Smooth muscle Extracellular matrix

1. External lamina is the basal lamina-type
structure that immediately surrounds the
sarcolemma of smooth muscle cells (except at gap
junctions). It is secreted by smooth muscle cells and
consists of type IV collagen, laminin, and perlecan
(heparan sulfate proteoglycan).
2. Argyryophilic reticulum. Adjacent to
external laminae, a delicate network of reticular
fibers are found between adjacent smooth muscle
cells. This network, which is argyrophilic with silver
stains, is composed principally of type III collagen
fibers. Some authors, particularly in the clinical
literature, sometimes use the term endomysium for
this layer.
3. Other matrix components. Smooth
muscle myocytes also secrete other proteoglycans
and elastin. In certain locations (e.g., blood vessels,
uterus), they can secrete large amounts of type I
collagen and elastin.
Other smooth muscle-like cells. Due to their
non-striated appearance, and presence of contractile
properties and proteins, several other cell types
share some structural and functional characteristics
with smooth muscle cells. They include the
myoepithelial cells that are found in certain
epithelial glands, myofibroblasts of CT, the
specialized epithelioid cells of the perineurium,
and the peritubular myoid cells surrounding the
seminiferous tubules in the testes.
Contraction of smooth muscle is slow and
sustained. Individual cells may contract completely
or a wave of contraction may propagate from one
end to the tissue to the other.
1. Mechanism of contraction. A sliding
filament mechanism of contraction occurs.
2. Initiation of contraction. Smooth
muscle cell contraction may be triggered by various
stimuli, including nervous and hormonal.
a. Nervous
i. Extrinsic. In some organs (e.g.,
ductus deferens), an extrinsic nerve supply to
smooth muscle is abundant.
ii. Intrinsic. In smooth muscle of the
alimentary tract, rhythmic depolarizations and
contraction are controlled by nerve fibers from
intrinsic enteric ganglia, which in turn may be
modulated by neural input from the ANS.