RF/Microwave Interaction Mechanisms in Biological Materials - RF/Microwave Interaction with Biological Tissues - page 63

2.1.3 Bioelectric Phenomena

All living cells exhibit bioelectric phenomena. However, a small variety only
produce potential changes that reveal their physiological function. There are
familiar bioelectric recordings. The three most prominent bioelectric effects,
those of heart, skeletal muscle, and brain, are recorded by the following: the
electrocardiogram (ECG), reflecting the excitation and recovery of the whole
heart; the electromyogram (EMG), reflecting the activity of skeletal muscle;
and the electroencephalogram (EEG), reflecting the activity of the outer
layers of the brain, the cortex. In these cases, the action potentials are used for
diagnostic purposes, and extracellular electrodes are used that are both large
and distant from the population of cells that become active and recover. The
depolarization and repolarization processes send small currents through the
conducting environmental tissues and fluids, resulting in a time-varying poten-
tial field. Appropriately placed electrodes can record the electrical activity of
the bioelectric generators. However, the waveforms of such recordings are
vastly different from those of the transmembrane action potentials. It has been
shown by Geddes and Baker that such extracellular recordings resemble the
second derivative of the excursion in the transmembrane potential [7]. Despite
the difference in waveform, extracellular recordings identify the excitation and
recovery processes very well [2]. Furthermore the eye, ear, sweat glands, and
many types of smooth muscles also produce action potentials that are used for
their diagnostic value [7].
Bioelectricity is extremely important in a living body. It has long been
shown that direct application of an externally generated voltage may have an
effect on bone and cartilage repair. Considerable animal and in vitro experi-
mentation suggests the clinical usefulness of electric currents for soft tissue
repair and possibly to enhance repair of nerve fibers that have sustained crush
or transsection injury [2]. There is no doubt that bioelectricity has to be taken
into account seriously when investigating possible medical applications of RFs
and microwaves as well as when wondering about possible hazards on human
beings and animals due to RF or microwave exposure. These issues will be
examined in more detail in subsequent sections of this chapter.

2.2 TISSUE CHARACTERIZATION

The classical topic by Michaelson and Lin, reviewing in 1987 the entire area of
biological effects due to RF radiation, is still a reference for those who want to
acquire a good knowledge of the field [8]. Thuery reviewed, in 1992, the indus-
trial, scientific, and medical applications of microwaves, with a number of basic
information sources on the interaction between microwave fields and the
nervous system [9]. In 1996, Polk and Postow reviewed the whole field of bio-
logical effects of EM fields [10]. More specifically, a summary of the action of
microwave EM fields on the nervous system has been published recently [11].