L3_NPB_101 - Lecture 3 •  SmartSite: –  Lec 3...

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Unformatted text preview: Lecture 3 •  SmartSite: –  Lec 3 Notes •  Announcements: –  None •  Membrane PotenDal •  The Neuron •  AcDon PotenDal •  Reading (Recommended): –  Chapter 3 (75 ­83) –  Chapter 4 (pp 87 ­99) 1 Membrane PotenDal 2 REV: Membrane PotenDal •  Plasma membrane of all living cells has a membrane potenDal (polarized electrically) •  SeparaDon of opposite charges across plasma membrane •  Due to differences in concentraDon and permeability of key ions 3 REV: Unequal DistribuDon of Ions Fig. 3 ­20, pg. 77 4 Membrane PotenDal •  ResDng membrane potenDal (Em) –  Constant membrane potenDal present in cells of non ­excitable Dssues and those of excitable Dssues when they are at rest •  Nerve and muscle cells –  Excitable cells –  Have ability to produce rapid, transient changes in their membrane potenDal when excited 5 Suppose a concentraDon gradient exists for K+ and A ­ •  Let’s make the plasma membrane IMPERMEABLE to A ­ –  A ­ are large organic anions •  K+ flows out of the cell Fig. 3 ­21, pg. 79 6 + The Equilibrium PotenDal for K •  A concentraDon gradient pushes K+ out of the cell •  An electrical gradient pulls K+ back into the cell •  Forces balance at equilibrium •  Electrochemical Equilibrium –  Ei (e.g., EK+) = Equilibrium PotenDal •  Nernst PotenDal (pg. 79) –  Eion = (61/z) log(Co/Ci) –  EK = 61 log(5 mM/150mM) = 61( ­1.477) =  ­90 mV –  Z=ion Charge –  Co = ConcentraDon Outside –  Ci = ConcentraDon Inside) Fig. 3 ­21, pg. 79 7 Equilibrium PotenDal •  We begin with unequal distribuDon 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 potenDal difference across the membrane When potassium channels open, potassium will flow out of the cell down its concentraDon gradient. (due to the chemical force). The cell will therefore become more negaDve inside, due to posiDve charges leaving the cell. As the cell becomes more negaDve inside, an electrical force is generated. This force agracts the posiDve 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 potenDal at which this occurs is called the equilibrium potenDal. •  •  Note: This image is available on SmartSite 8 Equilibrium PotenDal for Na+ •  A concentraDon gradient pushes 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 9 Real Cells are Permeable to Several Ions Fig. 3 ­23, pg. 81 10 Sodium ­Potassium Channels 11 The Na+/K+ ATPase Pump Fig. 3 ­17, pg. 72 12 The Neuron 13 Model of a Mammalian Neuron Fig. 4 ­8, pg. 95 14 Neuron •  Basic parts of neuron (nerve cell) –  Cell body –  Dendrites –  Axon 15 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. 16 Neuron •  Axon –  Nerve fiber –  Single, elongated tubular extension that conducts acDon potenDals away from the cell body –  ConducDng zone of the neuron –  Collaterals –  Axon hillock •  Side branches of axon •  First porDon of the axon plus the region of the cell body from which the axon leaves •  Neuron’s trigger zone •  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 associaDon 17 –  Axon terminals Neural CommunicaDon •  Membrane electrical states –  PolarizaDon •  Any state when the membrane potenDal is other than 0mV –  DepolarizaDon (More PosiDve) •  Membrane becomes less polarized than at resDng potenDal –  RepolarizaDon •  Membrane returns to resDng potenDal ajer having been depolarized –  HyperpolarizaDon (More NegaDve) •  Membrane becomes more polarized than at resDng potenDal 18 Basic Terminology Fig. 4 ­1, pg. 88 19 Neural CommunicaDon •  Two kinds of potenDal change –  Graded potenDals •  Serve as short ­distance signals –  AcDon potenDals •  Serve as long ­distance signals 20 AcDon PotenDals •  Brief, rapid, large (~100 mV) changes in membrane potenDal during which the membrane potenDal actually reverses polarity to become posiDve inside •  Involves only a small porDon of the total excitable cell membrane •  Do not decrease in strength as they travel from their site of iniDaDon throughout remainder of cell membrane (conducDon) 21 An AcDon PotenDal May Occur When Em Depolarizes Past A Threshold Level Fig. 4 ­4, pg. 91 22 ...
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This note was uploaded on 04/03/2010 for the course NPB 101 taught by Professor Fuller,charles/goldberg,jack during the Spring '08 term at UC Davis.

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