306
UNIT 2
Covering, Support, and Movement of the Body
9
caveolae open, Ca
2
1
influx occurs rapidly. Although the SR
does
release some of the calcium that triggers contraction, most Ca
2
1
enters through calcium channels directly from the extracellu-
lar space. Tis situation is quite different from what we see in
skeletal muscle, which does not depend on extracellular Ca
2
1
for excitation-contraction coupling. Contraction ends when cy-
toplasmic calcium is actively transported into the SR and out of
the cell.
In the
circular layer
, the fibers run around the circumference
of the organ. Contraction of this layer constricts the lumen of
the organ and elongates the organ.
Te alternating contraction and relaxation of these layers
mixes substances in the lumen and squeezes them through the
organ’s internal pathway. Tis propulsive action is called
peri-
stalsis
(per
0
ĭ-stal
9
sis; “around contraction”). Contraction of
smooth muscle in the rectum, urinary bladder, and uterus helps
those organs to expel their contents. Smooth muscle contrac-
tion also accounts for the constricted breathing of asthma and
for stomach cramps.
Smooth muscle lacks the highly structured neuromuscu-
lar junctions of skeletal muscle. Instead, the innervating nerve
fibers, which are part of the autonomic (involuntary) nervous
system, have numerous bulbous swellings, called
varicosities
(Figure 9.26)
. Te varicosities release neurotransmitter into
a wide synaptic cleF in the general area of the smooth muscle
cells. Such junctions are called
diffuse junctions
. Comparing
the neural input to skeletal and smooth muscles, you could say
that skeletal muscle gets priority mail while smooth muscle gets
bulk mailings.
Te sarcoplasmic reticulum of smooth muscle fibers is much
less developed than that of skeletal muscle and lacks a spe-
cific pattern relative to the myofilaments. Some SR tubules of
smooth muscle touch the sarcolemma at several sites, forming
what resembles half-triads that may couple the action potential
to calcium release from the SR.
± tubules are absent, but the sarcolemma has multiple
caveo-
lae
, pouchlike infoldings that sequester bits of extracellular fluid
containing a high concentration of Ca
2
1
close to the membrane
(Figure 9.27a)
. Consequently, when calcium channels in the
Small intestine
(a)
(b) Cross section of the intestine showing
the smooth muscle layers (one circular
and the other longitudinal) running at
right angles to each other.
Mucosa
Longitudinal layer of smooth
muscle
(shows smooth muscle fibers
in cross section)
Circular layer of smooth muscle
(shows longitudinal views of smooth
muscle fibers)
Figure 9.25
Arrangement of smooth muscle in the walls of hollow organs.
Smooth
muscle
cell
Varicosities
release
their neurotransmitters
into a wide synaptic
cleft (a diffuse junction).
Synaptic
vesicles
Mitochondrion
Autonomic
nerve fibers
innervate
most smooth
muscle fibers.
Varicosities
Figure 9.26
Innervation of smooth muscle.
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