Synaptic and neuro-muscular transmission

Minding the gap: discovering the phenomenon of chemical transmission in the nervous system

The neuron doctrine, according to which nerves consist of discontinuous neurons, presented investigators with the challenge of determining what activities occurred between them or between them and muscles. One group of researchers, dubbed the sparks, viewed the electrical current in one neuron as inducing a current in the next neuron or in muscles. For them there was no gap between the activities of neurons or neurons and muscles that required filling with a new type of activity. A competing group, the soups, came to argue for chemicals, subsequently referred to neurotransmitters, as carrying out the activities between neurons or between neurons and muscles. But even for them the conclusion that chemicals performed this activity was only arrived over time. I examine the prolonged period in which proponents of chemical transmission developed their account and challenged the sparks. My goal is to illuminate the epistemic processes that led to the discovery of a new scientific phenomenon-chemical transmission between neurons.

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.

Relationship between firing rate and recruitment threshold of motoneurons in voluntary isometric contractions

We used surface EMG signal decomposition technology to study the control properties of numerous simultaneously active motor units. Six healthy human subjects of comparable age (21 +/- 0.63 yr) and physical fitness were recruited to perform isometric contractions of the vastus lateralis (VL), first dorsal interosseous (FDI), and tibialis anterior (TA) muscles at the 20, 50, 80, and 100% maximum voluntary contraction force levels. EMG signals were collected with a five-pin surface array sensor that provided four channels of data. They were decomposed into the constituent action potentials with a new decomposition algorithm. The firings of a total of 1,273 motor unit action potential trains, 20-30 per contraction, were obtained. The recruitment thresholds and mean firing rates of the motor units were calculated, and mathematical equations were derived. The results describe a hierarchical inverse relationship between the recruitment thresholds and the firing rates, including the first and second derivatives, i.e., the velocity and the acceleration of the firing rates. This relationship describes an "operating point" for the motoneuron pool that remains consistent at all force levels and is modulated by the excitation. This relationship differs only slightly between subjects and more distinctly across muscles. These results support the "onion skin" property that suggests a basic control scheme encoded in the physical properties of motoneurons that responds consistently to a "common drive" to the motoneuron pool.

The brain on itself: Nobel laureates and the history of fundamental nervous system function

The Nobel Prize in Physiology or Medicine has been given in recognition of work in the neurosciences a number of times. Laureates have been awarded for work on both fundamental and more complex nervous system functions. This review is restricted to contributions by 20th century laureates to the understanding of fundamental nervous system function on the cellular level. In 1906, Camillo Golgi and Ramón y Cajal were awarded for their work on the microscopic structure of the nervous system. Their achievement and those of others within this field, coupled with technological progress, gradually allowed more complex physiological studies. In 1932, the prize was awarded to Charles Sherrington and Edgar Adrian for their discoveries of how neurons function. They were followed in 1944 by Herbert Gasser and Joseph Erlanger who uncovered the highly differentiated functions of single nerve fibers. Alan Hodgkin and Andrew Huxley were awarded for the detection of the ionic mechanism of the action potential and its mathematical explanation in 1963. In 1991, Erwin Neher and Bernd Sakmann were awarded for their work on single ion channels. Although the scientists who proved the hypothesis (Fridjof Nansen, Wilhelm His, and August Forel) were never awarded by the Nobel Committee, their studies gave rise to one of the most fundamental questions in 20th century neuroscience: How is information carried from one neuron to another or to an effector cell? This was first solved in the vegetative nervous system, and, in 1936, Henry Dale and Otto Loewi received the prize for their discoveries relating to chemical transmission of nerve impulses. In 1963, John Eccles was awarded the prize for his work on the physiology of synapses. In 1970, Bernhard Katz received the Nobel Prize for the discovery of quantal release. Katz shared the prize with Julius Axelrod and Ulf von Euler, who were central in finding that transmitters are stored in presynaptic vesicles and that the effect in many synapses is terminated by reuptake. This review does not include 21st century laureates, although the prize has already been given to neuroscientists twice this century; Arvid Carlsson, Paul Greengard, and Eric Kandel received the award in 2000 for their discoveries related to signal transduction, and Richard Axel and Linda Buck received the award in 2004 for their work in the field of odorant receptors and the organization of the olfactory system.

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.

Sir John Carew Eccles, A.C

Sir John Eccles, internationally recognized for his remarkable and outstanding impact on the neurosciences for more than six decades, died on 2 May 1997 at the age of 94. He performed his research in Oxford, Sydney, Dunedin, Canberra, Chicago and Buffalo from 1927 until 1975 (73)*. His numerous scientific papers and books, arising from pioneering experimental studies of synaptic mechanisms and the organization of neurons in the mammalian central nervous system, continue to have a major influence on brain research. Furthermore, his writings on the mind-brain interaction generated wide interest and debate. Eccles also made his mark as an administrator, particularly at the Australian National University and the Australian Academy of Science, of which he was a Foundation Fellow and the second President.

High shortening velocity of isolated single arterial muscle cells

Surprisingly high shortening velocities (less than 200 msec contraction-relaxation cycles) were found in isolated vascular muscle cells cultured from rat or chick aorta. All of the small fraction cells with such quick contractions had membrane excitation by short duration spikes, rather than the slower graded depolarization of the other cells which produced 20-fold slower contractions.