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Biomedical subjects

A Kossev

Publications and source records attributed to A Kossev.

At least 37 records · Page 2Linked to original sources
PubMed1990

Electrotonic potentials of myelinated nerve fibers.

The extracellular electrotonic potentials of a single myelinated nerve fiber in a volume conductor of infinite extent were studied. The spatial distribution of the transmembrane electrotonic potential was obtained by integrating the system of differential equations constituting the model of the activation of a myelinated nerve fiber. The stimulus was step-like. The present investigation was concerned with the steady-state conditions only. The spatial distribution of the extracellular potentials at various radial distances in the conducting medium was calculated using the line source model. Up to a certain radial distance the discontinuous structure of the myelinated fiber is reflected in the oscillatory nature of the extracellular potentials, while further in the volume conductor the potentials are smooth. The magnitude of the radial decline of the extracellular potentials were compared for myelinated fibers of various internodal distances.

Evoked Potentials, Somatosensory↗
PubMed1989

Extracellular potentials of a single myelinated nerve fiber in an unbounded volume conductor.

The extracellular potentials of a single myelinated nerve fiber in an unbounded volume conductor were studied. The spatial distribution of the transmembrane potential was obtained by integrating the system of partial differential equations characterizing the electric processes in the active myelinated nerve fiber. The spatial distribution of the extracellular potentials at various radial distances in the volume conductor were calculated using the line source model. Up to a certain radial distance (500 microns) the discontinuity of the action potential propagation is reflected in the extracellular potentials, while further in the volume conductor the potentials are smooth. The effect of the fiber diameter and the internodal distance on the volume conductor potentials as well as the changes in the magnitude of the extracellular potential (in the time domain) between two adjacent nodes at various radial distances were studied. The radial decline of the peak-to-peak amplitude of the extracellular potential depends on the radial coordinate r of the field point and increases with the increase of r.

Animals↗
PubMed1988

Comparison of the different variants of the current source density analysis methods in neurophysiological studies.

Computer calculations and theoretical studies of different variants of current-source density analysis (CSDA) for neurophysological studies were carried out. The different variants of the method are compared by estimating equiweight surfaces and equiweight lines in particular planes. The decrease in the distance between the different points of measurement has been found to result in more selective presentation of the neuronal activity. It is shown that this distance cannot be very small--50 micron seems a reasonable limit. Bidimensional and especially unidimensional CSD-analysis produces considerable volumes with negative weights, these weights being rather high in absolute value. Errors in the interpretation of the results are possible, such as incorrect calculation of the current density and even erroneous determination of its sign.

Mathematics↗
PubMed1987

Effect of the synchronization of the discharges of single motor units for the formation of the summated electromyogram.

The monosynaptic electromyographic reflex (EMG) response to muscle stretching represents a synchronized discharge from the single motor units (MUs). The role of this synchronization, taking into account the effect of the subsequent voluntary activity, has been investigated by calculating the probabilities for the appearance of impulse from a single MU for consecutive 10 ms time intervals. Experimental data on the firing of the single MUs under stationary conditions and before the front of the summated EMG for m. triceps surae are used. The results obtained are compared with the experimental data on the interaction of the stretch reflex and the voluntary movement in response to the stretch. It is shown that the synchronization in itself cannot explain the later responses in the EMG, which are with latencies longer than the latency of the monosynaptic reflex, but shorter than the reaction time of the subsequent voluntary activity.

Electromyography↗
PubMed1987

Optimization of the source derivations from the scalp surface.

The selectivity of source derivations (SD) is computer-calucla ted using different combinations of electrodes and different interelectrode differences, with a view to the optimization of the method. For this purpose, the method described by Nunez and Katznelson (1981) is used to calculate surfaces in the brain tissue from the points of which a single current source will determine the same potential in a given point on the scalp surface. The lines of these surfaces are given for different planes. The calculations show that the selectivity increases with the decrease of the interelectrode distances, as well as when more electrodes are used. With a small number of electrodes for SD and especially when one central and two lateral electrodes are used, there exist brain areas in which the generators cause a potential with opposite sign on the scalp surface.

Electrodes↗
PubMed1986

Changes in the firing of single motor units before and after voluntary movement.

The changes in the firing of single motor units (MUs), i.e. the probability for the appearance of an impulse and the instantaneous firing frequency, from m. tibialis anterior and m. triceps surae, were investigated when the respective muscle was agonist or antagonist of the movement, as well as when the movement was performed by the contralateral leg. The changes in MU firing show that the excitability of the alpha-motoneurones is changed 25-45 ms before the front of the ipsilateral voluntary electromyogram. The excitability is increased when the muscle is agonist of the movement, and decreased when it is antagonist. The period of changed MU firing, found by us, corresponds to the duration of the period of increased monosynaptic reflex excitability, preceding the voluntary motor activity (Gerilovsky et al., 1982). Therefore, there are no grounds for assuming that the mechanisms causing these changes differ in the two cases. The results obtained show that the observed changes are specific: they are connected with the organization of the voluntary movement and they do not occur in movement involving the contralateral leg to the MU investigated.

Action Potentials↗
PubMed1986

Motor unit potentials at high muscle activity recorded by selective electrodes.

The selectivity of different kinds of electrodes for recording of single motor unit potentials was theoretically evaluated. A new type of selective "branched", superficial and subcutaneous wire electrodes was described. The discharge of single alpha motoneurons during movements against elastic resistance was investigated studying motor unit potentials in m. biceps brachii and m.interosseus dorsalis l. Depending on the velocity of the movement, the following pattern of discharge was found: 1) At slow velocity the frequency of discharge increased and new motor units were recruited. 2) At higher velocity the frequency of discharge was constant. The muscle force increased only when new motor units were recruited. 3) At very quick ballistic movements the frequency of discharge decreased during the movement.

Adult↗
PubMed1983

Quantitative analysis of the glabellar reflex in patients with hemiplegic disorders.

The glabellar reflex (GR) was investigated in 15 patients with hemiplegia and in 10 control subjects. The reflex was elicited mechanically by a light tap on the glabella. The EMG-responses were recorded bilaterally from the orbicularis oculi muscles by pairs of surface electrodes. The EMG activities were averaged and integrated off-line. In addition, the latencies of the early and late components were determined. Two major types of the GR alterations could be distinguished. These two patterns probably indicate a lowered excitability of the brainstem trigeminal systems or brainstem facial systems respectively. A loss of facilitatory influences associated with the hemispheral lesion seems probable. The alterations of the early component showed essential linkage to those of the late component. The latencies of GR responses on both sides were frequently prolonged. This fact and the decrease of the GR responses on the unaffected side as well as on the affected side indicates that the glabellar reflex can be altered bilaterally in hemiplegic patients.

Adult↗
PubMed1982

Quantitative analysis of blink reflexes in patients with hemiplegic disorders.

Electrically evoked blink reflexes were investigated in 18 patients with hemiplegia and in 15 control subjects, using common electromyographic techniques. The EMG activities of the early and late components were quantitatively and integration. In addition, the latencies of the single components were determined. Regarding the EMG activity of the late components two major types of BR alteration could be distinguished. In type I stimulation of the clinically affected side evoked significantly decreased late components on both the affected and the unaffected sides. This pattern points to a lowered excitability of the brain stem trigeminal systems and may be associated with predominantly sensory disorders. In type II the decrease of the late components was confined to the affected side independent of the side of stimulation. This pattern may indicate a lowered excitability of the brain stem facial systems and/or of the lateral bulbar reticular formation and may be correlated with predominantly motor deficits. Both types are presumably due to a loss of facilitatory influences associated with the hemispheral lesion. Although the early component was frequently decreased on the affected side there was no consistent pattern and no relation to the alterations of the late components. The latencies of both responses, predominantly of the late ones were frequently prolonged, in particular following stimulation of the affected side. Comparison of the seemingly normal components in the patients with the corresponding control values pointed to a generally lowered blink reflex excitability in hemiplegic patients.

Adult↗
PubMed1982

Unilateral reduction of the early and late blink reflex component in hemiparkinson syndrome.

Electrically evoked blink reflexes were recorded in 10 patients with unilateral tremor and/or rigor, mostly diagnosed as hemiparkinson syndrome. Five of the patients could be investigated before and after stereotaxic thalamo-subthalamotomy. The EMG activity of the early and late components was quantified by means of averaging and integrations techniques. In addition, the latencies of the reflex components were determined. The major finding was a unilateral decrease of the EMG activity of both the early and late components confined to the clinically affected side. This pattern points to a lowered state of excitability of neurons in or close to the facial nucleus, probably due to a dysfunction of contralateral EPMS structures. A loss of facilitatory influences from EPMS centers, e.g. the nigro-striatal system, on brain stem neurons in the area of the facial nucleus appears most probable. The latencies of the early and late components were mostly normal. After stereotaxic surgery, the BR activity was decreased bilaterally. The difference between the affected and unaffected sides, however, was nearly unchanged. At present an unspecific postoperative effect cannot be excluded.

Adult↗
PubMed1977

Interactions between unloading reflex and voluntary muscle activity under different instructions.

Changes in the instantaneous firing frequency of a separate motor unit (MU) in the unloading reflex and voluntary movement according to instruction, triggered by external mechanical disturbance in the position of the arm, were studied. The activity of m. biceps brachii and m. pronator teres was recorded using surface bipolar electrodes with small area and small interelectrode distance. The correct position of the electrode with respect to the MU was controlled vectorelectromyographically. The probabilities of the appearance of an impulse in successive 10 ms intervals before and after the unloading were determined. The effect of the voluntary movement performed by the subject according to instruction on the reflex response is demonstrated, as well as the opposite effect, i.e. of the type of the external disturbing action on the voluntary movement. Certain problems related to the transcortical reflex response are discussed. It is assumed that some maxima in the summated electromyogram (EMG) which correspond in time to transcortical reflex influences are not due to a higher excitability of the alpha-motoneurones, but they are obtained by grouping of the impulses of many activated tonic MUs whose activity is recorded by recording the summated EMG.

Action Potentials↗
PubMed1977

Mathematical model of the summated electromyogram based on the probability characteristics of single motor units.

A probability model for synthesis of the summated electromyogram (EMG) is elaborated on the basis of previous experimental work describing the probability characteristics of the firing of single motor units (MUs) subjected to different external disturbances (e. g. stretch or unloading) and following different instructions for voluntary activation. The possibility is demonstrated some maxima in the summated EMG to be due not to increases excitability of the alpha-motoneurones but to synchronization of the impulses after application of external disturbance, whereby the probability for appearance of impulse in some time intervals will disturbance, whereby the probability for appearance of impulse in some time intervals will increase. The proposed model is used for qualitative studies as a number of factors influencing the value of the signal recorded upon leading off of summated EMG are not taken into account.

Electromyography↗
PubMedDate not supplied

Relation between the motor units recruitment threshold and their potentials propagation velocity at isometric activity.

The relationship between the propagation velocity of the excitation along the muscle fibers of the motor units (MUs) and their threshold of recruitment at different level of isometric voluntary contraction was investigated. The threshold of recruitment was measured by the value of the muscle force, expressed in percents from the maximal voluntary contraction (MVC) at which the first impulse of the MU appeared. A wire subcutaneous branched electrode was used to select the potentials from a single MU. The selected in this way MU impulses were used as a trigger to average two electromyographic (EMG) signals picked up by means of two monopolar surface electrodes with small leading-off areas mounted on a common plate at a distance of 10 mm from one another. The propagation velocity of the extraterritorial potentials of the MUs increased non-linearly with the increase of the recruitment threshold. The relationship was fitted as V = square root of a+b.theta, where v is the propagation velocity, theta is the threshold of recruitment and a and b are constants. The consideration of the velocity of propagation as a "size principle parameter" was discussed and the limitations of the latter are pointed out.

Action Potentials↗
PubMedDate not supplied

Influence of the interimpulse interval on the propagation velocity of the motor unit potentials.

The influence of the rate of firing of separate human motor units (MUs) from m. biceps brachii on the propagation velocity of the extraterritorial MU potentials was investigated. The experiments were carried out under normal physiological conditions--different level of isometric voluntary activity and different rate of firing--respectively. The subjects succeeded to keep 3-5 different levels of relatively constant rate of firing with mean value of the interimpulse intervals ranging from 67.8 to 183.2 ms. Wire subcutaneous branched electrodes and conventional bipolar wire electrodes were used to separate potentials from single motor units and as a trigger to average surface electromyographic (EMG) signals. MUs with superficially and deep lying muscle fibers were investigated. The propagation velocity of the potentials of superficially lying MUs was calculated from the time shift between the EMG signals picked up by means of two monopolar surface electrodes placed along the direction of the muscle fibers. For deep lying MUs the changes in the propagation velocity were estimated indirectly by the changes in the duration of the extraterritorial MU potentials. No systematic relationship between the rate of firing and the velocity of propagation was found.

Action Potentials↗