Smooth Myocytes - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 378

Chapter 8
Smooth Myocytes
Smooth muscle cells, or smooth myocytes (SMC), similarly to any myocyte, con-
tract and relax. Yet, unlike striated myocytes, they can withstand prolonged stretch.
Smooth myocytes can contract phasically with rapid contraction and relaxation such
as in the digestive tract, where they create peristaltism . Smooth myocytes can also
contract tonically with slow and sustained contraction, such as in the cardiovascular
and respiratory apparatus.
Transient and sustained contraction of smooth myocytes relies on 2 different
pathways. (1) Transient, calmodulin-dependent contraction results from activation
of G-protein-coupled receptors by their specific ligands, such as substance-P
and cholecystokinin, that trigger production of inositol (1,4,5)-trisphosphate and
calcium influx in the SMC cytosol. (2) Prolonged contraction is also caused by
chemical messengers, such as bombesin and ceramide, that prime extracellular
calcium influx and a protein kinase-C-dependent, calmodulin-independent axis.
Smooth myocytes pertain to involuntary (i.e., without conscious control),
non-striated myocytes. They control the dimension and mechanical function of
hollow organs and anatomical conduits. In particular, smooth myocytes lodge in
walls of both the vasculature (arteries, veins, and large lymphatic vessels) and large
and mid-sized segments of the ventilatory tract that experience large deformations
during the cardiac and respiratory cycles. In both structures, they reside beneath the
cell layer that lines the conduit lumen, i.e., the vascular endothelium and respiratory
epithelium, respectively.
Smooth myocytes are innervated by the autonomic nervous system. 1 According
to the unitary or collective innervation of SMC sheets or bundles, smooth myocytes
constitute 2 groups. (1) In single-unit smooth muscles, the autonomic nervous
system innervates a single cell within the SMC set. The command electrochemical
wave, or action potential, propagates through gap junctions to apposed cells, such
1 Involuntary and visceral nervous system, a part of the peripheral nervous system. Autonomous
means independent of cerebrospinal nervous centers.