Page 322 - Atlas of Histology with Functional Correlations

Unlike in skeletal and cardiac muscle, there are no T tubules in smooth

muscle, and the sarcoplasmic reticulum is not well developed for storing
much calcium. In addition, smooth muscles exhibit numerous vesicular
invaginations of the cell membrane called caveolae. These caveolae appear
to have similar functions of the T tubules of striated muscles by controlling
calcium release into the cells following stimulation. During stimulation and

contraction of the smooth muscle, calcium enters the sarcoplasm from the
sarcoplasmic reticulum and from the cell membrane caveolae, where it binds
to a protein called calmodulin, a calcium-binding protein that stimulates the

interaction of actin and myosin, inducing them to slide past each other. Both
actin and myosin contract by a sliding filament mechanism that is similar to
that in skeletal muscles. The contraction of the filaments pulls dense bodies
closer together, producing contraction and shortening of the smooth muscle.
Because the dense bodies of the neighboring smooth muscle cells are

connected, the force of contraction is transmitted to all connected smooth
muscle cells, allowing the smooth muscles to function as a unit.

Smooth muscle exhibits spontaneous wavelike activity that passes in a
slow, sustained contraction throughout the entire muscle, producing a

continuous contraction of low force that maintains tonus in hollow
structures. In ureters, the uterus, and digestive organs, contraction of smooth
muscle produces peristaltic contractions, which propel the contents along
the lengths of these organs. In arteries and other blood vessels, smooth

muscles regulate the luminal diameters.

Smooth muscle fibers are also in close contacts with each other via gap
junctions. These gap junctions allow rapid ionic communications between
the smooth muscle fibers, producing coordinated activity in smooth muscle

sheets or layers. Smooth muscles are also involuntary muscles. They are
innervated and regulated by nerves from postganglionic neurons of the
sympathetic and parasympathetic divisions of the autonomic nervous
system. These innervations influence the rate and force of contractility. In
addition, smooth muscle fibers contract and relax in response to nonneural

stimulation, such as stretching or exposure to different hormones.

Summary

Muscle Tissue

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