The Nervous System
Not all neurophysiologists were convinced that the simple nerve impulse or action potential was the sole basis of all nervous system function. While it could not be questioned that this mechanism did exist and did furnish an adequate basis for the transmission of information in a single neurone, many problems remained unanswered. Most important was the question of how all of the neurones integrated and worked together to produce a coherent functioning brain (perhaps the whole was greater than the sum of the parts). While most basic scientists avoided such questions, the clinical neurologists were convinced that something was lacking in the action potential only concept.
In the 1940's Gerard and Libet reported a particularly significant series of experiments on the DC electrical potentials measurable in the brain. (5).
Fig. 2.1. Schematic representation of a typical single neurone layer of cells in the frog brain used by Libet and Gerard to measure DC electrical gradients. Their concept of the DC gradient along a single neurone is shown in the lower figure. It is likely that the polarity was the result of the neurone being removed from the brain as the polarity of the neurone intact and in the nervous system is opposite to what they found.
In frogs, for example, some areas of the brain are only one neurone thick but are composed of many such neurones oriented all in one direction. In such areas steady or slowly varying potentials oriented along the axonodendritic axis were measured. These potentials changed in magnitude as the excitability level of the neurones was altered by chemical treatment. In other experiments using isolated but living frog brains, they found slowly oscillating potentials and "traveling waves" of potential change moving across the cortical layers of the brain at speeds of approximately 6 cm per second. These waves (which could be elicited by the application of a number of drugs, such as caffeine, that increased the excitability of the individual neurones) had some very important properties. If a cut was made on the cortical surface and the edges separated, the traveling wave could not cross the cut. 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.
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 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 postulate for the total field) was not well described by either worker, although Lund in one sentence comes close to a solid-state electronic idea when he mentions "electron transfer across the cytoplasm (in chain molecules)."
The work of Burr and Lund (as well as that of other workers) was mainly ignored by the scientific community. Their measurements were suspect due to insensitive and artifact-producing instrumentation, the potentials they measured were far below the "shock" level, and their theoretical concepts were "fuzzy," hinting at the now discredited vitalism. The fact still remained however, that they measured steady-state potentials on the surface of animals correlated with functional changes, very similar to the measurements made in the brain by the neurophysiologists. The usual explanation-that these were second-order phenomena, byproducts of underlying cellular metabolism-was unsatisfactory from a number of aspects. First, it was not clear how such metabolic activity was translated into electrical potentials, and second, a number of investigators had demonstrated that applied currents (well below the level of heating) did influence general growth patterns in a nonrandom fashion.
Thus by the mid-1950's serious doubts began to be expressed concerning the ability of the Bemstein semipermeable membrane hypothesis to explain all observed bioelectric phenomena both within the central nervous system and in the body as a whole.
In 1960 we repeated Burr's measurements of the DC field on the surface of the intact salamander using, however, the much more stable and sensitive instrumentation then available. Rather than a simple dipole field, we found a complex field pattern with an obvious relationship to the underlying anatomy of the central nervous system (17). Positive areas on the skin surface overlay areas of cellular aggregation with the CNS, such as the brain and the brachial and lumbar enlargements of the spinal cord, while the nerve trunks were increasingly negative as they proceeded distally away from the spinal cord. This suggested that the potentials were related to the DC potentials of the CNS rather than being generated by the total activity of all the cells of the organism. An immediate question was whether current flowing within such a structure embedded within the volume conductor of the body could produce such a field pattern on the surface of the animal. We found that when a CNS analog (built of copper wire with solder junctions as generating sources at the brain and spinal cord enlargements) was placed within a volume conductor of the same size and relative shape as a salamander, the same pattern of potentials was measurable on the surface as was measured on the living salamander. This indicated that the total CNS could be the source of the field potentials, but it did not confirm that it had such an activity.
Fig. 2.2. Plots of the surface DC electrical potential on the surface of the salamander as reported by Burr (left) and found by us (right). The central nervous system is diagrammed in the center. The relationship between the complex field and the nervous system is evident.
All of the previous neurophysiological studies on DC potentials had been made on the brain. The existence of similar electrical phenomena in the peripheral nerves could only be conjectured, but it was a necessity to relate the measured surface potentials to the total CNS. To investigate this further we measured the DC potentials along 1 cm segments of various peripheral nerves (18). Again, reproducible DC voltages were found; however, their polarity appeared to be dependent upon the direction of the normal nerve impulse travel. Sensory nerves were polarized distally positive, while motor nerves were polarized distally negative. Combined nerve trunks, with both sensory and motor components, demonstrated polarities and magnitudes of potentials that were related to the arithmetic addition of the potentials associated with each component. (It should be remembered that while all peripheral nerves are called axons, only those that are motor are truly so; the sensory fibers are in reality dendrites carrying information centrally.) These measured polarities seemed to indicate that each complete neurone was polarized in the same direction along its axono-dendritic axis. These observations almost exactly paralleled those made by Libet and Gerard on the cerebral neurones 30 years before. Thus the body surface fields measured by Burr and Lund, rather than being the result of the electrical activity of all the cells, appeared to be associated with some DC activity of the entire nervous system. The electrical potentials measured longitudinally along the peripheral nerves paralleled in magnitude the general state of the CNS "in toto," (i.e., they diminished with anesthesia and with section of the spinal cord). Since they could be measured constantly during periods of relatively normal CNS state, it was postulated that they had to be generated by a constant current flow.
Fig. 2.3. Measured DC electrical potentials along segments of peripheral nerves of the frog. The sensory nerves were found to be distally positive while motor nerves were distally negative. The arrangement of motor and sensory nerves in the typical reflex arc is shown in the center diagram. The conclusion reached is that neurones have an overall longitudinal electrical polarization as shown on the left.
As a means of further substantiating this idea and possibly determining the type of charge carriers involved, we observed the effect of freezing a segment of the nerve located between the two measurement electrodes. Charge carriers of the type proposed by Szent-Gyorgyi (electrons in a semiconducting lattice) would be enhanced in their movement by freezing, resulting in an increase in the current, while movement of ions would be inhibited by the freezing, causing the current to decrease and the potentials to decline. The experiment was carried out on the sciatic nerve of the bullfrog and enhancement of the voltage was found each time the segment of the nerve was frozen. If the voltage gradient was dropped to zero by section of the spinal cord or very deep anesthesia, freezing of the segment between the electrodes did not produce any measurable voltage. The freezing effect therefore seemed to be genuine and not the result of any artifact of the freezing process. However, the experiment, while suggestive, was not conclusive proof of the existence of a longitudinal current associated with the peripheral nerve fibers. A sensitive technique was required that could provide unequivocal evidence of the existence of the current while minimizing exposure and manipulation of the nerve. It seemed that the Hall effect could be such a technique.
The Hall effect consists of exposing a current-carrying conductor to a magnetic field oriented at 90° to the axis of the conductor. The field will produce some deviation of the charge carriers which can then be sensed as a steady voltage at the second 90° axis to the conductor. This is called the Hall voltage and its magnitude is very dependent upon the degree of mobility of the charge carriers (being almost undetectable with ionic currents, only slightly more detectable with electron flow in metallic conductors, but easily detected with semiconducting currents due to the high mobility of their charge carriers). Since the nerve currents were obviously not metallic conduction and since the detection of ionic Hall voltages was far beyond the capability of our equipment, we reasoned that if any Hall voltages were observed, they would indicate not only the existence of the current but also that it was probably analogous to semiconducting current.
The experiment was performed using the foreleg of the salamander as the current-carrying conductor (the nerve was not exposed, the intact limb being contacted only by the two soft measuring electrodes to obviate any injury effects). In 1961 we reported observing Hall voltages under these circumstances (19) . The magnitude varied inversely with the state of anesthesia of the test animal, indicating that the voltages were real, not artifactual, and directly related to the operational state of the CNS. There was now strong evidence that some component of the CNS generated and transmitted direct currents that produced the measurable voltages on the surface of animals, including humans.
It remained to determine if these voltages were related to the functions, as described by Libet and Gerard for the nervous system and Burr and Lund for the total organism. In our measurements of the surface potentials we had noted that they varied in a definite pattern with the level of consciousness of the subject, particularly a midline, occipito-frontal voltage vector across the head, which seemed to accurately reflect the level of anesthesia. In the conscious subject this vector was frontally negative, diminishing in magnitude and going positive as anesthesia was induced and deepened. This voltage on the skin surface exactly paralleled that observed by Libet and Gerard and by Caspers in the brain and we postulated that this current vector represented current flow in median unpaired structures of the brainstem area. If Libet and Gerard and Caspers were correct, this current should be the determiner of the level of excitability (hence consciousness) of the subject, and electrically reversing the normal frontally-negative potential should induce the same loss of consciousness as chemical anesthesia. Again using the salamander, we observed that currents as small as 30 µamp administered in this direction produced loss of consciousness and responsiveness to painful stimuli. In addition, they produced in the subject animal electroencephalographic patterns typical of the anesthetized state (high amplitude delta slow waves) (20). In the converse experiment, attempting to restore consciousness to a chemically anesthetized animal, the EEG evidence was suggestive but much less convincing.
Fig. 2.4. The effect of direct current administered longitudinally (fronto-occipital) through the brain of the salamander. Upper: the electroencephalogram when the current is oriented frontally positive (opposite to the normal awake pattern). The EEG demonstrates a typical delta wave pattern of deep anesthesia, with the magnitude of the delta waves proportional to the current magnitude. The animal demonstrated all the clinical signs of anesthesia. Lower: the reverse experiment. A deeply anesthetized animal is exposed to current oriented in the normal awake direction. Much more current is required to produce an objective response and while the animal did not recover consciousness as a result of the current passage, the EEG pattern did show signs (alpha waves) of an awake pattern.
Fig. 2.5. The effect of a magnetic field applied across the head of a salamander in a bitemporal direction. The interaction between the field and a longitudinal electrical current in the brain (if one was present) would lead to a decrease in the total current delivered along the original fronto-occipital vector. It was predicted that this would produce a state of anesthesia, and at the level of 3000 gauss the EEG pattern became one of deep anesthesia with delta wave forms This was accompanied by a loss of response to painful stimuli.
If, as these experiments seemed to indicate, there was a DC current flow organized in this fashion in the brain it should be of the same nature as the peripheral currents, and it should be subject to the same type of interaction with an applied magnetic field. The anatomical complexity of the head and brain ruled out the possibility of observing an unequivocal Hall voltage, but if the magnetic field were very high in strength and precisely oriented 90° to the fronto-occipital vector, then a sufficient number of charge carriers might be deviated from the original current vector to produce a significant decrease in the normal current along the fronto-occipital vector; possibly even sufficient to produce loss of consciousness. Even though fields of several thousand gauss were found to be necessary to produce the effect, the results were unequivocal. At field strengths exceeding 3000 gauss, the animals were not only nonresponsive to painful stimuli but they also demonstrated the large, slow delta wave patterns typical of deep anesthesia.
As mentioned earlier, the surest sign of actual electrical current flow in any biological structure would be the detection of the resulting magnetic field in space around the structure. Technology had long been quite inadequate to detect the extremely weak fields predicted. The first detection of a "biomagnetic" field was in 1963 by Baule and McFee who used classical techniques (a coil of million turns of wire and a ferrite core) to detect the field associated with the action of the human heart (21). The field intensity was five orders of magnitude lower then the earth's norma! field and it was necessary to conduct the experiment in a rural area as free of extraneous fields as possible. Two years later, Cohen, using the same technique coupled with a signal averaging computer, found evidence of a magnetic field around the human head of even weaker strength (about eight orders of magnitude lower than earth's normal field).
The invention of the SQUID (a superconducting quantum interference device based upon the Josephson junction) permitted detection of these and even lower intensity fields with relative ease. It was first applied by Cohen in 1970 for a more complete detection of the human magnetocardiogram, and in 1972 he reported that the magnetic field in space around the human head demonstrated a wave form pattern, similar to, but not identical with, the EEG as measured by skin electrodes (22). Cohen called this the magnetoencephalogram or MEG. Since then improvements in the technology have permitted detection of the magnetic fields associated with evoked responses, and correlations between the EEG and the magnetoencephalogram have shown that the low-frequency components of the EEG are well represented in the MEG, but that usually the high frequency components (i.e., sleep spindles) are missing (23, 25) . Most recently it has been possible to record the magnetic field associated with the nerve impulse itself, again using the SQUID and certain specialized techniques necessary because of the extremely small field strength (26).
Originally, it was considered likely that the MEG arose from the simultaneous action potential activity in large numbers of neurones arranged in parallel. However, this same concept has been advanced as the explanation of the EEG and as yet it remains unsubstantiated. While some aspects of the MEG are compatible with this concept-the magnetic evoked response for example-others have provided evidence supporting the existence of DC currents in the brain. The orientation and structure of the total magnetic field around the head is most compatible, according to Reite and Zimmerman (24), with a "longitudinally oriented current dipole within the head"-a concept identical to that proposed by Libet, Gerard and Caspers on the basis of their DC measurements in the brain and by us, based on our surface measurements and the results of low level direct currents injected along the same vector. Furthermore, the retina of the eye is actually a direct extension of the brain and it demonstrates a number of DC electrical phenomena as previously mentioned. The electroretinogram seems to be generated by a steady dipole field extending from the retina to the cornea. A DC magnetic field has been detected associated with it, indicating that an actual current flow is occurring along the vector. Most recently, DC magnetic fields have been detected from the brain itself. The evidence provided by the MEG has not only supported the much older concepts of DC activity in the brain outside of the neurones, but it has stimulated renewed interest in the entire area.
Taken all together, the evidence seems to be quite conclusive that there are steady DC electrical currents flowing outside of the neurones proper in the entire nervous system. The currents appear to be nonionic in nature and there is some evidence that they may be similar to semiconducting type currents. At present the perineural cells appear to be the most likely site in which the currents are generated and transmitted. One apparent function of the currents is that of governing the level of activity of the neurons proper; that is, the currents, via their polarity and magnitude, exert a biasing effect upon the neurone's ability to receive, generate and transmit action potentials.
Because the CNS pervades the entire body, these currents produce an organized total body field detectable with surface electrodes. It is not unlikely that this phenomenon constitutes a system for the transmission of very basic type data, and that it may well provide the integrative function postulated by Libet and Gerard on experimental grounds and by von Neumann on theoretical grounds. Thus the CNS can be viewed as a two-component system, with the DC system being the primitive, analog portion, possibly located in the perineural cell system (glia, Schwann cells), and the action potential system of the neurones proper being the more sophisticated but limited, digital system. This concept would seem to provide a fruitful frame of reference for further investigation of such higher nervous functions as memory, consciousness, and perception.