Psyc 6 week 2
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This 6 page Class Notes was uploaded by Sabrina Straus on Wednesday September 21, 2016. The Class Notes belongs to PSYC 6 at Dartmouth College taught by Catherine Cramer in Fall 2016. Since its upload, it has received 7 views. For similar materials see Introduction to Neuroscience in Psychology (PSYC) at Dartmouth College.
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Date Created: 09/21/16
5. 091916 Intro to Neuro Class Notes SYNAPTIC TRANSMISSION I. Types of celltocell communication Electrical synapses: very fast gap junctionsallow direct flow of ions Chemical synapses: more common / release of transmitter Neurotransmitters (synthesized depending on kindcan be synthesized in terminal}more abundant but large molecules ie peptides in cell body and move down axon in tubules) in synaptic vesicles diffusion across the synaptic cleft postsynaptic receptors II. Sequence of events in the presynaptic cell 1. Neurotransmitter synthesized and stored 2. AP reaches axon terminal 3. Voltagegated calcium channels open ~electrostatic pr2+ure and osmotic pushing Ca2+ in 4. Ca binds vesicle (via SNARESin active zone) to presynaptic membrane > exocytosis 5. Released neurotransmitter diffuses across the cleft 6. Receptors~ see below III. Sequence of events in the postsynaptic cell A. Ionotropic (fast) synapses: ligandgated (group of transmembrane ion channel proteins which open to allow ions such as Na , K , Ca , and/or Cl to pass through the membrane in response to the binding of a chemical messenger) Neurotransmitter binds with receptor on ion channel Ion channels open B. Metabotropic (slow) synapses ~ involves more step between receptor and ion channel opening Neurotransmitter binds with receptor coupled to Gprotein (has some effect on receptor) Intracellular message sent to ion channel via second messenger Ion channels open Ex: potassium channel would hyperpolarize as K+ goes out of the cell IV. Deactivation and reuptake of neurotransmittersto put a limit on how long a synapse can be active Enzymatic breakdown: break neurotransmitter down into components to inactivate it Reuptake by presynaptic transporters back into synapse *some presynaptic cell have autoreceptors to give feedback (effects opening of channels esp inhibitory) V. Synaptic integration in the postsynaptic cell Excitation (EPSPs) Inhibition (IPSPs) and shunting inhibition (inhibitory cancels excitatory because simultaneous Summation (temporal and spatial) revisited VI. Some specific neurotransmitters A. Acetylcholine (ACh, cholinergic): modified amino acid (also monoamines) neurojuscular junctions, autonomic NS Nicotinic (ionotropic) and muscarinic receptors (metabotropic) Acetyl Co A + choline (from diet) > ACh } synthetic pathway in terminal because only depends on 1 enzyme Monoamines: B. Catecholamines Dopamine (DA, dopaminergic) receptor subtypes Norepinephrine (NE, noradrenergic) C. Indoleamines: Serotonin (5HT, serotonergic) D. Amino acids Excitatory Glutamate Receptors: NMDA, AMPA, kainate **Inhibitory gammaaminobutyric acid (GABA), glycine E. Peptides: harder to synthesized (in cell body) Opioid peptides (e.g., endorphinact on the nervous system = pain killers) Oxytocin, vasopressin ~~voltage gated sodium channels are in axon hillock> action potentials are only there If EPSP and IPSP just makes action potential more or less likely could there be an inhibitory but still an action potential Ca Na K Cl Chapter Notes Chapter 5:Synaptic Transmission ~passing on info neuron to neuron occurring at synapses >electrical synapses: electrical flowing from one neuron to the next >chemical synapses: chemical neurotransmitters transfer info from one neuron to the next Types of Synapses ~synapse is where one part of a neuron contacts and communicates with another neuron (1st neuron: presynaptic and target cell is postsynaptic) ● Electrical synapses:allow the direct transfer of ionic current from one cell to the next ○ Occur at gap junctions (make oscillations and action potentials synchronized) ■ Between cells+interconnect nonneural cells ■ Function as electrical synapses ■ Contain connexins: proteins that combine to form a channel called a connexon which when two combine form a gap junction channel ● Channel allows ions to pass directly from the cytoplasm of one cell into another ○ Bidirectional ○ postsynaptic potential: When two neurons are electrically coupled, an action potential in the presynaptic neuron causes a small amount of ionic current to flow across the gap junction channels into the other neuron (when the second neuron generates an action potential it will induce a psp in the first neuron)} multiple occurring at the same time is a synaptic integration ○ Inferior olive: neurons in brainstem nucleus / can generate small oscillations of membrane voltage and action potentials ■ Send axons to cerebellum ■ Make gap junctions with one another ■ Time controlled by current that flows through gap junctions ● Chemical synapses ~characteristics: >pre and post synaptic membranes are separated by a synaptic cleft (filled with matrix of extracellular protein that binds pre and post together) ● Presynaptic element: axon terminal ○ Contains dozens of small membraneenclosed spheres (synaptic vesicles) which store neurotransmitter + large vesicles (secretory granules/densecore vesicles) which contain soluble protein ● Membrane differentiations: dense accumulations of protein by membrane and synaptic cleft ○ On pre side: protein pyramids (sites of neurotransmitter release) called active zones ○ Post side: postsynaptic density (contains neurotransmitter receptors which convert the intercellular chemical signal into an intracellular signal) ● CNS chemical synapses ○ Axosomatic: If the postsynaptic membrane is on the cell body ○ Axoaxonic: postsynaptic membrane is on another axon ○ Axospinous: When a presynaptic axon contacts a postsynaptic dendritic spine ○ Dendrodendritic synapses: dendrites form synapses with one another ~categories: 1. Asymmetrical synapses (Gray’s type I): Synapses in which the membrane differentiation on the postsynaptic side is thicker than that on the presynaptic side a. Usually excitatory 2. Symmetrical synapses (Gray’s type II): the membrane differentiations are of similar thickness a. Usually inhibitory ● Neuromuscular junction: synaptic junctions outside CNS ○ Ex: axons of autonomic nervous system+chemical synapses (between axons of motor neurons of spinal cord and skeletal muscle) ○ Fast and reliable ■ Very large ■ Pre contains a lot of active zones ■ post/ motor endplate has a lot of folds with neurotransmitter receptors Principles of chemical synaptic transmission ● Neurotransmitters 1. Amino acids: stored in and released from synaptic vesicles a. glutamate (Glu) , gammaaminobutyric acid (GABA) , or glycine (Gly) can help speed up synaptic transmission at CNS synapses b. acetylcholine (ACh) mediates fast synaptic transmission at all neuromuscular junctions 2. Amines: stored in and released from synaptic vesicles 3. Peptides: large molecules/chains of amino acids stored in and released from secretory granules a. Often exist in the same axon terminals that contain amine or amino acid neurotransmitters ● Neurotransmitter synthesis and storage ○ synthesizing enzymes for both amino acid and amine neurotransmitters are transported to the axon terminal, where they locally and rapidly direct transmitter synthesis Amino acids + amine ○ Synthesized in cytosol of axon terminal ○ Neurotransmitters are taken up by the synaptic vesicles } done by transporters Peptides ○ Occurs in rough ER ○ Split in golgi ○ One of smaller peptide fragments is the active neurotransmitter ○ Secretory granules cary peptide neurotransmitter to axon terminal } axoplasmic transport ● Neurotransmitter release: triggered by arrival of action potential ○ Depolarization causes voltagegated calcium channels to open (inward driving force of Ca2+> causes neurotransmitter to be released from synaptic vesicles) ○ Exocytosis: vesicles release contents and membrane of the synaptic vesicle fuses to the presynaptic membrane at the active zone so contents of the vesicle spill out into the synaptic cleft ■ Very quick (vesicles are already docked at active zones) ■ Reserve pool bound to cytoskeleton of axon terminal ■ Secretory granules also release peptide neurotransmitters/ requires highfrequency trains of action potentials ○ Endocytosis: recovers vesicle membrane ● Neurotransmitter receptors and effectors ~actions depend on the receptor the neurotransmitter binds to 1. transmitter gated ion channels: membranespanning proteins consisting of four or five subunits that come together to form a pore between them (opens w/ binding of neurotransmitter)} net effect=depolarize postsynaptic cell=excitatory a. Excitatory postsynaptic potential: transient postsynaptic membrane depolarization caused by the presynaptic release of neurotransmitter b. If the transmittergated channels are permeable to Cl net effect=hyperpolarize the postsynaptic cell from the resting membrane potential=inhibitory (brings membrane potential away from threshold) i. Inhibitory postsynaptic potential: transient hyperpolarization of the postsynaptic membrane potential caused by the presynaptic release of neurotransmitter 2. Gproteincoupled receptors/ metabotropic receptors a. Neurotransmitter molecules bind to receptor proteins in the postsynaptic membrane b. The receptor proteins activate small proteins (Gproteins) which are free to move along the intracellular face of the postsynaptic membrane c. The activated Gproteins activate “effector” proteins i. Gprotein gated ion channels ii. Enzymes that synthesize second messengers (diffuse in cytosol and can activate other enzymes that can regulate ion channel function and alter cellular metabolism) ● Autoreceptors: presynaptic receptors that are sensitive to the neurotransmitter released by the presynaptic terminal/ serve as safety valve ● Neurotransmitter recovery and degradation: after interaction with post receptors ~important because if [A] is too high > desensitization ○ Through diffusion of the transmitter molecules ○ Reuptake: reloaded into synaptic vesicles ○ Enzyme destruction ● Neuropharmacologydrugs can affect steps of synaptic transmission ○ Inhibitors: inhibit the normal function of specific proteins involved in synaptic transmission ■ Receptor antagonists: inhibitors of neurotransmitter receptors ○ Receptor agonists:mimic actions occurring in neurotransmitter Principles of synaptic integration ~synaptic integration: process by which multiple synaptic potentials combine within one postsynaptic neuron ● Integrations of EPSPs:caused by inward current through channels which depolarize the postsynaptic membrane ○ Quantal analysis of EPSPs: multiples of the quantum (reflects the number of transmitter molecules in a single synaptic vesicle & postsynaptic receptors) ~miniature post potential: size of the postsynaptic response to this spontaneously released neurotransmitter Quantal analysis: a method of comparing the amplitudes of miniature and evoked PSPs, can be used to determine how many vesicles release neurotransmitter during normal synaptic transmission ○ EPSP summation: neuromuscular junctions generate a large EPSP to make sure it works unlike CNS> summation represents the simplest form of synaptic integration in the CNS 1. Spatial:adding together of EPSPs generated simultaneously at many different synapses on a dendrite 2. Temporal: adding together of EPSPs generated at the same synapse if they occur in rapid succession ● Contribution of dendritic properties to synaptic integration because effectiveness of excitatory synapse depends on how far the synapse is from the spikeinitiation zone ○ Dendritic cable properties: two paths ■ Down the inside of dendrite: EPSP amplitude goes down ■ Across dendritic membrane ~depolarization falls exponentially (length constant: index of how far depolarization can spread down a dendrite or axon>depends on 2 factors) 1. Internal Resistance: the resistance to current flowing longitudinally down the dendrite a. depends on diameter of dendrite + electrical properties of cytoplasm} constant length constant 2. membrane resistance: the resistance to current flowing across the membrane a. Depends on number of open ion channels ~ length constant will increase as membrane resistance increases because more depolarizing current will flow down the inside of the dendrite rather than “leaking” out the membrane ○ Excitable dendrites: have voltagegated sodium, calcium, and potassium channelsact as amplifiers (esp in far out dendrites) + can carry signal in opposite direction to get rid of it ● Inhibition ~neuron: depends on number of coactive excitatory synapses, the distance the synapse is from the spikeinitiation zone, and the properties of the dendritic membrane ○ IPSPs and shunting inhibition: channels are permeable to only Cl ■ Shunting inhibition: Prevents current from flowing through soma to axon hillock by the inward movement of negatively charged chloride ions, which is formally equivalent to outward positive current flow and allows positive current to flow out the membrane instead of toward the spike initiation zone ○ Geometry of excitatory and inhibitory synapses ■ Inhibitory: Gray’s II / spread over dendrites, soma, and axon hillock ■ Excitatory: Gray’s I ● modulation ~some synapses with Gproteincoupled neurotransmitter receptors are not directly associated with an ion channel > synaptic activation modifies EPSPs
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