Covering, Support, and Movement of the Body
caveolae open, Ca
inﬂux occurs rapidly. Although the SR
release some of the calcium that triggers contraction, most Ca
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
for excitation-contraction coupling. Contraction ends when cy-
toplasmic calcium is actively transported into the SR and out of
, 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
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
. Te varicosities release neurotransmitter into
a wide synaptic cleF in the general area of the smooth muscle
cells. Such junctions are called
the neural input to skeletal and smooth muscles, you could say
that skeletal muscle gets priority mail while smooth muscle gets
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
, pouchlike infoldings that sequester bits of extracellular ﬂuid
containing a high concentration of Ca
close to the membrane
. Consequently, when calcium channels in the
(b) Cross section of the intestine showing
the smooth muscle layers (one circular
and the other longitudinal) running at
right angles to each other.
Longitudinal layer of smooth
(shows smooth muscle fibers
in cross section)
Circular layer of smooth muscle
(shows longitudinal views of smooth
Arrangement of smooth muscle in the walls of hollow organs.
into a wide synaptic
cleft (a diffuse junction).
Innervation of smooth muscle.