BISC 307L 1st Edition Lecture 5 Current Lecture Recording from Single K Channels o Measure current going through ion channels o 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 and the 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 affects single channel currents o Increase in probability of opening Depolarize the membrane Channels open more frequently still random but probability higher o Increase in amplitude of current When depolarize you can increase the amplitude o Ohm s Law I V R Current movement of charge ions Voltage the force causing the charge Resistance the property of material that impedes current flow membrane lipid bilayer has very high resistance o For Single Channel Currents i V m E ion R The 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 FORCE Not permeability but rather conductance the inverse of resistance o Multiplicity of ion channels Always more than one ion channel in a given cell membrane Even for one ion there are multiple channels Most cells have leakage channels K that are always open which are largely responsible for the membrane potential Right next to that may be a voltage gated channel there are several types Efective depolarization can increase probability of opening which can be fast or slow There are K channels that are activated by increase of Ca2 inside the cell There are also K channels that close when there is more depolarization Action Potentials Electrical Properties of Axons o Long distance electrical signals o How is it generated And how does it travel o Segment of unmyelinated axon Axon 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 r Useful to think of the membrane as an electrical model ECF 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 resistance Membrane R memb is inversely proportionally to the surface area if unmyelinated axon The 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 force The sodium channel has a resistance Current 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 cations Inward 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 directions All of the ions carry the current in proportion to the concentration When gets to branch point can go in either direction also Can also flow out through the capacitance of the membrane once in ECF the current drops of exponentially with distance Example EKGs Measuring 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 Potentials o How currents and potentials move down axons o 4 sharp microelectrodes on the axon o the tips are so small that you can poke this through the membrane of the axon and you don t kill the axon o using these electrodes to inject current that will turn on and turn of o other electrodes are measuring membrane potential o when injected with given current I we find Potentials are graded Vary with stimulus strength Potentials are local Decay exponentially with distance Length constant lambda is distance over which potential falls to 1 e 37 of the maximum value o Can predict the behavior of the whole axon by measuring individual increments of the axon Inject current in first electrode Current will flow down the axon in both directions and get to branch points in which some current flow out and some will continue o How does current flow through a capacitor Positive charges on the outer side of membrane and negative on inside capacitor is charged at rest Charges are held there by their mutual attraction This means there is a potential across the membrane Positive charge cancels negative charge and the current flows through the capacitor This 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 decrease Ionic Basis of the Action Potential o How the change in the Na K permeability underlie the action potential o Sudden increase in Na permeability and a delayed increase in K permeability drive this o What the membrane potential is in a little patch of axon what that patch of membrane is doing at 6 diferent times o What is the action potential doing not in one place at diferent times but what is the AP doing at diferent places at one time o