Biomedical Engineering Reference
In-Depth Information
Fig. 5.1
Structural and geometrical characteristics of smooth muscles: (
a
) layers of smooth muscle
cells; (
b
) isolated smooth muscle cell; (
c
) single, relaxed unit (myosin heads in skew position);
(
d
) same contracted unit (myosin heads in vertical position)
rooted on one basic unit, the SMC, see Fig.
5.1
(b). These spindle-shaped cells con-
tains a single centrally positioned elongated nucleus and vary notably in size, from
30 µm length in walls of small vessels to 200 µm length and 5 µm width in the intes-
tine, see Rhoades and Bell (
2008
). They are characterized by a fusiform shape. In
the mid-region they are thickest and tapered at each end. SMs are built up of layers
of cells (Fig.
5.1
(a)) that are linked together by various junctional contacts that serve
as points of cell to cell communication and mechanical linkages (dense plaque). The
mechanical contraction is caused by contractile units which consist of two filaments:
actin and myosin, see Fig.
5.1
(c). These filaments are present in large numbers and
roughly aligned with the long axis of the cell, see, e.g., Kuo and Seow (
2004
) and
Seow and Paré (
2007
). They are loosely associated into thin myofibrils. These my-
ofibrils consist of a centrally located myosin filament surrounded by multiple actin
filaments. In electron micrographs (e.g., Bond and Somlyo,
1982
; Hodgkinson et
al.,
1995
; Herrera et al.,
2005
) numerous dense staining regions, known as dense
bodies (Fig.
5.1
(d)), can be identified scattered throughout the cytoplasm of the cell.
In common with the Z-discs of skeletal muscles, these dense bodies contain the
actin-binding protein
α
-actinin and appear to serve as anchorage points for actin
filaments of myofibrils. Their association with the system of internal intermediate
filaments essential serve to integrate contractions over the entire cell and allow the
very high degree of shortening achieved by these cells. When actin filaments run
into the cell membrane, they connect the dense bodies and dense plaques. Based on
the coupling by pairs of opposed adjacent dense bodies located on neighboring cells
force transmission is accomplished across cell boundary. Thus, it appears that SMs
are composed on a huge number of contractile units in series as well as in parallel.
In comparison to experimental investigations there exist only a few approaches
describing parts of the biochemical-mechanical process in SM activation by means
of mathematical models. Looking at this type of models it stands out that a huge
number of these models is realized in a one-dimensional framework, see, e.g., Fay
