Human Physiology, 14th edition (2016)

Muscle 395

junction with a somatic motor axon, and the receptors for the
neurotransmitter are located only at the neuromuscular junction.
By contrast, the entire surface of smooth muscle cells contains
neurotransmitter receptor proteins. Neurotransmitter molecules
are released along a stretch of an autonomic nerve fiber that
is located some distance from the smooth muscle cells. The
regions of the autonomic fiber that release transmitters appear
as bulges, or varicosities, and the neurotransmitters released
from these varicosities stimulate a number of smooth muscle
cells. Since there are numerous varicosities along a stretch of an
autonomic nerve ending, they form synapses “in passing”—or
synapses en passant —with the smooth muscle cells. This was
described in chapter 9 (see fig. 9.9) and is shown in figure 12.36.

Excitation-Contraction

Coupling in Smooth Muscles

As in striated muscles, the contraction of smooth muscles
is triggered by a sharp rise in the Ca^2 1 concentration within
the cytoplasm of the muscle cells. However, this Ca^2 1 is not
derived primarily from the Ca^2 1 stored in the sarcoplasmic
reticulum (SR) of smooth muscle. The SR in smooth muscle
is less extensive than in striated muscle, and its roles in con-
traction are more variable and complex. The SR of smooth

muscle cells may release its stored Ca^2 1 in response to

Ca^2 1 entering from the extracellular fluid (calcium-induced

calcium release; see fig. 12.34 ) or in response to inositol

triphosphate produced at the plasma membrane due to stimu-

lation by a hormone (chapter 11, section 11.2). The release

of Ca^2 1 from the smooth muscle SR comes in different forms

(described as puffs, sparks, and waves) and serves varied

physiological roles in the smooth muscles of different organs.

Sustained smooth muscle contractions are produced in

response to extracellular Ca^2 1 that diffuses through the sarco-

lemma into the smooth muscle cells. This Ca^2 1 enters primarily

through voltage-regulated Ca^2 1 channels in the plasma membrane.

The opening of these channels is graded by the amount of depo-

larization; the greater the depolarization, the more Ca^2 1 will enter

the cell and the stronger will be the smooth muscle contraction.

The events that follow the entry of Ca^2 1 into the cytoplasm

are somewhat different in smooth muscles than in striated mus-

cles. In striated muscles, Ca^2 1 combines with troponin. Tropo-

nin, however, is not present in smooth muscle cells. In smooth

muscles, Ca^2 1 combines with a protein in the cytoplasm called

calmodulin, which is structurally similar to troponin. Calmodu-

lin was previously discussed in relation to the function of Ca^2 1 as

a second messenger in hormone action (chapter 11, section 11.2).

The calmodulin-Ca^2 1 complex thus formed combines with and

Figure 12.36 Single-unit and multiunit smooth
muscle. In single-unit smooth muscle, the individual smooth
muscle cells are electrically joined by gap junctions, so that
depolarizations can spread from one cell to the next. In
multiunit smooth muscle, each smooth muscle cell must be
stimulated by an axon. The axons of autonomic neurons have
varicosities, which release neurotransmitters, and which form
synapses en passant with the smooth muscle cells.

Single-unit smooth muscle

Digestive

tract

Varicosity

Varicosity

Gap

junctions

Smooth

muscle

cell

Multiunit smooth muscle

Eye

Autonomic

neuron

Synapses

en passant

Autonomic

neuron

Synapses

en passant