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Topics covered

Topics covered. Organization of the nervous system Regions / specialization of the neuron Resting membrane potential Especially ionic basis- Nernst, Goldman, Donnan, active transport Action Potential Especially ionic basis – voltage-gated channels, ionic current, electromotive force

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Topics covered

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  1. Topics covered • Organization of the nervous system • Regions / specialization of the neuron • Resting membrane potential • Especially ionic basis- Nernst, Goldman, Donnan, active transport • Action Potential • Especially ionic basis – voltage-gated channels, ionic current, electromotive force • Action Potential Conduction • Passive spread of current, local depolarization, length constant, role of myelin

  2. The text book • Only topics covered in class • Depth of topics covered in class • But know how to use the information I’ve given

  3. Electrochemical equilibrium • Equilibrium – no net movement • General state where the movement of ions is controlled by concentration gradients and electrical forces • Equilibrium potential • The electrical force that occurs at an electrochemical equilibrium

  4. Nernst Equation Ion Concentration Temp (K) Gas Constant Equilibrium Potential of X ion (eg. K+) Valence of ion (-1, +1, +2) Ion Concentration Faraday constant

  5. Nernst Equation • What if temperature changed? • What if valence of ion changed?

  6. Sample question • If two concentrations of KCl solution across a membrane give an equilibrium potential for K+ of -60 mV, what will the equilibrium potential be if the concentrations on each side are reversed • -120 mV • 0 • +60 mV • -30 mV Because if [in]>[out], log([out]/[in]) will be negative log([in]/[out]) will be positive

  7. What does it mean for an equilibrium potential to be positive or negative? • Indicates direction of electrical force • If negative, +ve charge flow inward • If positive, +ve charge flow outward

  8. Sample Question At rest Vm of this typical cell is -75 mV. What would Vm be if PNa >> Pk,PCl? K = 140 Na = 10 Cl = 30 K = 5 Na = 145 Cl = 110 Answer: Calculate ENa using Nernst equation. Assume Vm ENa = +67 mV

  9. Goldman equation Typical cell: Pk is 100X PNa therefore Vm  Ek If PNa >> Pk, then Vm  ENa

  10. What if permeability changes? • What if ion concentration changes?

  11. Electromotive force • Also called driving force • Difference between Eion and Vm • Determining force if ions flow or not • I = g(Vm-Eion)

  12. Explain the diagram showing the effect of low Na on action potential amplitude

  13. How do we know Na+ important for depolarization? Replace Na+ in extracellular bath with impermeable cation - choline Normal Low Sodium 0 mV -80 mV

  14. Ionic basis • Normal saline ENa = +50 mV • With reduced [Na], ENa will be lower • During AP, VmENa when voltage-gated Na channels open • Therefore if ENa, AP amplitude 

  15. How do we know Na+ important for depolarization? Normal ENa Low Sodium 0 mV ENa -80 mV

  16. in out Passive Distribution Equilibrium [K+]in = [A-]in + [Cl-]in A- [K+] > [K+] [K+]out = [Cl-]out [Cl-] < [Cl-] Since [A-]in is large, [K+]in must also be large +’ve = -’ve +’ve = -’ve

  17. Donnan Equilibrium Example Initial Concentrations I II A- = 0 K+ = 150 Cl- = 150 A- = 100 K+ = 150 Cl- = 50 Are these ions in electrochemical equilibrium? No, EK+ = 0 mV ECl-= -27 mV

  18. Let X be the amount of K+ and Cl- that moves Solve for X, 7500 + 200X + X2 = 22500 - 300X - X2 X=30

  19. Final Concentrations I II A- = 0 K+ = 120 Cl- = 120 A- = 100 K+ = 180 Cl- = 80 space-charge neutrality Are these ions in electrochemical equilibrium? Yes, EK+ = -10 mV ECl-= -10 mV

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