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Synaptic Transmission

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Synaptic Transmission Synapse specialized junction where an axon terminal contacts another neuron or cell type Types of synapses Electrical synapses – PowerPoint PPT presentation

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Title: Synaptic Transmission


1
Synaptic Transmission
  • Synapse specialized junction where an axon
    terminal contacts another neuron or cell type
  • Types of synapses
  • Electrical synapses
  • Chemical synapses
  • An understanding of synaptic transmission is
    necessary to understand the operations of the
    nervous system (ie. actions of psychoactive
    drugs, causes of mental disorders, neural basis
    of learning and memory)

2
Electrical Synapses
  • Allows the direct transfer of ionic current from
    one cell to the next.
  • Gap Junction is composed of 6 connexins that
    make up a connexon. (Pore size 2nm)
  • Ions can flow bidirectionally.
  • Cells are electronically coupled.
  • Conduction speed is very fast.
  • Found in neuronal pathways associated with
    escape reflexes or in neurons that need to be
    synchronized.
  • Common in non neuronal cells.
  • Important in development

3
Chemical Synapses
  • Synaptic cleft
  • 20 50 nm wide
  • Held together by a fibrous extracellular matrix
  • Synaptic bouton (presynaptic element)
  • Contains synaptic vesicles (50nm in diameter)
    and secretory granules (100 nm) called large,
    dense core vesicles.
  • Membrane differentiations accumulations of
    proteins on either side of the synaptic cleft
  • Active zones presynaptic site of
    neurotransmitter release
  • Postsynaptic density contains receptors to
    translate intercellular signal (neurotranmitter)
    into an intracellular signal (chemical change or
    membrane potential change)

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Synapses can be categorized by
  • Connectivity which part of the neuron is
    postsynaptic to the axon terminal
  • Synapse anatomy
  • Size and shape
  • Appearance of the pre and postsynaptic membrane
    differentiations.
  • Grays type I synapses asymmetrical
    (postsynaptic membrane is thick)
  • Grays type II synapses - symmetrical

7
CNS Synapses types of connections
Axodendritic
Axosomatic
Axoaxonic
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Synapses differentiated by size and shape.
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Categories of CNS membrane differentiations.
Grays type I synapses
Grays type II synapses
Usually Excitatory
Usually Inhibitory
10
  • Synaptic Junctions Exist Outside the Brain
  • Junctions between autonomic neurons and glands,
    smooth muscle, and heart.
  • The Neuromuscular Junction
  • Transmission is fast and reliable due to large
    size with many active zones and a motor end plate
    with specialized folds for more receptors.

11
Requirements of Chemical Synaptic Transmission.
  • Mechanism for synthesizing and packing
    neurotransmitter into vesicles.
  • Mechanism for causing vesicle to spill contents
    into synaptic cleft in response to action
    potential.
  • Mechanism for producing an electrical or
    biochemical response to neurotransmitter in
    postsynaptic neuron.
  • Mechanism for removing transmitter from synaptic
    cleft.
  • Must be carried out very rapidly.

12
Neurotransmitters
  • Amino Acids
  • synaptic vessicles
  • Amines
  • synaptic vessicles
  • Peptides
  • secretory granules.
  • Peptides may exist in the same axon terminals as
    amino acids and amines.
  • Fast transmission uses Amino acids or ACh.
  • Slow transmission may use any of the three types
    of neurotransmitters

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Neurotransmitter Synthesis and Storage
Synthesis of peptide neurotransmitters
Synthesis of amine and amino acids
15
Neurotransmitter Release
  • Action potential enters the axon terminal.
  • Voltage gated Ca channels open.
  • Ca activates proteins in the vesicle and active
    zone.
  • Activated proteins causes synaptic vesicles to
    fuse with membrane.
  • Neurotransmitter is released via exocytosis.
  • Note Peptide release requires high frequency
    action potentials and is slower (50 msec vs. 0.2
    msec).

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Neurotransmitter Receptors and Effectors
  • Neurotransmitters must bind to specific receptor
    proteins in the postsynaptic membrane.
  • Binding causes a conformational change in the
    receptor.
  • A change in structure equals a change in
    function.
  • Over 100 different types of receptors.
  • Two major categories of receptors
  • Transmitter (ligand) gated ion channels.
  • G-protein coupled receptors.

19
Ligand-gated Ion Channels
  • Structure Channel protein with a ligand binding
    domain.
  • Neurotransmitter binding causes channel to open.
  • Consequence depends on the specific ions that
    pass through the pore.
  • Na and K channels cause depolarization and are
    excitatory.
  • Cl- channels cause hyperpolarization and are
    inhibitory.
  • Activation is generally rapid and is mediated by
    amino acids and amines.

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Excitatory Postsynaptic Potential (EPSP)
Excitatory Neurotransmitters ACh and glutamate
21
Inhibitory Postsynaptic Potential (IPSP)
Inhibitory Neurotransmitters Glycine and GABA
22
G Protein-Coupled Receptors
  • Structure Receptor protein with a ligand binding
    domain and connected to G protein consisting of
    an alpha, beta and gama subunit.
  • Activation 1) Ligand binds to receptor 2)
    Receptor activates G-protein 3) G-protein
    dissociated 4) alpha subunit activates an
    effector protein.
  • Effectors G-proteins act in one of two ways
  • By opening ion channels
  • By activating enzymes that synthesize
    second-messenger molecules.
  • Tend to be slower, longer lasting and have
    greater diversity than ligand gated ion channels.
  • Ligand may bind to a family of receptors with
    different effects due to specific receptor type.

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Autoreceptors and Presynaptic Inhibition
  • Receptors are sometimes found on the presynaptic
    terminal.
  • Activation leads to
  • Inhibition of neurotransmitter release
  • Neurotransmitter synthesis.
  • Autoreceptors may act as a brake on the release
    of neurotransmitters.

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Neurotransmitter Recovery and Degradation
  • Neurotransmitters must be cleared from the
    synapse to permit another round of synaptic
    transmission.
  • Methods
  • Diffusion
  • Enzymatic degradation in the synapse.
  • Presynaptic reuptake followed by degradation or
    recycling.
  • Uptake by glia
  • Uptake by the postsynaptic neuron and
    desensitization.

26
Neuropharmacology
  • Synaptic transmission is a chemical process and
    therefore can be affected by drugs and toxins.
  • Neuropharmacology is the study of the effects of
    drugs on the nervous system
  • Receptor Antagonists inhibit the normal action
    of a neurotransmitter.
  • Curare blocks the action of ACh at the
    neuromuscular junction.
  • Receptor Agonists mimic the action of a
    neurotransmitter.
  • Morphine activates Mu-opiate receptors in the
    brain.
  • Nervous system malfunctions are often related to
    neurotransmission errors.

27
Synaptic Integration
  • Each neuron may receive thousands of inputs in
    the form of ion channel and G-coupled protein
    activation.
  • These complex inputs give rise to simple output
    in the form of action potentials.
  • Neural computation
  • Neurotransmitters are released in quanta.
  • EPSP Summation
  • Neurons do sophisticated computations by adding
    together EPSPs to produce a significant
    postsynaptic depolarization.
  • Types of Summation Spatial and Temporal
    Summation.

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Dendritic Cable Properties
-Triggering of an action potential depends on how
far the synapse is from the spike initiation zone
and the properties of the dendrite (ie. Internal
and membrane resistance.) -Some dendrites have
voltage gated channels that can help amplify
signals along dendrites.
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Inhibition
IPSP are generated when ion channels are opened
causing hyperpolarization of the membrane. Ie.
GABA or glycine opens Cl- channels Shunting
Inhibition inward movement of Cl- anions will
negate the flow of positive ions.
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