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WSU BIOLOGY 251 - Membrane Potential

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BIO 251 1nd Edition Lecture 4Outline of Last Lecture I. Membrane composition and functionII. Membrane adhesions between different cellsIII. Membrane transportIV. Electrochemical gradientsOutline of Current Lecture I. Membrane potentialII. Action potentialsIII. Neuron and the conduction of action potentialsIV. Graded PotentialCurrent LectureVI. Membrane PotentialA. Overview1. Definition: a separation of electrical charge across a membrane2. Because work is required to separate opposite charges already together, separated chargeshave the potential to do work3. Unit of measure is mV (1/1000 of a volt)B. All living cells have a membrane potential1. Slight excess of positive charges on outside of cell2. Slight excess of negative charges inside the cell3. Ions responsible: Na+, K+, Aa)b) Na+ and K+ are small enough to cross membrane through protein channelsc) A- is too big to fit into protein channel and so can NOT cross membrane4. Cells can have resting potentials between -5mV and -100 mVC. Causes of Membrane Potential in Living Cells1. Effect of K+ and A- (Fig 7.8)a) Concentration gradient of K+ from ICF to ECFb) So K+ diffuses out of cell through channels and A- stays in cellThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.c) Results in increase of electrical gradient as positive charges pile up outside and negative charges stuck insided) Continues until electrical gradient balances concentration gradient at -94 mV2. Effect of Na+ and Cl- (Fig 7.9)a) Concentration gradient of Na+ from ECF to ICFb) So Na+ diffuses into cell through protein channels and Cl- stays outsidec) Results in increase of electrical gradient as positve charges pile up inside and negative charges stuck outsided) Continues until electrical gradient balances concentration gradient at +60 mV3. Effect of Na+-K+-ATPase pump onlya) Pumps 3 Na+ out of cell for every two K+ pumped into cellb) Creates a small membrane potential because more positive charges leaving the cell than entering it4. All together now (Fig 7.10) Resting Membrane Potential in a Nerve Cella) Membrane is 50 to 75 times more permeable to K+ than Na+b) K+ wants to move out of cell down its concentration gradient but againstthe electrical gradient.c) Na+ wants to move into cell down its concentration gradient and with electrical gradientd) Membrane is more permeable to K+ so more K+ moves out than Na+ moving ine) However enough Na+ moves in to reduce membrane potential from -94 mV to -70 mV, the resting membrane potentialf) THIS IS NOT AT EQUILIBRIUM!!!!g) Na+- K+-ATPase pump counterbalances passive diffusion of K+ out of cell and Na+ into cell to maintain the -70 mV resting membrane potential.h) ALL PASSIVE FORCES BALANCED BY ACTIVE FORCES; NO NET DIFFUSION OCCURS WHEN MEMBRANE IS AT REST!!!!!5. Cl- play very little rolea) Because no active transport of Cl-(1) Does NOT influence membrane potential(2) IS influenced by membrane potentialb) So Cl- diffuses passively across membrane until its concentration gradient is balanced by overall electrical gradient established by membrane potentialA. All cells possess a membrane potential related to the non-uniform distribution of, and differential permeability of, Na+, K+ and A-.B. Nerve and muscle are excitable tissue that use this potential by undergoing controlled, transient, rapid changes in membrane potential. Such fluctuations in membrane potential serve aselectrical signals.C. Two kinds of such electrical signals1. Graded potentials are short distance signals2. Action potentials are long distance signalsII. Action PotentialsA. Terms (Fig 7.11)1. Polarization: separation of charges2. Depolarization: reduction in potential3. Hyperpolarization: increase in potential4. Repolarization: return to resting potentialB. Events in an Action Potential (Fig 7.16)1. Occurrence of a triggering event2. Slow depolarization from -70 mV to -55 mV3. -55 mV is threshold potential; as soon as it is reached, a very rapid depolarization to +30 mV occurs4. repolarization then occurs5. frequently overshoots to -80 mV before repolarizing at -70 mV; this is hyperpolarization6. in a nerve, this whole process takes 0.001 secondsC. Mechanisms underlying events in Action Potential (Fig 7.16, 7.17)1. Membrane contains Na+ channels and K+ channels. Both of these are voltage regulated channels, meaning that changes in voltage (i.e., membrane potential) open or close them.2. At rest (-70 mV) some K+ channels are open, fewer Na+ channels open; the “leaking” of these two ions across membrane balanced by Na-K-ATPase pump3. As depolarization begins in response to triggering event, the change in voltage causes additional Na+ channels to open4. Na+ concentration and electrical gradients are INTO cell, so Na+ begins to moveinto cell5. This causes more depolarization, which opens more voltage-gated Na+ channels6. Positive feedback loop established7. When threshold reached at -55 mV membrane becomes 600 times more permeable to Na+ than K+ because all voltage-gated Na+ channels opened at -55 mV8. Na+ rushes in, and inside of cell becomes positive (30 mV); note that this is close to the Na+ equilibrium potential9. Na+ voltage-gated channels close at 30 mV10. When membrane voltage is 30 mV, K+ voltage gated channels open wide and K+ rushes out of cell down both its concentration and electrical gradient.11. Positive charges leaving w/ K+ repolarizes the membrane back to resting potential of -70 mV; sometimes a few too many K+ leave and cause hyperpolarization.12. Relative to the total number of Na+ ions outside neuron very few Na+ ions move into cell during depolarization; likewise, relative to the total number of K+ ions inside neuron, very few K+ ions move out of the cell during repolarization.D. Restoration of Ion Gradients1. At completion, the resting membrane potential of -70 mV is restored, but concentrations of Na+ and K+ are not.2. However, very few K+ and Na+ actually cross the membrane during an action potential compared to total amount of these ions available (only about 1 out of every 100,000 ions crosses the membrane) so their overall concentration is not changed much.3. Eventually the original concentrations restored by Na+-K+ ATPase pump. Without this pump, repeated action potentials would eventually erode separation of Na+ and K+III. The Neuron and Conduction of Action PotentialsA. Neuron Structure (Fig 7.2)1. Three


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