That these direct currents could influence the behavior of neurones themselves was shown by
Terzuolo and Bullock, who used isolated neurones that spontaneously generated action potentials at a
steady rhythmic rate (10). They demonstrated that very small currents and voltages could modulate the
rate of firing without producing depolarization of the nerve cell membrane. They concluded that, "the
great sensitivity of neurones to small voltage differences supports the view that electric field actions
can play a role in the determination of probability of firing of units."
Clinically, direct currents were also being used to produce electronarcosis or electrical
anesthesia for surgery. While these studies were empirical, they frequently involved the passage of
current along the frontooccipital axis of the head, the same vector previously described by Libet and
Gerard as demonstrating a DC potential that seemed related to the state of consciousness. Somewhat
lower electrical parameters were used as "calming" or sleep-producing agents in psychiatric treatment
of various hyperactive states. These techniques are still in use in a number of countries. In 1976, Nias
reported positive results in a very carefully controlled study of the electrosleep technique, involving
double blind experiments using alternating currents as controls (11).
Finally, the role played by the glia, the "supporting" cells that constituted 90% of the total mass
of the brain, began to be questioned. Electron microscopy revealed close and involved associations
between the glia and the neurones as well as between the glia cells themselves (tight junctions, etc.).
The analog of the glia cell, the Schwann cell, was found to invest all peripheral nerve fibers outside of
the brain and spinal cord. They appeared to many investigators to be syncytial in nature; that is, to be in
continuous cytoplasmic contact along the entire length of each nerve. Biochemical changes were found
to occur in the glia concurrent with activity of their neurones (such as during repeated generation of
action potentials or cessation of activity as in sleep) (12). Evidence was even presented that these glia
cells were involved in the process of memory. In 1964 Kuffler and Potter reported electrophysiological
measurements on the glia cells of the leech which were very large and easily worked with (13). He
described DC potentials in these cells which spread through some low resistance couplings to many
other glia cells. The action potential of the neurone did not influence the glia cells but the reverse
appeared possible. Later Walker demonstrated that similar events occurred in mammalian glia cells
with transmission of injected direct currents between glia cells and some evidence that changes in the
electrical state of the glia did influence their associated neurones (14). It began to appear to be possible
that the extraneuronal currents originally described by Libet and Gerard could be associated with some
electrical activity in these non-neuronal cells themselves (15).
Another type of non-neuronal cell associated with the nervous system-the sensory receptor cell-
was found to have unusual electrical properties. In most instances the initial receipt of a stimulus is via
a specialized cellular "organ" called a sensory receptor that is located at the end of the nerve fiber, or
fibers, connecting it to the central nervous system. In some instances these are highly specialized, large
anatomical structures such as the eyes, which are sensitive to that portion of the electromagnetic
spectrum which we call light. Others are microscopic and specialized to receive mechanical stimuli,
such as the pressure-sensitive Pacinian corpuscles and the stretch-sensitive muscle spindles. In the
latter instance the receptor itself is clearly a modified muscle fiber that has a particularly intimate
connection to its nerve. These mechanical receptors produce an electrically measurable response when
stimulated by pressure or stretch. This so-called "generator potential" is quite different from the action
potential, being graded (i.e., varying in magnitude in direct relationship to the magnitude of the
mechanical stimulus) and regardless of its magnitude, nonpropagating (i.e., decreasing rapidly over
microscopic distances). Apparently, the action of the generator potential is to produce sufficient
depolarization of the associated nerve fiber membrane to start a propagated action potential which then
proceeds centrally along the associated nerve fiber carrying the sensory message. The mechanism of
ELECTROMAGNETISM & LIFE - 24