However, if the edges of the cut were simply brought into physical contact the waves crossed
unimpeded. If the edges were separated and the small gap filled with a physiological saline solution,
the waves again were prevented from crossing the gap. Gerard and Libet interpreted these findings to
mean that actual electrical currents of some type were flowing outside of the nerve cells through the
brain. The ability of the potential waves to cross the cut when the edges were placed in physical contact
ruled out the action potential as the source of the potential, and the inability of the wave to cross the
saline filled gap ruled out a chemical basis for the potential wave. They concluded that drugs that
increase or decrease the excitability of the neurones all act by similarly changing these electrical
currents, and that it is these extracellular electrical currents that exert the primary controlling action on
the neurones.
In a further series of studies on the intact brain, Libet and Gerard described the existence of a
steady DC potential existing longitudinal, from front to back in the brain, with the frontal (olfactory)
lobes being normally negative by several millivolts (mV) to the occipital lobe (6, 7). At about the same
time another neurophysiologist, Leao, conducted a series of experiments on the electrical changes
associated with localized physical injuries to the brain. He found that a positive-going DC polarization
spread out from the site of injury and that the neurones encompassed by this zone of "spreading
depression" stopped all activity and lost their ability to receive, generate and transmit nerve impulses as
long as the polarization remained. Thus the DC electrical activity generated by the injury produced a
functional loss in a larger area of the brain than that which was directly injured.
While these observations were interesting and seemed to have a more direct bearing on "higher
nervous functions" such as integration, most interest in the neurophysiological community was directed
toward the well-established nerve impulse and in relating it to the enormous anatomical complexity of
the brain that was becoming evident. Techniques were developed that enabled the neuroanatomist to
track the course of nerve fibers through complex areas of the brain, thus determining the connections
between various nuclei and brain areas. It became apparent that the "circuitry" of the brain was not a
simple "one on one" arrangement. Single neurones were found to have tree-like arborizations of
dendrites with input synapses from scores of other neurones. Dendritic electrical potentials were
observed that did not propagate like action potentials, but appeared to be additive; when a sufficient
number were generated the membrane depolarization reached the critical level and an action potential
would be generated. Other neurones were found whose action potentials were inhibitory to their
receiving neurones. Graded responses were discovered in which ion fluxes occurred across the
neuronal membranes and while insufficient in magnitude to produce an action potential, still produced
functional changes in the neurone. The complexity of function in the brain was found to be enormous.
At the same time the neurohistologists were finding that only about I0% of the brain was
composed of cells that could properly be called neurones. The remainder was made up of a variety of
"perineural" cells of which most were glia cells. Since they did not demonstrate any ability to generate
action potentials they were somewhat arbitrarily assigned the function of protecting and nourishing the
nerve cells proper. More interest was generated in the DC potentials from the point of view that they
may be generated by the neurones themselves. Several new investigators became involved in the area,
chiefly Bishop (8), Caspers (9), O'Leary and Goldring. Much new information was generated, all
indicating that DC potentials did play important functional roles in the activity of the brain. Caspers for
example, in 1961 measured DC potentials in unanesthetized, unrestrained animals engaged in normal
activity. He reported that increased activity, such as incoming sensory stimuli and motor activity, were
associated with negative potentials, while decreased activity such as sleep was associated with positive
DC shifts. Caspers proposed that these DC changes could be of diagnostic value similar to the EEG, if
they could be measured with precision.
ELECTROMAGNETISM & LIFE - 23