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Biomedical subjects

P Christova

Publications and source records attributed to P Christova.

28 records · Page 2Linked to original sources
PubMed1998

Discharge pattern of human motor units during dynamic concentric and eccentric contractions.

OBJECTIVES: A total of 45 motor units (MUs) from the human biceps brachii muscle were investigated during isovelocity concentric and eccentric movements performed by means of a device implementing an external torque in the direction of the extension proportionally to the elbow angle changes. The effects of movement velocity on the recruitment and decruitment thresholds (RT and DT) and the corresponding discharge patterns were determined. METHODS: A wire branched electrode placed subcutaneously was used to discriminate the potentials from a single MV. RESULTS: The majority of MUs (91%) were recruited at lower torque values with the increase of movement velocity. The decrease of RT was statistically significant for 47% of the investigated MUs. A typical discharge pattern of short first interspike interval (ISI) followed by a longer one was observed for 93% of all MUs. After the first 2-3 spikes the rate of the MU discharge was approximately constant regardless of the fact that the muscle force gradually increased until the end of the concentric movement. CONCLUSIONS: There are differences in the muscle force control during shortening and lengthening contractions. For 82% of the investigated MUs DT was smaller at faster movements and for 21 MUs (47%) the decrease of DT was significant. The gradually declined MU discharge rate throughout the entire movement with a very long last ISI was demonstrated for 93% of the investigated MUs.

Adult↗
PubMed1998

Discharge rate of selected motor units in human biceps brachii at different muscle lengths.

Action potentials of selected motor units (MUs) from biceps brachii muscle were recorded and analysed at three different elbow angles: 90, 120 and 150 degrees, corresponding to short, control and long muscle length, respectively. Using branched and conventional bipolar wire electrodes, superficial and deep-situated MUs were selectively recorded at relatively equal torques (the torque values were normalized to the corresponding maximal torque for a given muscle length). A total of 138 MUs (74 superficial and 64 deep) were investigated. The mean interspike intervals were significantly shorter at 90 degrees for the majority (52.2%) of the investigated MUs than at the other two angles. This increased discharge rate compensates for the reduction of twitch duration of evoked contraction at short muscle length. The other MUs were divided into three almost equal groups: two with significantly higher discharge rates at 120 and 150 degrees and one with discharge rates unaffected by the joint angle. No significant difference in the discharge rate of superficial and deep MUs at a fixed joint angle was found.

Action Potentials↗
PubMed1996

Thermal unfolding of the DNA-binding protein Sso7d from the hyperthermophile Sulfolobus solfataricus.

Thermal unfolding of the small hyperthermophilic DNA-binding protein Sso7d was studied by circular dichroism spectroscopy and differential scanning calorimetry. The unfolding transition can be described by a reversible two state process. Maximum stability was observed in the region between pH 4.5 and 7.0 where Sso7d unfolds with a melting temperature between 370.8 to 371.9 K and an unfolding enthalpy between 62.9 and 65.4 kcal/mol. The heat capacity differences between the native and the heat denatured states obtained by differential scanning calorimetry (620 cal/(molK)) and circular dichroism spectroscopy (580 cal/(mol K)) resulted in comparable values. The thermodynamic reason for the high melting temperature of Sso7d is the shallow stability curve with a broad free energy maximum, corresponding to the relatively small heat capacity change which was obtained. The calculated stability curve shows that Sso7d has, despite of its high melting temperature, an only moderate intrinsic stability, which reaches its maximum (approximately 7 kcal/mol) at 282 K. Sso7d is particularly poorly stabilized (approximately 1 kcal/mol) at the maximum physiological growth temperature of Sulfolobus solfataricus. Sso7d has furthermore untypically low specific enthalpy (0.99 kcal/(mol residue)) and entropy (2.99 cal/(mol K)) values at convergence temperatures. No significant differences in thermal stability of the partially methylated Sso7d from Sulfolobus solfataricus and the cloned non-methylated form of the protein expressed in Escherichia coli were observed.

Amino Acid Sequence↗
PubMed1994

Binding of heme-CO to bovine and porcine beta-lactoglobulins.

Two molecules of heme-CO bind to bovine or porcine beta-lactoglobulin (BLG) with an average affinity of 0.5 microM. The affinity increases with pH, with a transition near pH 7.5, indicating a conformational change in the protein. Heme-CO does not bind to the predominantly alpha-helix conformation of BLG, which occurs in solutions with more than 40% ethanol or methanol. Fluorescence energy transfer measurements have been made for the complex of BLG with retinol and/or heme-CO. Two species of BLG were used. While bovine BLG possesses two tryptophans (at positions 19 and 61) which are quenched by about a factor of 2 by either retinol or heme-CO, the porcine species has only one tryptophan (at position 19) whose fluorescence is decreased by a factor of 15 when both hemes are bound, indicating that at least one of the heme-binding sites is near (< 20 A) to this tryptophan. The fluorescence of retinol (complexed to BLG) is also quenched by the addition of heme-CO, indicating that BLG can bind both molecules simultaneously; a separation of 25 A between retinol and heme was calculated. The results suggest at least two hydrophobic pockets for this protein.

Animals↗
PubMed1992

Conduction velocity of antigravity muscle action potentials.

The conduction velocity of the impulses along the muscle fibers is one of the parameters of the extraterritorial potentials of the motor units allowing for the evaluation of the functional state of the muscles. There are no data about the conduction velocities of antigravity muscleaction potentials. In this paper we offer a method for measuring conduction velocity of potentials of single MUs and the averaged potentials of the interference electromiogram (IEMG) lead-off by surface electrodes from mm. sternocleidomastoideus, trapezius, deltoideus (caput laterale) and vastus medialis. The measured mean values of the conduction velocity of antigravity muscles potentials can be used for testing the functional state of the muscles.

Action Potentials↗
PubMed1992

The effect of muscle fiber length change on motor units potentials propagation velocity.

The influence of the muscle length on the propagation velocity of the extraterritorial action potentials was studied. Single low--threshold motor units (MUs) from m. biceps brachii, both with superficially and deeply situated fibers were investigated. The velocity of excitation propagation decreases with the muscle elongation. This dependence is not so prominent for MUs having more deeply situated fibers. In case of superficial MUs the experimentally obtained values are in better agreement with the core conductor model (v = k.square root of r), than with the linear dependence of the propagation velocity (v) on the muscle fiber radius (r) (v = k.r) observed by Håkansson (1957) on frog muscles. The observed difference between superficial and deep MUs was discussed. At least two factors may be responsible for greater changes of the excitation propagation velocity along the fibers of the superficially situated MUs during the muscle elongation. One possible factor is the essential change in the external resistance due to the decrease of distance from the fibers to the skin surface. The greater change of the muscle length during a passive muscle stretching might be another reason.

Action Potentials↗
PubMed1990

Optimal interelectrode distance with the method of source density analysis.

Three error sources which contribute to the total error of the second space derivative (SSD) used with the method of current source density analysis (CSD) were examined. The approximate formula usually used with CSD was the first error source. Another error originated from the finite interelectrode distance. The measurement error of the device with which the potential was recorded was the third error source herein considered. A relative SSD error was determined to estimate the effect of the errors examined. The experimental data (a section from the three-dimensional potential profile in annuran cerebellum produced by parallel fiber activation) were fitted by polynomials, Fourier expansion and cubic splines. Then a preferable interelectrode distance was obtained corresponding to the minimal relative error.

Animals↗
PubMed1984

Pyridoxal phosphate modified cytochromes c. Identification and electron transfer properties.

The preparation, purification and characterization of the three singly, three doubly and one triply substituted derivatives of cytochrome c modified by pyridoxal phosphate (PLP) at lysine residues are reported. The PLP positions in PLP derivatives were determined by the amino acid analysis and sequence of PLP peptides. The results identified the lysine at position 86 in one of the singly substituted, lysine 79 in the other singly substituted and lysines 86 and 79 in the third doubly substituted cytochrome c derivatives. The area surrounding phenylalanine 82 forms the predominant PLP binding site on the cytochrome c molecule. The visible, CD and proton NMR spectra, the full intensity of the conformation-sensitive 695 nm band and the oxidation-reduction properties provide evidence to confirm the conclusion that singly and doubly substituted PLP cytochromes c retain the native conformation. The ability to restore both succinate and ascorbate/TMPD oxidation in cytochrome c-depleted mitochondria decreases in the order: native cytochrome c greater than PLP-Lys-79-cytochrome c greater than PLP-Lys-86-cytochrome c greater than PLP-Lys-79,86-cytochrome c greater than triply substituted derivative.

Amino Acids↗
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↗