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USC BISC 307L - Electrical Signals in Neurons (Cont’d)
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Recording from Single K+ ChannelsMeasure current going through ion channelsCurrentMovement of chargeIons carry charge as they move through solution (carry current)Take a glass pipette with very small tip and press against the membrane of a neuron, the binding of the glass and those of the membrane become tightIf there is an ion channel and the seal between the membrane and the glass is tight then the current will flow into the electrode amplifierSmall current of 10 pA appears rapidly and shuts offCurrent turns on and off very fastMeasures the opening and closing of the ion channels (turning on and shutting off current)Time at which a channel opens is random, but you can measure the probability of whether or not it will be openDirectly demonstrates that this channel is voltage gatedDepolarization affects single channel currentsIncrease in probability of openingDepolarize the membraneChannels open more frequently (still random but probability higher)Increase in amplitude of currentWhen depolarize you can increase the amplitudeOhm’s Law: I=V/RCurrent = movement of charge (ions)Voltage= the force causing the chargeResistance= the property of material that impedes current flow (membrane)- lipid bilayer has very high resistanceFor Single Channel Currents: i=[V(m)-E(ion)]/RThe net force making an ion move is not the membrane potential but the difference between the membrane potential and the ion equilibrium potential = THE DRIVING FORCENot permeability but rather conductance (the inverse of resistance)Multiplicity of ion channelsAlways more than one ion channel in a given cell membraneEven for one ion there are multiple channelsMost cells have leakage channels (K+) that are always open which are largely responsible for the membrane potentialRight next to that may be a voltage gated channel- there are several typesEffective depolarization can increase probability of opening which can be fast or slowThere are K+ channels that are activated by increase of Ca2+ inside the cellThere are also K+ channels that close when there is more depolarizationAction PotentialsElectrical Properties of AxonsLong distance electrical signalsHow is it generated? And how does it travel?Segment of unmyelinated axonAxon defined by plasma membrane which separates ICF and ECF. Electrolyte solutions inside and outside (conduct electricity well, even though plasma membrane doesn’t)Has a radius, rUseful to think of the membrane as an electrical modelECF and ICF has resistance (not perfect conductors). Called R(ecf) and R(axoplasm)Inside the cell there is the R(axoplasm) which is inversely proportional to the square of the radius (the bigger the axon and bigger the radius, the lower the resistanceMembrane: R(memb) is inversely proportionally to the surface area if unmyelinated axonThe membrane can also store charge so has some capacitance too C(memb). In fact, this capacitance of the membrane is VERY high.The circuit is completed by the driving forceThe sodium channel has a resistanceCurrent flows in and flows down the axon (by convention the direction of current movement is the direction of the cation) Inward current = negative sign. Outward current = positive sign With cationsInward Cl- current means that the Cl- ions are moving out (for all anions this is the case)Once inside axon it will go in both directionsAll of the ions carry the current in proportion to the concentrationWhen gets to branch point can go in either direction alsoCan also flow out through the capacitance of the membraneonce in ECF the current drops off exponentially with distanceExample: EKGsMeasuring the blips of current that cardiac muscle that are flowing out into the ECF far enough to the surface of the skin since we conduct electricity so well. So we can measure this.Spread of Graded PotentialsHow currents and potentials move down axons4 sharp microelectrodes on the axonthe tips are so small that you can poke this through the membrane of the axon and you don’t kill the axonusing these electrodes to inject current that will turn on and turn offother electrodes are measuring membrane potentialwhen injected with given current I we find:Potentials are gradedVary with stimulus strengthPotentials are localDecay exponentially with distanceLength constant (lambda) is distance over which potential falls to 1/e (37%) of the maximum valueCan predict the behavior of the whole axon by measuring individual increments of the axonInject current in first electrodeCurrent will flow down the axon in both directions and get to branch points in which some current flow out and some will continueHow does current flow through a capacitor?Positive charges on the outer side of membrane and negative on inside- capacitor is charged at restCharges are held there by their mutual attractionThis means there is a potential across the membranePositive charge cancels negative charge and the current flows through the capacitorThis causes depolarization of the membrane“outward current depolarizes the membrane”as you go down the axon, a constant fraction of the current decides to go out and the rest continues down- this describes an exponential decreaseIonic Basis of the Action PotentialHow the change in the Na+/K+ permeability underlie the action potentialSudden increase in Na+ permeability and a delayed increase in K+ permeability drive thisWhat the membrane potential is in a little patch of axon- what that patch of membrane is doing at 6 different timesWhat is the action potential doing not in one place at different times, but what is the AP doing at different places at one time?BISC 307L 1st Edition Lecture 5Current Lecture- Recording from Single K+ Channelso Measure current going through ion channelso Current Movement of charge Ions carry charge as they move through solution (carry current)o Take a glass pipette with very small tip and press against the membrane of a neuron, the binding of the glass and those of the membrane become tight If there is an ion channel and the seal between the membrane andthe glass is tight then the current will flow into the electrode amplifier Small current of 10 pA appears rapidly and shuts of Current turns on and of very fast Measures the opening and closing of the ion channels (turning on and shutting of current) Time at which a channel opens is random, but you can measure the probability of whether or not it will be open Directly demonstrates that this channel is voltage gated- Depolarization


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USC BISC 307L - Electrical Signals in Neurons (Cont’d)

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