The academic lineage of Sir John Carew Eccles (1903-1997)

Control of Mammalian Locomotion by Somatosensory Feedback

When animals walk overground, mechanical stimuli activate various receptors located in muscles, joints, and skin. Afferents from these mechanoreceptors project to neuronal networks controlling locomotion in the spinal cord and brain. The dynamic interactions between the control systems at different levels of the neuraxis ensure that locomotion adjusts to its environment and meets task demands. In this article, we describe and discuss the essential contribution of somatosensory feedback to locomotion. We start with a discussion of how biomechanical properties of the body affect somatosensory feedback. We follow with the different types of mechanoreceptors and somatosensory afferents and their activity during locomotion. We then describe central projections to locomotor networks and the modulation of somatosensory feedback during locomotion and its mechanisms. We then discuss experimental approaches and animal models used to investigate the control of locomotion by somatosensory feedback before providing an overview of the different functional roles of somatosensory feedback for locomotion. Lastly, we briefly describe the role of somatosensory feedback in the recovery of locomotion after neurological injury. We highlight the fact that somatosensory feedback is an essential component of a highly integrated system for locomotor control. © 2021 American Physiological Society. Compr Physiol 11:1-71, 2021.

Pioneers in CNS inhibition: 2. Charles Sherrington and John Eccles on inhibition in spinal and supraspinal structures

This article reviews the contributions of the English neurophysiologist, Charles Scott Sherrington [1857-1952], and his Australian PhD trainee and collaborator, John Carew Eccles [1903-1997], to the concept of central inhibition in the spinal cord and brain. Both were awarded Nobel Prizes; Sherrington in 1932 for "discoveries regarding the function of neurons," and Eccles in 1963 for "discoveries concerning the ionic mechanisms involved in excitation and inhibition in central portions of the nerve cell membrane." Both spoke about central inhibition at their Nobel Prize Award Ceremonies. The subsequent publications of their talks were entitled "Inhibition as a coordinative factor" and "The ionic mechanism of postsynaptic inhibition", respectively. Sherrington's work on central inhibition spanned 41 years (1893-1934), and for Eccles 49 years (1928-1977). Sherrington first studied central inhibition by observing hind limb muscle responses to electrical (peripheral nerve) and mechanical (muscle) stimulation. He used muscle length and force measurements until the early 1900s and electromyography in the late 1920s. Eccles used these techniques while working with Sherrington, but later employed extracellular microelectrode recording in the spinal cord followed in 1951 by intracellular recording from spinal motoneurons. This considerably advanced our understanding of central inhibition. Sherrington's health was poor during his retirement years but he nonetheless made a small number of largely humanities contributions up to 1951, one year before his death at the age of 94. In contrast, Eccles retained his health and vigor until 3 years before his death and published prolifically on many subjects during his 22 years of official retirement. His last neuroscience article appeared in 1994 when he was 91. Despite poor health he continued thinking about his life-long interest, the mind-brain problem, and was attempting to complete his autobiography in the last years of his life.

One, no-one and a hundred thousand brains: J.C. Eccles, J.Z. Young and the establishment of the neurosciences (1930s-1960s)

Contemporary neurosciences have grown beyond the limits of a natural science. To its most vocal advocates, the study of the human brain can provide nothing short of the basis for a new science of man-the link between the "natural" and "human" sciences-as a simple consequence of the growing mass of facts relating to this most marvelous organ, accumulated in the last four decades. This straightforward picture of the growing import of the neurosciences simplifies and obscures the myriad different interpretations and images of "the brain" that have inspired the development of the neurosciences. Among them, this chapter will consider two deeply contrasting early images of the brain: the cellular-physiological brain proposed since the 1950s by John Carew Eccles, and the model-"whole" brain championed by John Zachary Young. Eccles' program was focused on the vertebrate synapse, and Young's on the whole brain of an "advanced" invertebrate (the octopus). The former was the programmatic extension of a long neurophysiological tradition, and the latter an outspoken attempt at providing a revolutionary model for the organization of an unprecedented research effort. One underscored continuity and scientific "soundness," and the other promised rupture and new, imaginative solutions to age-old problems. Nevertheless, they have been later lumped together into a single, marvelous and progressive history, or mythology, of the Science of the Brain. This chapter will show how the organizing principle of these two opposed (if almost equally successful) research efforts was not the foggy, ever-changing image of an experimental brain-in-becoming, but the clear, fixed horizon of a promised brain.

Memories of John Eccles

This article contains an account of the career of John Eccles that can be divided into two parts. The first extends from 1925, when he won a Rhodes scholarship to Magdalen College Oxford, to 1975, when he took voluntary retirement from the State University of New York at Buffalo. During this period, he set up six different laboratories in which he carried out research on synaptic mechanisms that provide the basis of neuroscience. The second period is the 20 years between his retirement and death in Switzerland, which he spent on the problem of the relationship between mind and brain.

The beginning of intracellular recording in spinal neurons: facts, reflections, and speculations

Intracellular (IC) recording of action potentials in neurons of the vertebrate central nervous system (CNS) was first reported by John Eccles and two colleagues, Walter Brock and John Coombs, in Dunedin, NZL in 1951/1952 and by Walter Woodbury and Harry Patton in Seattle, WA, USA in 1952. Both groups studied spinal cord neurons of the adult cat. In this review, we discuss the precedents to their notable achievement and reflect and speculate on some of the scientific and personal nuances of their work and its immediate and later impact. We then briefly discuss early achievements in IC recording in the study of CNS neurobiology in other laboratories around the world, and some of the methods that led to enhancement of CNS IC-recording techniques. Our modern understanding of CNS neurophysiology directly emanates from the pioneering endeavors of the five who wrote the seminal 1951/1952 articles.

Early history of glycine receptor biology in Mammalian spinal cord circuits

In this review we provide an overview of key in vivo experiments undertaken in the cat spinal cord in the 1950s and 1960s, and point out their contributions to our present understanding of glycine receptor (GlyR) function. Importantly, some of these discoveries were made well before an inhibitory receptor, or its agonist, was identified. These contributions include the universal acceptance of a chemical mode of synaptic transmission; that GlyRs are chloride channels; are involved in reciprocal and recurrent spinal inhibition; are selectively blocked by strychnine; and can be distinguished from the GABA_A receptor by their insensitivity to bicuculline. The early in vivo work on inhibitory mechanisms in spinal neurons also contributed to several enduring principles on synaptic function, such as the time associated with synaptic delay, the extension of Dale's hypothesis (regarding the chemical unity of nerve cells and their terminals) to neurons within the central nervous system, and the importance of inhibition for synaptic integration in motor and sensory circuits. We hope the work presented here will encourage those interested in GlyR biology and inhibitory mechanisms to seek out and read some of the “classic” articles that document the above discoveries.

Thomas Graham Brown (1882--1965), Anders Lundberg (1920-), and the neural control of stepping

Thomas Graham Brown (1882--1965) undertook experiments on the neural control of stepping in the University of Liverpool laboratory of Charles Sherrington (1857--1952) in 1910--13 and his own laboratory in 1913--15 at the University of Manchester. His results revealed the intrinsic capability of the spinal cord in the guinea pig and cat to generate a stepping output pattern whose timing did not depend upon descending or sensory inputs. This idea was then revolutionary because the prevailing viewpoint was that the stepping rhythm was generated by spinal reflexes. Sadly, Graham Brown's GBR peers gave little credence to this seminal accomplishment, except perhaps Sherrington, who waxed but largely waned on the potential significance of the work. It remained for the Swedish neuroscientist, Anders Lundberg (1920-), to rescue Graham Brown's concepts from obscurity: in seminars presented in several countries between 1957 and 1980, and in widely read articles and reviews (1965--1981). Graham Brown had proposed mutually inhibitory connections between a pair of intrinsically active flexor and extensor "half-centers" on each side of the spinal cord, with the rhythmic output modulated by sensory proprioceptive input. Lundberg, Elzbieta Jankowska (1930-), and their colleagues provided seminal, compelling evidence for spinal half-center interneuronal circuitry implicated in the control of stepping and Lundberg and Ingemar Engberg (1935--2005) made behavioral EMG observations on unrestrained cats that supported a central generation of the rhythm. Subsequently, models of the spinal pattern generators for mammalian locomotion have become progressively more complex but they mostly still include a half-center component.

Invertebrate preparations and their contribution to neurobiology in the second half of the 20th century

This review summarized the contribution to neurobiology achieved through the use of invertebrate preparations in the second half of the 20th century. This fascinating period was preceded by pioneers who explored a wide variety of invertebrate phyla and developed various preparations appropriate for electrophysiological studies. Their work advanced general knowledge about neuronal properties (dendritic, somatic, and axonal excitability; pre- and postsynaptic mechanisms). The study of invertebrates made it possible to identify cell bodies in different ganglia, and monitor their operation in the course of behavior. In the 1970s, the details of central neural circuits in worms, molluscs, insects, and crustaceans were characterized for the first time and well before equivalent findings were made in vertebrate preparations. The concept and nature of a central pattern generator (CPG) have been studied in detail, and the stomatogastric nervous system (STNS) is a fine example, having led to many major developments since it was first examined. The final part of the review is a discussion of recent neuroethological studies that have addressed simple cognitive functions and confirmed the utility of invertebrate models. After presenting our invertebrate "mice," the worm Caenorhabditis elegans and the fruit fly Drosophila melanogaster, our conclusion, based on arguments very different from those used fifty years ago, is that invertebrate models are still essential for acquiring insight into the complexity of the brain.

Cerebellar circuitry as a neuronal machine

Shortly after John Eccles completed his studies of synaptic inhibition in the spinal cord, for which he was awarded the 1963 Nobel Prize in physiology/medicine, he opened another chapter of neuroscience with his work on the cerebellum. From 1963 to 1967, Eccles and his colleagues in Canberra successfully dissected the complex neuronal circuitry in the cerebellar cortex. In the 1967 monograph, "The Cerebellum as a Neuronal Machine", he, in collaboration with Masao Ito and Janos Szentágothai, presented blue-print-like wiring diagrams of the cerebellar neuronal circuitry. These stimulated worldwide discussions and experimentation on the potential operational mechanisms of the circuitry and spurred theoreticians to develop relevant network models of the machinelike function of the cerebellum. In following decades, the neuronal machine concept of the cerebellum was strengthened by additional knowledge of the modular organization of its structure and memory mechanism, the latter in the form of synaptic plasticity, in particular, long-term depression. Moreover, several types of motor control were established as model systems representing learning mechanisms of the cerebellum. More recently, both the quantitative preciseness of cerebellar analyses and overall knowledge about the cerebellum have advanced considerably at the cellular and molecular levels of analysis. Cerebellar circuitry now includes Lugaro cells and unipolar brush cells as additional unique elements. Other new revelations include the operation of the complex glomerulus structure, intricate signal transduction for synaptic plasticity, silent synapses, irregularity of spike discharges, temporal fidelity of synaptic activation, rhythm generators, a Golgi cell clock circuit, and sensory or motor representation by mossy fibers and climbing fibers. Furthermore, it has become evident that the cerebellum has cognitive functions, and probably also emotion, as well as better-known motor and autonomic functions. Further cerebellar research is required for full understanding of the cerebellum as a broad learning machine for neural control of these functions.

Reflections on the interaction of the mind and brain

Problems associated with the topic of the mind-brain interaction are reviewed and analyzed. If there is an interaction, then the "mind" and "brain" are independent variables; the mind represents subjective experience and is therefore a non-physical phenomenon. This fact led to the need for a field theory, termed here the "cerebral mental field" (CMF). By definition, the CMF is a system property produced by the appropriate activities of billions of neurons. An experimental test of this theory is possible and a test design is presented. The most direct experimental evidence has been obtained by use of intracranial stimulating and recording electrodes. Important information has also been developed, however, with extracranial imaging techniques. These can be very fast (in ms), but the cerebral neuronal events that produce changes in physiological properties require a time delay for their processing. A number of surprising time factors affecting the appearance of a subjective somatosensory experience are described, and their wider implications are discussed. Among these is a delay (up to 0.5 s) in the generation of a sensory awareness. Thus, unconscious cerebral processes precede a subjective sensory experience. If this can be generalized to all kinds of subjective experiences, it would mean that all mental events begin unconsciously and not just those that never become conscious. In spite of the delay for a sensory experience, subjectively there appears to be no delay. Evidence was developed to demonstrate that this phenomenon depends on an antedating of the delayed experience. There is a subjective referral backward in time to coincide with the time of the primary cortical response to the earliest arriving sensory signal. The subjective referral in time is analogous to the well-known subjective referral in space. In conclusion, features of the CMF can be correlated with brain events, even though the CMF is non-physical, by study of subjective reports from the human subject.

John Eccles' studies of spinal cord presynaptic inhibition

Presynaptic inhibition is one of many areas of neurophysiology in which Sir John Eccles did pioneering work. Frank and Fuortes first described presynaptic inhibition in 1957. Subsequently, Eccles and his colleagues characterized the process more fully and showed its relationship to primary afferent depolarization. Eccles' studies emphasized presynaptic inhibition of the group Ia monosynaptic reflex pathway but also included group Ib, II and cutaneous afferent pathways, and the dorsal column nuclei. Presynaptic inhibition of the group Ia afferent pathway was demonstrated by depression of monosynaptic excitatory postsynaptic potentials and inhibition of monosynaptic reflex discharges. Primary afferent depolarization was investigated by recordings of dorsal root potentials, dorsal root reflexes, cord dorsum and spinal cord field potentials, and tests of the excitability of primary afferent terminals. Primary afferent depolarization was proposed to result in presynaptic inhibition by reducing the amplitude of the action potential as it invades presynaptic terminals. This resulted in less calcium influx and, therefore, less transmitter release. Presynaptic inhibition and primary afferent depolarization could be blocked by antagonists of GABA(A) receptors, implying a role of interneurons that release gamma aminobutyric acid in the inhibitory circuit. The reason why afferent terminals were depolarized was later explained by a high intracellular concentration of Cl(-) ions in primary sensory neurons. Activation of GABA(A) receptors opens Cl(-) channels, and Cl(-) efflux results in depolarization. Another proposed mechanism of depolarization was an increase in extracellular concentration of K(+) following neural activity. Eccles' work on presynaptic inhibition has since been extended in a variety of ways.

Plasticity from muscle to brain

Recognition that the entire central nervous system (CNS) is highly plastic, and that it changes continually throughout life, is a relatively new development. Until very recently, neuroscience has been dominated by the belief that the nervous system is hardwired and changes at only a few selected sites and by only a few mechanisms. Thus, it is particularly remarkable that Sir John Eccles, almost from the start of his long career nearly 80 years ago, focused repeatedly and productively on plasticity of many different kinds and in many different locations. He began with muscles, exploring their developmental plasticity and the functional effects of the level of motor unit activity and of cross-reinnervation. He moved into the spinal cord to study the effects of axotomy on motoneuron properties and the immediate and persistent functional effects of repetitive afferent stimulation. In work that combined these two areas, Eccles explored the influences of motoneurons and their muscle fibers on one another. He studied extensively simple spinal reflexes, especially stretch reflexes, exploring plasticity in these reflex pathways during development and in response to experimental manipulations of activity and innervation. In subsequent decades, Eccles focused on plasticity at central synapses in hippocampus, cerebellum, and neocortex. His endeavors extended from the plasticity associated with CNS lesions to the mechanisms responsible for the most complex and as yet mysterious products of neuronal plasticity, the substrates underlying learning and memory. At multiple levels, Eccles' work anticipated and helped shape present-day hypotheses and experiments. He provided novel observations that introduced new problems, and he produced insights that continue to be the foundation of ongoing basic and clinical research. This article reviews Eccles' experimental and theoretical contributions and their relationships to current endeavors and concepts. It emphasizes aspects of his contributions that are less well known at present and yet are directly relevant to contemporary issues.

Eccles' perspective of the forebrain, its role in skilled movements, and the mind-brain problem

Sir John Eccles' experimental life evolved from the "bottom" up: the synapse to the modular circuitry of the spinal cord, later the cerebellum and, less extensively, also the thalamus and hippocampus. He experimented quantitatively on basic properties of cell membranes, synapses, transmitters, cellular modules, reflexes, and plasticity. In parallel, he was also motivated to consider philosophical problems of mind-brain interactions. It was mostly during Eccles' "Swiss period" (1976-1997) that new experimental work advanced understanding of intentional motor actions and their preparation. For example, early brain imaging work suggested that the so-called "supplementary" motor area was rather a "supramotor" area, concerned with intentional preparation to move. Eccles also closely followed work on cortico-cerebellar integration and learning. His final contribution, in collaboration with the quantum physicist, Friedrich Beck, was a model of how specific neuronal modules interact with the mind. Being a declared dualist, Eccles encountered considerable resistance and skepticism among neuroscientists in accepting his experimentally untestable mind-brain theories. But one can only admire the remarkable continuity of effort in his search for modular operations of identified neurons in the central nervous system and their synaptic actions. This effort was facilitated by collaboration with the eminent anatomist, János Szentágothai, who had previously helped Eccles advance understanding of spinal and cerebellar circuitry. This review also includes some personal views on current understanding of the forebrain, with an emphasis on the multiplicity of cortical modules, all of which contribute in the mental preparation for forthcoming intentional actions.

Beginning at the end: repetitive firing properties in the final common pathway

Since the early 20th century, it has been recognized that motoneurons must fire repetitive trains of action potentials to produce muscle contraction. In 1932, Sir John Eccles, together with Hebbel Hoff, found that action potential spike trains in motor axons were produced by "rhythmic centres", which were within the motoneurons themselves. Two decades later, Eccles attended a Cold Spring Harbor Symposium in NY, USA entitled "The Neuron". Two of the many notable presentations at this symposium were juxtaposed: one by Eccles from the University of Otago, Dunedin, NZL, and the other by J. Walter Woodbury and Harry Patton from the University of Washington, Seattle, USA. Both presentations included data obtained using sharp microelectrodes to study the intracellularly recorded potentials of cat motoneurons. In this review, I discuss some of the events leading up to and surrounding this jointly accomplished advance and proceed to discussion of subsequent studies over 5+ decades that have made use of intracellular recordings from motoneurons to study their repetitive firing behavior. This begins with early descriptions of primary and secondary range firing, and continues to the discovery of dendritic persistent inward currents and their relation to plateau potentials, synaptic amplification, and motoneuronal firing. Following a brief description of the possible mechanisms underlying spike frequency adaptation, I discuss the modulation of repetitive firing properties during various motor behaviors. It has become increasingly clear that the central nervous system has exquisite control of the repetitive firing of motoneurons. Eccles' work laid the foundation for the present-day study of these processes.

Inhibitory circuits in the thalamus and hippocampus—An appraisal after 40 years

After scientific successes in the study of synaptic activation and inhibition of motoneurons and unraveling mechanisms underlying presynaptic inhibition, Sir John Eccles was interested in studying synaptic mechanisms governing the activity of neurons in the brain stem, cerebellum, and various cortical areas. In this new arena, his group discovered several principles, which have later been shown to generalize across brain structures and have substantial functional significance. Among these were the first identification and location of inhibitory synapses in the cerebral cortex and recurrent inhibitory systems in the hippocampus, cerebellum, and thalamus.

John Eccles’ pioneering role in understanding central synaptic transmission

This chapter deals with the central role that Sir John Eccles played in the elucidation of the mechanisms of synaptic transmission within the central nervous system during the three decades between the late 1930s and 1966. His seminal discoveries involved studies of synaptic input to spinal motoneurons using intracellular recording via glass micropipettes after their introduction in the late 1940s. After defending the hypothesis that electrical currents alone explained central synaptic events, his observations of reversal potentials and sensitivity to ion injections instantly converted Eccles to the idea that central synapses generate postsynaptic potentials, designated IPSPs and EPSPs, by liberating chemical transmitters. He and his collaborators used pharmacological manipulations of recurrent inhibition to support the idea that a given neuron liberates the same chemical transmitter substance at all of its synapses, which he called "Dale's Principle". His team worked out the mechanisms and spinal circuits underlying disynaptic and recurrent inhibition, as well as those of presynaptic inhibition. Not content with the view that central synapses were static entities, Eccles also made seminal observations on synaptic plasticity induced by alterations in use and disuse. Although his firmly held belief that the extensive dendritic trees of motoneurons were essentially irrelevant to synaptic events at the soma was later refuted by others in the mid-1960s, Eccles stands as a towering figure in the history of neuroscience. His prodigious energy and commanding intellect gave the field of central synaptic transmission the conceptual bases that have guided it for over 40 years.

Spinal reflexes, mechanisms and concepts: From Eccles to Lundberg and beyond

This review focuses on investigations by Sir John Eccles and co-workers in Canberra, AUS in the 1950s, in which they used intracellular recordings to unravel the organization of neuronal networks in the cat spinal cord. Five classical spinal reflexes are emphasized: recurrent inhibition of motoneurons via motor axon collaterals and Renshaw cells, pathways from muscle spindles and Golgi tendon organs, presynaptic inhibition, and the flexor reflex. To set the scene for these major achievements I first provide a brief account of the understanding of the spinal cord in "reflex" and "voluntary" motor activities from the beginning of the 20th century. Next, subsequent work is reviewed on the convergence on spinal interneurons from segmental sensory afferents and descending motor pathways, much of which was performed and inspired by Anders Lundberg's group in Gothenburg, SWE. This work was the keystone for new hypotheses on the role of spinal circuits in normal motor control. Such hypotheses were later tested under more natural conditions; either by recording directly from interneurons in reduced animal preparations or by use of indirect non-invasive techniques in humans performing normal movements. Some of this latter work is also reviewed. These developments would not have been possible without the preceding work on spinal reflexes by Eccles and Lundberg. Finally, there is discussion of how Eccles' work on spinal reflexes remains central (1) as new techniques are introduced on direct recording from interneurons in behaving animals; (2) in experiments on plastic neuronal changes in relation to motor learning and neurorehabilitation; (3) in experiments on transgenic animals uncovering aspects of human pathophysiology; and (4) in evaluating the function of genetically identified classes of neurons in studies on the development of the spinal cord.