Process of Synaptic Transmission

3 Key excerpts on "Process of Synaptic Transmission"

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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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  eBook - ePub

  Neuropsychotherapy

  How the Neurosciences Inform Effective Psychotherapy

  • Klaus Grawe(Author)
  • 2017(Publication Date)
  • Routledge

    (Publisher)

  The collaboration of specialized neurons in neural circuits is made possible by the transmission of action potentials among the individual neurons. This potential is transmitted via the synapses, which connect two neurons with one another. Before we turn to the transactions among neurons on the level of the entire brain, it is critical to elaborate on the processes transpiring within the neurons themselves and at the synapses during the transmission of action potentials. These microlevel processes are the basis of all other processes within the brain. They determine what is and is not possible within the brain. If therapeutic changes achieve their effects via changes of the brain, then they are ultimately effective—on a microlevel—because they modulate synaptic efficacy. Let us take a closer look at the fundamental process underlying the transmission of action potentials to see whether relevant implications for psychotherapy can be derived.

  2.2 WHAT EXACTLY HAPPENS DURING THE TRANSMISSION OF ACTION POTENTIALS BETWEEN NEURONS?

  The functions of individual neurons depend on three factors: their localization within the brain, their connections with other neurons, and their individual characteristics. Even the internal organization and appearance of neurons can differ considerably. Figure 2–1 illustrates some of the more common types of neurons.

  However, it is not only in their visible appearance (in the electron microscope) that neurons differ from one another. Neurons are designed to communicate with other neurons. They do this via electrical transmissions and chemical synapses. For the chemical transmission of signals, neurons use transmitters that they themselves produce. These are, narrowly defined, the neurotransmitters, neuromodulators, neuropeptides, and neurohormones—often all of these are collectively referred to as neurotransmitters. There are nine different kinds of just the classical neurotransmitters (glutamate, glycin, gamma amino butric acid [GABA], dopamine, norepinephrine, epinephrine, serotonin, histamine, and acetylcholine) and more than 50 types of neuroactive peptides. Until recently, it was assumed that each neuron produces only one specific neurotransmitter, but this assumption has by now been shown to be inaccurate. Most neurons produce multiple neurotransmitters, which is only reasonable, as this multiplies the neuron’s options to influence other neurons. Of importance, the neurotansmitters work on very different time schedules. The effects of glutamate and GABA transpire within milliseconds, those of dopamine and serotonin within seconds or minutes, and those of the neuropeptides and neurohormones often within hours, days, or even weeks.

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  eBook - ePub

  Handbook of Clinical Psychopharmacology for Psychologists

  • Mark Muse, Bret A. Moore(Authors)
  • 2012(Publication Date)
  • Wiley

    (Publisher)

  c fibers also convey information about the injury, but at a much slower and more prolonged rate. This latter communication stream reflects the ongoing state of the injured tissue, and the experience of pain will be prolonged if the injury is such that it requires attention over a period of weeks.

  SYNAPTIC TRANSMISSION

  In order for information to be conveyed past the boundaries of a single neuron, it must bridge the gap to the next neuron. The gap, or synapse, is about 20 nanometers (nm) wide. It is literally bridged in some cases and “forded” in others. The type of synapse between the presynaptic and postsynaptic membranes depends on its location. The most common set-up is an axon-to-dendrite connection, or axodendritic synapse. If, instead, the presynaptic axon synapses at the postsynaptic soma or another axon, these are labeled axosomatic or axoaxonic synapses. Other, less common relationships are axosynaptic , dendrodendritic , axoextracellular , and axosecretory .

  In order for a substance to be classified as a neurotransmitter (NT), it must fulfill four criteria (Werman, 1966):

  1. It must originate from a neuron, where it is stored and released via depolarization. 2. It must induce effects postsynaptically on a target cell by specific receptors. 3. It must be inactivated or cleared from the synapse through reuptake. 4. It must produce the same effects on nervous tissue in vitro as those produced in vivo.

  NTs cross the synapse to bind to receptors on the postsynaptic membrane. The nature of the message is “coded” by the type and amount of NT(s) released. NTs are synthesized in the presynaptic neuron, with the location dependent on the type of NT. Small-molecule NTs are synthesized locally in the terminal, whereas larger, peptide transmitters are synthesized farther upstream and transported along microtubules

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