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Lect. 2: Nervous System I - Resting Potential

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Lect. 2: Nervous System I - Resting Potential Organismal Lec 2 01/20/00 Lect. 2: Nervous System I - Resting Potential ASST.: READ CH. 5 and Ch 6 pp.163-174 I. Nervous System provided rapid communication throughout body and interaction with environment A. For rapid homeostatic control B. For conscious behavior II. Behavior is dictated by Nervous system A. Everything that is an animal’s behavior is determined by neural activity B. Basic unit is Nerve Cell 1. Pattern of connections with other nerve cells provides 2. Information processing 3. Patterns of activity for Behavior 4. Modulation of those connections Æ Modified behavior C. To understand how nervous system does all this must understand electrical properties of cells and their connections III. First some definitions A. Current (I) 1. Flow of electrical charge 2. Can be electrons (neg. current) 3. Can be positive ions (pos. current) 4. Although physicists generally refer to negative current flow, physiologists prefer to talk about positive ions 5. In analogy to flow of water through a hose, this is the water. B. Resistance (Ohms) 1. Resistance of current 2. In analogy to water, this is diameter of hose; i.e. how much hose restricts flow of water C. Voltage (V) 1. Force which drives current through a resistor 2. Force drops off as current goes through the resistor (IR drop) 3. Sometimes referred to as POTENTIAL DIFFERENCE; i.e. the potential force to Page 1 Organismal Lec 2 01/20/00 move current 4. In water analogy, this is difference in pressure between two points, can be due to difference in elevation between two points or pressure due to a pump a) Difference is low in gentle stream 5. Difference is very high at water fall 6. In voltage source there is charge separation as in a battery D. Conductor: Structure that allows current to flow easily E. Resistor: Element that reduces current flow F. Insulator: Element that prevents current flow (infinite resistance) G. Capacitor 1. Element that stores charge for current flow 2. In water analogy this is a weak spot in the wall of a hose 3. When pressure is increased all of it is not translated to end of hose 4. Some goes to bulge out wall 5. This stores pressure which can be used when pump is turned off to maintain pressure for short time H. Ohms law 1. Relationship of Current, Resistance and Voltage 2. V = IR IV. Nerve cells use Electrical signals for information A. Electrical potential already present across cell membrane of all cells 1. signals very fast 2. with amplification stations, can go long distances 3. various wave shapes can transport all kinds of information B. Source of MEMBRANE POTENTIAL 1. KCl placed in solution will eventually diffuse to uniform concentration a) Diffusion (1) Molecules are in constant motion (2) this is thermal energy; i.e. heat (3) In a concentration gradient (a) motion in both directions (b) more molecules to move down conc. grad. than up (c) so there is flux in both directions (d) however Net Flux is down conc. grad. Page 2 Organismal Lec 2 01/20/00 (e) Ultimately molecules will be uniformly distributed in equilibrium K K Net Flux (4) Rate of Diffusion (a) Fick's Law describes factors that affect diffusion rate (1) C1 - C2 (2) (A) = Cross-Sect. Area (3) Distance (L) (4) Temperature (5) Particle Size (D) Describe factors involved in Fick's law (i) Magnitude of conc. grad. (ii) Cross-sectional area through which molecules diffuse (iii) Distance over which diffusion takes place (iv) Rate of thermal motion - HEAT (v) Size of molecules - smaller particles move more rapidly than larger ones (vi) Net Flux = D A (C - C )/L 1 2 Page 3 Organismal Lec 2 01/20/00 (a) D = Diffusion coefficient (rate of thermal motion of a solute at standard temperature (b) A = Area available for diffusion (c) (C - C ) = Concentration gradient 1 2 (d) L = Distance diffusing particles must travel (e) Later in systems these factors will become very important 2. Divide into two chambers with MEMBRANE Membrane a) Lipid Bilayer b) Phospolipids associate in thermodynamically efficient way (1) Polar region out toward H2O (2) Non-polar regions tucked inside membrane c) Membrane has some selective effects on passage of different substances d) Permeability coefficient describes ease with which substance passes membrane (1) Non-polar molecules pass easily Page 4 Organismal Lec 2 01/20/00 (2) Polar molecules and ions not permeable (3) Oddly H2O goes right through membrane (a) small polar molecule (b) high kinetic energy (c) shoots through like a bullet going between phospholipid molecules e) In order to get ions or proteins through use Channels Channel (1) Protein pores (2) Imbedded in membrane (3) Acts as water tunnel through membrane (4) May have Selectivity filter (a) determines which substances can pass (b) e.g. K+ but not Na+ f) Can change permeability - Gated Channel (1) Voltage gated (2) Ligand gated 3. Channels in our membrane allow K through but NOT Cl 4. Equal amounts of KCl on both sides → NO VOLTAGE 5. Increase KCl on one side by 10x Page 5 Organismal Lec 2 01/20/00 10 K 1 K 10 Cl 1 Cl a) K wants to flow down concentration gradient but Cl cannot move b) For K to move down concentration gradient it would violate LAW OF ELECTRONEURTRALITY c) Chambers MUST remain ELECTRICALLY NEUTRAL (1) Positive charges = Negative charges 6. But because membrane is very thin, it can separate small amount of charge (SLIGHT violation of neutrality) a) A FEW K ions flow down concentration gradient b) But they are held against membrane by remaining Cl ions in left chamber + = - + = - + = - - - + + + - - - + + - + + + + - + - + - - - + - - + + + + - + - - + - - + + - - - + - + 7. Charge separation creates a voltage as in a battery Page 6 Organismal Lec 2 01/20/00 K Voltage 8. This voltage acts against further movement of K down gradient 9. When VOLTAGE = FORCE of K CHEMICAL GRADIENT system is in ELECTROCHEMICAL EQUILIBRIUM a) Stability requires that Voltage remains across membrane b) This is MEMBRANE POTENTIAL 10. Actually such a small amount of K ions move across membrane that CONCENTRATIONS are VIRTUALLY UNCHANGED a) So DON't try to ADD and SUBTRACT pos. and neg. CHARGES on either side to Æ voltage b) It will ALWAYS = ZERO 11. NOTE that since membrane potential OPPOSES chemical gradient, increasing K in left chamber makes left chamber MORE NEGATIVE with respect to right chamber 12. Can calculate membrane voltage with NERNST EQUATION: RT X E =×ln 1 X Fz X 2 R = gas constant z = valance of ion T = Absolute temp. ln = natural log F = Faraday's constant Ex = pot. side II - side I coulombs/g-equivalent charge) o a) At 18 C for MONOVALANT positive ion b) Equation reduces to Ex = 0.058 V log [x]1/[x]2 c) For NEGATIVE ion z = -1, so must switch to [x]2/[x]1 13. Membrane voltage for an ion at equilibrium is referred to as its Equilibrium + potential (EK is Equilibrium Potential for K ) Page 7 Organismal Lec 2 01/20/00 14. EK in most cells is ~ -90 mV a) This is because K is concentrated on inside of cell 15. ENA in most cells is ~+60 mV a) This is because Na is concentrated on outside of cell 16. Change concentration Æ predictable change in membrane potential 17. At equilibrium pot. no driving force for that ion 18. If membrane potential is displaced from the equilibrium pot., a driving force exists for that ion a) driving force is: (Memb. Pot. - EX) THIS IS IMPORTANT!!!!!!!!!!! Membrane Pot. Equilibrium Pot. for Ion X V = Driving Force on ion X Voltage → Driving Potential on Ion X Potential energy of (Memb. Pot. - EX) 19. In reality: a) Not just one ion permeable and b) Ions are not either 100% permeable or 100% impermeable c) The more permeable an ion is, the more it can move down its concentration gradient and the more dominant it is in membrane pot. 20. In a neuron: a) main permeable ions are: (1) K+ (2) Na+ (3) Cl- b) Resting pot. defined by extension of Nernst equation (1) Goldman equation: ++− RT PK[]++ P [ Na ] PCl [ ] E =×lnK out Na out Cl in m F PK[]++−++ P [ Na ] PCl [ ] K in Na in Cl out (2) Conc. differences maintained by Na-K pump (a) metabolic pump Page 8 Organismal Lec 2 01/20/00 (b) exchanges 3 Na+ for 2 K+ (c) Keeps ext. Na high and internal K high (d) Maintains Equilibrium potentials - (3) In most cells Cl moves passively so that ECl = Vm (4) Also permeability of Na+ ~ 0.01 x Perm of K+ (5) So Goldman equation reduces to: ++ RT PK[]+ 001. [ Na ] E =×ln K out out m F PK[]+++ 001. [ Na ] K in in + c) So RESTING POT. should approx. K equilibrium potential (EK) d) Hodgkin and Horowitz measured membrane pot. of frog muscle vs. Ext. K+ conc. 0 -20 Observed -40 -60 Calculated from Voltage -80 Nernst Equation -100 [K] o e) At lower end Vm deviates from EK because: (1) [K]o very low (2) [Na]o always high (3) So in this range 0.01 [Na]o starts to approach [K]o (4) So [Na] has some influence on membrane potential 21. Note that according to Goldman equation, membrane pot. Can be changed two ways a) Change CONCENTRATION of ions – this can be done experimentally but rarely occurs in animal because of homeostatic control of ion concentration b) Change in PERMEABILITY of ions – this is controlled by ion channels C. In most neurons at rest Vm ~ Ek ~ -60 to -90 mV Page 9 Organismal Lec 2 01/20/00 Penetrate Cell 0 mV -60 mV Ground + D.
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