell is linked to adjacent fi bers by
gap junctions, which allow action potentials occurring in one
cell to propagate to other cells by local currents. Therefore,
electrical activity occurring anywhere within a group of single-
unit smooth muscle cells can be conducted to all the other
connected cells (
Figure 9–38
).
Some of the cells in a single-unit muscle are pacemaker
cells that spontaneously generate action potentials. These
action potentials are conducted by way of gap junctions to the
rest of the cells, most of which are not capable of pacemaker
activity.
Nerves, hormones, and local factors can alter the con-
tractile activity of single-unit smooth muscles using the vari-
ety of mechanisms described previously for smooth muscles
in general. The extent to which these muscles are innervated
varies considerably in different organs. The nerve terminals
are often restricted to the regions of the muscle that contain
pacemaker cells. The activity of the entire muscle can be con-
trolled by regulating the frequency of the pacemaker cells’
action potentials.
One additional characteristic of single-unit smooth mus-
cles is that a contractile response can often be induced by
stretching the muscle. In several hollow organs—the stomach,
for example—stretching the smooth muscles in the walls of
the organ as a result of increases in the volume of material in
the lumen initiates a contractile response.
The smooth muscles of the intestinal tract, uterus, and
small-diameter blood vessels are examples of single-unit smooth
muscles.
Multiunit Smooth Muscle
Multiunit smooth muscles have no or few gap junctions. Each
cell responds independently, and the muscle behaves as mul-
tiple units. Multiunit smooth muscles are richly innervated
by branches of the autonomic nervous system. The contrac-
tile response of the whole muscle depends on the number of
muscle cells that are activated and on the frequency of nerve
stimulation. Although stimulation of the nerve fi bers to the
Figure 9–38
Innervation of a single-unit smooth muscle is often restricted to
only a few cells in the muscle. Electrical activity is conducted from
cell to cell throughout the muscle by way of the gap junctions
between the cells.
Varicosities
Gap junctions
Autonomic
nerve fiber
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