Computational Intelligence in Analysis of Electromyography Signals - Computational Intelligence in Biomedical Engineering - page 213

Smooth muscles are found in the linings of blood vessels, intestines, and hol-
low structures of organs. These are involuntary muscles, which have spindle-
shaped muscle fibers arranged in a sheet as seen in the electron micrograph in
Figure 5.1. There are two forms of smooth muscle. The first is visceral muscle
found in blood vessel linings and hollow parts of the organs such as the repro-
ductive organs. The second form is the multiunit muscle usually seen in large
blood vessels (valves), eyes (adjusting the lenses or the iris), and the base of
the hair follicle. These groups of muscles do not possess T-tubes, have minimal
sarcoplasmic reticulum, and hence have little mobility. The muscle fibers are
composed of thick and thin myofilaments but do not form sarcomeres. The
myofilaments do not contain troponin as in the skeletal muscles and rather
than the troponin-tropomyosin interaction activating the cross-bridge system,
a calcium-calmodulin complex system is in place.
Cardiac muscle, which was briefly discussed in Chapter 4 is the third class
of muscle. This muscle is special because it is an involuntary, mononucleated,
striated muscle found only in the heart. The cardiac muscle is myogenic in that
it does not require electrical impulses from the nervous system for contrac-
tion. The rate of contraction, however, is controlled by electrical nerve signals
originating from the SA node. When one cardiac muscle fiber is placed next to
another, contraction of one muscle will stimulate contraction in the adjacent
muscle fiber. This results in systematic propagation of the contraction action
throughout the heart. The T-tubes in cardiac muscles run along the Z-bands
and are shorter whereas the sarcoplasmic reticulum lacks terminal cisternae.
5.2.2 Muscle Contraction: Sliding Cross-Bridge Action
The sarcolemma or cell membrane of muscle fibers contains several hollow pas-
sages known as transverse tubes or “T-tubes” for short that wrap around the
myofibril and connect to other locations in the sarcolemma. They do not open
into the interior of the muscle because their sole function is to conduct action
potentials travelling on the surface of the sarcolemma deep into the muscle
core where the sarcoplasmic reticulum resides. The sarcoplasmic reticulum
is a hollow reservoir for storing Calcium ions (Ca 2+ ), which is a key chemi-
cal component for sustaining contraction. The membrane of the sarcoplasmic
reticulum as seen in Figure 5.4 is equipped with “pumps,” which work using
active transport in which energy is used to move ions across the membrane.
The membrane also has “gates,” which allow the ions through. Calcium is
continuously pumped into the sarcoplasmic reticulum from the cytoplasm of
the muscle fibers (sarcoplasm). In the relaxed state, the sarcoplasmic reticu-
lum has a high concentration of calcium ions compared to the muscle fibers
or myofibrils. The ionic gates are closed and calcium ions cannot flow back
into the muscle fibers creating a large ion diffusion gradient.
The motion of muscle fibers is due to the interaction between thick and
thin myofilaments. The myosin protein molecule or thick myofilament has a
headlike structure known as a cross-bridge, which forms connections with
actin protein molecules (Figure 5.5). This cross-bridge contains adenosine