Computational Intelligence in Analysis of Electromyography Signals - Computational Intelligence in Biomedical Engineering - page 218

and the peripheral nervous system giving us primary sensory information such
as touch and temperature. A motor unit consists of one alpha motoneurone,
a motor axon, and skeletal muscle fibers innervated by the terminals of the
axon (Preedy and Peters, 2002).
The motor neuron is an example of an efferent nerve in the body's nervous
system. Efferent nerves carry impulses away from the central nervous system
such as the spinal cord to effector units such as muscles and glands. In con-
trast, afferent nerves convey impulses to the CNS such as nerves from the skin
and other sensory organs. The motor neuron has its cell body in the CNS with
a long axon extending out toward the effectors; in this case the muscles. The
axon body has several branching dendrites and ends in a neuromuscular junc-
tion (NMJ) with the muscle. A diagram of this nerve connection is depicted in
Figure 5.8 and an electron micrograph of the NMJ is shown in Figure 5.9. The
axon of the motor neuron is usually sheathed in a myelin insulation, which is
typically composed of concentric spirals of Schwann cells. The myelin sheath
extends along the whole axon except at the nodes of Ranvier (gaps between
adjacent Schwann cells) and near the NMJ (Figure 5.10).
The main cell head of the motor neuron is also known as the anterior horn
cell and lies in the ventral gray matter of the spinal cord. The axonal body,
which extends out has an electrically active cell membrane, which is semiper-
meable to anions and cations and is always permeable to large negative ions.
FIGURE 5.9
An electron micrograph of the NMJ is shown. Label T indicates the axon
terminal and M indicates the muscle fiber. (Courtesy of http://www.nimh.
nih.gov/Neuroinformatics/shins04.cfm.)