RF/Microwave Interaction Mechanisms in Biological Materials - RF/Microwave Interaction with Biological Tissues - page 58

derline between these two types of compensation is obviously not always easy
to determine.
The radiation mechanism considered consists of a source that emits EM
energy. Part of the incident energy is absorbed and transformed within the
biological system. Hence, there is the sequence source-radiation-target. The
physical laws of EM field theory, reflection, diffraction, dispersion, interfer-
ence, optics, and quantum effects, must be applied to investigate and explain
the observed phenomena. This is true in general for the whole spectrum
of EM radiation. In this topic, however, the study is limited to RF and
microwaves.
The increasing industrialization of the world and the tendency to increase
the power of equipment raised the question of health risks first for personnel,
then for the general public. It gave an impetus to carry out large research proj-
ects and collect a vast amount of experimental data and clinical observations.
Before starting any interpretation of the results obtained, however, it is nec-
essary to survey the basic phenomena involved in the interaction of RF and
microwave radiation with living systems. The first step is to review basic bio-
electricity. Further reading on this subject can be found in a number of text-
books [e.g., 1]. An excellent summary is by Reilly et al., on which this section
is largely based [2].
Natural bioelectric processes are responsible for nerve and muscle func-
tion. Externally applied electric currents can excite nerve and muscle cells. The
nervous system is concerned with the rapid transfer of information through
the body in the form of electrical signals. It is conveniently divided into the
central nervous system (CNS) and the peripheral nervous system. The CNS
consists of the brain and spinal cord. The peripheral nervous system consists
of afferent neurons, which convey information inward to the CNS, and effer-
ent neurons, which convey information from the CNS to the body. The effer-
ent system is subdivided into a somatic nervous system and an autonomic
nervous system. The autonomic nervous system consists of neurons that
convey impulses to smooth muscle tissue, cardiac muscle tissue, and glands,
which are usually considered involuntary, that is, not under conscious control.
The autonomic nervous system has sympathetic and parasympathetic divi-
sions. Because they control opposite effects in various organs, they are usually
considered antagonistic to each other. The sympathetic nervous system tends
to mobilize the body for emergencies (secretion of adrenaline), whereas the
parasympathetic nervous system is concerned with vegetating functions of the
body, such as digestion. One responsibility of the autonomic nervous system
is to maintain body homeostasis.
Muscles can be stimulated directly or indirectly through the nerves that
enervate the muscle. Thresholds of stimulation of nerve are generally well
below thresholds for direct stimulation of a muscle. Hence, an understanding
of neuroelectric principles is a valuable foundation for investigating both
sensory and muscular responses to electrical stimulation.