Protoplasmic Action and Nervous Action

approaching each other from opposite directions undergo mutual extinction where they meet. It is clear that the processes determining transmission are in some manner compensated or nullified where the waves meet; and since each wave is associated with a bioelectric circuit, and the two circuits, being equal and oppositely oriented, must physically compensate each other when superimposed, it is to be expected that any transmission of electric influence and hence of stimulating effect beyond their intersection will be impossible. The blocking of excitation-waves at regions of injury is probably to be explained in a similar manner, as due to compensation of the action current by the injury current.^

The question of the part which interferences of this kind play in the intact living organism (e.g., in the central nervous system) is an open one. Recently there have been attempts to explain physiological interferences such as those of reciprocal inhibition, the Wedensky phenomenon, vagus inhibition, etc., on the basis of a lack of correspondence between the normal rhythm of response and recovery in the receiving irritable system and the rhythm of the series of nervous impulses entering it from without.^ The Wedensky effect is an example of such a phenomenon, the failure of the muscle to respond after the initial contraction depending on the coincidence of periods of increased decrement in the motor end-plate with the periods at which the

^ Mayor refers the mutual extinction of intersecting waves to the refractor}^ phase of the tissue; "tissue which has been in contraction cannot again contract until after an appreciable interval of rest." Electrical compensation must, however, also be present, as in the analogous cases of anelectrotonus and blocking at a region of injury. 2 Keith Lucas, Conduction of the Nervous Impulse, chaps, xii, xiii. successive waves of innervation are received. At each stimulation the decrement in the end-plate is increased temporarily. According to the degree of decrement an impulse received by the end-plate may penetrate the latter and reach the muscle cell with its normal ''intensity," or it may undergo a decrease which may entirely prevent penetration. A grading of the intensity of innervation, depending on the degree of coincidence of phase in the natural rhythms of the two interconnected systems, is thus possible. Forbes has recently appUed this conception to the case of sensory stimulation, and has* expressed the view ''that an unhmited range of sensory graduation might be based on the frequency with which the impulses follow one another in the sensory fibers."^ All of these possibilities should be fully studied and investigated, with a recognition of the probable dependence of excitation and transmission upon the bioelectric processes in the irritable elements. Further discussion of this problem is not possible in this place.^ The role of the bioelectric currents in other physiological processes (growth, cell-division, secretion, etc.) is only beginning to be understood, and much further investigation is required. It seems clear, however, that by means of these currents physiological influence may be transmitted rapidly to a distance in many protoplasmic systems other than nerve; and that correlations of activity and function may thus be effected which would otherwise be impossible. Appar-

' See Sherrington, "Some Aspects of Animal Mechanism," Nature, CX ently this general condition is illustrated in the phenomena of growth and development. There is evidence that many cases of form-correlation in both plants and animals have an electrical basis. Actively growing regions of organisms have usually been found electrically negative to more slowly growing regions. Hermann and Miiller-Hettlingen' observed that in seedlings the regions near the growing zones (terminal buds and roottips) were negative to those near the cotyledons; similarly the growing zones of planarians and annelids are negative to intermediate regions,^ and in hydroids regenerating hydranth heads are negative to the stems.^ In general the indications are that regions of active constructive metabolism — which are usually regions of active oxidation — are typically negative to less active regions. Miss Hyde's observations on fish eggs'* indicate that during cell-division the cell body undergoes a temporary negative variation, analogous to that accompanying excitation; the electrical negativity of regions in active proliferation may thus be accounted for.

The presence of bioelectric circuits between the rapidly growing regions of an organism and the regions adjoining is in all probability an important if not the chief factor in the controlling influence (''physiological dominance") which the former exerts upon the latter. In plants it has long been known that the removal of growing regions (terminal buds) initiates growth in the dormant regions adjacent (axillary buds, etc.); any arrest of growth by cold, anaesthetization, or removal of oxygen has the same effect.' Such facts show that pronounced activity of growth-processes in one region in some manner involves repression of similar processes in neighboring regions; and this effect has been shown in certain cases to be independent of the transport of special growth-inhibiting substances between the two regions.^ A similar inhibitory influence of one embryonic area on another is seen also in the development of animals.^

The hypothesis that the essential basis of this growthcontrolhng influence is electrical is consistent with the observation that in certain organisms the rate and direction of growth can be experimentally controlled by electric currents. We have seen that in hydroid stems the growing or regenerating hydranths are negative relatively to other regions of the stem and to the stolons. Apparently, therefore, a normal accompaniment of growth is the passage of electric currents in a constant direction through the organism; and the direction of the current through the galvanometer shows that the positive stream enters the living system (from the exterior) at the regions where growth is most rapid. If electric currents are in themselves a factor in the

growth process, it is to be expected that their passage through the organism from without should either promote or inhibit growth according to the direction of flow. Lund has in fact recently shown that the regeneration of new polyps from the cut stems of the hydroid Obelia may be controlled by weak electric currents passed lengthwise through the stems ;^ the formation of hydranths is promoted where the current passes so as to enter the protoplasm from the medium — i.e., at the cut end facing the positive pole of the battery — and inhibited at the other end. The normal polarity of a stem can thus be reversed by passing the current, a result in agreement with the view previously expressed by Mathews that morphological polarity in these organisms has an electrical basis.^ Bose has also found that the electric current influences growth processes in the higher plants in a polar manner, the anode enhancing and the cathode depressing the normal rate.^. Recently Ingvar has reported experiments in which the outgrowth of processes from embryonic nerve cells is influenced in a directive manner by the passage of weak currents through the culture medium. Here also a polar influence is seen, the processes growing toward the anode differing morphologically from those growing toward the cathode.^ All of these facts show clearly that growth processes resemble the processes of stimulation in irritable tissues in being subject to electrical control; further that in this

4 Ingvar, Proceedings of the Society oJ Experimental Biology and Medicine, XVII (1920), 198. control the polar influence is all-important.' If what is true of artificial electric currents is true also of the currents produced by the Kving system in its own activity, the conclusion seems unavoidable that the bioelectric currents exert a controlKng and co-ordinating influence in normal growth processes as well as in normal stimulation.^ Perhaps the most important general inference to be drawn from such experiments is that the electric current under appropriate conditions has a direct promoting influence on the synthetic reactions in Uving matter, i.e., those underlying grov/th and repair, as well as on the reactions involving oxidation and decomposition which yield the energy for normal activity. Many years ago, before the development of modern physical chemistry, Hering reached certain general conceptions of the relation of the current to protoplasmic action resembling closely in many respects those reached as the result of our present analysis.^ Hering regards the bioelectric currents as essentially an index of chemical

* In a recent paper, I have called attention to a number of analogies between organic growth and the growth of precipitation-structures on metals (Zn, Fe, Co, etc.) in ferricyanide solutions; control by electrical conditions is also characteristic of such precipitation-growths (Biological Bulletin [1917], loc. cit.; cf. pp. 162 ff.). 2 According to Kappers, the direction of outgrowth of nerve-tracts in the central nervous system is the expression of a directive electrical influence (analogous to galvanotropism) which he caUs "neurobiotaxis." Cf. Kappers, "On Structural Laws in the Nervous System," Brain, XLIV (192 1), 125, and earlier references there given. Cf. also Child's discussion in his Origin and Development of the Nervous System, University of Chicago Press (1921), chaps, x, xi.

reactions occurring in the protoplasm; conversely, electric currents passing through protoplasm from without alter its activity through their direct influence on its chemical processes. Hering also concludes, from the contrast between the physiological effects at the two electrodes, that where the current enters protoplasm from the surroundings it induces or promotes assimilatory (anaboHc) processes, and where it leaves, dissimilatory (catabolic) processes. In the typical irritable tissue the inhibitory effect at the anode is the expression of a predominance of anabolic processes, while the stimulation at the cathode results from chemical effects of the reverse kind (predominantly catabolic).

From our present point of view the above-described influences on growth and regeneration point to the conclusion that where the current enters the protoplasmic surface from the exterior it has the effect of promoting oxidation processes which form secondarily the condition of the syntheses required for the formation of new structure. Growth, repair, and recovery from stimulation are the result or expression of chemical reactions of the same general kind, apparently oxidative syntheses, which occur predominantly at the one polar region. At the other polar region reactions of the reverse kind are promoted; these form the condition for stimulation in irritable tissues or for cessation of growth or degression in growing regions.

The closest physico-chemical analogies to such processes are furnished by the chemical effects at electrodes, i.e., the phenomena of electrolysis; and the possibihty that electrolysis may underHe the physiological effects of the electric current was in fact early recognized by Du Bois-Reymond and other physiologists.^ Evidently the fundamental problem to be solved is the problem of the physico-chemical basis of electrical sensitivity in living matter. The various facts and considerations reviewed in the foregoing chapters indicate that this basis is to be found in the characteristic physical structure of protoplasm; in other words, that the polyphasic and film-partitioned feature of the system is the condition which makes it possible for the electric current to produce such definite chemical effects. The polar action of the current is a clear analogy to electrode action, and we have to inquire into the justification of regarding the protoplasmic surfaces as having properties like those of electrode surfaces in general. At the surface of a metalKc electrode, chemical action occurs when the current passes between metal and solution; and apparently the same occurs when a current passes from one protoplasmic phase to another; e.g., during electrical stimulation from the surface of the plasma membrane to the adjoining medium. As we have seen, the passage of the current (positive stream) in this direction is the condition of stimulation in an irritable cell or nerve fiber.

The parallels between the surface of a metallic electrode and the surface of a living cell or organic membrane have already been discussed in part, and certain resemblances have been pointed out. It remains to be seen whether the two are similar in the further respect that the passage of an electric current across ^ Cf. Du Bois-Reymond, Untersuchimgen uber tierische Elektricitat, II, 387 ("galvanische Reizung ist uns nichts mehr als die erste Stufe der Elektrolyse eines Nerven").

the surface is in both cases attended with chemical change. The chief pecuharity of the chemical reactions occurring under the influence of electric currents in electro-chemical circuits is that they are confined to the boundary region, where the current passes between the electrode and the electrolyte solution. No chemical change occurs in the interior of either phase; the reactions are surface reactions. Since the region of transition from the metallic conductor to the electrolytic conductor is the only region in the circuit where the passage of the current involves chemical change, the fundamental question relates to the general physical nature of the conditions in this region while the current is flowing. According to modern physico-chemical theory, the carriers of the current in the electrolyte solution are the anions and cations of the dissociated electrolyte; in the metal the carriers are free electrons. At the surface of contact there is a transfer of electrons between the electrode and the ions of the solution. For example, at the anode, ferrous ions are oxidized to ferric ions; on the electron theory this implies the transfer of an electron from each ferrous ion to the electrode; conversely, at the cathode electrons are transferred from the metal to the ions in solution, H ions becoming uncharged H atoms. To effect this transfer a certain potential gradient is required; it would appear therefore that the region of transition represents that portion of the circuit where the potential gradient is steepest and where the forces acting to displace electrons are greatest. That the contact of two dissimilar conductors, respectively metalHc and electrolytic, is not the essential condition, but rather the existence of a large fall of potential across a short distance,

is indicated by the facts of electrostenolysis and by the chemical effects produced by intense electrical discharges (sparking, etc.). The general fact that chemical effects are produced in protoplasm by the electric current and that protoplasm produces electric currents in its activity, when considered in conjunction with the further fact that these phenomena are dependent on the structure of the living system and disappear with the loss of semi-permeabihty (as at death) — as well as the various other facts reviewed above, which relate stimulation to membrane changes — indicates clearly that the electrical sensitivity of living protoplasm is intimately connected with the presence of the semipermeable partitions or surface-films. These partitions have high electrical resistance and are therefore highly polarizable; they are also extremely thin; hence when they are polarized by the passage of a current there is a correspondingly steep fall of potential between their opposite faces. The hypothesis naturally suggests itself that the existence of these steep gradients is the essential condition on which the chemical action of the electric current in living protoplasm depends.

We have recently attempted to put this hypothesis to an experimental test by passing electric currents through electrolyte solutions partitioned by artificial membranes, combining extreme thinness with high electrical resistance.^ Such a membrane, consisting of a thin film of rubber or other insoluble non-conducting material * R. S. Lillie and S. E. Pond, "Chemical Effects Produced by Passing Electric Currents through Thin Artificial Membranes of High Electrical Resistance," American Journal of Physiology (1923); Proceedings of the American Physiological Society, December (1922).

supported in a sheet of lens paper, is interposed between two salt solutions, one of which contains a readily oxidizable compound, ferrous chloride, together with an indicator (KCNS) to show the formation of ferric ions. With membranes of about sojJ- thickness and a P.D. of about II volts between the two faces, the red color of ferric thiocyanate appears rapidly at the surface facing the cathode; i.e., where the positive stream of the current passes from the membrane to the solution. The surface of the membrane under these conditions acts (in the quahtative sense) like the surface of a platinum anode. When the direction of the current is reversed, ferric ions are reduced to the ferrous state, as shown by the gradual disappearance of the color. In order to obtain these electrolysis-Kke effects, a certain minimal P.D. (i.e., steepness of gradient) across the membrane is required;^ for example, with a current giving 9 volts

* There is an interesting analogy here with the conditions of electrostenolysis, where also a critical P.D. is required for producing chemical effects. In the experiments of Braim (Ann. d. Physik, XLIV [189 1], N.F., 473) thin sheets of mica were used (ca 8o/x thick) in which fissures were cut; metallic silver separates out rapidly at the borders of the fissure when a strong current is passed through such a sheet separating two solutions of AgN03. Braun compares such chemical effects with Becquerel's " electrocapUlary reactions" (cf. Comptes rendus, LXXVI [1873], 1037) and suggests that they may have biological significance; he regards a narrow split or fissure in a thin layer of insulating material as acting essentially like an electrode; a certain critical intensity of current is required, -below which there is no separation of metal. Across the fissure there is a steep fall of potential, which he estimates at 700-900 volts per millimeter in currents effective with AgNOs.

P.D. across the membrane, no effect is obtained in half an hour or more. The effective gradient of about lo volts, across a partition 50 ju thick, is equivalent to a fall of about 2,000 volts per centimeter. From these experiments we may conclude that an arrangement of electrolyte solutions containing oxidizable materials and partitioned by thin films of water-insoluble material having high electrical resistance will be the seat of chemical change (oxidations and reductions) occurring at the surface of the partitions when the system is traversed by an electric current of sufficient intensity. It seems probable that this kind of structural arrangement is the one upon which the electrical sensitivity of Hving matter depends. We have already seen that this type of structure is characteristic of living protoplasm. The potentials in protoplasm are much smaller than those used in the experiment above; apparently 50 to 100 milUvolts is the usual order of the variations of potential in the bioelectric processes; but the protoplasmic films are much thinner than those used in our model; and since the steepness of the gradients rather than the absolute values of the potentials is the essential factor to be considered, the conclusion seems justified that conditions similar to the above exist in living protoplasm when the system is traversed by a current.

The parallel between the passive iron system and an irritable protoplasmic system such as a nerve axone may be described in general terms as follows. In both cases there are two electrically conducting phases separated by a thin impermeable film of chemically alterable material. In protoplasm both of the phases are electrolytic conductors. In the passive iron system only one phase is electrolytic, the other being metallic; but the chemical reaction, which is confined to the interfacial layer, does not depend directly on the internal composition and physical properties of either phase but only upon the conditions at the interface. When a current of sufficient intensity passes across the boundary at any region, in either system, it causes polarization and produces chemical effects ; and under the conditions already defined, these effects may be automatically transmitted over the whole surface.

Such a conception of protoplasmic structure and action is fully consistent with the views reached on the basis of histological research, and it has the further advantage of correlating the structural features of the living system with the special peculiarities of its chemical and physiological behavior. The great diversity exhibited by living organisms shows that the protoplasmic type of constitution permits the widest variation in the details of structure and activity. Yet the essential or fundamental structure common to all forms of protoplasm is apparently uniform; viz., a film-partitioned or filmbounded arrangement or organization of phases of different chemical composition. With this type of structure, of which an elementary model is an emulsion structure, the properties of growth, chemical activity, and irritability characteristic of Hving matter appear to be intimately bound up. Systems having this structure will give a maximum of polarization when traversed by electric currents, and hence a maximum of chemical effect. Most of the problems relating to the mode of action of such systems, especially the problem of the conditions of specific synthesis (the most characteristic

property of living matter), are still unsolved. The study of artificial systems having a similar type of physical constitution may be expected to throw further light on the nature of protoplasmic action, and also to indicate the directions in which further physiological research is most desirable. Alternating currents, activation by, 257; stimulating action of, Bioelectric potentials, membrane theories of, 302 ff.; relation to permeability of membranes, 301 ff., 311 ff.; resting, 299 ff.; variations with activity, 311 ff.

Circuits, bioelectric, as factors in transmission, 382 ff.; local, 63, Crystal forms, 32, 33, 34 Crystallization, relation to growth, Egg-cells, changes of permeability during activation, 360 ff. Environment, relations of organisms to, 25 ff.; as source of stimuli, 263 Lipoid-solubility, relation to physiological action of compounds, 189 ff. Nerve, 261, 270; bioelectric variations of, 328 ff.; changes of permeability during activity, 357 ff.; demarcation potential of, 303ff.; electrical stimulation of, 267 ff.; electro tonic currents of, 358; refractory period of, 339 ff . ; relation of salts to irritability of, 155; transmission,

379 ff. Nerve cells, rhythmical activity of, 332; structural changes in death, 64 Nervous system, directive factors in growth of, 402; integrative action of, 11; neurone theor}^ of, Non-electrolytes, physiological effects of solutions of, 151 ff. Organization, chemical, as dependent on film-partitioned structure of protoplasm, 22 Partition-coefiicients, relation to ■narcotic action, 83, 190 ff. Potential, electric, 219; variations during periodic catalysis, 249 ff.; variations during transmission in passive iron, 253, 298, 314; variations in living cells. See Bioelectric variations

Resistance, electrical, of plasma membranes, 103, 118. See Conductivity Structure, protoplasmic, relation to chemical activity of living matter, 52 ff., 63. See Protoplasm