Transmission Across a Synapse

10 Key excerpts on "Transmission Across a Synapse"

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  eBook - ePub

  Principles of Neurobiology

  • Liqun Luo(Author)
  • 2020(Publication Date)
  • Garland Science

    (Publisher)

  Signaling across Synapses

  Processes which go through the nervous system may change their character from digital to analog, and back to digital, etc., repeatedly.

  John von Neumann (1958), The Computer & the Brain

  In this chapter, we continue to explore neuronal communication. We discuss first how arrival of an action potential at the presynaptic terminal triggers neurotransmitter release and then how neurotransmitters affect postsynaptic cells. This process, called synaptic transmission, results in information transmission from the presynaptic cell to the postsynaptic cell across the chemical synapse. In the context of studying postsynaptic reception, we will also introduce the fundamentals of signal transduction and study how synaptic inputs are integrated in postsynaptic neurons. Finally, we will discuss the electrical synapse, an alternative to the chemical synapse. Intercellular communication mediated by chemical and electrical synapses is the foundation of all nervous system functions.

  HOW DOES NEUROTRANSMITTER RELEASE AT THE PRESYNAPTIC TERMINAL OCCUR?

  In Chapter 2 , we addressed the basic cell biological and electrical properties of neurons that are required to understand how molecules, organelles, and action potentials get to axon terminals. We now address the main purpose of these movements: to transmit information across synapses to postsynaptic targets, which can be other neurons or muscle cells. To illustrate general principles, we focus first on model synapses and neurotransmitter systems and then expand our discussion to other neurotransmitter systems.

  3.1 Arrival of the action potential at the presynaptic terminal triggers neurotransmitter release

  Neurotransmitters are molecules released by presynaptic neurons that diffuse across the synaptic cleft and act on postsynaptic target cells. The vertebrate neuromuscular junction (NMJ), the synapse between the motor neuron axon terminals and skeletal muscle, is a model synapse that has been used to understand basic properties of synaptic transmission, many of which were later found to apply to other synapses. The neurotransmitter at the vertebrate NMJ was identified in the 1930s to be acetylcholine (ACh) (Figure 3-1A ). An important advantage of studying the neuromuscular synapse is that the postsynaptic muscle cell (also called a muscle fiber) is a giant cell that can easily be impaled by a microelectrode for intracellular recording (see Section 14.21 for details); sensitive and quantitative measurement of synaptic transmission can be achieved by recording the resulting current or membrane potential changes in the muscle fiber. The NMJ is also an unusual synapse in that the motor axon spreads out to form many terminal branches that harbor hundreds of sites releasing ACh onto the target muscle. This property makes the NMJ a strong synapse that reliably converts action potentials in the motor neurons into muscle contraction (to be discussed in more detail in Section 8.1

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  Behavioral Neuroscience

  • George Spilich(Author)
  • 2023(Publication Date)
  • Wiley

    (Publisher)

  51 CHAPTER OUTLINE 3.1 The Synapse • The Presynaptic Mechanism • The Postsynaptic Mechanism • A Closer Look at Interneuron Communication • Chemical versus Electrical Synapses Check It Out: Electrical Synapses and Atrial Fibrillation 3.2 Receptors and Postsynaptic Effects • Ionotropic and Metabotropic Receptors • Downregulation and Upregulation Think About It: How Upregulation Explains Nicotine Cravings • Autoreceptors and Heteroreceptors 3.3 An Overview of Major Neurotransmitters • Four Key Neurotransmitter Systems Check It Out: The Sleepy Sickness Epidemic • Other Neurotransmitters of Note 3.4 Disorders and Treatments Related to Neurotransmission • Synaptopathies Advances in Behavioral Neuroscience: Dimitri Kullmann and the Silent Synapse • Alcohol, Drugs, and Addiction • Drug Treatments Targeting Neurotransmission LEARNING OBJECTIVES LO 3.1 Describe how information is transmitted between neurons through neurotransmitters, synapses, and receptor sites. LO 3.2 Outline the different regulatory processes involved in neurotransmission. LO 3.3 Identify the most common neurotransmitters, explaining the functions of each. LO 3.4 Integrate your understanding of synaptic processes to understand related disorders and their treatments. CHAPTER 3 Information transfer from one neuron to the next occurs across the synapse, like the gap between these two hands. PeopleImages/iStock/Getty Images Synapses and Neuro- transmitters 52 C H A P T E R 3 Synapses and Neurotransmitters Stories of the Brain Kurt was a professor of mathematics at a small college. He loved to teach his students about the many ways mathematics can solve real-world problems. After a long career, he retired to enjoy his hobbies of gardening and playing the violin in the town’s adult orchestra. One morning while he was eating breakfast, his coffee cup slipped out of his hand and fell to the floor, shattering into pieces. “How careless of me,” he thought and gave it no mind.

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  From Molecules to Networks

  An Introduction to Cellular and Molecular Neuroscience

  • John H. Byrne, Ruth Heidelberger, M. Neal Waxham, James L. Roberts(Authors)
  • 2003(Publication Date)
  • Academic Press

    (Publisher)

  CHAPTER 9 Pharmacology and Biochemistry of Synaptic Transmission: Classic Transmitters

  Ariel Y. Deutch, Robert H. Roth

  The study of the nervous system 100 years ago was a period of claim and counterclaim, confusion, and recrimination—not unlike politics today or, for that matter, science. The reason for the tempestuous entry into the 20th century was the radical overthrow of the idea that the brain is one large continuous melded network (a syncytium), with each cell in physical contact. In contrast to this view, the pioneering studies of Santiago Ramón y Cajal revealed a very different picture, in which neurons, the units of the brain, are independent structures (see Shepherd, 1991 , and Chapter 1 ). Although final confirmation of this view would await the development and application of electron microscopy, there soon became a generalized acceptance of neurons as the independent units of the nervous system. In turn, this acceptance brought about a new debate: What is the mode of communication between neurons? The answer is not static, but is evolving continuously. In this chapter we discuss briefly several means through which cells communicate with each other and then discuss in considerable detail one such mechanism—chemical synaptic transmission. Another means of communication, electrical transmission through gap junctions, is discussed in Chapter 15 .

  As discussed in Chapter 1 , neurons, although varying widely in both morphology and function, share certain structural characteristics. A cell body (soma) from which processes emanate is present, with the processes (axons and dendrites) representing polarized compartments of the cell. Axons can be short or long, and remain local or alternatively project to distant areas. In contrast, dendrites are local. The general concept arose that axons transmit information, which is conveyed to the dendrites or soma of follower cells. The critical gap between the transmitting element of the neurons (axon) and the recipient zone of the follower cell (e.g., the dendrite) is the area across which transmission of information occurs; this area was termed the synapse by Charles Sherrington (Shepherd, 1991 ). Thus, there were presynaptic and postsynaptic neurons. This general conceptual framework remains in place today, although there are many exceptions, including dendrites that release neuroactive substances and axons that receive inputs from other neurons. One other characteristic proposed by Sherrington that is central to the concept of chemical communication between neurons is that synaptic transmission does not follow all-or-none rules, but is graded in strength and flexible (Sherrington, 1906

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  From Molecules to Networks

  An Introduction to Cellular and Molecular Neuroscience

  • Ruth Heidelberger, M. Neal Waxham, John H. Byrne, James L. Roberts(Authors)
  • 2009(Publication Date)
  • Academic Press

    (Publisher)

  Chapter 9 Pharmacology and Biochemistry of Synaptic Transmission

  Classical Transmitters

  Ariel Y. Deutch and Robert H. Roth

  The study of the nervous system 100 years ago was a period of claim and counterclaim, confusion, and recrimination—not unlike politics today or, for that matter, science. The reason for this rowdy transition to the twentieth century was a body of accumulating data that argued against the prevailing wisdom, which was that the brain is a large continuous network, with each of its cells in physical contact. The remarkably sharp eye and interpretative powers of the Spaniard Santiago Ramón y Cajal revealed a very different picture, in which neurons, the units of the brain, are independent structures (see Shepherd, 1991 , and Chapter 1 ). Although it would take another 50 years for electron microscopic data to be obtained that provided the final confirmation of Cajal’s hypothesis, his view of neurons as the independent units of the nervous system had gained widespread acceptance much earlier. In turn, this acceptance brought about a new debate: how do neurons that are not in physical contact communicate? The answer is not static but is evolving continuously. In this chapter we discuss briefly several means through which cells communicate with each other and then discuss in considerable detail the major means of communication, chemical synaptic transmission. Another means of communication, electrical transmission through gap junctions, is discussed in Chapter 15 .

  As discussed in Chapter 1 , neurons vary widely in form and function but share certain structural characteristics. From the cell body emanates processes (axons and dendrites) representing polarized compartments of the cell. Axons can be short or long and remain local or project to distant areas. In contrast, dendrites are local. The general concept arose that axons transmit information, which is conveyed to the dendrites or soma of follower cells. The critical gap between the transmitting element of the neurons (axon) and the recipient zone of the follower cell (e.g., the dendrite) is the area across which transmission of information occurs; this area was termed the synapse by Charles Sherrington (Shepherd, 1991 ). Thus, there were pre synaptic and post synaptic neurons. This general conceptual framework remains in place today, although there are many exceptions, including dendrites that release transmitters and axons that receive inputs from other neurons. One other characteristic proposed by Sherrington is central to the concept of chemical communication between neurons: synaptic transmission does not follow all-or-none rules but is graded in strength and flexible (Sherrington, 1906

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  Molecular and Cellular Physiology of Neurons

  Second Edition

  • Gordon L. Fain, Margery J. Fain(Authors)
  • 2015(Publication Date)
  • Harvard University Press

    (Publisher)

  PART THREE Synaptic Transmission and Ligand-Gated Channels Part 3 of this book describes the fundamental basis of nerve cell communication: the physiology of synaptic transmission. Synapses are the specialized contacts between neurons and target cells where information is transmitted from one cell to the next. Understand-ing how they work is absolutely essential for understanding how signals travel from one cell to another and how information is pro-cessed within the CNS. Synapses are also the sites where neuromod-ulatory mechanisms act to store information during learning and memory formation. Finally, the proteins responsible for synaptic transmission are targets of many psychoactive drugs. Although several different kinds of cellular communication in the CNS have been postulated or described, we confine discussion to the two most common forms. These are electrical synapses involv-ing gap junctions, where ionic current can flow directly between cells; and chemical synapses, where a chemical transmitter is con-tained within membranous vesicles in the presynaptic terminal and is released at specialized sites to interact with receptors on the sur-face of the postsynaptic cell. I: Gap Junctions and Electrical Synapses The first clear demonstration of an electrical synapse was made by Furshpan and Potter (1959), who were studying the crayfish giant motor synapse. At this synapse, the axons of postsynaptic giant motor neurons innervating flexor muscles of the tail receive an ex-citatory input from presynaptic axons known as lateral giant fibers (see Fig. 8.1A). Because of the large size of both the presynaptic and postsynaptic regions, Furshpan and Potter were able to impale 8 Presynaptic Mechanisms of Synaptic Transmission 282 SYNAPTIC TRANSMISSION AND LIGAND-GATED CHANNELS cells on both sides of this synapse with microelectrodes to pass cur-rent and record changes in membrane potential and thus examine the properties of transmission.

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  Stahl's Essential Psychopharmacology

  Neuroscientific Basis and Practical Applications

  • Stephen M. Stahl(Author)
  • 2021(Publication Date)
  • Cambridge University Press

    (Publisher)

  This also happens very quickly when chemical neurotransmitters open ion channels that change the flow of charge into the neuron, and ultimately, action potentials in the postsynaptic neuron. Thus, the process of neurotransmission is constantly transducing chemical signals into electrical signals, and electrical signals back into chemical signals. SIGNAL TRANSDUCTION CASCADES Overview Neurotransmission can be seen as part of a much larger process than just the communication of a presynaptic axon with a postsynaptic neuron at the synapse between them. That is, neurotransmission can also be seen as 9 STAHL’S ESSENTIAL PSYCHOPHARMACOLOGY activation of otherwise “sleeping” and inactive molecules (see for example, Figures 1-9 through 1-19). An overview of such a molecular “pony express,” from first-messenger neurotransmitter through several “molecular riders” to the production of diverse biological responses, is shown in Figure 1-9. Specifically, a first- messenger neurotransmitter on the left activates the production of a chemical second messenger that in turn activates a third messenger, namely an enzyme known as a kinase that adds phosphate groups to fourth-messenger proteins to create phosphoproteins (Figure 1-9, left). Another signal transduction cascade is shown on the right with a first-messenger neurotransmitter opening an ion channel that allows calcium to enter the neuron and act as the second messenger for this cascade system (Figure 1-9, right). Calcium then activates a different third messenger on the right, namely an enzyme known as a phosphatase that removes phosphate groups from fourth-messenger phosphoproteins and thus reverses the actions of the third messenger on the left.

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  Keynes & Aidley's Nerve and Muscle

  • Christopher L.-H. Huang(Author)
  • 2020(Publication Date)
  • Cambridge University Press

    (Publisher)

  8 Synaptic Transmission in the Nervous System Central nervous system function heavily depends on synaptic inter- actions between its constituent nerve cells. In most cases, in common with neuromuscular transmission (Chapter 7), this synaptic transmission involves chemical mechanisms. The presynaptic cell releases a chemical transmitter which produces a response in the postsynaptic cell. Different chemically transmitting synapses vary in anatomical details, but all share particular features. The presynaptic terminal contains transmitter packaged in synaptic vesicles. Pre- and postsynaptic membranes are separated by a synaptic cleft into which the vesicle contents discharge. The postsynaptic membrane contains specific receptors for the neurotransmitter concerned. Acetylcholine (ACh) is only one of a range of different central neurotransmitters (Figure 8.1). It was initially thought that any one cell would only release one neurotransmitter, but cases involving simultaneous release of two transmitters are now known. Electrically transmitting synapses also exist, which we shall consider briefly. As with the neuromuscular junction, much of our understanding of synaptic physiology was derived from intracellular microelectrode recordings. Much such fundamental work was performed by J. C. Eccles and his colleagues on feline spinal motor neurons (Eccles, 1964), so we shall begin our account of synapses between neurons with these. 8.1 Synaptic Excitation in Motor Neurons α-Motor neurons, the nerve cells which directly innervate skeletal muscle fibres, provided the initial model for studies of mammalian synaptic physiology. Their cell bodies lie in the ventral horn of the spinal cord, from which their axons pass through the ventral roots to reach peripheral nerves. Their cell bodies, or somas, are ~70 μm across, and extend into a number of fine branching processes, dendrites, which are up to 1 mm long.

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  Environmental Physiology of Animals

  • Pat Willmer, Graham Stone, Ian Johnston(Authors)
  • 2009(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  The synapse is normally the only point of com-munication between two neurons, and somehow the signal from one cell has to pass to the other across the discontinuity of an inter-cellular gap. There are two ways in which this can happen: either by direct electrical signaling, or by a chemical message crossing the gap. Both systems occur, though the latter is more common and we will therefore consider it first. 9.3.1 Chemical synapses For chemical transmission to be useful in a nervous system it must have three characteristics: it must be fast, it must be point-to-point (rather than diffuse), and it must be simple (excitatory or inhibit-ory, but not complex or ambiguous). Chemical synapses are the main system for communicating between cells in a nervous system. They may occur between any two neurons, or occasionally between different parts of one neuron. A chemical synapse (Fig. 9.12) is a zone where two neuronal surfaces lie close together (commonly the axon terminals of one neuron abutting onto the dendrites or cell body of another) and a chemical transmitter diffuses across the gap between them. It is constructed by the close apposition of specialized regions of the plasma membranes of the two participating neurons, forming a clearly defined synaptic interface with a narrow interven-ing cleft. The presynaptic cell contains a cluster of transmitter-filled vesicles and may have a somewhat thickened membrane, while the postsynaptic membrane is usually clearly thickened due to an accumulation of receptor sites and the associated submembrane “scaffolding”. In essence, the functioning of a synapse requires that: 1 The spreading depolarization at the presynaptic terminal of neu-ron A brings about a specific release of chemical. 2 The chemical diffuses from the membrane into the synaptic cleft. 3 The diffusing chemical arriving at receptors on the postsynaptic terminal in neuron B initiates a new depolarization.

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  Nolte's The Human Brain E-Book

  Nolte's The Human Brain E-Book

  • Todd W. Vanderah, Douglas J. Gould(Authors)
  • 2020(Publication Date)
  • Elsevier

    (Publisher)

  8

  Synaptic Transmission Between Neurons

  Chapter Outline

  • There Are Five Steps in Conventional Chemical Synaptic Transmission,
    174
    • Neurotransmitters Are Synthesized in Presynaptic Endings and in Neuronal Cell Bodies, 175
    • Neurotransmitters Are Packaged Into Synaptic Vesicles Before Release, 175
    • Presynaptic Endings Release Neurotransmitters Into the Synaptic Cleft, 176
    • Neurotransmitters Diffuse Across the Synaptic Cleft and Bind to Postsynaptic Receptors, 177
    • Neurotransmitter Action Is Terminated by Uptake, Degradation, or Diffusion, 177
  • Synaptic Transmission Can Be Rapid and Point-to-Point, or Slow and Often Diffuse,
    178
    • Rapid Synaptic Transmission Involves Transmitter-Gated Ion Channels, 179
    • Slow Synaptic Transmission Usually Involves Postsynaptic Receptors Linked to Intracellular Proteins, 179
    • The Postsynaptic Receptor Determines the Effect of a Neurotransmitter, 181
    • The Size and Location of a Synaptic Ending Influence the Magnitude of Its Effects, 182
      • Synapses With Many Active Zones Have a Greater Effect,
        182
      • Synapses Closer to the Action Potential Trigger Zone Have a Greater Effect,
        182
      • Presynaptic Endings Can Themselves Be Postsynaptic,
        182
  • Synaptic Strength Can Be Facilitated or Depressed,

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  Brain Sciences in Psychiatry

  • David M. Shaw, A.M.P. Kellam, R.F. Mottram(Authors)
  • 2013(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  The chemical nature of neurotransmission 67 Drug A NaCI Figure 4.10 The techniques of micro-iontophoresis 1—40 jum Figure 4.11 The techniques of micro-iontophoresis continued These experiments, and many others giving quantitative measures of transmitter release and activity, are quite convincing. Acetylcholine is believed to be one of the transmitters between the thalamus and the neocortex. The experiments are difficult to conduct, however, and need to be repeated for each of the many pathways converging onto cortical cells, for all the cellular areas of the brain and for each of the chemicals that have been suggested as possible transmitters in the brain. This is a daunting prospect indeed, and explains why, at so many synapses, the neurotrans-mitters are still unidentified. Figure 4.12 is a diagram of the synaptic terminal under the electron microscope, which shows that terminals do not make contact with cells. There is a gap or 68 Transmission between neurons Microfilaments -Mitochondria Synaptic vesicles Synaptic cleft Thickening of the postsynaptic membrane Figure 4.12 The synaptic terminal (diagrammatic) synapse of 20 nm between the presynaptic and the postsynaptic membranes. The terminals contain many mitochondria which synthesize ATP among other things and it is concluded that many of the processes in axon terminals consume energy. Terminals usually contain synaptic vesicles, which are small round or oval sacs which cluster near to the synapse. Two experiments identify their function. Sucrose gradient ultracentrifugation was described on page 47. If homogenized brain is centrifuged in this way, a layer can be found which contains synaptosomes. These are the axon terminals and a small piece of the postsynaptic membrane. If these are subjected to osmotic shocks (i.e. are placed in ^ solution of low osmotic pressure), the synaptosomes rupture, spilling their contents.

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