Neuro Notes 9/15
Neuro Notes 9/15 NEUR 0010
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This 6 page Class Notes was uploaded by Eileen artigas on Thursday September 15, 2016. The Class Notes belongs to NEUR 0010 at Brown University taught by Michael Paradiso in Fall 2016. Since its upload, it has received 7 views. For similar materials see Intro to Neuroscience in Neuroscience at Brown University.
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Date Created: 09/15/16
Lecture 3: September 15, 2016 What do neurons use to talk to each other? Did you say… a cellular phone? WRONG. Resting Potential and Action Potentials Topics: Resting Potential Ions and Pumps Diffusion and electrical forces Equilibrium Nernst Equation Action Potentials Phases Voltage Gated Channels Action Potential Propagation The Neuronal Membrane at Rest Even when a cell is at rest, it is not electrically neutral Potential energy- stored energy Difference between inside and outside of membrane at rest At rest- no action potential Potential difference in electrical circuits (voltage= potential difference) Potential difference= energy to push electricity through wire, energy to drive ions across cell membrane Ion- molecules with electrical charge Membrane potential- about 65 mV- inside of neuron is 65 mV than outside Resting potential key points Vm= membrane voltage or membrane potential At rest, Vm= -65 mV Vm determined by distribution of ions inside and outside Ion concentrations in axoplasm and extracellular fluid Axoplasm, extracellular fluid Axoplasm (mM) Extracellular Fluid (mM) Potassium 100 5 Sodium 15 150 Chloride 13 150 Calcium 0.002 2 Concentration gradient- concentration changes across the membrane The process of diffusion causes particles to move from regions of high concentration, to regions of low concentration Concentration gradients established by ion pumps Ions pumped or pushed against concentration gradients Sodium potassium pump (uses an enormous amount of energy, over 50% of all of ATP) 3 sodiums out - 2 potassiums in Pump binds ATP and this changes the shape of the pump protein -pushing them against concentration gradient -membranes regulate flow of ions cell membranes block flow of ions Specialized proteins form ion channels that selectively allow ions to cross Ion channels can be open or closed Ion movement governed by 2 forces 1. Diffusion- process by which particles spread out or mix Diffusion causes particles to move from regions of high concentration to lower (driving them down a concentration gradient) - sodium and potassium always trying to independently balance each other out between the two groups 2. Electrical forces- particles can have a charge- positive or negative (basis of all electricity opposites attract, likes repel) Also trying to balance out charges on both sides Push from diffusion gets balanced by repulsion from the like charge The equilibrium potential occurs when the electrostatic forces caused by having a charge imbalance exactly balance out the force of diffusion that drives those same ions from where they existed in high concentration to where they exist in low concentration- Equilibrium when diffusion force pushing one way equals electrical force pushing the other way Equilibrium Potential and Ionic Driving Force Equilibrium for each ion= Eion +membrane potential at which force equal Ionic driving force= Vm- Eion - when membrane sitting on equiibrium potential of that ion, driving force is zero, when out of balance lots of driving force Key Points about Equilibrium 1 Charge difference is right at the membrane- small imbalance accumulates very close to the membrane, generates an electric field across membrane which results in electrical potential 2 Vm Membrane potential that you get is determined by a very large number of ions, but small percentage of ions Equilibrium Potentials for Different Ions Ion Eion (equilibrium potential for different ions) K+ -80mV Na+ +62mV Cl- -65mV Ca++ +123mV At rest, membrane is mainly permeable to K+ So Vm is near Ek 1 At rest Vm= -65mV 2 Na+ much more outside huge driving force pushing inward Ena= +62 mV Drving force= -65 - 62 = -127 -127 mV 1 K+ Ex= -80mV Driving force= -65-(-80)=15 Walther Nernst The Nernst Equation 1920 Nobel Prize (work at age 25) At body temp of 37 degrees c Eion= 61.5/z log [ion] out/[ion]in If concentration inside is greater than concentration of ion out (less than one total), then the log will be less than zero If ion concentration in equals out, log =0 If conc in less than out log greater than 0 Ion Concentrations and driving forces If membrane is only permeable to Na+, then Vm will move towards Ena (+62mV) If only permeable to potassium Vm moves towards Ek (-80mV) Meaning of Driving Force, conductance, current Ionic driving force= Vm- Eion Energy that is pushing an ion in and out, doesn't mean an ion will move, because it needs a channel open and force to push it Ionic conductance= gion Ability of ion to cross membrane Ionic current= conductance times driving force gion (vm-Eion) net movement of ion across membrane Depolarization An increase in Vm Hyperpolarization Decrease in Vm Action potentials- what are they? Rapid increase, then decrease in Vm Action potential = spike, nerve impulse Spike initiation zone= axon hillock Phases of the Action Potential Resting membrane potential Rising phase Peak/overshoot Falling phase Undershoot Action Potential Threshold When a certain potential (threshold) is reached, nerve signal initiates action potential An action potential is initiated when Vm> -40mV "threshold" How reach threshold 1 Sensory input e.g. eat salty chip, step on a tack, hear a loud sound 2 Neurotrasmitter, you get a signal coming from another neuron Refractory Periods Absolute refractory period, short period of time after action potential starts where its impossible for the cell to fire another action potential within about 1msec of previous action potential (While those channels are stuck in the inactive state, we're can't initiate another action potential) Relative Refractory Period- longer period of time in which greater depolarization is needed to get a second action potential (the voltage-gated sodium channels are active again, but the voltage gated potassium channels are still open.) Three Ion Channels involved in action potentials Action potentials require voltage gated ion channels 1 K+ channels not voltage gated 2 K+ channels voltage gated….. 3 Na+ Voltage gated Na+ channels- - able to open and close very quickly - are more likely to be open when the membrane potential is more positive - usually closed when cell is at resting potential voltage gated Na+ channels- voltage alters shape of protein Channels open when Vm> -40mV Positive feedback loo- the membrane potential which used to be at -65 mV, is positive, reversing polarity, the “ball and chain” mechanism inactivates the open channel, stops influx of sodium into cell, it takes potassium channels a bit of time to open in response to more positive membrane potential, but when they do, membrane potential comes back down and even undershoots for a bit Ball and Chain model of Na+ channels Protein plug that can block ion flow Pore can be blocked two ways 1 Channel closed 2 Channel open but plugged Three States of the voltage gated Na+ channel 1 Closed state (de-inactivated) 2 Open state (activated) 3 Inactivated-(open but plugged) Voltage gated K+ channel Voltage gated- open or closed Opening takes 1 milisecond after threshold is reached "delayed rectifier"- brings membrane potential down with a delay
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