Chapter 5: Synaptic Transmission
Chapter 5: Synaptic Transmission NEUROSC 3000 - 020
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NEUROSC 3000 - 020
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This 10 page Class Notes was uploaded by Victoria Gonzalez on Thursday October 1, 2015. The Class Notes belongs to NEUROSC 3000 - 020 at Ohio State University taught by Robert Boyd in Summer 2015. Since its upload, it has received 18 views. For similar materials see Introduction to Neuroscience in Neuroscience at Ohio State University.
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Date Created: 10/01/15
1 Chapter 5: Synaptic Transmission Victoria Gonzalez Learning Objectives: Understand differences between electrical and chemical synapses Know the basic structure of chemical synapses Understand principles of synaptic transmission Know the basic structures of ligand-gated and G protein coupled receptors Understand basic principles of synaptic integration 1. Introduction a. Synaptic transmission: information transfer between cells at synapses i. Charles Sherrington: named synapses b. Electrical synapses: electrical current flowing from one neuron to the next (evolutionarily old) i. Explains the speed of synaptic transmission ii. By Furshpan and Potter c. Chemical synapses: chemical neurotransmitters transfer information from one neuron to another at the synapse i. By Loewi 2. Types of synapses a. Information flows in one direction; from neuron to target cell i. Presynaptic neuron: sends message ii. Postsynaptic neuron: receives message b. Electrical synapses: in the form of ions i. Gap junctions: sites where electrical synapses occur 1. Electrically coupled: cells that communicate through gap junctions 2. The membrane of presynaptic and postsynaptic cells is covered with connexins 3. Six connexin units combine to form a channel called a connexon 4. Two connexons (one presynaptic, one postsynaptic) meet and combine to form a gap junction channel a. Channels allow small ions to pass through, big proteins cannot 2 ii. Electrical synapses are bidirectional and very fast iii. Common in mammalian CNS, in glia, cardiac muscle cells, smooth muscle, epithelial cells, and liver cells iv. Postsynaptic potential (PSP): a change in the postsynaptic membrane potential by the presynaptic action of an electrical synapse 1. Because the process is bidirectional, when the second neuron generates an action potential, it will in turn induce a PSP in the first neuron c. Chemical synapses: most synaptic transmissions i. Universal characteristics: 1. Synaptic cleft: between presynaptic and postsynaptic neurons; filled with a matrix of fibrous extracellular protein a. Matrix helps organize the synapse; the glue b. Synaptic cleft: 20-50 nm 2. Synaptic vesicles: membrane-enclosed spheres in the presynaptic terminal a. Vesicles store neurotransmitters 3. Secretory granules (dense-core vesicles): contain soluble proteins 4. Membrane differentiations: a dense accumulation of protein adjacent to and within the membrane on either side of the synaptic cleft a. Active zone: presynaptic membrane protein coat where neurotransmitters are released b. Postsynaptic density: thick protein around the postsynaptic membrane i. Contains receptor proteins 3 ii. Chemical synapses in the CNS 1. Various configurations a. Axodendridic: axon terminal to dendrite b. Axosomatic: axon terminal to cell body c. Axoaxonic: axon terminal to axon d. Axospinous: axon to dendritic spine e. Dendodendritic: dendrite to dendrite 2. Various sizes a. Gray’s type I (asymmetrical synapses): a membrane differentiation on the postsynaptic side is thicker than on the presynaptic side i. Usually excitatory ii. Big and round presynaptic terminals b. Gray’s type II (symmetrical synapses): b membrane differentiations are of similar thickness i. Usually inhibitory ii. Small and oval presynaptic terminals 4 iii. Neuromuscular junctions 1. Between motor neurons and muscles 2. Similar to CNS synapses 3. Easier to study than CNS synapses a. Much of what we know about synapses was first learned with neuromuscular junctions 4. Fast, large, and reliable synapses a. One of the largest synapses in the body b. Action potentials in motor axons always cause action potentials in the muscle cell innervated 5. Motor end plate (postsynaptic membrane): contains shallow folds so that many neurotransmitter molecules are released onto a large surface area of membrane 3. Neurotransmitters a. Neurotransmitter function depends on where it binds b. Categories: i. Amino acids: small, organic, at least one nitrogen 1. Stored in and released from synaptic vesicles ii. Amines: small, organic, at least one nitrogen 1. Stored in and released from synaptic vesicles iii. Peptides: large chains of amino acids 1. Stored in and released from secretory granules iv. All three types often exist in the same axon terminals c. Neurotransmitter synthesis and storage i. Various transmitters have distinct synthetic pathways ii. Specific enzymes in neurons synthesize unique neurotransmitters iii. Amino acid and amine neurotransmitters are synthesized in the cytosol of the axon terminal 5 1. Transporters: get neurotransmitters into synaptic vesicles iv. Peptides are synthesized in the rough ER 1. Packaged into secretory granules by the Golgi apparatus d. Neurotransmitter release i. Neurotransmitter release is triggered by the arrival of an action potential in the axon terminal 1. Depolarization of the terminal membrane opens voltage-gated calcium channels a. Ca 2+will flood into the cytoplasm of the axon terminal releasing the neurotransmitters from synaptic vesicles through exocytosis (synaptic vesicle membrane fuses with the presynaptic membrane) i. Speed of exocytosis suggests that some vesicles may already be docked at the membrane ii. Some vesicles come from a “reserve pool” on the axon’s cytoskeleton iii. Vesicle membrane is recycled by endocytosis 2. Peptide neurotransmitters require high- frequency trains of action potentials a. Respond to higher Ca 2+i b. The release is slow 6 e. Neurotransmitter receptors i. 100 different neurotransmitter receptors ii. 2 types: 1. Transmitter (ligand) gated ion channels: a. Span the membrane b. Made of four or five subunits with a pore between them c. The pore opens in response to a neurotransmitter binding to a site on the outside of the channel d. Do not show the same degree of ion selectivity as voltage-gated channels e. Excitatory postsynaptic potential (EPSP): a postsynaptic membrane depolarization caused by the release of a neurotransmitter i. Acetylcholine and glutamate f. Inhibitory postsynaptic potential (IPSP): a change in the postsynaptic membrane making action potentials less likely; caused by the release of a neurotransmitter i. Glycine and GABA 2. G-protein-coupled (metabotropic) receptors: a. Slower and longer lasting b. Step 1: neurotransmitter molecules bind to receptor proteins embedded in the postsynaptic membrane c. Step 2: the receptor proteins activate small proteins (G-proteins), which are free to move along the intracellular face of the postsynaptic membrane d. Step 3: The activated G-proteins activate “effector” proteins (G-protein gated ion channels or enzymes called second messengers) 7 3. Auto-receptors: receptor in the presynaptic membrane that is sensitive to neurotransmitter released by presynaptic membrane a. Auto-receptors are often G-protein-linked receptors f. Neurotransmitter reuptake and degradation i. Neurotransmitters are destroyed or removed from the synaptic cleft when signaling terminates 1. Diffusion: through the extracellular fluid away from the synapse a. Reuptake: neurotransmitter is allowed back into the presynaptic membrane with help of transporters. i. These neurotransmitters can then be put back into vesicles and reused. 2. Enzymatic destruction of neurotransmitters in the synapse ii. Desensitization: transmitter-gated channels remain closed even in the presence of the neurotransmitter 1. Desensitization is prevented by enzymatic destruction 4. Neuropharmacology: studies the effect of drugs on nervous system tissue a. Many chemicals, diseases, and drugs can affect these steps i. Receptor antagonists: inhibitors of neurotransmitter receptors 1. Curare 2. Cobra venom ii. Receptor agonists: bind to receptors mimicking the action of the natural neurotransmitter 1. Nicotine 8 iii. Organophosphates: irreversible inhibitors of acetylcholine enzymes (AchE) 1. Prevents the degradation of acetylcholine 2. Death by desensitization of acetylcholine 5. Synaptic integration: multiple synaptic potentials combine within one postsynaptic neuron a. EPSP: inward current through channels that depolarize the postsynaptic membrane i. Quantum: number of neurotransmitter molecules in a vesicle (several thousand) ii. Miniature postsynaptic potential: produced by spontaneous release of the contents of one vesicle iii. There are thousands of channels 1. The number that open depend of quantity of neurotransmitter released iv. Quantal analysis: compare miniature and evoked potentials to decide how many vesicles of neurotransmitters were released 1. Neuromuscular junction: 200 vesicles (-40 mV) 2. CNS: as little as one vesicle (0.1 mV) v. EPSP summation: synaptic integration in the CNS 1. Spatial summation: adding EPSPs that happened at different synapses at the same time 2. Temporal summation: adding EPSPs generated at the same synapse over a period of time (1-15 msec) 6. Dendritic cable properties a. Length constant: an index of how far depolarization can spread down a dendrite or axon i. The longer the length constant, the more likely it is that EPSPs generated at distant synapses will depolarize the membrane at the axon hillock ii. Depends on: 1. Internal resistance: depends on the diameter and electrical properties of the cytoplasm a. Greater internal resistance = smaller length constant ( 2. Membrane resistance: depends on synaptic activity and how many ion channels are open a. Greater membrane resistance = larger length constant (no leakage) iii. Length constant is largest when there is a wide dendrite (small internal resistance) and few open membrane channels (large membrane resistance) 9 7. Excitable dendrites a. Usually can’t fire action potentials b. Opening sodium and potassium channels here can add current which allows the propagation of an EPSP 8. Inhibition a. Inhibitory synapses: move membrane potential away from the action potential threshold b. Shunting inhibition: a form of synaptic inhibition in which the main effect is to reduce membrane resistance, thereby shunting depolarizing current generated at excitatory synapses 10 9. Modulation: neurotransmitters that do not directly evoke PSPs, but rather modify cellular response to EPSPs and IPSPs generated by other synapses a. Not directly associated with ion channels
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