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3492. Brachistochrona - problém stále živý
- Creator:
- Slavíček, Josef and Musilová, Jana
- Format:
- print, bez média, and svazek
- Type:
- model:article and TEXT
- Subject:
- Černohorský, Martin, 1923-, Bernoulli, Johann, 1667-1748, teoretická fyzika, mechanika, variační počet, theoretical physics, mechanics, calculus of variations, 6, and 53
- Language:
- Czech
- Description:
- V matematice a fyzice nejsou výjimkou situace, kdy řešení problému, jehož praktický význam se někomu může jevit i sporným, vede ke vzniku rozsáhlé obecné teorie či k rozvoji nové účinné metodiky. Je tomu tak i v případě problému brachistochrony, který sehrál klíčovou roli při vzniku variačního počtu. Lze jej formulovat takto: Jaký tvar má mít hladká skluzavka spojující dva zadané body ležící ve svislé rovině, aby tělísko volně vypuštěné z výše položeného z nich sklouzlo vlivem tíže do druhého bodu v nejkratším možném čase? V první části tohoto článku se věnujeme historickým aspektů problému brachistochrony a komentujeme jednotlivá významnější řešení, která byla v průběhu doby předložena. Druhá část sahá do současnosti. Uvádíme v ní tři téměř elementární a navzájem zcela odlišné způsoby přístupu k nalezení brachistochrony: fyzikální, geometrický a variační. Příspěvek věnujeme k významnému životnímu výročí profesoru RNDr. Martinu Černohorskému, CSc., učiteli několika generací brněnských fyziků, který se ve svém vědeckém díle zabýval na mimořádné fyzikální úrovni mj. i historií Newtonovy mechaniky., Josef Slavíček, Jana Musilová., and Obsahuje seznam odkazů a literatury
- Rights:
- http://creativecommons.org/licenses/by-nc-sa/4.0/ and policy:public
3493. Bracketing paradoxes - are there any?
- Creator:
- Štekauer, Pavol
- Format:
- Type:
- model:internalpart and TEXT
- Language:
- English
- Description:
- Příspěvek se zabývá složitými otázkami závorkových paradoxu. Po přehledu nejvlivnějších teorií následuje stručný náčrt základních principů onomaziologické teorie slovo-tvorby, která tvoří základ nového přístupu k problematice závorkových paradoxů. Tento jev je doložen více příklady, které ilustrují, že pojem závorkových paradoxů se váže na určitá teoretická východiska a není nezbytným a inherentním znakem anglické slovo-tvorby.
- Rights:
- http://creativecommons.org/licenses/by-nc-sa/4.0/ and policy:public
3494. Braggovské mřížky ve vláknových laserech se spontánním rozmítáním vlnové délky
- Creator:
- Peterka, Pavel, Honzátko, Pavel, Koška, Pavel, Todorov, Filip, Aubrecht, Jan, Navrátil, Petr, Podrazký, Ondřej, Kašík, Ivan, and Čtyroký, Jiří
- Format:
- bez média and svazek
- Type:
- model:article and TEXT
- Subject:
- fiber lasers, mode instability, fiber Bragg gratings, spontaneous laser wavelength sweeping, vláknový laser, nestabilita módů, vláknová braggovská mřížka, and spontánní rozmítání vlnové délky
- Language:
- Czech
- Description:
- Fiber lasers may exhibit instabilities and self-pulsed regimes that can have catastrophic consequences on their components. One of the self-pulsing regimes is the recently observed self-induced laser line sweeping (SLLS). The SLLS is characterized by periodic wavelength drift over broad spectral interval of several nanometers followed by quick bounce backward. The sweeping rate is relatively slow, of the order of nm per second. Thanks to narrow line width and simple construction, the SLLS fiber lasers are attractive sources for testing of photonics components, interrogation of optical fiber sensor arrays and for laser spectroscopy. The SLLS can be explained by creation of a standing wave built up by laser longitudinal mode and consequent spatial-hole burning and refractive-index grating inscription. We review an evaluation of reflection spectra of transient fiber Bragg gratings that are created in SLLS fiber lasers. The evaluation is shown using example of ytterbium fiber laser in Fabry-Perot configuration of the laser cavity. We discuss also the effect of the laser wavelength self-sweeping on the triggering of the self-Q-switched regime and the generation of giant laser pulses. and Vláknové lasery mohou vykazovat nestabilní pulzní režimy. Jedním z režimů se samovolnou generací impulzů je nedávno pozorovaný jev spontánního rozmítání vlnové délky vláknového laseru. Tento jev je doprovázen charakteristickým periodickým posunem vlnové délky laseru v širokém rozsahu vlnových délek několika nanometrů. Rychlost rozmítání je relativně pomalá, řádově nanometry za sekundu. Díky úzké šířce čáry a jednoduché konstrukci jsou tyto lasery atraktivními zdroji záření, mj. pro systémy optických vláknových senzorů, charakterizaci optických komponent a laserovou spektroskopii. Samovolné rozmítání vlnové délky lze vysvětlit vytvořením stojaté vlny určené podélným módem rezonátoru. Vznik stojaté vlny vede k podélné modulaci indexu lomu - zápisu mřížky. V tomto příspěvku podáváme shrnutí dosud prezentovaných prací týkajících se modelování odrazivosti braggovských mřížek ve vláknových laserech se spontánním rozmítáním vlnové délky. Popisujeme experimentální ytterbiový vláknový laser s tímto typem nestability, následovaný teoretickým popisem braggovské mřížky a ukázkou výpočtu odrazivosti pro laser odpovídající experimentu. Je též diskutován význam spontánního rozmítání na spouštění režimu samočinného Q-spínání a generaci gigantických laserových pulzů.
- Rights:
- http://creativecommons.org/licenses/by-nc-sa/4.0/ and policy:public
3495. Brain activation during volitional control of breathing
- Creator:
- Václav Šmejkal, Rastislav Druga, and Jaroslav Tintěra
- Format:
- print, bez média, and svazek
- Type:
- article, články, model:article, and TEXT
- Subject:
- Fyziologie člověka a srovnávací fyziologie, sluch, magnetická rezonance, hearing, respiratory control, brain activation, magnetic resonance, 14, and 612
- Language:
- English
- Description:
- Functional magnetic resonance imaging (fMRI) was used to demonstrate the brain activation during volitional control of breathing in nine healthy human subjects. This type of breathing was induced by acoustic stimuli dictating the respiratory frequency. During the period of dictated breathing not only the frontal and temporal lobes of the brain, but also the parietal lobes were bilaterally activated. The frontal lobe was activated bilaterally in all subjects, with frequent activation of Brodmann areas 4 and 6. In the parietal lobe, activation could mostly be demonstrated in gyrus postcentralis and the same was true for area 22 in the temporal lobe., V. Šmejkal, R. Druga, J. Tintěra., and Obsahuje bibliografii
- Rights:
- http://creativecommons.org/licenses/by-nc-sa/4.0/ and policy:public
3496. Brain cell RNA: sexual differences in the rat
- Creator:
- Šterc, J. and Nováková, V.
- Type:
- article, model:article, and TEXT
- Subject:
- laboratory rats, sexual differences, brain cells, ribosomal RNA, ontogeny, and hormonal influence
- Language:
- English
- Description:
- Sexual differences in the total content of ribosomal RNA, established by cytophotometric measurements in neurones from selected brain regions, were studied in rats of the Wistar strain. In females of reproductive age, cyclic changes of RNA were synchronized with their oestrous cycle, the values being higher in the oestrous phase than in dioestrus. These changes were observed in pyramidal cells of the hippocampus and of the frontal cortex, in cells of anterior thalamus, of ventromedial and lateral hypothalamus and of tuberculi olfactorii. However, cycling cells were not disclosed in septum and thalamus posterior. A dependence upon the actual level of ovarial hormones was found in ventromedial hypothalamic cells only. In general, the RNA values in males of the same age corresponded to values of dioestrous females. The differences between newborn and 7-day-old pups were not marked enough and did not allow to define the critical period responsible for initiation of this sexual difference. In senescent rats, this difference persisted. During the stable phase of long-lasting dioestrus, the total RNA content in cells of the frontal cortex, hippocampus and hypothalamus was higher in females than in males of the same age which may suggest a faster reduction of this substance in aged males. The prolonged influence of oestrogens in the oestrous phase of the climacteric period (preceding the permanent dioestrus) decreased the RNA values in hippocampal and hypothalamic neurones even below the level established during the permanent dioestrus (and thus reached male values). On the contrary, in frontal cortical neurones, the female values remained higher in the permanent dioestrus as well as during long-lasting oestrus. A discussion concerns the possible participation of genetic determination and of the actual state of ovarial hormones in the manifestation of sexual differences in brain cells of the rat.
- Rights:
- http://creativecommons.org/licenses/by-nc-sa/4.0/ and policy:public
3497. Brain fluid channels for metabolite removal
- Creator:
- Maloveská, Marcela, Humeník, Filip, Vikartovská, Zuzana, Hudáková, Nikola, Almášiová, Viera, Krešáková, Lenka, and Čížková, Daša
- Format:
- počítač and online zdroj
- Type:
- model:article and TEXT
- Subject:
- brain ISF, CSF, meningeal lymphatics, and lymphatic drainage
- Language:
- English
- Description:
- The adult human brain represents only 2 % of the body's total weight, however it is one of the most metabolically active organs in the mammalian body. Its high metabolic activity necessitates an efficacious waste clearance system. Besides the blood, there are two fluids closely linked to the brain and spinal cord drainage system: interstitial fluid (ISF) and cerebrospinal fluid (CSF). The aim of this review is to summarize the latest research clarifying the channels of metabolite removal by fluids from brain tissue, subarachnoid space (SAS) and brain dura (BD). Special attention is focused on lymphatic vascular structures in the brain dura, their localizations within the meninges, morphological properties and topographic anatomy. The review ends with an account of the consequences of brain lymphatic drainage failure. Knowledge of the physiological state of the clearance system is crucial in order to understand the changes related to impaired brain drainage.
- Rights:
- http://creativecommons.org/licenses/by-nc-sa/4.0/ and policy:public
3498. Brain lipid binding protein (FABP7) as modulator of astrocyte function
- Creator:
- Kipp, M., Clarner, T., Gingele, S., Pott, F., Amor, S., Valk, P. van der, and Beyer, C.
- Format:
- Type:
- article, články, model:article, and TEXT
- Subject:
- Fyziologie člověka a srovnávací fyziologie, roztroušená skleróza, regenerace (biologie), multiple sclerosis, regeneration (biology), oligodendrocyte, astroglia, remyelination, 14, and 612
- Language:
- English
- Description:
- Over a century ago, hyperplasia and hypertrophy of astrocytes was noted as a histopathological hallmark of multiple sclerosis and was hypothesized to play an important role in the development and course of this disease. However until today, the factual contribution of astrocytes to multiple sclerosis is elusive. Astrocytes may play an active role during degeneration and demyelination by controlling local inflammation in the CNS, provoking damage of oligodendrocytes and axons, and glial scarring but might also be beneficial by creating a permissive environment for remyelination and oligodendrocyte precursor migration, proliferation, and differentiation. Recent findings from our lab suggest that brain lipid binding protein (FABP7) is implicated in the course of multiple sclerosis and the regulation of astrocyte function. The relevance of our findings and data from other groups are highlighted and discussed in this paper in the context of myelin repair., M. Kipp ... [et al.]., and Obsahuje bibliografii a bibliografické odkazy
- Rights:
- http://creativecommons.org/licenses/by-nc-sa/4.0/ and policy:public
3499. Brain natriuretic peptide and NT-proBNP levels reflect pulmonary artery systolic pressure in trekkers at high altitude
- Creator:
- Woods, D. R., Mellor, A., Begley, J., Stacey, M., John O'Hara, Hawkins, A., Yarker, J., Foxen, S., Smith, C., and Boos, C.
- Format:
- Type:
- article, články, model:article, and TEXT
- Subject:
- Fyziologie člověka a srovnávací fyziologie, hypoxie, hypoxia, altitude, BNP, NT-proBNP, pulmonary artery systolic pressure, 14, and 612
- Language:
- English
- Description:
- Our objective was to evaluate the utility of the natriuretic peptides BNP (brain natriuretic peptide) and NT-proBNP as markers of pulmonary artery systolic pressure (PASP) in trekkers ascending to high altitude (HA). 20 participants had BNP and NT- proBNP assayed and simultaneous echocardiographic assessment of PASP performed during a trek to 5150 m. PASP increased significantly (p=0.006) with ascent from 24±4 to 39±11 mm Hg at 5150 m. At 5150 m those with a PASP ≥ 40 mm Hg (n=8) (versus those with PASP<40 mm Hg) had higher post-exercise BNP (pg/ml): 54.5±36 vs. 13.4±17 (p=0.012). Their resting BNP at 5150 m was also higher: 57.3±43.4 vs. 12.6±13 (p=0.017). In those with a pathological ( ≥ 400 pg/ml) rise in NT-proBNP at 5150 m (n=4) PASP was significantly higher: 45.9±7.5 vs. 32.2±6.2 mm Hg (p=0.015). BNP and NT-proBNP may reflect elevated PASP, a central featur e of high altitude pulmonary oedema, at HA., D. R. Woods ... [et al.]., and Obsahuje bibliografii a bibliografické odkazy
- Rights:
- http://creativecommons.org/licenses/by-nc-sa/4.0/ and policy:public
3500. Brain-derived neurotrophic factor treatment increases the skeletal muscle glucose transporter 4 protein expression in mice
- Creator:
- Suwa, M., Yamamoto, K.-I., Nakano, H., Sasaki, H., Radak, Z., and Kumagai, S.
- Format:
- print, bez média, and svazek
- Type:
- article, články, model:article, and TEXT
- Subject:
- Fyziologie člověka a srovnávací fyziologie, svaly, muscles, BDNF, GLUT4, hypophagia, skeletal muscle, 14, and 612
- Language:
- English
- Description:
- The purpose of the present study was to investigate whether peripheral brain-derived neurotrophic factor (BDNF) treatment induced metabolic adaptations in mouse skeletal muscle. BDNF (20 mg/kg/day) was injected subcutaneously for successive 14 days. BDNF treatment significantly reduced the total food intake and inhibited the weight gain in comparison to the control group. The glucose transporter 4 (GLUT4) protein expression in the gastrocnemius muscle was significantly increased by BDNF treatment in comparison to the control and pair-fed groups. Neither the oxidative nor the glycolytic enzyme activities in the gastrocnemius muscle changed after the BDNF treatment. These results suggest that the peripheral BDNF treatment promotes the skeletal muscle GLUT4 protein expression as well as hypophagia., M. Suwa ... [et al.]., and Obsahuje bibliografii a bibliografické odkazy
- Rights:
- http://creativecommons.org/licenses/by-nc-sa/4.0/ and policy:public