1. Propagation of the Action Potential The Central Dogma Of Excitable Tissues

2. Resting Membranes: Potential: The inside of the cell is negative relative to the outside Na + /K + Pumps: 3:2 exchange keeps more + charges outside Internal Na + runs the pump Membranes generally permeable to K +

3. QUESTION: If the cell membrane is permeable to K +, And inside of the cell is supposed to have fewer + charges than the outside….

4. What if…. You bathe excitable tissue (nerves and muscles) in a high K + solution?

5. Banana-Mongers want to know: The following scenario results: 1. K + passively diffuse into cells 2. The inside becomes more + 3. This “depolarization” cause an Action potential in all excitable cells (ouch!) 4. The cells won’t “repolarize” and no further actions will occur….

6. Action Potentials: Key Events: Depoloarization 1. Depolarization of membrane to “Threshold” level 2. Threshold “voltage” OPENS Na + channels rapid “flood” of + charges into the cell 3. Causes a Charge Reversal: Depolarization

7. Time: msec Membrane Potential (inside) mV -65 TH 0 Depolarization RMP Threshold voltage Na + Channels Open

8. Action Potential: Key Events: Repolarization 1. The new “membrane potential” voltage (+) causes Na + Channels to CLOSE 3. The new voltage causes K + Channels to OPEN 2. Concentration gradient causes K + to DIFFUSES OUT

9. mV Time Repolarization Na channels CLOSE K channels OPEN K moves OUT Refractory Period

10. Action Potential FACTS: All or None Principle Refractory Period: During “recovery” from AP, cell cannot be re-stimulated Conduction along membrane is like “dominos” Entire Cell Depolarizes

11. Signal Propagation 2 Local ion changes cause next cell to depolarize 2 Wave spreads along nerve axon + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

12. Myelination & Conduction 2 Myelin sheaths cover nerve except for Nodes of Ranvier 2 At these nodes depolrization currents escape 2 The signal “Jumps” to the next node & causes depolarization 2 Known as “Saltatory Conduction”

13. Saltatory Conduction Conduction velocity is much faster Medium Myelination: Conduction Velocity = 3-15 m/s Heavy Myelination: 15 - 120m/s

14. SUMMARY: Negative Resting Potentials are Necessary for Excitable Tissues Depolarization causes an Action Potential when the voltage reaches a “Threshold” voltage and Na+ Channels OPEN

15. Summary: Continues Repolarization is caused when Na+ Channels CLOSE and voltage gated K+ Channels OPEN allowing K+ to flood out of the cell Refractory Period: short Hyperpolarization when cells cannot be stimulated

16. ENOUGH! Quiz: Information Through Action Potentials on Wednesday