Cellular Neurophysiology and Integration

An Interpretive Introduction

W. R. Uttal

• Publisher: Psychology Press
• Year: 2014
• ISBN:
  9781317768708
• Edition: 1
• Topic: Psychology
• Subtopic: History & Theory in Psychology
• Index: Psychology

First published in 1975. Because of its general importance to a number of related disciplines, students of the modern science of neurophysiology have benefited from time to time from an introductory survey presented at a more elementary level than is usually found in advanced textbooks. The dynamism of the field is such, however, that more up-to-date statements incorporating many of the exciting new findings concerning cellular neurophysiology are required periodically. This text is aimed at filling that need. It is an outgrowth of a part of a course on the neurophysiology of sensory processes taught by the author at The University of Michigan during the last ten years. This book is an attempt to present the subject matter at a level appropriate for advanced undergraduate students and first year graduate students whose knowledge of chemistry, physics, and mathematics is limited to introductory courses.

1
Introduction

A. The Contemporary Significance of Neurophysiology

An extraordinary scientific development has been underway for the last 30 years. After centuries of speculation and frail theories devoid of empirical foundation, it is now possible to examine directly the cellular basis of the action of the nervous system, and thus to begin considering the true relation between the neural substrate and behavior. Perhaps no science holds such intrinsic excitement or hope for the long-range solution of both theoretical and practical human problems as does modern cellular neurophysiology—the inquiry into the basic nature and function of neurons. It is this class of specialized cells that ultimately defines the very basis of human nature, and it is at this level that many of the solutions to the social and personal problems afflicting people may ultimately be found.

Each neuron, the basic building block of the nervous system, is a member of a class of cells that includes a remarkable variety of cellular units found throughout the peripheral and central portions of all multicellular organisms. The essential feature of the neuron is its greatly heightened ability to react to events external to itself, and thus to interact with events occurring in neighboring neurons, as well as with the external environment. The reactive capacity of this class of highly specialized single cells has evolved from the primitive excitability, elicited by relatively gross amounts of physical stimuli, characteristic of all cells, to a sensitivity that allows neurons to respond to stimuli whose energy content approaches, in many cases, the absolute limits of physical energy. A photoreceptor in the eye, for example, is sensitive to a single quantum of light when conditions are optimized, and the ear is sensitive to a mechanical oscillation an order of magnitude smaller than the diameter of a hydrogen atom.

The highly economical power levels required by the individual neuron allow an information-processing system to be built into the human brain that is probably the most complicated single structure to have evolved in the history of the universe. Only at levels of cosmological interactions do we find comparable complexities, but even at that level there is no evidence for the emergence of any sentient awareness.

This economy of energy, however desirable from an evolutionary point of view, is nevertheless not the essential characteristic of nervous action. The same events that are represented by patterns of neural action could conceivably be constructed from completely different kinds of components, which are far less efficient, without any change in the nature of thought. The logical operations could remain invariant in an information-processing system, no matter what sort of parts were used in its construction. The essential aspect of neural processes is that they are able to synthesize complex events, including thoughts and self-awareness, by concatenating a huge number of quite simple processes. While the individual neuron can perform only a few simple logical processes, logicians have established that a multiply connected net of neurons can represent virtually any concept, no matter how complicated.

The presumption that a net of neurons ultimately gives rise to cognitive behavior is based on a great logical leap, a statement of belief, or more formally, an axiomatic premise, rather than a rigorously established “proof” of the relationship between brain structure and mind. Nevertheless, this premise is based on a compelling set of arguments. Many developments in a variety of sciences have coalesced on this issue in the last 30 years in a way that cannot be ignored any longer. Indeed, we may now be in the midst of a psychobiological revolution, comparable to the great religious and industrial revolutions of the past, that will have profound effects on society in the centuries to come. The notion of brain-mind identity may have as powerful an influence on social development as the notions of personal freedom and democracy have had in the last few centuries.

The notion that complex interactions between simple neural units may underlie much more complex cognitive processes can be understood only if the problem of neural activity is approached from a point of view that emphasizes networks of neurons. Yet for many technological and historical reasons, our knowledge is still limited to the actions and interactions of a relatively few neural units. We know quite a bit about the nature of the membrane of the neuron and the processes that allow the membrane to perform its specialized functions. We know quite a bit about how neurons interact with each other at their points of synaptic contact. However, the limits of our knowledge are rapidly reached when attempts are made to understand how even as few as three neurons interact collectively. Of course a far greater number must be involved in even the simplest information-processing act in the mammalian nervous system.

It is for these reasons that researchers often study relatively simple structures to elucidate some aspect of neural interaction. Invertebrates are commonly used, not so much because of any intrinsic interest in their function (how few, indeed, worry about the problem of the innervation of the muscles that masticate food in the stomach of the lobster for its own sake?) but because these simpler creatures model some aspect of nervous function that may be generally applicable to the solution of the mind-brain problem in man. Even when working with higher animals, including man, we tend to study the function of relatively simple, well-ordered sensory receptor structures to find out about some special aspect of a more general nervous function. Much of what is known about inhibition in nervous nets, for example, has arisen from studies of crustacean neurons and vertebrate sensory systems. But obviously inhibition is a process of great significance throughout the phylogenetic tree, including all parts of the vertebrate nervous system, and we can gain important understanding by generalizing from the primitive to the more complex.

As more is learned about the basic mechanisms that underlie complex processes, the focus of laboratory attention has shifted perceptibly in emphasis. During the 1950’s and early 1960’s an observer at a neuro-physiological conference would have seen an almost complete absorption with the study of action potentials produced by single neurons. In the last 10 years, however, increasing attention has been directed at interactions among neurons.

In light of this modern emphasis on information processing in nets of neurons, it is important not to overlook another vital function of neurons, which is clearly interwined with the processing aspect, but which in many regards is quite different. This, of course, is the function of communication. The necessity for neuronal communication links is obvious. Receptors and effectors are located at the periphery of the body. Yet the complicated central information processing that links inputs from receptors to the biologically appropriate, adaptive effector responses generally occurs in the most centralized, interior portion of the organism. It is to and from the brain, in particular, that information must be communicated. In man this distance may be five or more feet, and in other animals even greater distances must be traversed. To depend upon the slow spread of excitable responses along an unspecialized cellular net would be behaviorally catastrophic; thus there is strong evolutionary pressure toward the development of rapid communication links in multicellular animals.

To make the distinction between the processes of communication and integration more specific, let us define these terms more explicitly at this point:

• Communication—The process by which information is transmitted from one point to another in the nervous system. The prime criterion for excellence of communication is that the transmitted information be the same when it arrives at its destination as when it left the source.
• Integration—The process by which information from several different sources interacts at some processing point to produce an output that is improved in its utility, economy of representation, or biological effectiveness. The prime criterion for excellence of integration is that multiple inputs result in outputs of increased biological value.

It must be realized that there is a basic incompatability between the requirements for good communication and the requirements for good information processing in the nervous system. The communication aspects of the nervous system should be ideally designed to have the highest possible fidelity. The communicated information pattern should be accurately reproduced. On the other hand, integrative information-processing mechanisms function to alter, to convert, or to transform the incoming signal from one pattern to another, and thus they run counter to the demands for transmission fidelity. The ability to adapt to the great variety of environmental stimuli encountered by organisms is critically dependent upon this integrative processing of information. From the evolutionary point of view it is virtually useless for an organism to spew out the same information pattern that was taken in. Without integration, interaction, and synthesis, without the deletion of irrelevant information and the enhancement of incoming information patterns by assimilation with old memories, behavioral evolution would probably come to a standstill as animals failed to modify their responses to environmental stresses.

The analogy between neural integration and the genetic combination process in sexual reproduction is clear. Both processes achieve variety, flexibility, and adaptability because they combine a large number of components into a virtually infinite number of combinations, in a way that leads to continuous improvement in the species. Just as genetic combination is the essence of evolutionary development, so neural integration is certainly the really unique aspect of an animal’s mental life. Though it may seem far-fetched to some, it is reasonable to argue that the essence of human consciousness is largely this ability to combine and permute input signals at the neuronal level.

Older models that view the nervous system as simply a switching network (such as classic conditioning models or more recent computer models) miss this essential point. In the view of most psychobiological philosophers today, man cannot be analogized by a telephone system or a computer. The manner in which man is able to program his behavior and invent new responses generates surprising results in a way that still does not have the remotest artificial analog.

B. Plan and Organization of the Book

This book is directed toward those students at the university level who have already had some introduction to the biological and physical sciences. It is assumed that the reader has a basic, introductory knowledge of electricity and cellular biology. The foundations of cellular neurophysiology will be presented in a way that will allow students of specialities other than physiology to acquire an understanding of the basic concepts of this elegant and intricate body of knowledge. The main effort in this book is to interpret extensively the meaning of the various aspects of neurophysiology to be considered. The historical, philosophical, and conceptual aspects of cellular neurophysiology will be emphasized.

Modern neurophysiology is unquestionably a difficult body of material to master. Much of it is not only highly technical but also involves a number of interacting concepts that make some aspects of the subject quite complex. Unfortunately there is no way to avoid this detail without losing a solid understanding of the subject. But it is a body of knowledge of increasing importance that grows almost daily in new findings, and the time has come for a statement that attempts to extract the important concepts and general principles to a greater degree than usual. Methods and findings will necessarily play an important role in our discussion, but the author’s principal intent is to use these materials more to help in achieving conceptual understanding than as goals in themselves.

The only hope for grasping the fundamentals of cellular neurophysiology is to keep rigid control over the sequence of ideas as they are presented. The problem is in deciding along which of the alternative dimensions this sequence of control should be exerted. There are two obvious candidates. One possible thread along which the discussion might be organized is that of increasing levels of cellular anatomy and complexity. This thread passes from the basic biochemical properties of the neuronal membranes to the interactive mechanisms that allow cells to communicate with each other. An alternative sequence of discussion is one imposed by the intrinsic directionality of information flow within the nervous system. Receptors “transduce” physical energy. The long axons of certain specialized neurons then transmit that information through synaptic junctions to the central nervous system, where it is integrated into output patterns that are again transmitted through a similar channel—but in a reverse direction, to the periphery. From this point of view, the study of cellular neurophysiology is dominated by its directionality, ratherd than by levels of anatomical complexity.

In this book the author has tried to maintain both threads of the discussion simultaneously. In a few instances, the thread must deviate from a strictly parallel route, in order to avoid premature discussion of material for which the prerequisites have not yet been introduced. But in general we shall be progressing from the microscopic to the more macroscopic while simultaneously following the stream of information flow from the periphery to the central nervous system and then to the periphery once again.

Chapters 2 and 3 contain the first digressions. In order to set the stage for the cellular neurophysiological details to follow, two bodies of background information must first be considered. The first, presented in Chapter 2, is a brief historical survey, to place the discussion in the context of its intellectual origins. Though some of the basic ideas of modern neurophysiology are surprisingly antique (for example, the notion of the cell membrane was originally proposed around the turn of the century), much supporting evidence for some of the most widely accepted theories did not appear until the 1940’s. Chapter 3 briefly presents some details of the gross anatomy of the nervous system, in only sufficient detail to introduce the reader to the locus of various tissues that are discussed in later chapters. A brief anatomy of the microstructure of neurons and receptor cells is then presented. Chapter 3 then concludes, at an even finer level of micro-anatomy, with a discussion of the anatomy of the cell or plasma membrane, a tissue of the utmost importance to our later discussion.

Chapter 4 presents a discussion of modern neurophysiological research techniques, information required for understanding the experimental data to be discussed in later chapters. Chapter 5 first specifically introduces what is meant by the neuron theory, the now universally accepted notion that the nervous system is made up of cytoplasmically separate cellular units rather than an interconnected reticulum or network. The discussion then turns to a consideration of how the distributions of ions across the membrane are produced by the various passive and active forces that operate on chemical ions and how those distributions of ions give rise to potential differences, particularly the so-called resting potential. Finally, a taxonomy of membrane potentials is presented.

Once it has been established how resting potentials can be developed by metabolic engines within the membrane, several subsequent chapters are dedicated to explorations of how the resting potential can be perturbed to carry information to other parts of the nervous system. Chapter 6 discusses the initiation of membrane action potential by external physical energies. This, of course, is the function generally known as transduction. Transductive mechanisms are considered separately for each of the senses, after a brief introduction to the general nature of the process. This is necessary because each of the sensory receptors has evolved a somewhat different solution to its own biological transduction problem.

The transduction processes are the main means through which neural information patterns begin to course through the nervous system. Once the process has started, the main question is: How is this neural information transmitted from place to place? Over short distances, passive and decremental signal spread appears to be used, but over longer distances some other mechanism is required. The need is filled by the propagating spike action potential, a response class evolved specifically for communication over the long axons of the body. Chapter 7 is entirely devoted to the topic of transmission, including both general qualitative and quite specific explanations of how decremental and propagating action potentials occur. These explanations are phrased in terms of ionic and membrane properties.

In Chapter 8 we consider how single cell responses can be mixed to produce compound or pooled action potentials. These classes of electrical signals reflect the action of many cells, rather than one. A particularly important issue is whether or not the group statistics of mixed neurophysiological responses truly represent the characteristics of individual cells. A consideration of some appropriate research shows that the two are highly correlated, and thus some important links can be made between the two kinds of response.

Having completed the discussion of transmission properties along neurons and mixed, but noninteracting, neuronal responses, we then are prepared to consider, in Chapter 9, the problem of transmission between neurons. The synapse is a highly specialized region between neurons that allows the electrochemical activity in one cell to initiate a comparable pattern of activity in a nearby cell. Both chemically and electrically mediated synapses are discussed, as well as some important topics concerning the ionic basis of synaptic action.

Chapter 10 also emphasizes interactions among neurons, but from a very different point of view. Rather than dealing with the ultramicroscopic action of the synapse, we are concerned with the interactions of highly idealized neurons in networks. ...

Table of contents

• Cover
• Title
• Copyright
• Dedication
• CONTENTS
• Preface
• Acknowledgments
• Chapter 1. Introduction
• Chapter 2. A Brief History of Cellular Neurophysiology
• Chapter 3. Neuroanatomy
• Chapter 4. Electrophysiological Techniques
• Chapter 5. The Chemical Basis of Neuroelectric Activity: The Resting Potential and a Taxonomy of Action Potentials
• Chapter 6. Cellular Aspects of the Transduction Process
• Chapter 7. The Transmission Process
• Chapter 8. Compound and Pooled Action Potentials
• Chapter 9. The Synapse
• Chapter 10. Neuronal Integration
• Chapter 11. The Aplysia: Anatomy and Physiology of a Model Preparation
• Chapter 12. A Summary of Emerging Principles
• Bibliography
• Author Index
• Subject Index