Ratones Wistar 6 meses de edad se dividieron en dos grupos: The quantitative changes promoted by endurance training on so-matic nerves are well established. There is an increase in the area of myelin sheath MA , unmyelinated axons UA and myelinated fibers. The vagus nerve is one of the most important visceral nerve controlling the function of thoracic and abdominal viscera and maintai-ning homeostasis under changing environmental and behavioral con-ditions.
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In this study we have used electron microscopy to check the in-fluence of aerobic training on quantitative aspects from myelinated and unmyelinated fibers of the vagus nerve using the Wistar rat as an animal model. The effects of treadmill training on vagus nerve structure were assessed by comparison of two sets of rats: Therefore, this study aimed to analyze the effect of endurance training on the vagus nerve morphology and ultrastructure of rats.
Male Wistar rats 6 months of age were acclimated during one week before the initiation of the experiment. It received commercial food and water ad libitum , and housed in a light- and temperature--controlled room. After acclimatization, rats were randomly divided into two groups: AT group was submitted to a treadmill exercise training program according to previously described.
The he-arts were excised in diastole, heparinized and the myocardium was perfused through the aorta using a 2. The left ventricle, including the septum, was then isolated and weighted. A perpendicular section to the long axis of the left ventricle including the entire thickness of the left ventricle LV was cut at the level of the papillary muscles. Then, the thickness of the LV wall and internal diameter were obtained by measuring the widths of four of uniformly spaced sites along the length of each section using a computerized program Axio Vision, Carl Zeiss AG, Jena Germany.
Under a dissecting microscope, the vagus nerve trunk was exposed at mid-cervical level and bathed in fixative solution 2. In order to obtain transverse sections, the nerve was oriented longitudinally in a silicone mold and embedded in Araldite. To determine the transversal section area of the nerve, semi-thin sections of 0. The ratio of axon diameter to the total fiber diameter g ratio diameter - a measure of degree of myelination was also calculated for the two groups.
Only myelinated axon MA and Schwann cell SCs whose contour was completely within each field was counted. In addition, the number of unmyelinated axons UMs present in each section was obtained with electron micrographs at a magnification of x on 10 randomly chosen fields per animal.
Five electron photomicrographs from myelinated fibers per animal were taken x50 and neurotubules and neurofilaments were counted over a representative unit rectangular sampling area 2. Photomicrographs of representative semithin sections of the va-gus nerves obtained from CG and AT group rats appear in figure 2.
The endoneural space contained numerous nerve fibers and several blood vessels. The diameter of the nerve seems to be greater in AT than in CG rats. Note the cervical vagus nerves are enveloped by a well- defined perineurium arrowheads. The endoneural space contains hundreds fibers and several blood vessels arrows accompanying the longitudinal axis of the fascicles. Diameter difference was observed between CG and AT rats. Electron photomicrographs of cross sections of the nerves showed myelinated fibers of various diameters, intermingled with unmyelinated ones and several capillaries within the endoneural space.
Other endoneural space components were Schwann cells, interstitial tissue and fibroblasts figure 3.
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The average value of the g ratio was 0. The mean number of microtubules and neurofilaments per unit area was greater in myelinated axons in the AT rats than in the controls table 2.
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Figure 4 shows the histograms for the myelinated axon diameters in the two groups of rats. Myelinated axon diameters were unimodally distributed for both groups of rats and a right-ward shift of the distribution was observed for the AT group of rats.
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These histograms include all nerves measured: Note the unimodal distribution for both groups of rats and the right-ward shift of the distribution for the AT group of rats. There are reports on the effects of endurance training on peripheral nerves in rats, but the vagus nerve has never been investigated from this point of view. There are two major findings in the present work. First, rats sub-jected to chronic endurance training for 12 weeks exhibited a significant increase in the diameter of the axon and of the myelin sheath of the vagus nerve fibers compared to sedentary controls.
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The diameter of the unmyelinated fibers increased significantly in trained rats. Second, an increase in the number of microtubules and neurofilaments per unit area was detected in myelinated axons from trained rats. In the present study, no significant differences were observed be-tween body weight of sedentary and trained rats and AT animals have larger LV internal diameter compared to CG.
These results were similar to those described in the literature. Consequently, the LV must adapt to both volume and pressure loads. The LV responds by increasing its internal diameter. The results of the present study show that in rats submitted to endurance training, the diameter of the axon and of the myelin sheath of the vagus nerve fibers increases. The diameter of the unmyelinated fibers also increases. Furthermore, the histograms of the frequency distribution of axon diameter of the vagus nerve showed a unimodal distribution for both groups of rats with a right-ward shift of the distri-bution for the AT group of rats.
No significant difference was observed between the average values of the g ratio for the two groups. The quantitative nerve changes observed in the present study could be induced by the increase of the vagus nerve function pro-moted by training. Increase in the nerve function with training is related to factors such as increased production of growth factors and nucleic acids, which result in a series of chemical events leading to gradual increase in neuron sizes.
The exposure to training probably induces production of proteins, lipids Brain-derived neurotrophic factor, also known as BDNF. This is a secreted protein that, in humans, is encoded by the BDNF gene.
These factors are found in the brain and the periphery; they support the survival of neurons and the growth of new ones. Exercise has been shown to increase the secretion of BDNF as a myokine at the mRNA and protein levels in the rodent hippocampus, suggesting the potential increase of this neurotrophin after exercise in humans. The present work showed correlation between the myelin sheath and the diameter of the respective axon, i. This means that training promotes a similar growth in myelin sheath and axon.
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