Local Potential (“Passive” Depolarization) Depolarization to Threshold

1. Local Potential (“Passive” Depolarization) • Depolarization to Threshold • depolarization produced by the stimulus • chemical, electrical, mechanical • depolarization due to what’s done to (received by) this part of the membrane • Caused by, for example • an action potential upstream • a graded potential upstream • postsynaptic potentials • receptor potentials rest rest Fig. 12.13a Action Potential Events #1

2. Depolarization • “Active”/“Rapid” Depolarization • depolarization produced in response to the stimulus • depolarization due to what this part of the membrane does Acceleration indicates a process that has positive feedback. Depolarization continues to about +35 mV. rest rest Fig. 12.13a Action Potential Events #2

3. Membrane potential becomes even more negative, falling a little bit below resting membrane potential, and then gradually returns back to resting level. Membrane potential returns to resting level. Repolarization Hyperpolarization (= after-hyperpolarization) rest rest Fig. 12.13a Action Potential Events #3

4. Thresholdfiring level The production of an action potential is an all-or-none response. Fig. 12.13a Moffett, Moffett and Schauf, Human Physiology

5. Electrical Changes in Excitable Membranes squid giant axon Moffett, Moffett and Schauf, Human Physiology First sighting of a living giant squid: http://news.nationalgeographic.com/news/2005/09/0927_050927_giant_squid.html

6. ConductancesThe action potential is caused by changes in gNa and gK. • conductance = g = 1/resistance • measured in siemens (formerly mhos) • (cf. resistance: ohms) • “passive” depolarization • no significant changes compared to resting state • “active” depolarization • gNa rapidly increases • repolarization • gNa rapidly decreases • gK rapidly increases • after-hyperpolarization • gNa at resting level • gK slowly decreases to resting levels after-hyperpolarization Guyton, Medical Physiology

7. Moffett, Moffett and Schauf, Human Physiology Electrochemical Gradients I = g x V • When studying action potentials you must always keep in mind the electrochemical gradients and the equilibrium potentials for Na+ and K+. • When studying the cardiac action potential, you must also consider the Ca++ gradient. • The action potential is caused by passive movements of Na+ and K+. • In an action potential the cell makes use of the electrochemical gradients for Na+ and K+ to produce rapid changes in membrane potential. There is no direct use of energy from ATP.

8. Electrical Views of the Membrane from Mountcastle, Medical Physiology Changes in conductances and currents during an action potential

9. Action Potentials • Electrically speaking, the action potential is caused by changes in conductances of Na+ and K+. • Molecularly speaking, changes in conductance are caused by the opening and closing of voltage-gated channels.

10. Channels • Channels are transmembrane proteins. • Channels are specific for a certain ion or ions, or for water. • the hourglass shape of a channel pore serves as a selectivity filter Ion Channel Alberts et al., Molecular Biology of the Cell Fig. 3.8

11. The Action Potentialthe importance of channels Fig. 12.14 (altered; channel cartoons from Katzung and Alberts)