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Unformatted text preview: Lecture 3 SmartSite: Lec 3 Notes Animated figure Announcements: None Membrane PotenEal The Neuron AcEon PotenEal Reading (Recommended): Chapter 3 (75-83) Chapter 4 (pp 87-99) 1 REV: Membrane PotenEal Plasma membrane of all living cells has a membrane potenEal (polarized electrically) SeparaEon of opposite charges across plasma membrane Due to differences in concentraEon, charge and permeability of key ions 2 REV: Unequal DistribuEon of Ions Fig. 3-20, pg. 77 3 Membrane PotenEal (Em) ResEng membrane potenEal Constant membrane potenEal present in cells of non-excitable Essues and those of excitable Essues when they are at rest Nerve and muscle cells Excitable cells Have ability to produce rapid, transient changes in their membrane potenEal when excited 4 Suppose a concentraEon gradient exists for K+ and A- Let's make the plasma membrane IMPERMEABLE to A- A- are large organic anions PA- = 0 K+ flows out of the cell PK+ > 0 Fig. 3-21, pg. 79 5 + The Equilibrium PotenEal for K A concentraEon gradient pushes K+ out of the cell An electrical gradient pulls K+ back into the cell Forces balance at equilibrium Electrochemical Equilibrium Eion (e.g., EK) = Equilibrium PotenEal Em at which ion is in electrical & chemical equilibrium Nernst PotenEal (pg. 79) Eion = (61/z) log [ion]o / [ion]i EK = 61 log (5 mM/150mM) = 61 (-1.477) = -90 mV Z = ion Charge [ion]o = ion ConcentraEon Outside [ion]i = ion ConcentraEon Inside Fig. 3-21, pg. 79 6 Equilibrium PotenEal 1. We begin with unequal distribuEon of ions inside and outside the cell (due to sodium/ potassium pump). No channels are open, so no current can flow, so there is no potenEal difference across the membrane 2. When potassium channels open, potassium will flow out of the cell down its concentraEon gradient. (due to the chemical force). The cell will therefore become more negaEve inside, due to posiEve charges leaving the cell. 3. As the cell becomes more negaEve inside, an electrical force is generated. This force alracts the posiEve potassium ions back into the inside of the cell. When the electrical force is equal and opposite to the chemical force, there is no net flow of ions across the membrane. The potenEal at which this occurs is called the equilibrium potenEal. Note: This image is available on SmartSite 7 Equilibrium PotenEal for Na+ A concentraEon gradient drives Na+ into the cell An electrical gradient pulls Na+ out of the cell Forces balance at equilibrium ENa = 61 log (150/15) = 61 log (10) = 61 mV Fig. 3-22, pg. 80 8 + and K+ Channels Na 9 The Na+/K+ Electrogenic ATPase Pump Fig. 3-17, pg. 72 10 Real Cells are Permeable to Several Ions Fig. 3-23, pg. 81 11 DeterminaEon of Em Membrane potenEal (Em) can be determined by: Direct measurement of voltage across membrane CalculaEon Goldman-Hodgkin-Katz (GHK) EquaEon (Pg. 81) Considers RelaEve Permeability & ConcentraEon Gradient of all permeable ions PK [K+]o + PNa [Na+]o Em = 61 log ---------------------------- PK [K+]i + PNa [Na+]i 12 The Neuron 13 Model of a Mammalian Neuron Basic parts of neuron (nerve cell) Dendrites Cell body Axon Fig. 4-8, pg. 95 14 Neuron Cell body Houses the nucleus and organelles Dendrites Project from cell body and increase surface area available for receiving signals from other nerve cells Signal toward the cell body Dendrite and cell body serve as the neurons input zone. 15 Neuron Axon Nerve fiber Single, elongated tubular extension that conducts acEon potenEals away from the cell body ConducEng zone of the neuron Axon hillock Collaterals First porEon of the axon plus the region of the cell body from which the axon leaves Neuron's trigger zone Side branches of axon Output zone of the neuron to communicate with the next cell Release chemical messengers that simultaneously influence other cells with which they come into close associaEon 16 Axon terminals Neural CommunicaEon Membrane electrical states PolarizaEon DepolarizaEon (More PosiEve) RepolarizaEon Any state when the membrane potenEal is other than 0mV Membrane becomes less polarized than at resEng potenEal Membrane returns to resEng potenEal aoer having been depolarized Membrane becomes more polarized than at resEng potenEal HyperpolarizaEon (More NegaEve) Fig. 4-1, pg. 88 17 Neural CommunicaEon Two kinds of potenEal change AcEon potenEals Serve as long-distance signals Graded potenEals Serve as short-distance signals 18 AcEon PotenEals Brief, rapid, large (~100 mV) changes in membrane potenEal during which the membrane potenEal actually reverses polarity to become posiEve inside Involves only a small porEon of the total excitable cell membrane Do not decrease in strength as they travel from their site of iniEaEon throughout remainder of cell membrane (conducEon) 19 An AcEon PotenEal May Occur When Em Depolarizes Past A Threshold Level Fig. 4-4, pg. 91 20 ...
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- Fall '08