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Biomedical subjects

A L Zefirov

Publications and source records attributed to A L Zefirov.

At least 55 records · Page 3Linked to original sources
PubMed1992

[Effect of exogenous acetylcholine on potassium currents of motor nerve endings in the frog].

Mechanisms of the exogenic acetylcholine (ACh) effect on ionic currents of nerve endings (NE) were investigated by extracellular recording of NE at the cutaneous pectoris muscle of frog. It was found that ACh in concentrations 0.1-0.6 mmol/l increased the third phase of the nerve ending response, while in concentrations 0.7-2.0 mmol/l-decreased it. Tubocurarine and atropine did not alter the ACh effect. It has been also revealed that growth of the third phase of NE response, induced by ACh, occurs in the presence of calcium channel blockers and disappears under the influence of 4-aminopyridine and tetraethylammonium. In calcium-free solution or when the calcium current is blocked the suppressive effect of ACh disappears. The mechanisms of exogenic and endogenic ACh effects on the potential-sensitive and calcium-activated potassium NE currents are discussed.

4-Aminopyridine↗
PubMed1991

[The early postdenervation changes in the properties of the postsynaptic membrane of amphibian muscle fiber].

The amplitude-temporal characteristics of miniature end-plate currents (MEPC) were studied in denervated frog cutaneous-pectoral muscles. After the nerve cutting, the initial shortening of MEPC decay time was followed by its prolongation. In 6-8 days after the denervation, the MEPC sensitivity to membrane potential decreased, while the average open time of ACh-activated channels increased 2-fold. The autocorrelation function of ACh-induced current noise revealed two components at some of the synapses. The denervation of the frog cutaneous-pectoral muscle seems to make the postsynaptic membrane to contain two types of ACh receptors (AChR): normal synaptic AChR which gradually disappear, and new synthesized AChRs which open the channels with much longer mean time and a lesser sensitivity to membrane potential changes.

Acetylcholine↗
PubMed1991

[The postsynaptic effects of substance P in the frog neuromuscular synapse].

The effect of substance P on the end-plate currents (EPC) and miniature EPC (MEPC) was studied in the "cut" sartorius muscle of the frog using voltage-clamp technique after acetylcholinesterase inhibition. Substance P in the concentration 5.10(-7)-1.10(-6) mol/l had no effect on the amplitude and time course of the single EPC and MEPC, but promoted significant prolongation of EPC decay during repetitive nerve stimulation (10/s), which indicated development of postsynaptic potentiation. Elevation of the substance P concentration to 5.10(-6) mol/l has led to the shortening of single EPS decay and more significant depression of the EPC amplitude in trains. This effect was connected with a decrease of the postsynaptic membrane sensitivity to acetylcholine, i. e. development of desensitization.

Animals↗
PubMed1990

[The detection of the sites of mediator release in a motor nerve ending].

A model of the postsynaptic current generation in response to a release of the quantum mediator from the nerve terminal is suggested. In its terms the law of the current density attenuation is determined as j = I/rb, where I is the current density in the site of generation, while j--current densities at the distance r from the site of generation. Experiments with extracellular recording have shown that coefficient b equals approximately 1. Assuming that sites of the quantum release and a site of the postsynaptic current generation are spatially identical, the new method is suggested to determine coordinates of the transmitter release sites in the motor nerve terminal. This method consists in the measuring of a uniquantal signal amplitude by three extracellular microelectrodes, arranged at a distance of 5-10 microns from each other. The construction of spatial pictures of the transmitter secretion on the basis of the analysis of several hundreds of signals in the cutaneous pectoris frog muscle has shown that release sites are organized in groups transversal to the nerve terminal. It is supposed that these groups of sites reflect the transmitter secretion in the active zones of the nerve ending. Advantages, shortcomings and errors of the method are shown.

Animals↗
PubMed1990

[An analysis of mediator secretion in the active zone of the motor nerve ending].

The topography of transmitter release sites at the motor-nerve terminal of the cutaneous-pectoris frog muscle has been determined using three extracellular electrodes. It is shown that release sites are united in groups arranged transversally to the nerve endings and reflecting the transmitter release in the active zones (AZ) of the nerve terminal. The quantitative analysis of revealed groups has permitted concluding that the maximal level of secretion is at the centre of AZ, decreasing to the edge and aside from AZ. At the low extracellular Ca2+ concentration all the AZ take part in the spontaneous release process, while in the evoked one--only some of AZ. Advantages of the three-microelectrode method over the two-microelectrode one are analyzed. It is found that the transmitter secretion in spatially isolated AZ leads to the polymodality in uniquantal signal amplitude distribution at extracellular recording. The role of AZ in the transmitter release process is discussed.

Animals↗
PubMed1990

[The formation of nerve endings in the phasic muscles of the frog].

Synapses with the complex nerve endings consisting of several terminal branchings of a single axon divided by myelin segments were studied at the cutaneous-pectoris muscles of a frog by histological and electrophysiological methods. It has been revealed that the number of the synapses with complex nerve endings rises during the postnatal development. The relation between the nerve ending complexity, total terminal length and size of the muscle fibres is described. Some of branches in complex nerve endings arise from the Ranvier nodes. They are characterized by a low level of quantal secretion and a peculiar fall of the sodium current along nerve terminals. Mechanisms that induce growth of nerve endings, branching and peculiarities of the transmitter release are discussed. It is supposed that the processes of growth and myelinization of nerve endings take place simultaneously.

Aging↗
PubMed1990

[The generation and spread of excitation in the motor nerve ending of the frog].

In experiments on the frog cutaneous-pectoris muscle under the visual control the evoked responses of nerve endings were recorded in proximal, central and distal parts of the terminal. At the fixed position of the recording electrode the local iontophoretic application of TTX to different terminal parts and Ranvier's nodes was performed. It was concluded, that local currents at more proximal terminal parts provide the depolarization of the located parts and conduction of excitation to the nonexcitable parts. Inward currents at behind located and already excited parts are shortened and decreased by local currents from more distal parts during the propagation of excitation. It results in shortening of the action potential and decreasing of the transmitter release in more proximal parts. This effect disappeared along the nerve terminal due to decreasing of inward Na current at the end parts.

Animals↗
PubMed1987

[Calcium and calcium-activated potassium currents of motor nerve endings in the frog].

Evoked electrical responses of nerve endings were recorded in experiments on frog cutaneous-pectoris muscle under visual control. During superfusion by Ca2+-free solution with tubocurarine a late inward current was found in the evoked response of nerve endings recorded by CaCl2 filled electrode. With addition of the external solution of 4-aminopyridine the late current was changed into an outward one. This outward current depended on Ca2+ concentration in the electrode, decreased after local increase of K+ concentration and was abolished by Co2+ ions. Local ionophoretic application of tetraethyl-ammonium eliminated the outward current and revealed a strong and prolonged inward current. Similar currents were recorded by microelectrodes filled with SrCl2, BaCl2 and MgCl2. It is concluded that the late inward current is carried through potential-dependent calcium channels, while the late outward one--through Ca2+-activated K+ channels. The ratio of calcium and Ca2+-activated potassium currents in motor nerve endings and their role in the transmitter release are discussed.

Animals↗
PubMed1987

[Characteristics of the time course of miniature endplate currents in different parts of the neuromuscular junction in the frog].

Extracellular glass microelectrodes were used for recording miniature endplate currents (MEPC) from proximal and distal parts of frog sartorius synapse. The amplitude and half-time of decay (t1/2) of MEPC in proximal parts exceeded those in distal parts. These differences were eliminated by tubocurarine and increased under the influence of armin. In 80% of experiments the positive correlation between the amplitude and t1/2 of MEPC was revealed, which reflects repetitive binding of acetylcholine (ACh) molecules with cholinoreceptors (ChR). Positive correlation was observed in all investigated proximal parts of terminal, and only in half of distal ones. The electron microscopic study showed that postsynaptic folds in proximal parts occur more frequently, are poorly branching and rather shallow. A conclusion is made that the more prolonged time course of MEPC in proximal synapse parts is due to the more pronounced repetitive binding of ACh molecules with ChR of the postsynaptic membrane, which may be explained by high density of ChR and special, features of the synaptic cleft geometry in proximal parts.

Acetylcholine↗
PubMed1987

[The single nerve ending of the frog sartorius muscle: its ultrastructural characteristics and mediator secretion].

Electron microscopy and extracellular recordings were used for the investigation of structural and functional peculiarities of single frog sartorius muscle nerve terminal. It has been found that the diameter, length of the synaptic contact and quantity of synaptic vesicles decreased from proximal to distal parts of the nerve terminal. A number of varicosities, separated from each other by schwann cells, have been revealed along the course of the nerve terminal. This indicates the existence of an interrupted synaptic contact. Both the evoked and spontaneous transmitter release decreased from the initial to the end parts of the nerve terminal. The data obtained suggest that there is a correlation between structural heterogeneity and the differences in the transmitter release.

Animals↗
PubMed1985

[Ion currents of a nerve ending in the sartorius muscle of the frog].

Nerve terminal responses evoked by motor nerve, stimulation were recorded from frog sartorius nonmyelinated endings by external electrodes. At proximal parts of nerve terminal a triphasic waveform (+ - +) was observed. Ionophoretic application of tetrodotoxin, tetraethylammonium and 4-aminopyridine indicated that the negative phase reflected inward sodium current while the third positive phase--outward potassium current. Using solutions with tubocurarine from which calcium was omitted and an electrode filled with CaCl2 a late slow negative response component was recorded. This component depended on Ca2+ concentration in the electrode, increased with addition of tetraethylammonium and 4-aminopyridine and was abolished by Co2+. A similar component was recorded by microelectrodes filled with solutions containing Sr2+ and Ba2+ ions. It is concluded that the slow component of the response reflects the current across potential-dependent calcium channels in the presynaptic membrane. The time courses of calcium current and transmitter release in the synapse are compared.

Animals↗
PubMed1985

[Distribution and functioning of mediator release sites in the neuromuscular junction of the frog].

Two microelectrodes were used to record end plate potentials (EPPs) and miniature EPPs (MEPPs) in a single nerve-muscle synapse of frog sartorius muscle. Under conditions of extracellular recording on the scattering diagrams of MEPPs amplitude some groups of signals (fields) were revealed which reflect the transmitter secretion in spatially distinct sites of the nerve terminal. It was found that the release sites in the synapse were grouped in certain areas and have different probability of quantum transmitter secretion. The comparison of fields on the scatter diagrams of MEPPs amplitude and that of uniquantal EPPs at low [Ca]0 (0.2-0.4 mmol/l) showed that the probability of the evoked and spontaneous transmitter release is different in certain secretion sites and sometimes a release site does not take part in evoked secretion. Simultaneous recording of synaptic potentials by extra- and intracellular electrodes showed that the transmitter secretion in spatially distinct groups of release sites leads to the appearance of multimodality on amplitude distribution of intracellular MEPPs and EPPs.

Animals↗
PubMed1985

[Periodic changes in evoked mediator secretion at the neuromuscular synapse].

Extra- and intracellular recordings of the frog m. sartorius postsynaptic potentials with low concentration of Ca2+ ions (0.2-0.6 mmol/l) aided to study the dynamics of quanta transmitter release in prolonged motor nerve stimulation (0.5-10.0 imp/s). The statistical analysis showed that with intracellular recording only, time intervals between transmitter release were distributed in an evenly manner. Estimation of the correlation function and the spectral density revealed the periodical fluctuations of evoked transmitter release with periods from a few seconds to 1-2 min. With intracellular recording, oscillations of the transmitter release occurred in about 1/3 of cases whereas with extracellular recording they were always present. Analyses of a single release site activity by means of two extracellular electrodes inserted into a single nerve terminal, showed considerable changes of the cyclic transmitter secretion. The evoked transmitter release in a single release site seems to fluctuate periodically.

Animals↗