Protoplasmic Action and Nervous Action

We have seen that an irritable system with a highly developed general sensitivity, e.g., a muscle or a nerve, may be excited by a variety of stimulating agents, and the question arises why such physically dissimilar agents produce the same physiological effect. The general sequence of events when such a tissue is stimulated may be briefly described as follows. Some local change, whose precise nature is determined by the nature of the stimulating agent, occurs at the site of stimulation; a state of ''excitation" is there initiated which, however, does not remain confined to this region, but spreads or is propagated to a distance, often at a high velocity.

This propagated effect, or '' propagated disturbance" (Keith Lucas' term), as it appears at a distance from the point of stimulation, has its own definite peculiarities, which are independent of the nature of the initiatory local change or stimulus. For example, any single nerveimpulse in a normal frog's nerve, by whatever means it is initiated, travels at a constant velocity (assuming the state of the tissue to be normal and the temperature and surrounding conditions constant), and its most readily observed physical accompaniment, the bioelectric variation, has a definite range of potential change and definite time-relations. In other words, the propagated disturbance differs from the local change in exhibiting constant and specific features, qualitative and quantitative, whose nature is determined by the special or inherited constitution of the tissue.

Almost any kind of sufficiently rapid local alteration may initiate such a wave of physical and chemical disturbance. The distinction between the local change and the propagated effect is a fundamental one in any theory of stimulation. The former is the ''releasing" event and follows upon some simple physical change produced in the tissue by the stimulating agent; the latter is the distinctively physiological process, and as such has specific characters of a complex kind, dependent on the nature of the irritable system and as yet imperfectly analyzed.

Especially significant is the fact that all irritable elements, apparently without exception, respond to electrical stimuli, or are influenced in their already existing activity by the electric current. The electrical sensitivity of highly irritable tissues, such as vertebrate motor nerve, is extreme; the frog's sciatic nerve may be stimulated by a current of .000001 ampere or less; it is well known that the neuro-muscular apparatus of the frog was used by Galvani as the most sensitive means known to him by detecting variations in the electrical state of bodies; in fact it is to this property of living tissues that we owe the discovery of current electricity by Volta. Galvani's experiments also showed • — although their significance was disputed at the time — that electric currents are produced in the activity of living tissues. Thus two of the most fundamental properties of living matter, its electrical sensitivity, and its production of electrical currents during activity, were early observed; and many of the chief problems of general physiology at the present time relate to the physico-chemical conditions and physiological significance of these properties. That they are among the chief factors controlling normal cell-processes seems certain.

It is remarkable that complete stimulation, involving a change in the activity of the entire cell, is produced in many if not all irritable elements by agents which affect directly only the surface-layer of protoplasm. Some local modification of surface conditions seems to be all that is required to set in motion the whole complex process of stimulation. This is best shown in the mechanical stimulation of single cells; thus in a ciliated protozoon like Paramcecium a slight touch is sufficient to call forth the characteristic motor reaction, involving a reversal of the direction of ciliary activity over the whole surface of the organism. The extraordinary

sensitivity of many blood cells to mechanical contact illustrates the same phenomenon; a slight touch with a capillary needle is often sufficient to cause a rapid and complete disintegration of a leucocyte or a red blood corpuscle;^ i.e., a wave of alteration involving the " breakdown of the semi-permeable surface-film is propagated over the entire protoplasmic surface. The breakdown of the explosive corpuscles in Crustacea^ and of nematocytes in coelenterates are other examples of the same kind of process. Such facts suggest that in normal cases of stimulation, as in nerve, where a local stimulus initiates a temporary disturbance, which passes like a wave over the entire irritable element, a similar disintegration of the surface-protoplasm occurs, with the difference that this change is immediately and rapidly reversed by the formation of a new surface layer.

That some such process occurs during the transmission of the excitation-wave in a nerve or other irritable tissue is indicated by the character of the local bioelectric variation. A reversible surface-change is known to accompany the transmission of the activationwave along a passive iron wire immersed in nitric acid; the passivating surface-film is removed by electrolytic reduction in the neighborhood of each active area, and is then immediately reformed by the oxidizing action of the acid and of the local electric current (at the anodal areas); and the destruction and re-formation of the film are associated with definite and rapid variations of potential. In the stimulated living system a similar

reversible variation of potential accompanies the passage of the excitation-wave. The resemblance of the transmission phenomenon in passive iron to the protoplasmic type of transmission is in fact so detailed as to confirm strongly the hypothesis that in the latter case also the essential feature of the transmission-process is the breakdown and reconstruction of the thin protoplasmic surface-film under the influence of the local bioelectric circuits.

If the processes in the living system and in the simple inorganic model are in fact similar in their dependence on surface-changes of this kind, it becomes evident at once why protoplasm is so readily excited by conditions acting upon its surface layer. A mechanical agent, by interrupting the continuity of the surface-film, or by otherwise altering it so as to give rise to a local circuit, may initiate a wave of electromotor variation and disintegration which travels automatically over, the whole surface, and in so doing alters the physiological activity of the whole system. Any other sufficient local alteration (chemical, thermal, etc.) may produce the same effect. The critical factor in the local process of excitation thus appears to be the alteration of the protoplasmic surface-film in such a manner as to change locally the potential difference between the protoplasm and the external medium, to a sufficient degree and at a sufficient rate. The local circuit arising between this altered region and the as yet unaltered regions adjoining forms the next link in the chain of events; and if this local current has sufficient intensity and local density to break down electrolytically the film over a certain area of the adjoining region, an indefinite wave of propagation

may be initiated, since the same effect will be repeated at each newly formed boundary between the active and the inactive areas. According to this conception the primary action of a local stimulating agent is to change rapidly the electrical condition of the cell-surface at its point of application. The parallel between the irritable protoplasmic system and the passive iron model is obvious, since in the latter case also a local alteration of the surface-film involving a local change of potential is the initiating condition for the propagated wave of chemical and electromotor disturbance.

The present view, therefore, regards stimulation as conditioned by surface processes of the foregoing definite kind. If this is true, the structural arrangements providing for the transfer of excitation from one irritable element to another should exhibit features of a character to correspond. In fact, many peculiarities of the structure of the central nervous system — especially of the synaptic junctions — and of the structure and arrangement of nerve-endings, such as the myoneural junctions, are in harmony with this conception. Nerve end-plates spread out over the surface of the muscle cell, effecting intimate contact but not penetrating; the junctions between neurones are eft'ected by brushlike interlacing terminals, or by end-feet and similar structures which are applied to the cell surface with a closeness that apparently admits of variation. That transmission by contact, through the influence which the cell-process exerts upon adjoining processes or upon the cell body, is the chief mode of transmission in the nervous system is one of the corollaries of the neurone theory of the

structure of this system. The well-known experiments ("rheoscopic frog") in which one active muscle or nerve stimulates another which is in close contact with it, by means of the bioelectric currents accompanying activity, show that excitation can be transmitted from one irritable element to another without direct protoplasmic continuity, through a purely electrical influence. Similarly in the passive iron nitric acid system, activation is readily transmitted from one wire to another by contact, and the basis of this transmission is also electrical.

According to these conceptions, all forms of stimulation are electrical; or, more exactly expressed, electric currents resulting from local alterations of the cell surface form a necessary part of the sequence of processes constituting stimulation. Such a view implies further, since the activity of the whole cell is altered by stimulation— e.g., all of the fibrils in a muscle cell contract when the excitation-wave travels over its surface — that the intracellular processes, including the chemical or metabolic processes which furnish the energy for the activity, are largely controlled by processes having their origin at the cell surface.

This inference is confirmed by a study of the conditions of electrical stimulation. The work of Nernst and his successors^ has shown that the electric current does not act by penetrating the living cell (which in fact is a poor conductor) but by changing its surfacepolarization. And the semi-permeability of the plasma membranes of irritable cells (e.g., muscle-cells) with reference to the inorganic salts of the medium — which, without penetrating the cell, nevertheless influence profoundly its properties and activity — shows further how closely cellular activities are dependent on surfaceconditions. Among these surface-conditions the state of electrical polarization appears to be of primary importance.

The facts of electrical stimulation, now to be considered, show that variations in electrical surfacepolarization have a far-reaching control over the metabolic and other processes occurring in the cell interior. The means by which this polarization may be altered are of three chief kinds: (i) changes in the structure or composition or permeability of the surface-film; (2) external electrical influences, especially the influence of electric currents traversing the cell or its medium; and (3) changes in the composition of the medium or internal protoplasm. Since, according to the present view, an inseparable feature of stimulation, the transmission of the excitation-state, is a direct result of electrical activation by the currents of local bioelectric circuits, it is clear that the problem of stimulation resolves itself largely into the problem of the general conditions under which the electric current stimulates living matter. Electrical stimulation is in fact the primary form of stimulation.

In general the physiological studies of the last two decades have shown that the stimulation of an irritable living system by the constant electric current is subject to definite quantitative laws; they indicate also that the current acts primarily through its polarizing action. Upon this polarizing action follows chemical action as a secondary consequence. The essential conditions under which the current causes stimulation may be briefly summarized as follows :

1. The current must exceed a certain minimal intensity; this ''threshold" intensity varies widely for different irritable tissues and for the same tissue under different conditions; e.g., during states of fatigue, narcosis, sensitization, etc. 2. A current of threshold or greater intensity must traverse the tissue for a certain minimal time; its stimulating action thus depends not only upon its intensity but also upon the duration of its flow. The rule that for equal stimulating action the product of the intensity into the root of the duration is constant {il/t = K) appears to hold for most tissues within a considerable range of intensities.' For the current of threshold intensity this critical duration varies widely in different tissues (from a few thousandths to several seconds) ; it is the expression of a time-factor (chronaxie), which is specific for the tissue in question.''

These general statements apply to the case of currents which reach their full intensity rapidly or instantaneously; e.g., when the stimulating circuit is suddenly closed. Such a condition, however, is apparently not a normal or ''physiological" one, since the bioelectric current — which, according to our present conception, is the factor arousing the local excitation at each successive region in a transmitting element (nerve, etc.) — attains its full intensity not instantaneously but in a rising curve of more or less gradual slope, varying from tissue to tissue, and subsides in a similar manner. Stimulation by currents of varying intensity is thus the typical condition prevailing in the organism, and to which, therefore, especial attention must be directed. The general empirical rule governing the action of such currents is as follows:

3. In order to stimulate, a current rising continuously and uniformly from zero to full intensity must change its intensity at a certain minimal rate which is characteristic for the tissue; i.e., —= const, (assuming temperature and other conditions normal). Hence we find that a slowly increasing current may fail to stimulate, while one rising to the same intensity at a more rapid rate stimulates. The same rule applies to stimulation by the decrease of a current already flowing through a tissue. The rate of change, in either direction, must exceed a minimal value which is specific for the tissue. A time-factor enters, closely related to that already referred to above (under 2) as "chronaxie."

4. The stimulating action of the current is characteristically polar; i.e., the current produces its primary physiological effects chiefly at its regions of entrance and exit, and the effects at the two regions are typically opposite or antagonistic. Typically, when the current is made, it initiates excitation at the cathode (i.e., where the positive stream of the stimulating circuit passes from the tissue to the appHed electrode), and inhibits

activity (at the same time depressing irritability) at the anode. When a current already flowing through the tissue is broken, stimulation also results, but the polar relations are reversed; i.e., stimulation is then at the anode, inhibition at the cathode. This summarized statement is the usual form of the ''law of polar stimulation." It is important to note, however, that a polar action is seen in many other physiological processes occurring under the influence of the current; e.g., electrotonus, polar disintegration of cells, galvanotropic growth, and galvanotaxis. All of these phenomena show that the direction of the current, relatively to the cell surface, determines the nature of its physiological action. The parallel to electrolysis, at the surface of any electrode, is especially clear in phenomena of this class; it is well known that where the positive stream passes from the metallic electrode to the solution (at the anode) it produces chemical effects (in general of an oxidative kind) of the reverse nature to those produced where it passes from solution to electrode (cathode); here the general chemical action is reducing.

5. That a variation in the electrical state of the irritable elements, sufficient in degree and rate, is the determining factor in the physiological action of the current is seen in the fact that a change in either direction, i.e., make or break, increase or decrease, may stimulate or produce other characteristic physiological effects. 6. Finally, summation effects are highly characteristic; i.e., two or more electric stimuli (induction shocks) which, acting singly, are ineffective, may cause stimulation if sent in sufflciently rapid succession into the tissue. The interval between the successive single stimuli must

be less than a certain critical time, or no summation results. This ''summation time" is closely related to the characteristic time-factor of the tissue, being brief in tissue with brief chronaxie and vice versa/ Since the stimuli normally acting in the intact organism are largely repetitive or rhythmical, summation processes are of special physiological interest. An exhaustive discussion of the effects of electricity on living organisms is not possible within the limits of space; but those features of electrical stimulation which indicate its dependence on surface-alteration are of fundamental theoretical significance and will be considered in some detail.

Chief among these features are the time-relations of electrical stimulation. A stimulating current of a given intensity must flow uninterruptedly for more than a certain time through the tissue or it produces no apparent efl'ect. When the relations between the intensity of a stimulating current and its minimal duration are investigated, a highly characteristic relation appears, indicating that the action of the current depends upon the transport of ions to or from the semi-permeable surfaces of the irritable tissue. The resulting change of electrical surface-polarization forms the primary condition of stimulation. This was first clearly shown by Nernst,^ in a paper on the relation between the stimulating action of alternating currents and the rate of alternation. In a living tissue, wliich, considered from a simplified physico-chemical point of view, represents an electrolyte solution partitioned by membranes not readily permeable

to ions, there is during the flow of the current a movement of cations with the positive stream and of anions with the negative stream; at the semi-permeable membranes interposed in their path the movement of ions is impeded ; the cations then undergo an increase of concentration at those surfaces of the membranes which face toward the anode, simultaneously w^ith a decrease of concentration at the opposite faces; the reverse relations hold with the anions. A gradient of ionic concentration is thus set up between the layer of solution in immediate contact with the membrane and the layer at some distance. This process of concentration at the semi-permeable surface continues until a condition of equihbrium is reached at which the rate of diffusion back from the membrane into the interior of the solution is equal to the rate at which the ions are transported to the membrane. Assuming the existence of these two opposed processes, transport to the surface by current and backdiffusion, it can be shown that to produce a definite change in concentration at the surface, the product of the current-intensity into the root of its time of flow should be constant {i\/t=K).

This result was reached by Nernst from the consideration of the case of a single membrane interposed in the path of a current. It is evident ,that such a system offers conditions much simpler than those of an irritable tissue, which typically consists of a bundle of cells or fibrils. HilP and Keith Lucas^ have pointed out that in considering the case of the living cell or nerve-fiber, with its small linear dimensions, it is necessary to take into account the processes at the two opposite surfaces of each element, since if these surfaces are close enough together, there is mutual interference of the two processes, and more complex conditions have to be assumed. Yet the fundamental condition assumed by Nernst's theory — a membrane partitioning an electrolyte solution — exists in the living tissue, hence polarization effects must result when a current is passed; and it has been found that the foregoing law relating the current-intensity to the duration required for a constant polarizing effect holds true also for the stimulating effect of the current within a considerable range of intensities and durations, and especially for higher intensities. This general result, that polarizing effect and stimulation run closely parallel, indicates that stimulation is a consequence of the polarizing action of the current.

Hermann' and others had previously referred the stimulating action of the current to its polarizing action, since in any tissue the presence of a reverse current (polarization current) can always be demonstrated immediately after the passage of a brief constant current. A high degree of polarizability is characteristic of living tissues, and this peculiarity is undoubtedly dependent on the semi-permeable properties of the cell membranes, since the polarization current is greatly diminished at death, at which time the membranes lose semipermeability, as already pointed out. Both polarizability and semi-permeability are thus manifestations of the same condition, hindrance to diffusion of ions. Lapicque has shown that the polarization currents obtained from dead partitions (parchment or bladder

membranes) are related to the intensity and duration of the polarizing current in the same manner as the stimulating effect of a current traversing a living irritable tissue; i.e., to produce a constant polarization current, the product of the intensity of the polarizing current into the root of its duration must be constant.' It thus appears certain that the primary or initiatory process in electrical stimulation is the production of a certain critical degree of polarization at the semi-permeable membranes of the irritable tissue. In other words, the current stimulates by means of its polarizing action, i.e., by producing a potential difference (or by altering an already existing potential difference) between the external and the internal faces of the semi-permeable plasma membranes.

It should be noted that in itself this result throws little light upon the special physiological nature of the stimulation-process; it merely defines the physical conditions under which this process is initiated. The process itself, as just pointed out, has its specific peculiarities which are independent of the nature of the exciting agent. It is, however, an important theoretical advance to recognize that polarization changes are involved in all forms of stimulation. That this is the case is further shown by the invariable participation of bioelectric currents in stimulation processes^; these currents, like any others traversing the tissue, must cause changes of polarization at the cell surfaces. According to the present view, the spread of excitation is due to the secondary stimulation-effects resulting from the polarizing action of such currents.

We are thus led to consider the kinds of effect which changes in the electrical polarization across the cell surface may have upon chemical processes occurring in this region. It should first be noted that various phenomena occurring at metallic surfaces and involving electrolysis have been shown to follow the same '' square root law" as the stimulation process. Bredig and Kerb found this to be true for the influence of alternating currents in initiating the characteristic rhythmical action in the mercury hydrogen peroxide system, which, as we have seen, resembles closely the passive iron model in its mode of activity; the same was found by Wilke and Meyerhof in the electrolytic oxidation and reduction of chromic salts and chromates at platinum electrodes.'