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Biomedical subjects

A S Alexandrov

Publications and source records attributed to A S Alexandrov.

15 recordsLinked to original sources
PubMed2006

Simulating focal demyelinating neuropathies: membrane property abnormalities.

Membrane properties such as potentials (intracellular, extracellular, electrotonic) and axonal excitability indices (strength-duration and charge-duration curves, strength-duration time constants, rheobasic currents, recovery cycles) can now be measured in healthy subjects and patients with demyelinating neuropathies. They are regarded here in two cases of simultaneously reduced paranodal seal resistance and myelin lamellae in one to three consecutive internodes of human motor nerve fiber. The investigations are performed for 70 and 96% myelin reduction values. The first value is not sufficient to develop a conduction block, but the second leads to a block and the corresponding demyelinations are regarded as mild and severe. For both the mild and severe demyelinations, the paranodally internodally focally demyelinated cases (termed as PIFD1, PIFD2, and PIFD3, respectively, with one, two, and three demyelinated internodes) are simulated using our previous double-cable model of the fiber. The axon model consists of 30 nodes and 29 internodes. The membrane property abnormalities obtained can be observed in vivo in patients with demyelinating forms of Guillain-Barré syndrome (GBS) and multifocal motor neuropathy (MMN). The study confirms that focal demyelinations are specific indicators for acquired demyelinating neuropathies. Moreover, the following changes have been calculated in our previous papers: (1) uniform reduction of myelin thickness in all internodes (Stephanova et al. in Clin Neurophysiol 116: 1153-1158, 2005); (2) demyelination of all paranodal regions (Stephanova and Daskalova in Clin Neurophysiol 116: 1159-1166, 2005a); (3) simultaneous reduction of myelin thickness and paranodal demyelination in all internodes (Stephanova and Daskalova in Clin Neurophysiol 116: 2334-2341, 2005b); and (4) reduction of myelin thickness of up to three internodes (Stephanova et al., in J Biol Phys, 2006a,b, DOI: 10.1007/s10867-005-9001-9; DOI: 10.1007/s10867-006-9008-x). The membrane property abnormalities obtained in the homogeneously demyelinated cases are quite different and abnormally greater than those in the case investigated here of simultaneous reduction in myelin thickness and paranodal demyelination of up to three internodes. Our previous and present results show that unless focal demyelination is severe enough to cause outright conduction block, changes are so slight as to be essentially indistinguishable from normal values. Consequently, the excitability-based approaches that have shown strong potential as diagnostic tools in systematically demyelinated conditions may not be useful in detecting mild focal demyelinations, independently of whether they are internodal, paranodal, or paranodal internodal.

Animals↗
PubMed2006

Normal-state diamagnetism of charged bosons in cuprate superconductors.

Normal-state orbital diamagnetism of charged bosons quantitatively accounts for recent high-resolution magnetometery results near and above the resistive critical temperature T(c) of superconducting cuprates. The parameter-free descriptions of normal-state diamagnetism, T(c), upper critical fields, and specific heat anomalies support the 3D Bose-Einstein condensation of preformed real-space pairs with a zero off-diagonal order parameter above T(c) at variance with phase fluctuation scenarios of cuprates.

Journal Article↗
PubMed2006

Phase coexistence and resistivity near the ferromagnetic transition of manganites.

Pairing of oxygen holes into heavy bipolarons in the paramagnetic phase and their magnetic pair breaking in the ferromagnetic phase (the so-called current-carrier density collapse) has accounted for the first-order ferromagnetic-phase transition, colossal magnetoresistance, isotope effect, and pseudogap in doped manganites. Here we propose an explanation of the phase coexistence and describe the magnetization and resistivity of manganites near the ferromagnetic transition in the framework of the current-carrier density collapse. The present quantitative description of resistivity is obtained without any fitting parameters, by using the experimental resistivities far away from the transition and the experimental magnetization, and is essentially model-independent.

Journal Article↗
PubMed2005

Magnetic quantum oscillations in nanowires.

Analytical expressions for the magnetization and the longitudinal conductivity of nanowires are derived in a magnetic field, B. We show that the interplay between size and magnetic field energy-level quantizations manifests itself through novel magnetic quantum oscillations in metallic nanowires. There are three characteristic frequencies of de Haas-van Alphen (dHvA) and Shubnikov-de Haas (SdH) oscillations, F = F(0)/(1 + gamma)(3/2), and F(+/-) = 2F(0)/|1 + gamma +/- (1 + gamma)(1/2)|, in contrast with a single frequency F(0) = S(F)plankc/(2pie) in simple bulk metals. The amplitude of oscillations is strongly enhanced in some magic magnetic fields. The wire cross-section area S can be measured using the oscillations as S = 4pi(2)S(F)plank(2)c(2)/(gammae(2)B(2)) along with the Fermi-surface cross-section area, S(F).

Journal Article↗
PubMed2005

Differences in potentials and excitability properties in simulated cases of demyelinating neuropathies. Part I.

OBJECTIVE: The aim of this study is to investigate the potentials (intracellular, extracellular, electrotonic) and excitability properties (strength-duration and charge-duration curves, strength-duration time constants, rheobases, recovery cycles) in three cases of uniform myelin wrap reduction (20, 50 and 70%) along the fibre length. METHODS: The internodally systematically demyelinated cases (termed as ISD1, ISD2 and ISD3) are simulated using our previous double cable model of human motor fibres. RESULTS: In the more severely demyelinated cases, the intracellular potentials are with significantly reduced amplitude, prolonged duration and slowed conduction velocity, whereas the electrotonic potentials show greater increase in the early part of the hyperpolarizing responses. The radial decline of the extracellular potential amplitudes depends on the radial distance of the field point and increases with the increase of the distance and demyelination. The time constants and rheobasic currents increase with the increase of the degree of demyelination. In the recovery cycles, the more severely demyelinated cases have greater refractoriness (the increase in threshold current during the relative refractory period), supernormality and less late subnormality than the normal case. CONCLUSIONS: The myelin thickness has significant effects on the potentials and axonal excitability properties of the simulated demyelinated human motor fibres. The obtained abnormalities in the potentials and excitability properties can be observed in Charcot-Marie-Tooth disease type 1A (CMT1A). SIGNIFICANCE: The study provides new information about the pathophysiology of human demyelinating neuropathies.

Demyelinating Diseases↗
PubMed2005

The electrophysiological profile of hereditary motor and sensory neuropathy-Lom.

OBJECTIVE: To make electrophysiological observations on a large kindred with hereditary motor and sensory neuropathy-Lom (HMSN-L) containing 27 affected individuals. CLINICAL FINDINGS: Onset was in early childhood with gait difficulty related to progressive lower limb weakness. Upper limb weakness developed later. Bulbar involvement was present in one third of the patients, and deafness appeared during the second or third decades. ELECTROPHYSIOLOGICAL FINDINGS: Electromyographic evidence of denervation was progressive, more severe distally, and greater in the legs, being total in distal lower limb muscles in most patients. Sensory action potentials were absent and motor nerve conduction was severely slowed. This included proximal upper limb (musculocutaneous and axillary), hypoglossal, and facial nerves. The severity of slowing increased during childhood. M waves, often multiple, were recorded in all affected individuals. The blink reflex showed an unusual three component response. The latencies of all three components were prolonged. CONCLUSIONS: HMSN-L is shown to be a demyelinating neuropathy involving severe and early axonal loss. The progressive slowing of nerve conduction during childhood differs from the static reduction seen in type I HMSN.

Adolescent↗
PubMed2004

Nernst effect in poor conductors and in the cuprate superconductors.

We calculate the Nernst signal in disordered conductors with the chemical potential near the mobility edge. The Nernst effect originates from the interference of itinerant and localized-carrier contributions to the thermomagnetic transport. It reveals a strong temperature and magnetic field dependence, which describes quantitatively the anomalous Nernst signal in high-Tc cuprates.

Journal Article↗
PubMed2003

Lorenz number in high T(c) superconductors: evidence for bipolarons.

The strong electron-phonon interaction in cuprates has gathered support over the last decade in a number of experiments. While phonons remain almost unrenormalized, electrons are transformed into itinerant bipolarons and thermally excited polarons when the electron-phonon interaction is strong. We calculate the Lorenz number of the system to show that the Wiedemann-Franz law breaks down because of the interference of polaron and bipolaron contributions in the heat flow. The model fits numerically the experimental Hall Lorenz number, which provides direct evidence for bipolarons in the cuprates.

Journal Article↗
PubMed2003

Elevated plasma endothelin as an additional cardiovascular risk factor in patients with Cushing's syndrome.

BACKGROUND: Recently the pathophysiological role of endothelin (ET) has been presumed in a number of adrenal disorders such as primary hyperaldosteronism, pheochromocytoma and adrenocortical insufficiency. AIM: The aim of the present study was to evaluate circulating ET-1 levels in patients with endogenous Cushing's syndrome. METHODS AND RESULTS: Plasma ET-1 levels were determined by highly sensitive RIA. Thirteen untreated subjects with Cushing's syndrome were studied: eight women and five men of mean age 44.2+/-9.5 Years (s.d.). In ten of them, Cushing's disease had been diagnosed and three had adrenal adenomas. ET-1 was 3-fold higher in the patient group than in age-matched healthy controls (n=13): 1.59+/-0.78 vs 0.46+/-0.20 pmol/l respectively, P<0.001. In adrenal adenoma patients, ET-1 was not significantly higher than in the Cushing's disease subjects (1.84+/-0.67 vs 1.51+/-0.83 pmol/l respectively, P>0.05). In three patients who died of severe cardiovascular complications, plasma ET-1 was significantly higher than in the remaining patients (2.34+/-0.35 pmol/l, P<0.05). A positive correlation was found between the total cholesterol (6.94+/-1.75 mmol/l) and ET-1 levels in the patients with Cushing's syndrome: r=+0.73, P<0.02. No correlation was observed, however, between the levels of ET-1 and blood pressure (183+/-37/106+/-18 mmHg), plasma cortisol levels (455.2+/-74.5 nmol/l) or urinary cortisol excretion (1463+/-726 nmol/24 h). The successful treatment and correction of hypercortisolism in seven patients led to insignificant reduction in plasma ET from 1.34+/-0.69 to 0.73+/-0.53 pmol/l, P>0.05. CONCLUSION: Our results clearly demonstrate that the ET system is activated in Cushing's syndrome. Elevated plasma ET-1 levels probably play a role in the pathogenesis of accelerated and early atherosclerosis development in this disorder.

Adenoma↗
PubMed2001

A-waves in patients with novel hereditary motor and sensory neuropathy Lom.

OBJECTIVES: To determine A-waves of a family with autosomal recessive form of demyelinatig hereditary motor and sensory neuropathy Lom (HMSNL). METHODS: A-waves were investigated during conventional F-wave study of family members with HMSNL which had genetic testing. RESULTS: During routine F-wave studies A-waves were observed in all tested nerves. They appeared between M-responses and F-wave. The A-waves were with low amplitude, shorter duration and constant shape and latency than F-waves. They appeared independently of F-waves. A-waves were more often recorded as multiple waves in lower and upper extremities. More than three A-waves per nerve were found mostly in ulnar and facial nerves. In 16 cases A-waves were found in the absence of F-waves. CONCLUSION: It is assumed that the A-waves could be seen as additional signs of pathology because they were not observed in unaffected members of the family and in healthy subjects.

Action Potentials↗
PubMed2001

Comparative analyse of single motor unit pattern in healthy subjects and patients with neuromuscular disorders.

OBJECTIVE: The motor unit (MU) spike trains in human muscle contractions are an object to new mathematical processing. The aim is to identify the interspike interval relations characterizing normal and changed physiological states (neuro-muscular disorders). METHODS: MU activities in healthy subjects and patients with Schwartz-Jampel syndrome, neuromiotonia and Parkinson disease were investigated. Motor unit action potentials (MUAPs) were recorded by surface multielectrode with small leading-off area without provoking a burst of activity, as usually was observed during the needle electromyography study in patients with Schwartz-Jampel syndrome and neuromiotonia. RESULTS: Different single motor unit activity in healthy subjects and patients was observed. Discharge pattern of patients was basically changed. Disturbance of different parts of the motor system leads to the changes of temporal order of interspike interval. This permit us to suppose that the pattern of repetative neuronal discharges suggests an influence at a higher level than the muscle in all investigated patients but the reason of multiple discharges with an interimpulse interval of 2 to 10 ms probably originated in the muscle membrane. CONCLUSIONS: The comparative analysis of MU patterns in healthy subjects and patients with neuro-muscular disorders can help us to disclose inapparent connections between cortical and spinal level of motor control.

Action Potentials↗
PubMed1999

Peripheral late waves in patients with hereditary motor sensory neuropathy.

Patents with different forms of hereditary motor sensory neuropathy (HMSN) were investigated. Peripheral late waves (PLWs) were recorded when determining F wave at supramaximal stimulation. We registered them most frequently in patients with demyelinating neuropathies (HMSN1--58% and HMSN3--100%) and in patients with HMSN2--24% and HMN--13%. In patients with HMSN1 and HMSN3 the peripheral late waves sometimes were more than one--two or three. They had a consistent appearance above a maximal threshold of stimulation, an invariable latency, amplitude and wave-form. Their latency times were in parallel with the M-response latency. These PLWs can be explained by collateral regeneration in case of axonal neuropathy. The ephaptic transmission might be taken in consideration when interpreting data from patients with demyelinating processes.

Adolescent↗
PubMed1999

Comparison between normal and parkinsonian pattern of motor unit firing.

There are many approaches to the study of the activity of separate motor units (MU) in man. In humans alpha motoneuron activity consists in generating spike trains that innervate groups of muscle fibers thus building MU. In the present study we used a new evolution of the joint interval density analysis in patients with Parkinson disease.

Adult↗
PubMedDate not supplied

Comparative analysis between Duchenne and Becker types muscular dystrophy.

Duchenne and Becker types of muscular dystrophy are usually differentiated according to age of onset and rate of progression criteria which are not sufficient. The aim of this paper was to re-establish the clues for distinguishing Duchenne from Becker types of muscular dystrophy. According to the onset and progression of the disease, one hundred and eleven patients were subdivided into two groups. First group--Becker muscular dystrophy--consisted of 40 patients and second one of 71 patients with Duchenne type of muscular dystrophy. Clinical data confirm some well known differences between Duchenne and Becker muscular dystrophy concerning the age of onset, severity of disease and rate of progression. Electromyographic signs of myopathic changes and spontaneous activity were found in both diseases. Spontaneous activity--bizarre and fibrillation potentials, as well as sharp waves are more common for Duchenne type. The differences between the Becker from Duchenne type of muscular dystrophy can be described on the basis of complex investigations (clinical, electromyographical, histological and biochemical).

Action Potentials↗