the sensory receptor itself seems to be an excellent example of an analog transducer, with the generator
potential being the DC output signal.
While the generator potential is often postulated to be produced by ionic movement through a
semipermeable membrane as in the action potential, this view is not supported by the same kind of data
as for the action potential. There are, for example, several conditions that abolish the action potential
and leave the generator potential undiminished (e.g., low concentrations of tetrodotoxin and reduced
sodium concentration in the tissue fluids around the receptor). In addition, the electrical response of the
Pacinian corpuscle is quite unusual. Not only is it graded and nonpropagating, but it is also biphasic,
with a potential of one polarity and magnitude upon application of the pressure and a potential of equal
magnitude but opposite polarity upon release of the pressure. This is an action usually associated with a
piezoelectric material that will be discussed later in this chapter. In addition, Ishiko and Lowenstein
have been able to demonstrate that the rate of rise and the amplitude of the generator potential of a
Pacinian corpuscle increased markedly with temperature while such a temperature increase had no
effect upon the action potential of the associated nerve fiber (16). Such temperature sensitivity is one of
the characteristics of a solid-state electronic process.
The situation in regard to the eye is particularly complex, involving a change in state of the
visual pigment as an intermediary step in the light-sensing process. In addition, the eye demonstrates a
steady (DC) corneo-retinal potential (electroculogram) and a DC potential associated with the
impingement of light on the retina (electroretinogram). These phenomena are also not well understood
and similarly difficult to explain on the basis of the ionic hypothesis.
Thus by the mid-decades of this century, much new evidence had been obtained indicating that
both the anatomical complexity and the electrical activity of the brain were much more complicated
than first thought when the nerve impulse had been discovered. It seemed quite possible that Libet and
Gerard had been right when, in the 40's, they described electrical currents of nonionic nature flowing
outside of the neurones of the brain. To some investigators this appeared to be a mechanism of coding
and data transmission that related to the problem of integrating the entire activity of the brain. Support
for this view came from theoretical analysis of the nervous system by the cyberneticists. It was evident
to von Neumann that the action potential system was in essence a digital type information system,
similar to the binary coded computers (2). Analyzing the functions of the brain, he concluded that this
system alone was inadequate to explain brain functions and theorized that there had to be an underlying
simpler system that regulated large blocks of neurones grading and regulating their activity. He again
seemed to propose an analog system similar to that of Libet and Gerard.
While these studies were going on in relation to the DC electrical activity of the brain and its
integrative function, other investigators were working on the integration of the total organism and were
convinced that a similar DC electrical system was in operation (surprisingly, they seemed to be
unaware of the work of the neurophysiologists and the support that it would have given them). Lund at
the University of Texas (3) and Burr at Yale (4) published many articles in the 40's and 50's reporting
electrical measurements on the surface of a variety of intact living organisms which could be correlated
with a number of physiological variables. Both investigators arrived at the concept of a "bioelectric" or
"electrodynamic" field; a DC potential field that pervaded the entire organism, providing integration
and direction for morphogenetic and growth processes, among other functions. The fields they
observed were simple dipoles, oriented on the head-tail axis of the animal and they considered the
source of the field to be the summation of the individual fields of all of the cells of the organism. While
they conceived of currents flowing within the cells, they excluded total currents of any organized
nature existing outside of the cells. The source of the internal cellular current (which was a necessary
ELECTROMAGNETISM & LIFE - 25