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Biomedical Signal and Image Processing
entirely made of smooth muscles of different size and function. As mentioned in the
previous chapter, the smooth muscle of the heart is called cardiac muscle and plays
a central role in synchronization of cardiovascular activities. Smooth muscle tissue
is generally found in the abdomen and in arteries. Even though smooth muscles are
not controlled voluntarily, they are controlled by the autonomic nervous system.
Unlike in the skeletal muscles, the depolarization wave front is not delivered to the
entire smooth muscle system at once. Rather, the smooth muscle cells transmit the
depolarization signal from cell to cell, to produce a wavelike contraction mechanism. The transmission between the individual cells takes place through intercalated disks, which are incorporated in the smooth muscle cell itself. This process
was previously described in Chapter 10 with more details for the smooth muscles
of the heart.
Regardless of the exact type of the muscle, every muscle has the following
four characteristics: excitability, contractility, extensibility, and elasticity. The
excitability represents the phenomenon of the effect of an external stimulus to
contract. The contractility means that the muscles have the ability to contract
or shorten themselves. The extensibility represents the ability of the muscles to
stretch by external force and to extend their length. Finally, the elasticity signifies
the fact that muscles have the ability to return to the original shape after contraction or extension.
In principle, a nerve cell provides a train of impulses delivered to a muscle or a
group of muscles. These impulses depolarize muscle cell(s) and cause the muscles
to contract. The frequency of the nerve impulses determines the process of muscle
depolarization and muscle contraction. Due to the central role of this nervous excitation process and in order to better describe the nature of the EMG, next we will focus
on the concept of a “motor unit” as part of the driving mechanism in the operation
of the muscle contraction.
11.2.1 MOTOR UNIT
Several muscle fibers are innervated by only one single motor neuron or motoneuron.
The structure containing the motor neuron and its connected muscle fibers is known
as the motor unit. In other words, a motor unit is composed of a single nerve fiber
(neuron) and all of the muscle fibers it innervates. All the motor neurons innervated
by the same nerve cell fire in concert whenever an action potential is conducted
along the neuron. Each muscle is composed of a number of motor units, and each
motor unit, as mentioned earlier, is driven by a neuron. Depending on the collective
effects of the information coded in the nerves deriving a muscle, a particular action
(e.g., contraction or relaxation) is performed by the muscle. Absence of any electric
impulses will make the muscle relax.
The chemical process under which the neuron stimulates the fibers in a motor
unit, illustrated in Figure 11.1, can be briefly described as follows. As can be seen
in Figure 11.1, under the influence of an external action potential transmitted by
a nerve cell, the axon of the nerve cell ending in a synapse releases a chemical
substance called a neurotransmitter (acetylcholine: ACh) that travels the distance
between the nerve cell and the muscle cell. The ACh attaches to ACh receptors
Biomedical Signal and Image Processing
entirely made of smooth muscles of different size and function. As mentioned in the
previous chapter, the smooth muscle of the heart is called cardiac muscle and plays
a central role in synchronization of cardiovascular activities. Smooth muscle tissue
is generally found in the abdomen and in arteries. Even though smooth muscles are
not controlled voluntarily, they are controlled by the autonomic nervous system.
Unlike in the skeletal muscles, the depolarization wave front is not delivered to the
entire smooth muscle system at once. Rather, the smooth muscle cells transmit the
depolarization signal from cell to cell, to produce a wavelike contraction mechanism. The transmission between the individual cells takes place through intercalated disks, which are incorporated in the smooth muscle cell itself. This process
was previously described in Chapter 10 with more details for the smooth muscles
of the heart.
Regardless of the exact type of the muscle, every muscle has the following
four characteristics: excitability, contractility, extensibility, and elasticity. The
excitability represents the phenomenon of the effect of an external stimulus to
contract. The contractility means that the muscles have the ability to contract
or shorten themselves. The extensibility represents the ability of the muscles to
stretch by external force and to extend their length. Finally, the elasticity signifies
the fact that muscles have the ability to return to the original shape after contraction or extension.
In principle, a nerve cell provides a train of impulses delivered to a muscle or a
group of muscles. These impulses depolarize muscle cell(s) and cause the muscles
to contract. The frequency of the nerve impulses determines the process of muscle
depolarization and muscle contraction. Due to the central role of this nervous excitation process and in order to better describe the nature of the EMG, next we will focus
on the concept of a “motor unit” as part of the driving mechanism in the operation
of the muscle contraction.
11.2.1 MOTOR UNIT
Several muscle fibers are innervated by only one single motor neuron or motoneuron.
The structure containing the motor neuron and its connected muscle fibers is known
as the motor unit. In other words, a motor unit is composed of a single nerve fiber
(neuron) and all of the muscle fibers it innervates. All the motor neurons innervated
by the same nerve cell fire in concert whenever an action potential is conducted
along the neuron. Each muscle is composed of a number of motor units, and each
motor unit, as mentioned earlier, is driven by a neuron. Depending on the collective
effects of the information coded in the nerves deriving a muscle, a particular action
(e.g., contraction or relaxation) is performed by the muscle. Absence of any electric
impulses will make the muscle relax.
The chemical process under which the neuron stimulates the fibers in a motor
unit, illustrated in Figure 11.1, can be briefly described as follows. As can be seen
in Figure 11.1, under the influence of an external action potential transmitted by
a nerve cell, the axon of the nerve cell ending in a synapse releases a chemical
substance called a neurotransmitter (acetylcholine: ACh) that travels the distance
between the nerve cell and the muscle cell. The ACh attaches to ACh receptors