Computational Intelligence in Analysis of Electromyography Signals - Computational Intelligence in Biomedical Engineering - page 220

gradient and the concentration of potassium ions inside the axonal body is
lower than the exterior. During this time, the potassium channels open and
potassium ions flow back into the axonal body along the concentration gradi-
ent returning the voltage to negative and restoring the cell membrane to its
resting equilibrium potential.
The series of depolarizations continue along the axonal body until it reaches
the NMJ at which the electrical impulse crosses using an electrical-chemical-
electrical interface from nerve to muscle. The interface is formed by two special
membranes, one on the nerve end and one on the muscle, which are separated
by a synaptic cleft (Figure 5.9). As the action potential reaches the nerve mem-
brane, voltage-gated-calcium channels activate causing calcium ions to enter.
The consequent increase in calcium ion concentration activates the release
of the neurotransmitter acetylcholine, which diffuses across the synaptic cleft
and binds with receptors on the muscle membrane. The binding causes sodium
gates on the muscle side to open and initiates a depolarization followed by
propagation of an action potential in the muscle. This impulse is the initiator
of the muscle fiber motion described earlier.
A MUAP occurs when depolarization in a motor neuron causes action
potentials in all muscle fibers of the motor unit. Conduction velocity is depen-
dent on the diameter of the axonal body, with higher speeds in larger axonal
bodies. If the axon is unmyelinated, the conduction speed rarely exceeds
3-5 m/s as opposed to myelinated axons like the motor neuron where speeds of
35-75 m/s can be achieved. Depolarization only occurs at each node of Ranvier
and the current jumps between the myelin gaps to cause depolarizations in
adjacent nodes, known as saltatory conduction . The nervous system controls
the level of muscle contraction by altering the firing rate of the motor neurons
and varying the number of motor units recruited for a particular movement.
5.3 Neuromuscular Disorders
Neuromuscular disorders are generally diseases of the peripheral nervous sys-
tem, for example, nerve disorders and NMJ disorders. These disorders can
be categorized depending on the location or point of origin. Neuropathies are
disorders of the nerves themselves whereas myopathies are complications with
the muscle usually caused by muscle degradation or muscle death. In this sec-
tion, we introduce the various disorders that affect the neuromuscular system
by highlighting their clinical manifestations, etiologies, symptoms, and where
appropriate their prognosis. Table 5.1 summarizes some of the common dis-
orders, whereas a more extensive set of muscular diseases can be found, for
example, at the Muscular Dystrophy Association (MDA) ∗ homepage.
∗ http://www.mdausa.org/disease/