Posttetanic hyperpolarization of the isolated nerve fiber of Rana ridibunda

The parameters of posttetanic hyperpolarization (PTH) of a single node of Ranvier are unchanged by an increase of duration of stimulation of over 0.1 sec. PTH was not sensitive to strophanthin but was abolished by tetraethylammonium. The membrane resistance of the Ranvier node, measured by the amplitude of anodic shocks, was reduced by 50% during PTH. It is concluded that PTH of isolated nerve fibers is due to preservation of increased potassium permeability during and after the end of tetanus. An increase in the duration of tetanization does not cause the appearance of PTH connected with active ion transport.     

Peculiarities of nerve conduction block produced by lidocaine and ajmaline

The development of tonic and use-dependent block of nerve conduction under the effect of antiarrhythmic drug ajmaline and local anesthetic lidocaine was examined on sciatic nerve of Rana ridibunda. Ajmaline and lidocaine attenuated the amplitude of action potential by half during 120 and 13 minutes, respectively. Ajmaline produced virtually immediate use-dependent block of conductance, while it was minor in lidocaine-treated nerve preparation. __________ Translated from Byulleten’ Eksperimental’noi Biologii i Meditsiny, Vol. 143, No. 3, pp. 262–266, March, 2007     

The effect of changes in the pH of the external solution on the afterpotentials of individual frog nodes of Ranvier]

A decrease in pH to 5 elicited a small increase in afterdepolarization of a single Ranvier node and an increase in pH to 9 had no effect. Post-tetanic hyperpolarization decreases at pH 5 and its duration somewhat increases. An increase in pH to 9 also had no effect. Similar changes in afterdepolarization and post-tetanic hyperpolarization were observed in potassium free solutions. A conclusion is made that changes in afterpotentials are determined by changes in kinetics of the membrane potassium permeability.     

Confirmation of the hypothetical diffusion barrier in the region of the Ranvier node of myelinated nerve fibers

Experiments were carried out on single nerve fibers with a covered node. The nerve fiber was isolated together with one to three fragments of neighboring fibers adjacent to the node and which were firmly connected to the nodal ends of the myelin sheath by means of numerous connective-tissue membranes, thereby strengthening the node and protecting it against stretching during dissection. In potassium-free solution after-hyperpolarization of the nerve fiber membrane with an amplitude of 2.7 mV develops immediately after the end of the spike. Its origin is connected with maintenance of the increased potassium permeability of the membrane. During repetitive stimulation of the covered node the amplitude of after-hyperpolarization falls successively, as a result of a decrease in the outward potassium current caused by a reduction in the potassium electrochemical gradient. These observations are regarded as confirmation of the hypothesis expressed previously on the existence of a diffusion barrier in the region of the Ranvier node due to accumulation of K⁺ in the juxtamembranous space.Department of Anatomy and Physiology of Man and Animals, Ul'yanovsk Pedagogic Institute. (Presented by Academician of the Academy of Medical Sciences of the USSR V. N. Chernigovskii.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 86, No. 11, pp. 517–519, November, 1978.     

Accumulation of K+ Ions in the Perinodal Space as a Factor Regulating the Functional State of Myelinated Nerve Fibers

Experimental studies on intact nodes of Ranvier in isolated nerve fibers led to the conclusion that the accumulation of K⁺ ions released into the perinodal space during excitation performs a series of important functions: it decreases K⁺ release from the axoplasm, induces prolonged post-spike depolarization and its associated exaltation phase, underlies repeated and rhythmic activity, activates the electrogenic membrane Na⁺,K⁺-pump, and determines the generation of the prolonged component of post-tetanic hyperpolarization, which is an additional factor responsible for reabsorption of potassium ions into the cytoplasm of nerve fibers, etc.     

Effect of carbon dioxide on the excitable membrane of the single Ranvier node

Addition of 5% CO₂ to the atmosphere in the chamber with the specimen caused no significant change in the recorded parameters of the Ranvier node membrane. Increasing the CO₂ concentration to 50% (50% CO₂+50% O₂) caused a significant increase in the resistance and critical depolarization level of the membrane, a decrease in amplitude of the spike and maximal steepness of its rise. Spike generation ceased 2 min after the beginning of exposure to the gas mixture. The action of CO₂ was completely reversible. After-potentials of the node were virtually unchanged by the action of CO₂ in these concentrations.Department of Anatomy and Physiology of Man and Animals, Ul'yanovsk Pedagogic Institute. (Presented by Academician of the Academy of Medical Sciences of the USSR V. N. Chernigovskii.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 86, No. 10, pp. 389–391, October, 1978.     

Effect of exogenous nitric oxide on electrogenesis in myelinated and unmyelinated nerve fibers

Extracellular recording was used to study the effect of sodium nitroprusside, a donor of NO, on parameters of action potential and ionic currents in single sciatic nerve fibers and unmyelinated nerve terminals in the sternal muscle in frogs. Sodium nitroprusside significantly decreased the duration of action potential in Ranvier node and the amplitude of afterdepolarization. In motor nerve terminals bathed in low Ca²⁺ saline, sodium nitroprusside increased phase III amplitude of the nerve terminal response corresponding to outward potas-sium currents. Blockade of voltage-dependent potassium channels with 4-aminopyridine abolished the effects of NO. These data indicate that exogenous NO reduced the duration of action potential and afterdepolarization through enhancement of voltage-dependent potassium currents.     

The inhibitory effect of carbon dioxide gas on the post-tetanic hyperpolarization of frog nerves]

CO2 as well as its mixture with O2 were passed through the Ringer solution with nerve in it or through the chamber with the preparation. This caused a cessation of posttetanic hyperpolarization of the nerve. An inhibitory effect of CO2 on the active ionic transport through the membrane of myelinated nerve fibres mediated by change in their internal pH is supposed.