Psych 3313 Behavioral Neuroscience Notes Week 4
Psych 3313 Behavioral Neuroscience Notes Week 4 PSYCH 3313
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This 10 page Class Notes was uploaded by Casey Kaiser on Saturday September 17, 2016. The Class Notes belongs to PSYCH 3313 at Ohio State University taught by Dr. Supe in Fall 2016. Since its upload, it has received 21 views.
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Date Created: 09/17/16
Psych 3313 Week 4 Class Notes 9/12 The Resting Potential Electrical Potential An electrical charge measured in volts, the ability to do work through the use of stored potential electrical energy Volt: a measure of a difference in electrical potential Voltmeter: a device that measures the difference in electrical potential between two bodies Tools for measuring a Neuron's Electrical activity Oscilloscope Squids have REALLY BIG axons They don’t have myelination around their axons o So they increase the diameter of theirs o We can see it with plain sight - makes for really great research o This larger size will make it faster, but not faster than our myelinated axons When tested we found that inside the cell has a more negative charge o Sodium ions and chloride ions were more concentrated outside the cell o Potassium ions were more concentrated inside the cell Basic Chemistry Water - h2O o Polar o Hydrophilic Ions o Charged particles o Cations (+) Potassium: K+ Sodium: Na+ Calcium: Ca++ o Anions (-) Chloride: Cl- Basic Setup Inside the cells we have a lot of proteins and potassium Remember bananas in the ocean - to remember the ions at rest Bananas are really high in potassium - inside, just like the inside of the cell Use the force: diffusion Molecules move from areas of high concentration to areas of low concentration o Moving down the concentration gradient Electrical Charge Opposite charges attract o If you have a negative charge it is going to be drawn toward the positive charge Like charges repel each other Electrical gradient - moving towards the opposite charge Equilibrium Balance that occurs when the concentration gradient = voltage gradient o Think of it like a tug of war, chemical force against electrical force Barriers to equilibrium Certain ions cannot pass the cell membrane Overcoming barriers If we have a channel, chloride is allowed through sodium is not, some of the chloride molecules will flow through the passage to the other side of the barrier - because of concentration gradient. But they won't evenly split, some negative ions will go through the barrier and others will attach to the positive charges that could not get through the barrier Selective Permeability We can allow some ions to pass and stop others Different channels can gate certain ions Ion channels and pumps Channels allow for ions to pass, because of protein structure o Voltage-dependent will be based on electrical charges o Ligand-gated will be based on chemicals, so we may need neurotransmitters Ion pumps require energy and ATP Opening the gate Can be voltage-dependent o Opens or closes in response to local electrical environment Can be ligand-gated o Responds to chemical messengers, when certain chemicals are present the gate will open The Resting Potential Electrical charge across the cell membrane in the absence of stimulation A store of negative energy on the inside of a cell relative to the outside of the cell The inside of the membrane at rest potential at rest is -70 millivolts relative to the extracellular side Diffusion and electrical forces at rest Protein is negatively charge and too large to pass through the channel Potassium is positively charged but resting inside the cell Chloride is negatively charged and resting outside of the cell Sodium is concentrated outside of the cell, and based on the charge it wants to be pushed into the cell, but we are trying to keep it out to maintain a difference until we are ready to let it in Permeability at rest Resting membrane is permeable to Potassium Because cells aren't perfect, some sodium leaks into the cell Equilibrium Potential and Potassium Even though the large concentration difference is there we don't let the ions move Maintaining the Resting Potential We want to try to keep the resting potential maintained so we are ready for action potential The membrane is not perfect, there are leaks o There are mechanisms to get the leaked sodium out The sodium-potassium pump, trap three ions and eject it and bring in calcium Pump 3 sodium ions out for every 2 potassium ions it brings in Energy demanding, lots of ATP This pump is tricky Why does it matter? What is the state of the neuron at rest? High sodium outside, high potassium inside Sodium wants inside and potassium wants outside Why is it important? The pump maintains the -70 millivolts, the voltage difference needed for resting potential, it counteracts the molecular leak At rest… What would happen if we opened a channel? o A sodium channel opened briefly would lead to a lot of sodium ions trying to get into the cell It would flow down the concentration gradient into the cell, it is also attracted by negative potential in the cell o The neuron becomes more positive - depolarized o A chloride channel would lead to a flow down its concentration gradient but it is also repelled by the negative potential in the cell The cell may become slightly more negative - hyperpolarized Local (graded) potentials Postsynaptic potentials - very small regional differences in charge Depends on the specific ions that come in Can be excitatory (EPSP) or inhibitory (IPSP) Gradual Graded Potentials IPSP o More negative - hyperpolarization, less likely for action potential o Increased electrical charge across a membrane o Usually due to inward flow of chloride ions or outward flow of potassium ions More likely to be the chloride than the potassium flow EPSP o More positive - depolarization o Decrease electrical charge across the membrane o In small regions of dendrites or cell body o Usually due to inward flow of sodium 9/14 The Action Potential Small local graded membrane potentials IPSPs and EPSPs IPSP - hyperpolarizing (more negative), inward flow of chloride outward flow of potassium EPSP - depolarizing (more positive), inward flow of sodium When multiple EPSPs build on each other, we can reach "threshold", this threshold is where the decision for an action potential becomes YES Spatial Summation Has to do with place, proximity, location Combines all EPSPs and IPSPs occurring near in time at different locations on the dendrite and cell body o If two excitatory signals are stimulated, the signal is larger o If two inhibitory signals are stimulated, the signal is smaller o If one of each are stimulated the signal is flat The Axon Hillock is the decision point The signals that are closer to the axon hillock have a greater effect than those signal further away Generally we see more inhibitory signals closer to the hillock Temporal Summation Has to do with time Combines all EPSPs and IPSPs occurring near in place at different times on the dendrite and cell body SO overall we can look at where the signal comes from and how fast it comes Action potential - binary, all or none, will fire or won't Mediated primarily by voltage gated sodium or potassium channels Role of the Axon Hillock The decision point of the cell Connection point of cell body and axon Contain many voltage-gated channels Threshold of excitation Membrane potential necessary to trigger action potential Approximately -60mV Causes opening of voltage-gated Na+ and (later) voltage gated K+ channels o When this gate is open for sodium, it rushes in because it wanted in so bad. This rush of sodium means the voltage-gated potassium channel will open, the potassium is pulled in and pushed out because the environment is super positive and potassium is too Voltage-gated channels Sodium channels o Activating gate opens at threshold o Inactivating gate closed at peak of AP Potassium channels o Open near peak of AP o Closes during after-hyperpolarization Action potential Starts at the axon hillock when we reach threshold Large, brief reversal in polarity of an axon Inside of the cell becomes positive relative to the outside Lasts about 1-2 milliseconds Mediated by opening and closing of voltage-gated ion channels The action potential at the start of the axon will be the same electrical signal at the end of the axon (non-decremental) All-or-none Frequency coding, the only thing we can change is how often it occurs Metaphor with the firing of a gun, you cannot change the intensity of the fire, you fire, you fire Action potential graph is important ***** Describe the graph, very helpful There are refractory periods Absolute refractory period - you cannot fire another AP Relative refractory period - you can fire again but you need more positive ions Membrane Permeability A measure of ion conductance - measure of allowed to flow Rising phase - high permeability to sodium Falling phase - high permeability to potassium Refractory Periods Limits how frequently the cell can fire Maintains the one way, unidirectionality, of signals Absolute refractory period No matter how much stimulation there is there is NO fire Sodium channels that are not voltage-sensitive are closed Relative Refractory period Can fire again but with stronger stimulus, so AP is less likely Potassium channels are still open Ways to affect the Action Potential Tetrodotoxin Found in pufferfish, other sea creatures o "Fugu" if prepared correctly can be tasty but if not it can kill you Binds to voltage-gated sodium channels, preventing the flow of sodium In turn blocks the rising phase of action potential Can also be used to create a temporary lesion of the brain Local Anesthetics Novocain, lidocaine, etc.. Selectively binds to voltage-gated sodium channels Block sodium conductance, stop action potential propagation TEA Tetraethylammonium Selectively blocks voltage-gated potassium channels Action potential is longer, no afterhyperpolarization Potassium Chloride Lethal injection cocktail Increases concentration of potassium in extracellular fluid - change the resting potential o Raises resting potential o Cells become inactive o Cardiac arrest The Domino Analogy The axon has a lot of voltage-gated sodium channels o When you open one it has enough positive charge to open the next one, and the next one, and so on You cannot change the amplitude of an action potential - ALWAYS THE SAME How does action potential go so fast? Myelin sheath - layer of glial cells covering the axon that speeds the transmission of info within neurons Saltatory Conduction Fast, signal can jump from node to node and propogate down the axon Only thing we can change about an action potential is how often it occurs - the frequency Stimulus intensity coded by firing rate o More intense stimulation will continue to show stronger messages down the line, more release of neurotransmitters 9/16 The Synapse Electrical Synapses "gap junctions" Atypical, not the usual synapse Purely electrical, found in other cells throughout the body not usually in the nervous system Fused presynaptic and postsynaptic membrane - direct action potential propagation Very small Can only delivery an excitatory signal - only action potentials Bi-directional Once again - NOT THE NORMAL SYNAPSE THAT WE WILL SEE Chemical Synapses These are the junctions where neurotransmitters are released Excites or inhibits other neurons The gap between the presynaptic side and the postsynaptic side is called the synaptic cleft Discovery of Chemotransmission This scientist had a dream about how to measure the chemical signals in the brain He proved that neurons used chemicals to communicate o He used frog hearts to test this The Synapse Includes Presynaptic terminal Mitochondrion Synaptic vesicles Synaptic cleft Neurotransmitter molecules Presynaptic membrane Synapse can be directed or nondirected Directed means NT released toward single postsynaptic neuron Non-directed means NT diffuses over a wider area to effect more neurons, not just one o AKA volume transmission Think of neurotransmitters like bubbles - if there are a lot released there is a higher chance of an effect taking place but it is a probability statement Varieties of Synapses - location Axondendritic synapse is the most common one we will see There are others though.. Dendrodendritic - dendrites send to other dendrites Axoectracellular - terminal with no specific target Axosomatic - axon terminal ends on cell body Axosynaptic Axoaxonic Axosecretory Neuromodulation Axo-axonic synapses between an axon terminal and another axon fiber have an effect of the release of neurotransmitter by the target axon o One way to change a signal that emerges from a cell Varieties of Synapse - type I and type II Know how one works and know the other is the exact opposite Type I synapse Excitatory Typically located on dendrite Round vesicles Dense material on membranes Wide cleft Large active zone Type II synapse Inhibitory Typically located on cell body Vesicles are more pancake shaped Sparse material on membranes Narrow cleft Small active zones Storage and Substance of a Terminal Vesicle - membrane enclosed oraganelles Neurotransmitter - chemical substance that a neuron releases to communicate with other neurons What makes a neurotransmitter? It has to have the ability to be made within the cells It has to respond to a triggered release Once it is released it has to do something - cause an effect on receptors There has to be a method to shut it down, important step Synthesis of NT Can be made in the axon terminal itself o Ingredients can typically be found in our food We take the materials and brings them into the cells to be made They can also be made in the cell body o DNA to RNA to proteins and transported by the microtubles to the axon terminal We also need to store them until there is a triggered release We store them inside of vesicles o There is a triggered release - the action potential Exocytosis results in the release of neurotransmitters Actions potential will come down and open voltage gated calcium channels There are different types of release Calcium is critical for bringing the vesicles down to the synapse As vesicles get closer to the synapse they are kind of "roped" in by proteins until they are close enough to the synapse that they can release a quanta of neurotransmitters Receptors in General Proteins that are embedded in the membrane They respond to chemical messengers - they contain recognition molecules Post-synaptic receptors Usually on the dendrite or dendritic spines This is where we get the EPSPs or IPSPs They can open the ion channels or any other chemical reactions Pre-synaptic autoreceptors Means self receptor Used for feedback and regulation and recycling This is important so we can monitor how much neurotransmitter has been released Also helpful in recycling / reuptake Receptors are like a lock and neurotransmitters are like a key Types of postsynaptic receptors Can be divided into ionotropic and metabotropic o Ionotropic focuses on those electric ions Opens ion channels directly Immediate reactions for muscle activity and sensory processing Fast and short o Metabotropic Focuses on energy Can still open channels but through indirect means Relatively slow and long-lasting effect Involved G-proteins as a second messenger These G-proteins are used to activate certain channels to open for ions Life cycle of NT Made in the cell Stored in the cell Released after action potential Mechanism for ending signals Fates of Neurotransmitters Diffusion Broken down by enzyme Taken up by glial cells o Astrocytes play key roles Reuptake into synaptic neuron via transporter
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