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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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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Introduction Synaptic Transmission –Information transfer at a synapse –Plays role in all the operations of the nervous system –1897: Charles Sherrington- “synapse” Cajal vs Golgi (1906) –Chemical and electrical synapses 1921- Otto Loewi 1959- Furshpan and Potter
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins 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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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins
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Type of Synapses Golgi – Continuous synapse Cajal – Contiguous synapse
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins –Gap junction Channel Connexon- formed by six connexins –Cells are said to be “electrically coupled” Flow of ions from cytoplasm to cytoplasm Electrical Synapse
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins INSERT FIG. 5.1 (Yes, deliberately out of order)
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Electrical Synapse –Very fast transmission Postsynaptic potentials (PSPs) –Synaptic integration: Several PSPs occurring simultaneously to excite a neuron (i.e. causes AP)
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Chemical Synapse
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Chemical Synapse
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins CNS Synapses (Examples) –Axodendritic: Axon to dendrite –Axosomatic: Axon to cell body –Axoaxonic: Axon to axon –Dendrodendritic: Dendrite to dendrite Types of Synapses
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins
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CNS Synapses (Examples) –Gray’s Type I: Asymmetrical, excitatory –Gray’s Type II: Symmetrical, inhibitory Types of Synapses
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins The Neuromuscular Junction (NMJ) –Studies of NMJ established principles of synaptic transmission Types of Synapses
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An Actual Neuromuscular Synapse Goodsell, D. S. (2009), Neuromuscular synapse. Biochem. Mol. Biol. Educ., 37: 204–210. doi: 10.1002/bmb.20297
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Principles of Chemical Synaptic Transmission 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
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Synaptic Transmission at Neuromuscular Junction
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Principles of Chemical Synaptic Transmission 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
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins –Amino acids: Small organic molecules Neurotransmitters
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins –Amino acids: Small organic molecules –Amines: Small organic molecules Neurotransmitters
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins –Amino acids: Small organic molecules –Amines: Small organic molecules –Peptides: Short amino acid chains (i.e. proteins) stored in and released from secretory granules Neurotransmitters
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins –____ ____: Small organic molecules e.g., Glutamate, Glycine, GABA –____ : Small organic molecules e.g., Dopamine, Acetylcholine, Histamine –____ : Short amino acid chains (i.e. proteins) stored in and released from secretory granules e.g., Dynorphin, Enkephalins Neurotransmitters
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Neurotransmitter Synthesis and Storage Loading Neurotransmitters
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Dense Core Vesicles Goodsell, D. S. (2009), Neuromuscular synapse. Biochem. Mol. Biol. Educ., 37: 204–210. doi: 10.1002/bmb.20297
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Exocytosis: Process by which vesicles release their contents Neurotransmitter Release
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Process of exocytosis stimulated by release of intracellular calcium, [Ca 2+ ]i Proteins alter conformation - activated Vesicle membrane incorporated into presynaptic membrane Neurotransmitter released Vesicle membrane recovered by endocytosis Mechanism of Neurotransmitter release (Clathrin-mediated endocytosis: http://biochem.web.utah.edu/iwasa/projects/clathrin.html)
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins - Ionotropic: Transmitter-gated ion channels Neurotransmitter Receptors and Post- synaptic Response
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins –Metabotropic: G-protein-coupled receptor Neurotransmitter Receptors and Post- synaptic Response
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins EPSP:Transient postsynaptic membrane depolarization by presynaptic release of neurotransmitter IPSP: Transient hyperpolarization of postsynaptic membrane potential caused by presynaptic release of neurotransmitter Reversal potential Excitatory and Inhibitory Postsynaptic Potentials
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins –Diffusion: Away from the synapse –Reuptake: Neurotransmitter re-enters presynaptic axon terminal –Enzymatic destruction inside terminal cytosol or synaptic cleft –Desensitization: despite continued presence of ACh, transmitter-gated channels close Neurotransmitter Recovery and Degradation
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins –Effect of drugs on nervous system tissue –Receptor antagonists: Inhibitors of neurotransmitter receptors Curare –Receptor agonists: Mimic actions of naturally occurring neurotransmitters Nicotine –Defective neurotransmission: Root cause of neurological and psychiatric disorders Neuropharmacology
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Principles of Synaptic Integration –Process by which multiple synaptic potentials combine within one postsynaptic neuron
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins –Synaptic vesicles: Elementary units of synaptic transmission. Almost fixed amount of nerotransmitter 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 a millivolt Quantal Analysis of EPSPs
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins –Allows for neurons to perform sophisticated computations –EPSP summation: EPSPs added together to produce significant postsynaptic depolarization –Spatial: EPSP generated simultaneously in different spaces –Temporal: EPSP generated at same synapse in rapid succession Synaptic Integration - EPSP Summation
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Synaptic Integration - EPSP Summation
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins –Assume: Dendrite as a straight cable (~spinal motor dendrite) Membrane depolarization falls off exponentially with increasing distance V x = V o /e x/ Dendritic length constant (): at which, 37% of Vo –In reality, dendrites are very elaborate structures that contribute to more complex integrative properties The Contribution of Dendritic Properties to Synaptic Integration
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins V x =V o * e -x/λ The Contribution of Dendritic Properties to Synaptic Integration
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins –Dendrites of neurons: voltage-gated sodium, calcium, and potassium channels Can act as amplifiers (vs. passive) –Dendritic sodium channels: May carry electrical signals in opposite direction, from soma outward along dendrites (Cajal?) Excitable Dendrites
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins –Action of synapses to take membrane potential away from action potential threshold –Exerts powerful control over neuron output Inhibition
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins –Excitatory vs. inhibitory synapses: Bind different neurotransmitters, allow different ions to pass through channels IPSPs and Shunting Inhibition
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins –Excitatory vs. inhibitory synapses: Bind different neurotransmitters, allow different ions to pass through channels –Membrane potential more negative: hyperpolarizing IPSP –E Cl = -65 mV ? Shunting Inhibition: Inhibiting current flow from soma to axon hillock IPSPs and Shunting Inhibition
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Shunting Inhibition
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins –Excitatory synapses Gray’s type I morphology Spines: Excitatory synapses –Inhibitory synapses Gray’s type II morphology Clustered on soma and near axon hillock The Geometry of Excitatory and Inhibitory Synapses
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Concluding Remarks Chemical synaptic transmission –Rich diversity allows for complex behavior –Provides explanations for drug effects –Defective transmission is the basis for many neurological and psychiatric disorders –Key to understanding the neural basis of learning and memory
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Question of the Day How stepping on a rusty nail may result in muscle spasms and even eventual death? Hint: It involves synaptic transmission. -In general, what molecular/cellular events happened? Are there effective treatments? -Does the behavior a result of (mostly) defects in inhibitory or excitatory synapse? How do these defects affect synaptic integration?
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Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins Principles of Synaptic Integration Modulation –Synaptic transmission that modifies effectiveness of EPSPs generated by other synapses with transmitter- gated ion channels –Example: Activating NE β receptor
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