Electrotonus

ELECTROTONUS, changes in the physical and physiological state of tissue under the action of a more or less prolonged electric current. Physical electrotonus, described by du Bois-Reymond (1848), and physiological electrotonus, described by Pflüger (1859), are especially studied for the nerve. Physical electrotonus is a consequence of polarization changes introduced by the external electric current into the tissue. The external current, insofar as it is conducted by the nerve, produces the movement of ions in it, first in the interpolar section. As the original equilibrium in the distribution of ions in the conductor is increasingly disturbed, the resistances in the interpolar section grow, and thereby the current begins to spread into the extrapolar sections, preserving in them its former direction (current loops). If a constant current is applied to the middle section of the nerve, very soon galvanometers at both ends of the nerve detect currents of the same direction as the one applied in the middle. Voltages of the opposite direction are created by the external current and at the same time oppose it. These secondary compensating voltages, which accumulate during the polarization of the nerve by the external current, immediately after the external current is opened manifest themselves as secondary currents of the opposite direction with respect to the opened current. These depolarization currents will manifest themselves until the nerve returns to its original, neutral distribution of ions. During polarization, the currents of the extrapolar sections increase with the strengthening of the polarizing current and the growth of the interpolar section, but weaken with distance from the place of polarization. There are no extrapolar currents when the polarization is transverse and when the physiological continuity of the nerve is disturbed. Finally, the extrapolar current on the anode side is always stronger than on the cathode side. These phenomena were reproduced by Matteucci (1863) and even more fully by Hermann (1872–73) on physical models consisting of a liquid and a solid conductor with a more or less significantly polarized interface between them. A convenient model is a core of platinum wire surrounded by a saturated solution of ZnSO₄. The reproduction of the phenomena of physical electrotonus on such a "Kernleiter" served as the beginning of the theory that the nerve is also a Kernleiter—a heterogeneous conductor with polarization structures. In the model, the active role belongs to the contact potential between platinum and ZnSO₄, which combines with the electrolytic polarization from the external current. Since the contact potential has positive charges in the liquid, on the side of the polarizing anode the voltages add up, while on the side of the polarizing cathode a decrease in voltage is observed. As long as the nerve is alive, the described physical-electrotonic currents can be algebraically summed with physiological "action currents": on the side of the polarizing anode, nerve impulses increase, on the side of the polarizing cathode they are inhibited. On a living nerve, it is noteworthy that the potential on the anode side can continue to increase for a long time with continued polarization, while on the cathode side it progressively falls. On the physical model and on narcotized nerve, the limit to these polar changes in potential is reached quickly. This indicates that in a living nerve the matter is not a constant contact potential from which the polarization by the external current starts, but rather a variable membrane potential, which can increase on the anode as the barrier becomes more compacted, and fall on the cathode as the permeability of the barrier increases (Ebbekes, 1922). The study of physical electrotonus especially emphasized the functional significance of morphological architecture, active surfaces, and permeability in the internal parts of tissue for its electrical activity. Physiological electrotonus is expressed in changes in excitability near the electrodes and in the so-called Pflüger's polar law. As the polarization of the nerve by a galvanic current increases, regularly changing effects are observed in the muscle: as long as the currents have a threshold strength, they give rise to excitations only upon their closure in the ascending or descending direction; with strengthened currents, excitations are obtained both upon closure and upon opening in both directions; with further strengthening of the currents, the nerve impulse reaches the muscle only upon opening the ascending current and only upon closing the descending current. This sequential evolution of effects is explained by the fact that the excitatory action belongs to the emerging cath-ET (catheto-electrotonus) and the disappearing an-ET (aneto-electrotonus). Under equal conditions, the excitatory action from the emerging cath-ET is greater than from the disappearing an-ET. The fall in excitability in the area of the emerging strong an-ET and disappearing strong cath-ET leads to a local delay of conduction (blocking of excitation waves). Regarding the place from which the excitation originates, one can make a generalization: it is the area of cath-ET or the place where the current leaves the nerve fibers (Peltier-Verigo's rule). But excitation in the area of cath-ET quickly passes into inhibition (Verigo's cathode depression, 1883). Once a local block formed under the local action of a galvanic current can then be maintained by a minimal remainder of the same current (Vvedensky's minimal polarization method, 1884). The effects corresponding to the classical law of Pflüger are characteristically perverted when polarization falls on tissue with already altered functions. M. I. Vinogradov (1917) showed that the emerging (closing) an-ET, falling on an area of nerve previously made electro-negative, no longer inhibits, but, on the contrary, releases (deblocates) the state of parabiosis here (also Vasiliev, 1922, Thorner, 1924, Vorontsov, 1924–25, Mackuth, 1926). This is what the recently famous "Wendungs-effekt" of Sheminsky (1929–32) consists of, having nothing in common with "fatigue" and "recovery from fatigue." When unilateral polarization forms a local accumulation of ions of a certain sign, the rest of the nerve participates in restoring equilibrium and smoothing out this local condensation of ions; and if a newly applied external current acts in the same direction, the tissue will quickly return to a neutral position ready for work. The expression of the fact that the rest of the nerve participates in smoothing the local unilateral accumulation of charges is the electro-negativity of the excited area with respect to any point of the same nerve and, on the other hand, "electrotonic long-range actions" along the nerve, noted by N. E. Vvedensky under the name of peri-electrotonus (1920–1921). Electrotonic long-range action along the nerve, changing and evolving in its expressions depending on time, initially spreads upon closing the current with extreme speed, overtaking the wave of excitation (Hermann and Weiss, 1898, Turner, 1922). It is very interesting that the same area of electrotonic block, under equal conditions, proves impassable for excitation waves running from the cathode side, but passable for them from the anode side (Verigo, 1920, Samoylov and Kiselev, 1925). From the edge of the parabiotic section where lability and polarizability are higher due to the proximity of the anode or taken by some other measures, arriving waves acquire the possibility to preserve their rhythm, penetrating into the block area. The block area is capable of assimilating the rhythm of the arriving impulses when there is a factor present that increases lability on the path of the impulses entering this area (Ukhtomsky, 1928). At different moments of its development, the effects of an-ET and cath-ET can be similar or opposite. If one does not take into account in the comparison which moment is observed in the effects of an-ET and cath-ET, one can encounter complex contradictions. Registering the effects with an oscillograph on a film with intervals in Viek, Schmitz and Schafer traced that the anodic increase in the amplitude of action currents in nerve fibers occurs already in the first interval, then increases during 1/2 sec., so as to in the next half second pass into a block continuing after polarization. The cathode immediately lowers the amplitudes, after 1/4 sec. depression and a fall in polarizability set in. From the point of view of abstract oppositions, the triple "anode, Ca, cold" was considered the physiological antipode of the triple "cathode, K, heat": if the second contains factors of excitation, then the first contains factors of inhibition. But on the anode the resting potential grows for quite a long time and, as we saw, the discharges of action currents are increased. At the same time, refractory phases are lengthened under the action of K (Graham, 1933) and cath-ET (Bishop and Erlanger, 1926), but are shortened under the action of Ca (Graham, 1933) and an-ET (Bishop and Erlanger, 1920). N. E. Vvedensky attributed the origin of refractory phases to electrotonic influences of excitation waves on one another (1886). The subsequent impulse's active participation in the formation of the refractory state after the previous impulse was noted by Kato (1929).

Schaefer acknowledges that any stimulus acting during the refractory phase of the first impulse and sending out current lines through the refractory membrane will, if not excite, at least delay the restitution after the preceding impulse. -- The physiological effects of electrotonus were studied especially on the nerve: in view of the extreme complexity of the effects, a simple substrate was required as far as possible for exact analysis. The question remains far from finished for the nerve even in our time.