science of synaptic

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Copyright 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins

Neuroscience: Exploring the Brain, 3e

Chapter 5: Synaptic Transmission


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Introduction

Synaptic Transmission

Information transfer at a synapse

Plays role in all the operations of the nervous system

1897: Charles Sherrington-synapse

Chemical and electrical synapses

1921- Otto Loewi

1959- Furshpan and Potter


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Types of Synapses

Direction of Information Flow

In one direction: Neuron to target cell

First neuron = Presynaptic neuron

Target cell = Postsynaptic neuron


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Electrical Synapses

Gap junction

Channel

Connexon- formed by six connexins

Cells are said to be electrically coupled

Flow of ions from cytoplasm to cytoplasm

Types of Synapses


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INSERT FIG. 5.1 (Yes, deliberately out of order)


7/37Copyright 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins

Types of Synapses

Electrical Synapses (Contd)

Very fast transmission

Postsynaptic potentials (PSPs)

Synaptic integration: Several PSPs occurringsimultaneously to excite a neuron (i.e. causes AP)



Types of Synapses

Chemical Synapses



CNS Synapses (Examples)

Axodendritic: Axon to dendrite

Axosomatic: Axon to cell body

Axoaxonic: Axon to axon

Dendrodendritic: Dendrite to dendrite

Types of Synapses



CNS Synapses (Examples)

Grays Type I: Asymmetrical, excitatory

Grays Type II: Symmetrical, inhibitory

Types of Synapses



The Neuromuscular Junction

(NMJ) Studies of NMJ established

principles of synaptictransmission

Types of Synapses



Principles of Chemical SynapticTransmission

Basic Steps

Neurotransmitter synthesis

Load neurotransmitter into synaptic vesicles

Depolarization Vesicles fuse to presynaptic terminal

Neurotransmitter spills into synaptic cleft

Binds to postsynaptic receptors

Biochemical/Electrical response elicited in postsynaptic cell

Removal of neurotransmitter from synaptic cleft



Neurotransmitters

Amino acids: Small organic molecules

e.g., Glutamate, Glycine, GABA

Amines: Small organic molecules

e.g., Dopamine, Acetylcholine, Histamine

Peptides: Short amino acid chains (i.e. proteins)stored in and released from secretory granules

e.g., Dynorphin, Enkephalins




Neurotransmitters




Neurotransmitter Synthesis and Storage

Amines, amino acids, peptides




Neurotransmitter Release

Exocytosis: Process by which vesicles release theircontents




Neurotransmitter Release (Contd)

Mechanisms

Process of exocytosis stimulated by release ofintracellular calcium, [Ca2+]i

Proteins alter conformation - activated

Vesicle membrane incorporated into presynapticmembrane

Neurotransmitter released

Vesicle membrane recovered by endocytosis




Neurotransmitter receptors:

Ionotropic: Transmitter-gated ion channels




Metabotropic: G-protein-coupled receptor



Excitatory and Inhibitory Postsynaptic Potentials:

EPSP:Transient postsynaptic membrane depolarization bypresynaptic release of neurotransmitter

IPSP: Transient hyperpolarization of postsynaptic membranepotential caused by presynaptic release of neurotransmitter

Reversal potential concept




Neurotransmitter Recovery and Degradation

Diffusion: Away from the synapse

Reuptake: Neurotransmitter re-enters presynapticaxon terminal

Enzymatic destruction inside terminal cytosol orsynaptic cleft

Desensitization: e.g., AChE cleaves Ach to inactivestate



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Neuropharmacology

Effect of drugs on nervous system tissue

Receptor antagonists: Inhibitors of neurotransmitterreceptors

Curare

Receptor agonists: Mimic actions of naturallyoccurring neurotransmitters

Nicotine

Defective neurotransmission: Root cause ofneurological and psychiatric disorders



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Principles of Synaptic Integration Synaptic Integration

Process by which multiple synaptic potentialscombine within one postsynaptic neuron


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Quantal Analysis of EPSPs

Synaptic vesicles: Elementary units of synaptic

transmission Quantum: An indivisible unit

Miniature postsynaptic potential (mini)

Quantal analysis: Used to determine number of

vesicles that release during neurotransmission

Neuromuscular junction: About 200 synaptic vesicles,EPSP of 40mV or more

CNS synapse: Single vesicle, EPSP of few tenths of amillivolt

Principles of Synaptic Integration


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EPSP Summation

Allows for neurons to perform sophisticatedcomputations

Integration: EPSPs added together to producesignificant postsynaptic depolarization

Spatial: EPSP generated simultaneously in differentspaces

Temporal: EPSP generated at same synapse in rapidsuccession



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The Contribution of Dendritic Properties to SynapticIntegration

Dendrite as a straight cable Membrane depolarization falls off exponentially with

increasing distance

Vx = Vo/ex/

Dendritic length constant ()

In reality, dendrites are very elaborate structuresthat contribute to more complex integrativeproperties



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The Contribution ofDendritic Properties to

Synaptic Integration


Vx=V

o* e-x/


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Excitable Dendrites

Dendrites of neurons: voltage-gated sodium,

calcium, and potassium channels

Can act as amplifiers (vs. passive)

Dendritic sodium channels: May carry electricalsignals in opposite direction, from soma outwardalong dendrites



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Inhibition

Action of synapses to take membrane potential away

from action potential threshold

Exerts powerful control over neuron output



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IPSPs and Shunting Inhibition

Excitatory vs. inhibitory synapses: Bind different

neurotransmitters, allow different ions to pass throughchannels

Membrane potential more negative than -65mV =hyperpolarizing IPSP

Shunting Inhibition: Inhibiting current flow from soma toaxon hillock



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Shunting Inhibition: Inhibiting current flow from soma toaxon hillock

Principles ofSynaptic Integration


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The Geometry of Excitatory and Inhibitory Synapses

Excitatory synapses

Grays type I morphology

Spines: Excitatory synapses

Inhibitory synapses

Grays type II morphology

Clustered on soma and near axon hillock



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Concluding Remarks

Chemical synaptic transmission

Rich diversity allows for complex behavior

Provides explanations for drug effects

Defective transmission is the basis for manyneurological and psychiatric disorders

Key to understanding the neural basis of learningand memory


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End of Presentation


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Modulation

Synaptic transmission thatmodifies effectiveness ofEPSPs generated by othersynapses with transmitter-gated ion channels

Example: Activating NE receptor

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