Muscle 393Also unlike skeletal muscles, where there is direct excitation-
contraction coupling between the transverse tubules and sarco-
plasmic reticulum (see fig. 12.16 ), in myocardial cells the voltage-
gated Ca^2 1 channels in the plasma membrane and the Ca^2 1 release
channels in the sarcoplasmic reticulum do not directly interact.
Instead, the transverse (or T) tubules come very close to a region
of the sarcoplasmic reticulum. Depolarization of the T tubules dur-
ing an action potential opens voltage-gated Ca^2 1 channels in their
plasma membrane, and the Ca^2 1 that diffuses into the cytoplasm
interacts with the nearby Ca^2 1 release channels in the sarcoplasmic
reticulum. This causes them to open and release the stored Ca^2 1
into the cytoplasm to stimulate contraction ( fig. 12.34 ), a process
termed calcium-induced calcium releas e. Thus, Ca^2 1 serves as
a second messenger from the voltage-gated Ca^2 1 channels to the
Ca^2 1 release channels. As a result, excitation-contraction coupling
is slower in cardiac than in skeletal muscle.
Diffusion of Ca^2 1 through the plasma membrane of the
transverse tubules ( fig. 12.34 ) serves mainly to open the chan-
nels in the sarcoplasmic reticulum. The Ca^2 1 release chan-
nels in the sarcoplasmic reticulum are 10 times larger than the
voltage-gated Ca^2 1 channels in the plasma membrane, and so
are primarily responsible for the rapid diffusion of Ca^2 1 into
the cytoplasm, which then binds to troponin and stimulates
contraction. In order for the muscular chambers of the heart to
relax, the Ca^2 1 in the cytoplasm must be actively transported
back into the sarcoplasmic reticulum ( fig. 12.34 ).Smooth Muscle
Smooth (visceral) muscles are arranged in circular layers in
the walls of blood vessels and bronchioles (small air passages
in the lungs). Both circular and longitudinal smooth muscle
layers occur in the tubular digestive tract, the ureters (which
transport urine), the ductus deferentia (which transport sperm
cells), and the uterine tubes (which transport ova). The alter-
nate contraction of circular and longitudinal smooth muscle
layers in the intestine produces peristaltic waves, which pro-
pel the contents of these tubes in one direction.
Although smooth muscle cells do not contain sarcomeres
(which produce striations in skeletal and cardiac muscle), they
do contain a great deal of actin and some myosin, which pro-
duces a ratio of thin to thick filaments of about 16 to 1 (in
striated muscles the ratio is 2 to 1). Unlike striated muscles, in
which the thin filaments are relatively short (extending from a
Z disc into a sarcomere), the thin filaments of smooth muscle
cells are quite long. They attach either to regions of the plasma
membrane of the smooth muscle cell or to cytoplasmic proteinFigure 12.34 Excitation-
contraction coupling in cardiac
muscle. Depolarization of the plasma
membrane during action potentials,
when voltage-gated Na^1 channels are
opened, causes voltage-gated Ca^2 1
channels to open in the transverse
tubules. (1) This allows some Ca^2 1 to
diffuse from the extracellular fluid into
the cytoplasm, which (2) stimulates
the opening of Ca^2 1 release channels
in the sarcoplasmic reticulum. This
process is called Ca^2 1 -stimulated Ca^2 1
release. (3) The Ca^2 1 released from
the sarcoplasmic reticulum binds to
troponin and stimulates contraction.
(4) A Ca^2 1 (ATPase) pump actively
transports Ca^2 1 into the (5) cisternae
of the sarcoplasmic reticulum, allowing
relaxation of the myocardium and
producing a concentration gradient
favoring the outward diffusion of Ca^2 1
for the next contraction.CytoplasmZZSarcomereNa+Na+Ca^2 +Ca^2 +Ca^2 + Ca^2 +Ca^2 +Transverse
tubuleVoltage-gated
Na+ channelVoltage-gated
Ca^2 + channelCa^2 +^ release
channelExtracellular fluidTroponinSarcoplasmic
reticulumCa^2 +
ATPase
1 pump2345Figure 12.33 Cardiac muscle. Notice that the cells are
short, branched, and striated and that they are interconnected by
intercalated discs.
NucleusIntercalated discs