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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 (Ei) •  Membrane potenBal at which electrical and chemical concentraBon forces are in equilibrium –  Equilibrium PotenBal for an ion (Ei) –  Calculated as Nernst PotenBal for an “ion” •  Ei = 61 x log (Co / Ci) [units in mV] •  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 (Em) •  Membrane potenBal (Em) determined by: •  Two types of membrane proteins determine Em –  [Na+] and [K+] in ICF and ECF –  Na+ and K+ Ion membrane permeability (Pi) –  SelecBve ion “leak”channels for Na+ and K+ (Passive) –  Na+/K+ Electrogenic ATPase Pump (AcBve) –  PK > Pna (~25:1), thus Em closer to EK than ENa –  Na+/K+ ATPase Pump •  ResBng membrane potenBal (RMP; Em ~  ­70 mV) •  Membrane potenBal (Em) at any given Bme is calculated by GHK equaBon: –  Considers ion Permeability (Pi) and ConcentraBon Gradient (Co/ Ci) for all permeable ions 3 AcBon PotenBal 4 An AcBon PotenBal May Occur When Em Depolarizes to A Threshold = Action potential = After hyperpolarization Na+ equilibrium potential Threshold potential Resting potential Triggering event K+ equilibrium potential 5 See also Fig. 4 ­4, pg. 91 AcBon PotenBals (AP) •  Rapid stereotyped change in Em, which occurs in response to a sBmulus depolarizing Em to threshold. •  There are 6 primary characterisBcs of an AP: –  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 se Rising pha e Falling phas d by N + lux by K eff in a TEMPORAL MANNER Cause •  The Change Occurs Threshold potential Resting potential 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/AlerhyperpolarizaBon 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 ...
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