Browsing by Subject "muscle"
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Item Autonomic nervous control of cardiovascular function during prolonged exercise in humans(2014-05-01) White, Daniel W.; Peter B. RavenThe importance of physical activity is well established as a means to maintain good health. However, under certain conditions and in some individuals, heavy exercise leads to catastrophic failure of the cardiovascular system. This is especially true during early recovery from exercise. This may be due in part to an improper response of the autonomic nervous system; that is, an imbalance of the sympathetic and parasympathetic nervous systems. The purpose of the investigations presented in this dissertation was to: i) re-evaluate the commonly accepted model of autonomic influence on control of heart rate during exercise; ii) study the effects of posture on recovery from heavy exercise; and iii) determine the effect of muscle pump activity on cardiorespiratory control of the cardiovascular system during the transition from active to inactive recovery following heavy dynamic two legged cycling. In the first investigation we examined previously reported and newly collected data and determined a fine balance exists between the sympathetic and parasympathetic nervous systems throughout all intensities of exercise. Our conclusions led to the development of a new model of autonomic balance during exercise. In the second investigation we concluded that unloading of the cardiopulmonary baroreceptors by upright posture significantly increases baroreflex control of heart rate during rest and during recovery from heavy dynamic leg cycling exercise. We also show that steady-state blood pressure and the baroreflex control of blood pressure is not significantly different based on orthostatic posture before or after exercise. In the third investigation we concluded that loading of the cardiopulmonary baroreceptors by muscle pump activity during active recovery from heavy exercise diminishes the respiratory induced changes in cardiovascular function observed during inactive recovery. Overall, these investigations highlight the importance of the autonomic nervous system during exercise and during recovery from heavy exercise. Collectively, these conclusions should influence the decision making process regarding mode of recovery from heavy exercise, especially in an “at risk” population, because recovery is the time when most adverse events take place.Item Resetting of the Carotid Arterial Baroreflex during Dynamic Exercise(1997-08-01) Bryant, Kristin Hannah Norton; Peter B. Raven; James Caffrey; Thomas YorioByrant, Kristin, Resetting of the Carotid Arterial Baroreflex during Dynamic Exercise Doctor of Philosophy (Biomedical Sciences), August, 1997; 121 pp; 8 tables; 20 figures, bibliography, 82 titles. Following the initial response to the onset of dynamic exercise, prolonged exercise at a constant workload is characterized by a progressive decrease in stroke volume (SV) and mean arterial pressure (MAP) and concomitant rise in heart rate (HR). These data raise the question as to whether there is a loss of baroreflex regulation of arterial blood pressure during prolonged dynamic exercise. However, we propose that the carotid barareflex (CBR) is continually reset during prolonged exercise, with the operating point being shifted toward the reflex threshold, in relation to a progressive increase in central command activity as motor fibers are recruited in response to muscle fatigue. Therefore, the baroreflex is unresponsive to the fall in MAP. In order to investigate the hypothesis, volunteer subjects performed one hour of dynamic leg cycling exercise at 65% of maximal oxygen uptake (VO2max) with: I) no intervention; and ii) maintenance of cardiac filling volume via continuous infusion of a 6% dextran in saline solution to counteract the fall in SV. At 10 and 50 minutes of exercise, CBR stimulous-response curves were generated using the neck pressure/neck suction technique. The maintenance of cardiac filling volume and thus SV resulted in a diminished drift in MAP. However, indices of central command such as HR, VO2 and ratings of perceived exertion (RPE) increased to the same extent regardless of exercise condition. Furthermore, there was augmented resetting of the CBR at 50 minutes of exercise as compared to 10 minutes under both exercise conditions. In order to further investigate the effects of central command on baroreflex control of blood pressure, a second investigation was designed to demonstrate the effects of exercise type and intensity on CBR function. Stimulus-response relationships were compared during dynamic exercise at a wide range of exercise intensities performed with either leg exercise alone or leg exercise combined with arm exercise. Increases in exercise intensity to maximal exercise resulted in increases in indices of central command such as HR and VO2 as well as an augmentation of the magnitude of the lateral shift in the CBR stimulus-response curve (with the operating point being shifted further toward the threshold of the reflex) relative to the activation of central command. In addition, the performance of combined arm and leg exercise elicited an augmented shift in the carotid-vasomotor stimulus-response relationship as compared to leg exercise alone at the same exercise intensity. As arm exercise compared to leg exercise performed at the same absolute VO2 results in an increased lactate accumulation in the venous system, the augmented resetting of the CBR is likely due to a disproportionate activation of the muscle metaboreflex component of the muscle pressor reflex. Therefore we propose that the central command is the primary mechanism by which the CBR is reset at the onset of dynamic exercise through feed-forward control. However, additional, feed-back modulation can be exerted by the muscle pressor reflex upon the development of mechanical or chemical error signals in the exercising muscle.Item The Effect of Dietary Loading on Structural Determinants of Force Production in the Rat Masseter(2020-05) Rossiter, Jeffrey A.; Menegaz, Rachel A.; Maddux, Scott D.; Reeves, Rustin E.Rossiter, Jeffrey A., The Effect of Dietary Loading on Structural Determinants of Force Production in the Rat Masseter. Master of Science in Medical Sciences - Anatomy, May 2020. Biomechanical loading associated with feeding is known to direct cranial bone growth, however less is known about its effects on masticatory muscle growth and performance. Peak muscle contractile forces are determined by a combination of factors including total muscle mass, fiber length, and fiber type. Here, we test two hypotheses: that mechanically challenging diets will (1) increase the physiological cross-sectional area (PCSA), an estimate of maximum contractile force at tetanus, and (2) increase the number and proportion of type II (fast-twitch) muscle fibers in the masseter of the rat. Sprague-Dawley rats were raised on either a hard/tough (overuse) diet or a soft (underuse) diet (n=5/cohort). The superficial masseters were dissected and photographed using a trifocal stereo microscope, and muscle fiber length (6/individual) were measured using ImageJ. Muscle volumes were calculated from in-situ diffusible iodine-based contrast-enhanced μCT scans. Muscles were stained using an IHC protocol for the fast isoform of myosin heavy chain, allowing the number and areas of type II (stained) and type I (unstained) fibers to be quantified in ImageJ. Results from this study do not support our hypotheses, most likely due to the small sample sizes (n=5/treatment group) available for this study. Paradoxical results were found, with rats raised on a soft diet tending to have longer superficial masseter muscle fibers and more type II muscle fibers with larger cross-sectional areas in the posterior masseter. Rats raised on a hard diet tend to have larger masseter muscle volumes. However, these trends were not statistically significant (p > 0.05). Mechanically challenging diets tend to be associated with greater masticatory muscle volumes and thus increased PCSA. The fiber type results from the posterior masseter (with more deep masseter fibers) were the opposite of those previous results from the middle masseter (with more superficial masseter fibers) in the same animals. Future studies with increased sample sizes are needed to better understand the structural determinants of force production in the rat masseter.