Neural Control Of Circulation

Neural Control of Circulation presents an in-depth view of specialized areas in the neural regulation of the circulatory system that have been the subject of intensive research, the historical basis and theory from which those investigations evolved, and directions for future studies. Special emphasis is placed on critical evaluation of the experimental data in each field of research. This volume is comprised of seven chapters and begins with a synthesis of a large number of studies undertaken using conscious animals, particularly those that focuses on the behavioral and cerebral control of cardiovascular function. The second chapter explores the role of the brain stem and cerebellum in cardiovascular control. Next, specific research areas concerning bulbospinal control of sympathetic nerve discharge are discussed. This is followed by a chapter devoted to the nucleus tractus solitarii and experimental neurogenic hypertension. A concept in potential hypertensive mechanisms involving long-term transsynaptic regulation of adrenal medullary function is also described, and the neural control of the circulation during hypoxia is considered. Finally, aspects of central nervous system pharmacology and regulation of circulation are examined. This book is designed for individuals who are interested in the cardiovascular system and its function, and should also prove useful to students and researchers in physiology and individuals in other ancillary areas of bioscience.

This e-book will review special features of the cerebral circulation and how they contribute to the physiology of the brain. It describes structural and functional properties of the cerebral circulation that are unique to the brain, an organ with high metabolic demands and the need for tight water and ion homeostasis. Autoregulation is pronounced in the brain, with myogenic, metabolic and neurogenic mechanisms contributing to maintain relatively constant blood flow during both increases and decreases in pressure. In addition, unlike peripheral organs where the majority of vascular resistance resides in small arteries and arterioles, large extracranial and intracranial arteries contribute significantly to vascular resistance in the brain. The prominent role of large arteries in cerebrovascular resistance helps maintain blood flow and protect downstream vessels during changes in perfusion pressure. The cerebral endothelium is also unique in that its barrier properties are in some way more like epithelium than endothelium in the periphery. The cerebral endothelium, known as the blood-brain barrier, has specialized tight junctions that do not allow ions to pass freely and has very low hydraulic conductivity and transcellular transport. This special configuration modifies Starling's forces in the brain microcirculation such that ions retained in the vascular lumen oppose water movement due to hydrostatic pressure. Tight water regulation is necessary in the brain because it has limited capacity for expansion within the skull. Increased intracranial pressure due to vasogenic edema can cause severe neurologic complications and death.

The kidney is innervated with efferent sympathetic nerve fibers reaching the renal vasculature, the tubules, the juxtaglomerular granular cells, and the renal pelvic wall. The renal sensory nerves are mainly found in the renal pelvic wall. Increases in efferent renal sympathetic nerve activity reduce renal blood flow and urinary sodium excretion by activation of α1-adrenoceptors and increase renin secretion rate by activation of β1-adrenoceptors. In response to normal physiological stimulation, changes in efferent renal sympathetic nerve activity contribute importantly to homeostatic regulation of sodium and water balance. The renal mechanosensory nerves are activated by stretch of the renal pelvic tissue produced by increases in renal pelvic tissue of a magnitude that may occur during increased urine flow rate. Under normal conditions, the renal mechanosensory nerves activated by stretch of the sensory nerves elicits an inhibitory renorenal reflex response consisting of decreases in efferent renal sympathetic nerve activity leading to natriuresis. Increasing efferent sympathetic nerve activity increases afferent renal nerve activity which, in turn, decreases efferent renal sympathetic nerve activity by activation of the renorenal reflexes. Thus, activation of the afferent renal nerves buffers changes in efferent renal sympathetic nerve activity in the overall goal of maintaining sodium balance. In pathological conditions of sodium retention, impairment of the inhibitory renorenal reflexes contributes to an inappropriately increased efferent renal sympathetic nerve activity in the presence of sodium retention. In states of renal disease or injury, there is a shift from inhibitory to excitatory reflexes originating in the kidney. Studies in essential hypertensive patients have shown that renal denervation results in long-term reduction in arterial pressure, suggesting an important role for the efferent and afferent renal nerves in hypertension.

This presentation describes various aspects of the regulation of tissue oxygenation, including the roles of the circulatory system, respiratory system, and blood, the carrier of oxygen within these components of the cardiorespiratory system. The respiratory system takes oxygen from the atmosphere and transports it by diffusion from the air in the alveoli to the blood flowing through the pulmonary capillaries. The cardiovascular system then moves the oxygenated blood from the heart to the microcirculation of the various organs by convection, where oxygen is released from hemoglobin in the red blood cells and moves to the parenchymal cells of each tissue by diffusion. Oxygen that has diffused into cells is then utilized in the mitochondria to produce adenosine triphosphate (ATP), the energy currency of all cells. The mitochondria are able to produce ATP until the oxygen tension or PO2 on the cell surface falls to a critical level of about 4–5 mm Hg. Thus, in order to meet the energetic needs of cells, it is important to maintain a continuous supply of oxygen to the mitochondria at or above the critical PO2 . In order to accomplish this desired outcome, the cardiorespiratory system, including the blood, must be capable of regulation to ensure survival of all tissues under a wide range of circumstances. The purpose of this presentation is to provide basic information about the operation and regulation of the cardiovascular and respiratory systems, as well as the properties of the blood and parenchymal cells, so that a fundamental understanding of the regulation of tissue oxygenation is achieved.

This book is an attempt to indicate to researchers and clinicians a simple way to approach the complexity of cardiovascular neural regulation. A conceptual pillar like homeostasis is contrasted with instability and a continuous interaction of opposing mechanisms that have negative and positive feedback characteristics, and is considered to subserve the multitude of patterns pertaining to physiology. However, in pathophysiological conditions the final design is most often replaced by largely purposeless neural mechanisms. The complexity of cardiovascular neural regulation, reflected by the state of sympathovagal balance, is also assessed in the frequency domain. Power spectrum analysis of heart rate and arterial pressure variability, a sophisticated but simply explained approach, provides an unprecedented tool to evaluate this interaction in both physiological and pathophysiological conditions. The elementary characteristics of nonlinear dynamics are also outlined. Finally, the need for an ethical structure for science and medicine is analyzed.

Research centering on blood flow in the heart continues to hold an important position, especially since a better understanding of the subject may help reduce the incidence of coronary arterial disease and heart attacks. This book summarizes recent advances in the field; it is the product of fruitful cooperation among international scientists who met in Japan in May, 1990 to discuss the regulation of coronary blood flow.

Reflex Control of the Circulation presents an interdisciplinary discussion of concepts in the reflex control of circulation. This volume describes aspects of autonomic receptor physiology, central pathways of reflex control, the electrophysiology of cardiovascular afferents, the interaction between reflexes, the autonomic control of regional blood flows, the autonomic control of fluid and electrolyte balance, and neurohumoral control of the circulation through normal and pathological states (e.g., hypertension, congestive heart failure). In addition, the regulation of regional blood flow during exercise and developmental aspects of reflex control are examined. Any researcher interested in the autonomic system and its role in circulation will find this book fascinating reading.