Biological Sciences
Types of Synapse
Types of synapse include electrical and chemical synapses. Electrical synapses allow for direct electrical communication between neurons through gap junctions, while chemical synapses involve the release of neurotransmitters to transmit signals across the synaptic cleft. These two types of synapses play crucial roles in the communication and integration of signals within the nervous system.
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11 Key excerpts on "Types of Synapse"
eBook - PDF- A.C. Damask(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
4.2 A few synaptic types: (a) two Types of Synapse between axon terminals and a dendrite; (b) synapse between an axon terminal and a dendritic spine; (c) two Types of Synapse between axon terminals and a neuron soma; (d) a synapse whose postsynaptic spine is in-vaginated into the presynaptic terminal. [From Whittaker and Gray (1962).] 90 4. CHEMICAL AND ELECTRICAL PROPERTIES OF SYNAPSES FIG. 4.3 The spiral ending of a nerve fiber upon a sympathetic ganglion neuron in the frog. Note the presence of many boutons (presynaptic knobs) near the origin (hillock) of the axon. [After Taxi (1965).] neuron. The small circles are the axon terminals' endings, called presynaptic knobs, or boutons. Approximately 10% of the large cell body is covered by these boutons, as estimated from electron micrographs (see Fig. 2.19). In the cerebral cortex approximately 2 % of the synapses are on cell bodies and large dendritic trunks, approximately 50% are located on dendritic spines, and virtually all the rest terminate on fine dendritic processes. Although there are numerous Types of Synapses, we shall find it convenient to divide them into two classes, depending on the specific mechanism em-ployed for the transmission of the nerve impulse from one neuron to another. These two classes are (a) chemical synapses and (b) electrical synapses. C H E M I C A L S Y N A P S E S The chemical synapse is characterized by a narrow gap of 200-300 A, termed the synaptic cleft, which separates the two cells. The presynaptic terminal consists not only of synaptic vesicles, both coated and uncoated (see p. 125), but it also contains an elaborate, intricate network of energy-generating mitochondria, voidlike structures called vacuoles, and elongated tube structures, called microtubules and neurofilaments. The names of the latter two structures arise because of their appearance in electron micro-graphs; microtubules are 200-300 A in diameter, whereas neurofilaments are about 100 A in diameter.
eBook - PDFColor Atlas of Neuroscience
Neuroanatomy and Neurophysiology
- Ben Greenstein, Adam Greenstein(Authors)
- 2011(Publication Date)
- Thieme(Publisher)
From electron microscope studies of synapses on spines, synapses have been classified by the width of the synaptic cleft and the distribution of synaptic density. Type 1 synapses , for example, are characterised by a wider synaptic cleft and a denser re-gion of synaptic thickening than occurs at Type 2 synapses . The classification may have functional significance, since in the pyramidal cells of the cerebral cortex, Type 1 synapses occur on spines projecting from apical or basal dendrites, whereas Type 2 synapses occur on the soma of the pyramidal cells of the cerebral cortex. Synapses may also be complex, where a single spine may form several synapses with an incoming axon, as occurs most commonly in certain cell types in the cen-tral nervous system; complex synapses of this type have been described in, for ex-ample, pyramidal cells of the hippocam-pus. Cellular Structures 97 Cellular Structures 98 The Neuromuscular Junction The neuromuscular junction (NMJ) is the synapse where electrical information is chemically transmitted from nerve to skeletal muscle. The nerve is the mo-toneuron , which has its cell body in the spinal cord, and whose axon terminates at the motor end plate of the muscle. The NMJ has been extensively studied. It was one of the first synapses to be character-ized anatomically, physiologically, and bi-ochemically, because the muscle cell is large enough to take several electrodes, and because the NMJ can be seen under the light microscope. The electrical information (action potential) is transduced to a chemical sig-nal in the form of a neurotransmitter, acetylcholine (ACh; see also p. 101), which is transduced back to an action potential in the muscle. The action poten-tial is then transduced to mechanical work as a muscle twitch. Usually, one axon in-nervates one fiber. Shortly before the axon arrives at the end plate, it loses the myelin sheath and forms several thin branches.
eBook - PDF- 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.
eBook - PDF- George Spilich(Author)
- 2023(Publication Date)
- Wiley(Publisher)
Chemical versus Electrical Synapses MCAT So far we have been discussing synapses that are chemical in nature; that is, they transmit data through a neurotransmitter, which serves as a chemical messenger. In these neurons, when the presynaptic neuron is sufficiently activated by an action potential that has traveled down the axon, a neurotransmitter is released via the activation of a voltage-gated calcium channel in the presynaptic terminal button. This neurotransmitter travels across the synapse and ide- ally binds to a specific receptor site in the postsynaptic neuron. When a neurotransmitter suc- cessfully binds to the postsynaptic receptor, that binding initiates an electrical impulse, which then activates the neuron to either excite or inhibit the postsynaptic neuron. (Remember for a moment that neurons can either be gas or brake, excitatory or inhibitory.) Most synapses are chemical in nature and function as we have just described. However, neurons that are very close together can communicate through a direct electrical connection. A typical chemical synapse has a gap, or synaptic cleft, of 20 to 40 micrometers (µm), whereas an electrical synapse has a gap of about 3.5 µm. In electrical synapses, the space between the pre- and postsynaptic membranes is not called a synaptic cleft but instead a gap junction, and that very close gap junction allows an electrical signal to be transmitted easily from the presynaptic neuron to the postsynaptic neuron (Figure 3.3). The channels between the two synaptic membranes are hydrophilic, which means they permit the passage of water and ions. When that hydrophilic channel opens, a direct connection of 1–2 µm allows transfer of the ions and even smaller molecules between the pre- and postsynaptic neurons. There are several key differences between chemical and electrical synapses.
eBook - PDFLanguage Learning and the Brain
Lexical Processing in Second Language Acquisition
- Ulf Schütze(Author)
- 2016(Publication Date)
- Cambridge University Press(Publisher)
49 3 Synaptic Connections Neurons and Synapses Neurons are nerve cells that can transmit information. A neuron is like a relay station containing three parts: dendrites, a nucleus, and an axon. The nucleus contains the genetic code that predetermines whether the cell has the capacity to transmit information. A synapse is the point of contact between two neurons. Where the neurons do not meet, the gap or space between them is called a syn- aptic gap. Information comes in the form of an electrical signal. The electric signal, known as a nerve impulse, is passed from one neuron to another at the synapse. Sometimes the signal can be quickly transferred at the synapse of two adjacent neurons, but if not it can then be carried across the gap through a biochemical process with the help of neurotransmitters. In the process of neu- rotransmission, the neuron releases a chemical – the neurotransmitter – which drifts across the synaptic gap and transports the message from the axon of one neuron to the dendrites of another. The dendrites of the receiving neuron convert the chemical message back into an electronic nerve impulse that can then be read by the nucleus of the receiving neuron and passed on to the axon. This is how nerve impulses can travel between neurons. Through this process, each neuron can communicate with innumerable others and this facilitates the interconnectivity of our brain. The biochemical reaction between two neurons can provide either a tempo- rary or permanent bridge between the synaptic gap. If it is permanent, a new synapse, or bridge, is formed. Where these synapses can be built is predeter- mined by the genetic code. Whether or not they are built, however, is a matter of stimulation. Bridging the gap temporarily can be compared to the action of a boat that ferries the information across. Unfortunately, the boat only runs for a few minutes or, in some cases, for a few hours.
eBook - PDFDiscovering Behavioral Neuroscience
An Introduction to Biological Psychology
- Laura Freberg(Author)
- 2018(Publication Date)
- Cengage Learning EMEA(Publisher)
In contrast, chemical synapses allow for both excitatory and inhibitory messages to be sent. In inhibition, the next cell is told not to produce an action potential. Another major advantage of chemical synapses over gap junctions is that a very small presynaptic neuron using chemical messengers can still influence a very large postsynaptic neu-ron. In the case of a gap junction, it takes a very large presynaptic neuron to influence a tiny postsynaptic neuron, because the strength of the signal decreases as it moves from one cell to the next. Chemical Synapses There are two types of intercellular chemical communication in the central nervous system (Agnati & Fuxe, 2014). In wiring transmission, chemicals diffuse from one cell to impact an adjacent cell or cells through private, highly localized channels. The classic chemical synapse, in which a presynaptic axon terminal releases neurochemical molecules that cross the synaptic gap and bind with postsynaptic receptors, is the primary mechanism through which neurons exchange information and control behavior (see ● Figure 3.21). This classic synapse will be the main model of neural communication featured in this dis-cussion. Technically, a gap junction also uses wiring transmission to share ions and neurochemicals. The second type of chemical communication is volume transmission, in which neurochemicals diffuse through the extracellular fluid and CSF to influence cells located some distance away from the releasing cell. We will discuss both types of com-munication further in Chapter 4 in the context of the particular types of neurochemi-cals released during the two processes. Signaling at chemical synapses occurs in two steps. The first step is the release of neurochemicals by a presynaptic cell. The second step is the reaction of a postsynaptic cell to neurochemicals. NEUROCHEMICAL RELEASE In response to the arrival of an action potential at a terminal, a new type of voltage-dependent channel will open.
eBook - PDFExcitable Cells
Monographs in Modern Biology for Upper School and University Courses
- F. A. Miles(Author)
- 2013(Publication Date)
- Butterworth-Heinemann(Publisher)
CHAPTER 5 The Synapse The information processing carried out in the CNS involves the transfer of signals across intricate networks of neurones. In the course of this data handling, fresh information is introduced into the system from the receptors and is sorted, weighed against other factors and ultimately shaped into some pattern of activity which will provide appropriate action. Sophisticated processing of this kind requires an immensely elaborate organization, and structur-ally, the system is a profusion of interconnexions which defies formal description. The function of the neurone in this, is to provide an integrating surface where signals can be brought together, sifted and fashioned into an output signal. The axon, of course, provides for the immediate dispatch of this output signal to the next integrating centre. Thus, the individual unit functions like a nervous system in miniature. The extensive stretch of membrane investing the body of the neurone and its projecting dendrites—the soma-dendritic mem-brane—composes the receiving area for the whole cell, and is covered with the swollen end-feet of hundreds or even thousands of axon terminals which deliver the input signals for processing. A narrow gap separates each of these axon terminals from the underlying membrane and presents such a high resistance to the local circuit currents associated with the incoming spikes that they fail to invade it. Transmission across these junctions, or synapses, is therefore discontinuous, and the functional link between neurones is provided by chemical transmitter processes. These involve a chemical transmitter substance which is stored in the presynaptic axon terminal and is released into the gap on the arrival of an impulse. Diffusion then carries the transmitter on to the postsynaptic membrane.
eBook - PDF- James Kalat(Author)
- 2018(Publication Date)
- Cengage Learning EMEA(Publisher)
Copyright 2019 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 41 I f you had to communicate with someone without sight or sound, what would you do? Chances are, your first choice would be a touch code or a system of electrical impulses. You might not even think of passing chemicals back and forth. Chemicals are, however, the main way your neurons communicate. They communicate by transmitting chemicals at specialized junctions called synapses. Module 2.1 The Concept of the Synapse Properties of Synapses Relationship among EPSP, IPSP, and Action Potentials In Closing: The Neuron as Decision Maker Module 2.2 Chemical Events at the Synapse The Discovery of Chemical Transmission at Synapses The Sequence of Chemical Events at a Synapse Hormones In Closing: Neurotransmitters and Behavior After studying this chapter, you should be able to: 1. Describe how Charles Sherrington used behavioral observations to infer the major properties of synapses. 2. Relate the activities at a synapse to the probability of an action potential. 3. List and explain the sequence of events at a synapse, from synthesis of neurotrans-mitters, through stimulation of receptors, to the disposition of the transmitter molecules. 4. Discuss how certain drugs affect behavior by altering events at synapses. 5. Contrast neurotransmitters, neuropep-tides, and hormones. Chapter 2 Synapses Chapter Outline Learning Objectives Opposite: This electron micrograph, with color added artificially, shows branches of an axon making contact with other cells. (Eye of Science/Science Source) Copyright 2019 Cengage Learning.
eBook - PDFBrain-mind Machinery: Brain-inspired Computing And Mind Opening
Brain-Inspired Computing and Mind Opening
- Gee-wah Ng(Author)
- 2009(Publication Date)
- World Scientific(Publisher)
In 2008, the word “synapse” turned 111 years old. The word synapse was first used in a book called, A Textbook of Physiology, part three: The Central Nervous System , by Michael Foster and assisted by Charles S. Sherrington, in 1897. It is commonly cited that Charles S. Sherrington coined the term “synapse”. The word “synapse” comes from the Greek word, meaning “to clasp together”. Fig. 2.2. Heinrich Wilhelm Gottfried von Waldeyer Hartz (1836–1921). (Picture from http://en.wikipedia.org/wiki/Heinrich_Wilhelm_Gottfried_von_ Waldeyer-Hartz) 20 Brain-Mind Machinery This chapter presents the following: • How do neurons communicate and affect our learning and memory? Neurons have specialized functions. How is information transmitted and stored? How is information represented? How does the neuron’s receptive field affect our visual scene? • How does synaptic plasticity give rise to learning and memory? Synaptic plasticity. Synaptic spike and normalization. • Modeling the neurons and synapses Historical background of artificial neural networks. Modeling at the cellular level. How Do Neurons Communicate and Affect Our Learning and Memory? The fundamental building blocks of the nervous system are the neurons. Fig. 2.3. Charles S. Sherrington (1857–1952). (Picture from http://nobel-prize.org/nobel_prizes/medicine/laureates/1932/sherrington-bio.html.) Neurons and Synapses: The Key to Memory and Learning 21 The neuron (Fig. 2.4) consists of a cell body called the soma, which contains the nucleus for sustaining the life of the cell, den-drites that serve to receive information from other neurons (via synapses), and an axon that carries information away from the soma to the terminal buttons where the information is transmitted to other neurons. At the end of the terminal buttons are gaps known as synapses. Neurotransmitters are secreted from the terminal but-tons.
eBook - PDFHistophysiology of Synapses and Neurosecretion
International Series of Monographs on Pure and Applied Biology: Modern Trends in Physiological Sciences
- Eduardo D.P. De Robertis, P. Alexander, Z. M. Bacq(Authors)
- 2013(Publication Date)
- Pergamon(Publisher)
S U M M A R Y In this chapter, a brief review of the morphological aspects of the synaptic region, as revealed by the light microscope, is presented. Synapses are classified by their relationship with the postsynaptic component and external morphology. The existence of specific synaptic patterns in mono and poly-synaptic systems and their complexity in higher integrative centresis emphasized. This synaptic mosaic is apparently maintained on the neural surface by the firm adhesion of the synaptic endings. The finding of a special fibrillary attachment of macromolecular dimen-sions may explain this phenomenon. The great number of endings and their relative density on the surface of certain neurons is mention-ed and the variations in size and distribution are stressed. Some of the scanty results given by various light microscope methods on the structure of the synapse are described. The presence of argyrophilic loops or rings and mitochondria is mentioned and the so-called synaptolema discarded as a confusing term. The problem of contact at the synapse is analyzed and several hypotheses dealing with intervening materials at synapses are discussed. The conclusion is reached that although the classical methods for study of the synapse may still give important information related to the complex synaptic fields and connectivity pattern in the CNS, they do not provide a structural background for the interpretation of synaptic function. C H A P T E R 3 G E N E R A L U L T R A S T R U C T U R E O F T H E S Y N A P T I C R E G I O N ALTHOUGH by 1947 the electron microscope had reached a limit of resolution below 10 A and advances had been made in the analysis of isolated macromolecules, this instrument could not be applied to the study of thick biological specimens. The great progress made since 1950 in ascertaining the submicroscopic organization of cells and tissues has mainly resulted from improvements made in the techniques of fixing, embedding and sectioning.
eBook - PDF- David M. Shaw, A.M.P. Kellam, R.F. Mottram(Authors)
- 2013(Publication Date)
- Butterworth-Heinemann(Publisher)
Chapter 4 Transmission between neurons Introduction In Chapter 3 the typical neuron of the central nervous system (CNS) was shown to have an axon which divides into several terminal branches. At the end of each branch a terminal swelling was closely applied to another neuron. There are many variations on this basic theme. Sometimes an axon will be repeatedly applied to several cells. Each time the axon approaches another cell, it swells into a terminal but then continues on to form several more synaptic contacts. Terminals are applied to many different regions of cells. They are found on dendrites, on the cell body and also on the area where the axon forms from the cell body. The terminal may be applied to a flat portion of a cell membrane or may lie on a projection from the surface known as a spine. There are also axo-axonic contacts where one terminal makes contact with another {Figure 4.1). Considerable variation can be -4: Figure 4.1 These schematic diagrams illustrate the considerable variation found in synapses within the central ner-vous system. Terminals may be: 1, axonal somatic synapses; 2, axonal-dendritic synapses; 3, axon-hillock synapses; 4, den-dritic spine synapses; 5, axo-axonal synapses; tight, or electrical, synapse (note the narrow gap between presynap-tic and postsynaptic neurons) 58 Excitatory transmission 59 detected in the fine structure of the terminal regions, the most striking being the so-called tight junctions where particularly close contact between two cells occurs and the terminal is lacking some of the special features usually associated with them. This confusing variation is such an essential feature of neurotransmission in the CNS that it is necessary to bear in mind that the account which follows is typical but cannot be applied in a simple way to all neurotransmitter systems. Excitatory transmission In the mammal neurotransmission has been studied most extensively on the motoneuron (anterior horn cell).
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