Cardiovascular Physiology: Heart, Peripheral Circulation and Methodology

Proceedings of the 28th International Congress of Physiological Sciences, Budapest, 1980

A. G. B. Kovách, E. Monos, G. Rubányi

• Publisher: Pergamon
• Year: 2014
• ISBN:
  9781483189963
• Topic: Médecine
• Subtopic: Physiologie

Advances in Physiological Sciences, Volume 8: Cardiovascular Physiology: Heart, Peripheral Circulation and Methodology presents the proceedings of the 28th International Congress of Physiological Sciences, held in Budapest, Hungary, on July 13–19, 1980. This book examines some of the significant arterial pressure control mechanisms. Organized into 43 chapters, this compilation of papers begins with an overview of the origin of the notion of a vasomotor center controlling the arterial blood pressure. This text then discusses the concept of long-term arterial pressure regulation and some of the basic mechanisms of hypertension. Other chapters consider the sites and mechanisms of capillary control. This book discusses as well the performance of the heart during its excitation–contraction cycle, which is influenced by the movements of ions across the cell membrane. The final chapter deals with the interpretation of in vivo NADH fluorescence measurements. This book is a valuable resource for physiologists and clinicians.

CENTRAL NERVOUS ORIGIN OF VASOMOTOR TONE

S.M. Hilton, Department of Physiology, University of Birmingham, England

Publisher Summary

This chapter discusses the central nervous origin of vasomotor tone. The neuronal organization within the so-called pressor area is exceedingly complex as it contains elements of the baroreceptor reflex pathway and others with inhibitory influences on individual vascular beds. The baroreceptor reflex is thought to play the key role in blood pressure regulation. The evidence from many different sources points to the important role of the arousal system in setting the general level of arterial blood pressure. The effect of blocking the pathway connecting the brainstem regions that organize the pattern of response characteristic of this system with the preganglionic effector neurons is so profound as to suggest that the arousal system plays a key role in the generation of vasomotor tone in the awake state. This is a visceral activating system that intensifies all the visceral components of the defense response as a part of a fully-fledged and coordinated reaction when suddenly or strongly engaged, and it also shows a moderate, though fluctuating, level of engagement at all times in the waking state, and this is what mainly determines the general level of arterial blood pressure in daily life.

The commonly accepted ideas about the origin of vasomotor tone are still influenced by nineteenth century concepts which led physiologists to imagine an assembly of centres in the brain, each controlling a single physiological variable. The origin of the notion of a vasomotor centre controlling arterial blood pressure is thus buried deep in history. This is largely why the idea persists, in spite of several attempts made over the past fifteen years to show it up as a naive notion which has hardly any experimental support.

The concept of a vasomotor centre – a source of confusion

Most of you probably know that the vasomotor centre controlling arterial blood pressure was placed in the medulla on the basis of the work over a 100 years ago of Dittmar (1870; 1873) and Owsjannikow (1871) : this comprised three separate studies of the effect on blood pressure of successive transections of the brainstem starting in the mid-brain and working through the medulla. The finding in these classical experiments was that blood pressure was only seriously affected when the section encroached upon the rostral medulla. More caudal sections reduced the blood pressure still further until a point was reached, about the caudal third of the medulla and some 3–4 mm rostral to the obex, at which the maximum fall was produced.

If findings such as these were made today, they would not be given the same significance; for we are so familiar with the syndrome of spinal shock. Indeed, all the attempts to analyse experimental information of this kind has led physiologists to be wary of assuming that it can be used simply to locate a specific centre whose influence has been removed. Nevertheless, in this particular case, and even more so following the discovery by Ranson and Billingsley (1916), on electrical stimulation, of pressor and depressor points in the floor of the 4th ventricle, the idea came about not only that the vasomotor centre exists but also that it is dorsally placed within this region of the brain stem. Much careful work followed, in which pressor and depressor regions were mapped in the medulla, (Ranson and Magoun, 1939) culminating in the maps made by Alexander (1946), which are those usually reproduced in textbooks, in which extensive pressor and depressor medullary areas are shown, the former being more dorsal and lateral, and the latter being a rather smaller ventromedial zone.

We now know that the neuronal organisation within the so-called pressor area is exceedingly complex (Hilton, 1974; 1975); for it contains elements of the baroreceptor reflex pathway and others with inhibitory influences on individual vascular beds. Nevertheless, the idea that this is the vasomotor centre persists as the established view. A very recent investigation based upon it was that of Kumada et al. (1979) where, in the course of a study on the cerebral ischaemic reflex in rabbits, large lesions were made bilaterally in the dorsal medulla. Because of a marked fall in arterial blood pressure in 4 out of 15 of these rabbits, the suggestion was made by these authors that they may have located the site of the vasomotor centre. Reference back to the early work of Dittmar (1873), however, shows that this cannot be so; for in his experiments, which were also on rabbits, very careful extirpations of the dorsal medulla had been made with a knife specially constructed for the purpose, and the blood pressure found to be hardly affected by lesions which removed almost the dorsal two-thirds of this region of the brainstem. The area of the dorsal medulla common to all the lesions made by Kumada et al. (1979) was caudal to the site located by Dittmar (1873) as being of special significance for maintenance of blood pressure. He put it in the ventrolateral reticular formation near the facial nucleus — a conclusion to which I will return later. For now, however, it is sufficient to note that many investigators have made large lesions in the dorsal medulla and observed only small falls of blood pressure (e.g. Manning, 1965; Chai & Wang, 1968) which has led to the rather confusing conclusion that neurones with tonic vasomotor effects are diffusely distributed throughout the medulla. Although I have called this conclusion confusing, it appears to be supported when one looks at the results of experiments carried out as a search for specific vasomotor neurones within the medulla. A painstaking study of this kind was made by Hukuhara (1974) who looked for medullary neurones with a firing pattern which was correlated with the discharge in sympathetic efferent nerves, chiefly those to the kidney. His map of the location of these neurones show them to occupy almost all of the medullary reticular formation.

Something seems wrong with the conventional wisdom and this doubt is heightened by experimental evidence from a variety of sources. In a series of papers, published in the Chinese Journal of Physiology before the last war (Chen et al., 1936; 1937; Lim et al., 1938) stimulation experiments similar to those of Ranson and Billingsley (1916) were repeated, but in which measurements were made not only of arterial blood pressure, but also of heart-rate, pupil-size, the size of the spleen, tone of the bladder, small intestine and colon, and the level of the blood sugar. Piloerection, sweat gland activity, and effects on the bronchioles were also looked for. Such was the ubiquity of the changes found that there would appear to be no reason at all for isolating the arterial blood pressure from any of the other variables mentioned, or from any that could be measured or recorded, and hence for isolating a region that could be called specifically a pressor region. The clear implication of these findings is that if a section of the cord is made at the level that causes a profound fall in arterial blood pressure, every autonomic response organised at a lower level of the neuraxis would be expected to be profoundly depressed, and there is much clinical and experimental evidence that this is so. The problem of interpretation has arisen because of the original assumption that any physiological variable which can be isolated experimentally, for independent measurement or registration, must have its appropriate controlling region in the brainstem, and that each of them can be regarded as a separate physiological and anatomical entity. As we now see, this assumption leads again and again to the unlikely conclusion that widespread, overlapping regions control each physiological variable.

Sympathetic rhythms

One way of attempting to deal with the dilemma has been to ignore the seemingly intransigent problems of topography and to speak instead of oscillating circuits. This is the approach of Gebber and his colleagues (Taylor and Gebber, 1975; Barman and Gebber, 1976) who have started out from the long-known observation (Adrian et al., 1932) that the discharges in sympathetic efferent nerves show several rhythmic variations. There is firstly a slow rhythm which is close to the respiratory rhythm but also apparently generated independently of it, as it often persists during hypocapnia of a degree sufficient to abolish the rhythmic discharge in the phrenic nerves and as it displays shifting phase relationships with the bursts of phrenic nerve discharge (Cohen and Gootman, 1970; Barman and Gebber, 1976). This rather capricious rhythm is superimposed upon a faster rhythm at 3–5 c/sec which is very close to the cardiac rhythm. However, as it persists after sino-aortic denervation, it in its turn is apparently independent of the baroreceptor input (Taylor and Gebber, 1975). These two rhythms have a supraspinal origin, whereas another at 8–12 c/sec (Green and Heffron, 1967; Cohen and Gootman, 1970) can be generated in the spinal cord (McCall and Gebber, 1975).

In Gebber’s papers the hope is expressed that this approach will provide information on the intrinsic organisation of the brainstem circuits responsible for generating the background discharges of pre-ganglionic sympathetic neurones; but it is not easy to see how this may be realised. Even a close correlation between the discharge of a brainstem neurone and that in some sympathetic efferent nerve several synapses away is no proof that the former has actually generated the latter. Secondly, extensive investigations by Langhorst and his group of the activity in brainstem neurones, using cross-correlation techniques and power spectral analysis of their discharges in relation to potential inputs and outputs, has shown the shifting relations, both with time and the condition of the animal preparation, of the discharges recorded. This has led Langhorst (1980) to assert that there is nothing other than a “common brainstem system” of multiple potentiality. Indeed, in the light of all the arguments and data quoted already it seems unlikely that there can be any brainstem neurones which are dedicated to a single system — sympathetic, parasympathetic or somatic — or even to a single output.

Organisation of patterns of response

It seems to me that a straightforward way out of the dilemma which all these results appear to present is to regard the central nervous system as a mechanism for initiating and integrating patterns of response (Hilton, 1974; 1975). There is a number — but only a small number — of readily recognisable and biologically significant patterns of this kind, such as those comprising alimentary and sexual reactions, temperature regulation, and fear and rage reactions (usually described in physiology as the defence reaction). All of these include important cardiovascular components, so if different but separate regions of the brainstem are responsible for the organisation of each pattern, there can be no separate brainstem region responsible for any si...

Table of contents

• Cover image
• Title page
• Table of Contents
• ADVANCES IN PHYSIOLOGICAL SCIENCES
• Copyright
• FOREWORD
• Chapter 1: CENTRAL NERVOUS ORIGIN OF VASOMOTOR TONE
• Chapter 2: A COMPUTER AND EXPERIMENTAL ANALYSIS OF ARTERIAL PRESSURE REGULATION AND HYPERTENSION
• Chapter 3: MECHANISMS OF CAPILLARY RECRUITMENT: RELATION TO FLOW, TISSUE PO2 AND MOTOR UNIT CONTROL OF SKELETAL MUSCLE
• Cation regulation in the myocardium
• Factors regulating myocardial substrate metabolism
• Biomechanical properties of arteries
• Cardiovascular foetal physiology
• Non-invasive cardiovascular and metabolic techniques
• INDEX