Page 199 - Anatomy and Physiology of Farm Animals, 8th Edition

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smooth muscle does not lose its contractile function (e.g., smooth muscle in the walls
of blood vessels).
ability. The reverse occurs upon emptying
VetBooks.ir of a visceral organ when the stretched
muscle shortens back to its original length:
all tension is lost at first but returns shortly. Role and Sources of Calcium
Plasticity is believed to be due to changes As in skeletal muscle, contraction and
in the arrangement or binding of the myo- relaxation of smooth muscle are linked to
sin and actin filaments upon stretching or the Ca concentration in the cytosol of
2+
shortening.
smooth muscle cells. However, how this
concentration is regulated and the role of
Contraction and Relaxation Ca in the contraction process is quite
2+
different in the two types of muscle. Some
All skeletal muscle contraction depends calcium is stored in the sarcoplasmic retic-
on ACh release at a neuromuscular junc- ulum of smooth muscle cells, but many
tion and the generation and propagation types of smooth muscle cells also contain a
of action potentials on the cell membrane. significant number of calcium channels
This is not the case for smooth muscle. in the outer cell membrane (Fig. 9‐9). These
The stimuli that bring about smooth mus- calcium channels may be either voltage‐
cle contraction and relaxation are quite gated or ligand‐gated, and the smooth
variable. Thus, smooth muscle is a much muscle within a given organ may have
more functionally diverse tissue than both types of channels or primarily only
skeletal muscle. one type of channel. When these channels
The contraction and relaxation of most open in response to the appropriate stimu-
smooth muscle are much slower events lus (changes in membrane potential or
than the rapid muscle twitch that is char- presence of specific ligands), calcium can
acteristic of skeletal muscle. This permits diffuse into the cell to initiate contraction
the maintenance of a relatively constant (Fig. 9‐9). Receptors that are specific for
pressure with the use of little cell energy. many different ligands (e.g., hormones
This is characteristic of smooth muscle in and neurotransmitters) are found on the
organs that require a constant state of tone smooth muscle of different organs. This is
or some degree of contraction for normal important functionally, because a given

Extracellular fluid
Calcium Cell membrane of
channel Ca 2+ Ca 2+ smooth muscle cell

Ca 2+ Ca + calmodulin
2+
Other enzymes which
Inactive kinase dephosphorylate myosin
Ca 2+ Ca 2+ calmodulin
Phosphorylated
Activated kinase + actin
myosin
Sarcoplasmic
reticulum Contraction

Figure 9-9. Sources and role of calcium ions in the initiation and regulation of smooth muscle contrac-
tion. Dotted lines associated with calcium ions indicate movement by diffusion. Solid lines associated
with calcium ions indicate removal of calcium from the cytosol to promote relaxation. Relaxation can also
be promoted by the action of other intracellular enzymes that dephosphorylate myosin.