L4_NPB_101

L4_NPB_101 - Lecture 4 •  SmartSite: –  Lec 4...

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Unformatted text preview: Lecture 4 •  SmartSite: –  Lec 4 Notes •  Review –  Nernst PotenBal –  Membrane PotenBal •  Announcements: –  None •  Reading (Recommended): –  Reading: Ch 4 (pp 87 ­94) •  AcBon PotenBal •  ConducBon of AP 1 REV: Equilibrium PotenBal •  Membrane potenBal at which electrical and chemical concentraBon forces are in equilibrium •  Equilibrium PotenBal for an ion (Ei) –  Nernst PotenBal for an “ion” –  Eion = 61 x log (Co / Ci) [units in mV] •  C = [ion] = ConcentraBon •  o is ECF, i is ICF •  EK = 61 x log (5mmol)/(150 mM) = 61 x  ­1.5 =  ­90 mV •  ENa = 61 x log (150mmol)/(15 mM) = 61 x 1 = 61 mV 2 REV: Membrane PotenBal •  Membrane potenBal (Em) determined by: –  [Na+] and [K+] in ICF and ECF (defines Ei) –  Na+ and K+ Ion membrane permeability (Pi) •  Two types of membrane proteins determine Pi –  Ion selecBve channels for Na+ and K+ (Passive) –  Na+/K+ ATPase Pump (AcBve) •  ResBng membrane potenBal (RMP; Em ~  ­70mV) –  PK > Pna (~75:1) –  Na+/K+ ATPase Pump 3 REV: An AcBon PotenBal May Occur When Em Depolarizes to Threshold = Action potential = After hyperpolarization Na+ equilibrium potential Threshold potential Resting potential Triggering event K+ equilibrium potential 4 See also Fig. 4 ­4, pg. 91 AcBon PotenBal 5 AcBon PotenBals 1.  Rapid stereotyped change in Em, which occurs in response to a sBmulus. 2.  There are 6 primary characterisBcs of an AP: a)  b)  c)  d)  e)  f)  Rapid depolarizaBon of Em Rapid repolarizaBon of Em Every AP followed by 2 refractory periods Every AP is All ­or ­None AP conducted over the axon AP are conducted rapidly 6 AcBon PotenBals •  When membrane reaches threshold potenBal –  Voltage ­gated channels in the membrane undergo conformaBonal changes –  First  ­ increase PNa •  Flow of sodium ions into the ICF reverses the membrane potenBal from  ­70 mV to +30 mV –  Second  ­ decrease PNa and increase PK •  Flow of potassium ions into the ECF restores the membrane potenBal towards the resBng potenBal 7 AcBon PotenBals •  AddiBonal characterisBcs –  Sodium channels open during depolarizaBon by posiBve feedback. –  When the sodium channels become inacBve, the channels for potassium open. This repolarizes the membrane. –  As the acBon potenBal develops in one segment of the plasma membrane, it regenerates an idenBcal acBon potenBal in the next segment of the membrane. –  Therefore, it travels along the plasma membrane undiminished. 8 The AcBon PotenBal Results from PosiBve Feedback Fig. 4 ­6, pg. 91 9 AcBon PotenBals are Stereotypical 10 What’s Happening During an AcBon PotenBal? •  Ion Permeability CHANGES a + influ x –  Voltage Gated Na+ and K+ Channels Caused Rising pha d by N •  The Change Occurs in a TEMPORAL MANNER Threshold potential Resting potential se e Falling phas + lux by K eff Cause Fig. 4 ­7, pg. 94 11 Current Flow During a Graded PotenBal Fig. 4 ­2, pg. 89 12 Sub Threshold Responses Graded potential (change in membrane potential relative to resting potential) Threshold Resting potential Time Magnitude of stimulus Stimuli applied 13 Graded PotenBal •  Occurs in small, specialized region of excitable cell membranes •  Magnitude of graded potenBal varies directly with the magnitude of the triggering event Fig. 4 ­3, pg. 90 14 Na+ Influx Depolarizes the Cell and Opens More Voltage Gated Channels Na+ activation gate opens At resting potential Threshold reached Action potential begins Depolarizing triggering event 15 Opening of Voltage Gated Na+ Channels Depolarizes the Cell Fig. 4 ­5, pg. 92 16 The Peak of the AcBon PotenBal Na+ inactivation gate begins to close K+ gate opens Peak of action potential; potential reversed 17 How do We Stop a PosiBve Feedback Process? Fig. 4 ­5, pg. 92 18 + Channel The Voltage ­Gated K •  This channel also opens with depolarizaBon –  A bit slower –  Has a single acBvaBon gate Fig. 4 ­5, pg. 92 19 RepolarizaBon/AmerhyperpolarizaBon Na+ inactivation gate opens; Na+ activation gate closes Repolarization begins Action potential complete; after hyperpolarization begins After hyperpolarization is complete; return to resting potential 20 AcBon PotenBal (AP) Summary Fig. 4 ­7, pg. 94 21 Fig. 4 ­7, pg. 94 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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