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N A Dimitrova

Publications and source records attributed to N A Dimitrova.

At least 37 records · Page 2Linked to original sources
PubMed1992

Use of surface potential spectral characteristics for solving the inverse problem in electroneurography.

The changes in the power spectra of single-fibre extracellular action potentials (SFEAPs) generated in an infinite anisotropic frequency-dependent volume conductor, which occurred as a result of alterations in the propagation velocity v and duration T(in) of the intracellular action potential (IAP) were analytically determined. Effects of the temporal and spatial dispersions of almost synchronously activated fibres on the power spectrum of compound extracellular potentials (CEPs) were analysed for different shapes and sizes of the activated fibres' territory. It was found that, as a result of desynchronisation in the fibres' activation, dips existed in the CEP power spectra and that the frequencies of the dips depended on the degree of desynchronisation but did not depend on the velocity. It was shown that the hypothetical power spectrum of compound IAP was sensitive to the variations in the desynchronisation in the fibres' activation and in the risetime and duration of IAP even at a great fibre electrode distance typical for surface recordings.

Action Potentials↗
PubMed1992

Radial changes of extracellular potential amplitude and integral characteristics and the inverse problem in electroneurography.

The possibility of solving the inverse problem in electroneurography, i.e. of estimating the main parameters specifying the activated fibre's functional state, using the amplitude and integral characteristics of the surface potentials generated by infinite homogeneous fibres, has been analysed. An analytical expression has been found for the amplitude of the negative phase Anph of the single fibre extracellular action potential (SFEAP) as a function of the wavelength b, the fibre-electrode distance y and a scale factor Ao proportional to the intracellular action potential amplitude Vm, to the square of the fibre radius a and to the ratio of the axoplasm conductivity sigma a and volume conductor conductivity sigma e. For a large fibre-electrode distance, typical of surface recordings, an analytical expression of the integral of the negative phase Inph of the SFEAP as a function of Ao, b, y and the propagation velocity v was also found. Simple methods are proposed for estimating v, the location of the electrical centre of the activated fibres' territory and the product of the number of activated fibres N, duration T(in) of the intracellular action potential and of the factor Ao. The estimation errors due to the temporal and spatial dispersion of the activated fibres were analysed as a function of the fibre-electrode distance and the territory shape.

Action Potentials↗
PubMed1992

Threshold stimulation and accommodation of the Hodgkin-Huxley axon.

The charge-duration and strength-duration relations for just threshold rectangular stimuli were numerically investigated for the Hodgkin-Huxley axons of different lengths and different membrane capacitances under normal conditions and blockage of the development of accommodative processes. Two linear portions could be distinguished on the charge-duration curve. One of them followed the Weiss law. The other one represented a portion of a straight line passing through the zero point of the coordinates. The slope of the second portion was determined by the charge for very short stimuli (Q0), the slope of the first portion, and the maximum time to excitation (tau max). The rheobase reflected the slope of the second portion. Upon varying the fibre length the slope of the first and the second linear portions and the rheobase changed. The membrane capacitance substantially affected both the value of Q0 (as in the case of myelinated fibres) and the rheobase. The accommodative processes affected the Q0, the slope of the first line, tau max, and, consequently, the rheobase. The effect of potassium activation was stronger than that of sodium inactivation. The slope of the first line, tau max, and the rheobase might be considered more comprehensive indicators of the accommodative processes than the usually used indicators.

Action Potentials↗
PubMed1992

Effect of stimulus (postsynaptic current) shape on fibre excitation.

Effects of variation of the stimulus pulse shape on the excitation of a nonmyelinated nerve fibre were studied using a mathematical model based on the Hodgkin-Huxley equations. Efficiency of smoothly changing pulses was compared with that of rectangular pulses. For pulses shorter than the time to excitation, the rate of the stimulus rise did not determine the ability of a smoothly changing pulse to excite the fibre. For a given stimulus duration, the main factor was the pulse area or the charge delivered by the pulse. The strength-duration curve for smoothly changing pulses was a nonmonotonic function, in contrast to the curve for rectangular pulses. The dependence of latency on changes in the pulse area was non-linear. It would be nonmonotonic when the pulse area variation were due to the stimulus duration or the stimulus rise duration. More that one propagating intracellular action potential (IAP) could arise upon fibre activation by a long smoothly changing threshold stimulus. Upon activation of relatively short fibres the IAP could arise not at the site of the smoothly changing stimulus injection. The rectangular pulses of long duration were more efficient than the corresponding smoothly changing ones. Irrespective of the shape, the pulses whose duration at the foot is 1-2 ms, are more suitable for a prolonged threshold fibre activation.

Action Potentials↗
PubMed1991

Difference in excitability along geometrically inhomogeneous structures and occurrence of "hot spots".

The differences in excitability along geometrically inhomogeneous, electrically excitable structures as well as the possibility of occurrence of "hot spots" at certain branch points were theoretically analysed on the basis of the Hodgkin-Huxley model assuming uniform specific membrane parameters along the structure length. It was shown that the "hot spots" conditioned by geometrical inhomogeneities should be not only morphological but also functional formations. The excitability at the branch point could be higher than that at the rest of the structure when the branch point was an electrical equivalent of a step decrease in the cable diameter. The stronger the diameter decrease, the higher the excitability at the branch point and thus the higher is the possibility of observation of "hot spots" in the nerve cells whose dendrites have a profuse branching. The realization of the "hot spots", however, depended on the distance from the site of the stimulus application (synapse) to the branch point and on the stimulus (synaptic current) strength, as well. The closer the synaptic current strength to the threshold value, and the shorter the synapse-branch point distance, the higher was the possibility of a propagating action potential origin at the branch point but not at the site of the stimulus application and thus the higher was the possibility of realization of "hot spots". The conclusion that the geometrical position of the initial segment contributes to its higher excitability (as compared to the rest of the cell) in the case of orthodromic activation of the neuron was also made.

Action Potentials↗
PubMed1990

Power spectra of extracellular potentials generated by an infinite, homogeneous excitable fibre.

The power spectra of the extracellular potentials (EPs) generated under activation of an infinite, homogeneous excitable fibre immersed into an infinite, resistive, isotropic and homogeneous volume conductor are theoretically analysed. The changes in the power spectrum related to the changes in the propagation velocity v, amplitudes Vm and duration Tin of the intracellular action potential (IAP) are analytically determined. It is found that in the ultra-low-frequency region the EP spectral power follows the course of alteration in the square of the modified Bessel function of the second kind and order zero multiplied by the fourth power of the frequency, and the Tin can be assessed by the deviation of the EP power spectrum from this function. It is shown why the sensitivity of the spectral characteristics depends substantially on the radial distance yo from the activated fibre to the point of observation; why the total spectral amplitude depends directly on the IAP wavelength but the total spectral power depends on the IAP wavelength as well as on its duration and propagation velocity; and why the EPs are not proportional to the IAP second spatial derivative even in close proximity to the fibre.

Action Potentials↗
PubMed1990

Power spectra of single infinite fibre extracellular potentials recorded by a bipolar electrode.

The power spectra of bipolarly recorded extracellular action potentials (EAPs) generated by an infinite, homogeneous, excitable fibre in an infinite, resistive, isotropic and homogeneous volume conductor were theoretically analysed. The changes in the power spectrum of EAP, which occurred as a result of alterations in the propagation velocity v and duration Tin of the intracellular action potential IAP, were analytically determined for bipolar parallel and radial electrodes with a small interpole distance. It was found that the sensitivity of the spectral characteristics to alterations in v, Tin and/or the IAP asymmetry substantially depends on the fibre-electrode distance; information on the IAP fast changes, that seems to be lost in unipolar recording as a result of the filtering effect of the fibre-electrode distance, can be restored. The orientation of the recording electrode need not be taken into account when a qualitative analysis is carried out, but when a quantitative analysis has to be performed, then the electrode orientation has a significant influence. A method is suggested for determination of the fibres' orientation by means of the spectrum of EAPs recorded bipolarly. The selectivity of the bipolar electrodes is analysed.

Action Potentials↗
PubMed1988

Effect of electrical stimulus parameters on the development and propagation of action potentials in short excitable fibres.

Intracellular action potentials (IAPs) produced by short fibres in response to their electrical stimulation were analysed. IAPs were calculated on the basis of the Hodgkin-Huxley (1952) model by the method described by Joyner et al. (1978). Principal differences were found in processes of activation of short (semilength L less than 5 lambda) and long fibres under near-threshold stimulation. The shorter the fibre, the lower was the threshold value (Ithr). Dependence of the latency on the stimulus strength (Ist) was substantially non-linear and was affected by the fibre length. Both fibre length and stimulus strength influenced the IAP amplitude, the instantaneous propagation velocity (IPV) and the site of the first origin of the IAP (and, consequently, excitability of the short fibre membrane). With L less than or equal to 2 lambda and Ithr less than or equal to Ist less than or equal to 1.1Ithr, IPV could reach either very high values (so that all the fibre membrane fired practically simultaneously) or even negative values. The latter corresponded to the first origin of the propagated IAP, not at the site of stimulation but at the fibre termination or at a midpoint. The characters of all the above dependencies were unchanged irrespective of the manner of approaching threshold (variation of stimulus duration or its strength). Reasons for differences in processes of activation of short and long fibres are discussed in terms of electrical load and latency. Applications of the results to explain an increased jitter, velocity recovery function and velocity-diameter relationship are also discussed.

Action Potentials↗
PubMedDate not supplied

Fundamentals of power spectra of extracellular potentials produced by a skeletal muscle fibre of finite length. Part II: Effect of parameters altering with functional state.

The reasons for dependence of the power spectra of extracellular potentials (EPs) produced by a skeletal muscle fibre of finite length, on parameters altering with functional state was analysed. The sensitivity of the EP power spectra to alterations in the parameters depends on the distance of the observation point from the fibre. At large distances the sensitivity can change with longitudinal position as well. The differences in the sensitivity are due to the changes in the inter-relations between the power spectra of the input signal (the first temporal derivative of the intracellular action potential) and of the impulse response (IR) of the fibre of finite length as a linear system of EP generation. It was shown that not only the parameters affecting the IR (propagation velocity of the waves of depolarisation), but also the parameters determining the input signal (intracellular action potential duration and after-potential) can affect the characteristic frequencies of the EP power spectra.

Action Potentials↗
PubMedDate not supplied

Effect of electrode dimensions on motor unit potentials.

Different effects of longitudinal and transversal electrode dimensions on nerve or muscle single fibre action potentials detected monopolarly, were reported in the literature. The results were contradictory. We studied motor unit potentials (MUPs) detected at a large distance (typical of surface recording) on the basis of a mathematical model without source simplification. The MUPs were calculated as a single convolution of the first temporal derivative of a realistic intracellular action potential and MU impulse response. The spatial averaging of the MUPs by rectangular plate electrodes was performed through analytical integration of the MU impulse response over the electrode area. The effects of longitudinal dimension of the electrode were stronger than those of a transversal one. The effects were distance dependent. The longitudinal dimension of the electrode influenced the main phases (that reflected the excitation origin and propagation) more than the terminal phases (that reflected the excitation extinction at the muscle fibers' ends). This was due to differences in the character of the potential fields (quadrupole or dipole) during generation of individual MUP phases. It was shown that the relative weight of the individual MUP phases could be stressed or suppressed by a proper choice of electrode dimensions, position and orientation.

Action Potentials↗
PubMedDate not supplied

Longitudinal variations of characteristic frequencies of skeletal muscle fibre potentials detected by a bipolar electrode or multi-electrode.

We aimed to reveal reasons for longitudinal variations of characteristic frequencies of electromyographic signals detected by surface longitudinal multi-electrodes. Since the terminal phases were reduced in bipolar recordings, we tested whether the frequency variations reflected the effects of the excitation origin and extinction as in monopolar recordings. A precise and fast convolution method to calculate the signals detected by a multi electrode was suggested. The contribution of different electrode poles was introduced in the impulse response. When a longitudinal multi-electrode with an even number of poles was positioned above the end-plate of asymmetrical fibres, the signal mainly reflected the processes of the excitation extinction This increased the signal mean and median frequencies Although the effects of origin and extinction of the excitation were significantly reduced in the spatially filtered signals, the frequency variations along the fibre reflected these intrinsic features of any skeletal muscle fibre of finite length.

Electromyography↗
PubMedDate not supplied

Effect of parameters altering with muscle fibre functional state on power spectra of spatially filtered extracellular potentials.

We aimed to analyse the effect of parameters altered with muscle fibre functional state on power spectra (PS) and spectral characteristics of the signals detected from skeletal muscle fibres of finite length by bipolar or one-dimensional multi-electrodes oriented in parallel to the muscle fibres. The PS were calculated as the product of the power spectrum of the input signal (the first temporal derivative of the intracellular action potential (IAP)) and spatially filtered impulse response. A multi-electrode with an even number of poles, located above the end-plate or fibre end, reduces the effect of alterations in the propagation velocity and increases that of the IAP duration and after-potentials. Detection of the signals far from the end-plate and fibre ends reduces the effect of the IAP spike duration and increases that of the propagation velocity. Increasing the number of poles reduces the effect of after-potentials. A proper multi-electrode arrangement and position could help to separate and assess the relative alteration of individual parameters.

Action Potentials↗
PubMedDate not supplied

Area-to-amplitude ratio of the terminal phase of the belly-tendon detected motor unit potentials could be used to recognize reinnervated motor units.

Besides the increased number of fibres, the reinnervated motor units (MUs) are characterised by an increased scattering the end-plates, greater desynchronization in the fibres' activation, greater dispersion in the diameters of the MU fibres and thus in propagation velocities along them. As a result, desynchronization in the moments, at which the excitation waves reach the fibres' ends, increases in reinnervated MUs. The possibility to recognize reinnervated MUs in short (hand) muscles on the basis of changes in duration of the terminal (second) phase of the belly-tendon detected motor unit potentials (MUPs) was examined by numerical experiments. A convolution model that took into account the finite fibre length, was used to calculate MUPs for distances typical of surface detection. It was shown that the ratio between the area of the terminal phase and its amplitude, as a measure of duration of the terminal phase, was sensitive to desynchronisation of the waves of excitation. The ratio was independent of the distance from the MU axis and of the volume conductor properties. Basing on the results obtained, we can conclude that the ratio reflects main functional compensations in reinnervated MUs and could be used for discrimination between reinnervated and normal MUs.

Electromyography↗
PubMedDate not supplied

Influence of the fiber length on the power spectra of single muscle fiber extracellular potentials.

Influence of the fiber length on the power spectra of the single muscle fiber extracellular action potentials (SMFEAPs) as well as on the spectral changes under alterations in the propagation velocity (v) and intracellular action potential duration (Tin) was studied theoretically for a fiber-electrode distance typical of surface recordings. It was shown that the magnitude and distribution of the SMFEAP spectral power depend on the fiber length. The shorter the fiber, the wider the frequency region in which the total spectral power is distributed and the smaller the spectrum max amplitude. The fiber length affects the spectrum changes that are due to alterations in v or Tin, and, consequently, the spectrum characteristic frequencies--Fmax, Fmed and Fmean, as well as the total spectral amplitude (TSA) and total power (TP). Whereas for a relatively long fiber Fmax depends linearly on v and Fmed and Fmean increase with v increasing, for a short fiber the linearity of the relation between Fmax and v can be disturbed, and Fmed and Fmean can decrease with v increasing. Under the condition of a relatively short fiber, a change in Tin can cause changes not only in Fmed and Fmean, but in Fmax as well. It was shown, that as a result of the finite length of the muscle fibers as well as of the asymmetry of the end-plate location in relation to the fiber ends, dips can be observed in the spectrum even in a case of monopolar recording. Positions of the dips in the power spectra depend on propagation velocity.

Action Potentials↗
PubMedDate not supplied

Integral characteristics of extracellular single fibre action potentials.

The changes in the integral of the extracellular action potentials (EAPs) generated by an infinite homogeneous fibre in an infinite homogeneous and isotropic volume conductor were studied at different radial distances (yo) from the fibre axis, depending on the propagation velocity (v), duration (Tin) and asymmetry of the intracellular action potential (IAP). The method used was a mathematical modelling. It was obtained that: the EAPs generated by an infinite fibre have no DC component, i.e. the integral value of the negative phase (INPh) is equal to that of the two positive phases (IPPh); the character of the integral dependences is different at different radial distances; INPh is related both to the EAP amplitude and to the temporal parameters of the EAP; at small radial distances INPh normalized in respect of the negative phase amplitude is in generally linear relationship with the Tin, it decreases when the IAP asymmetry increases (with the Tin being unchanged), and it is not practically affected by the changes in the yo (up to 0.2 mm) and by the changes in the v; at large yo the normalized INPh is almost inversely proportional to the v and is not practically affected by the changes in the Tin and in IAP asymmetry.

Action Potentials↗