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

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

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

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

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

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

7. 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. Moffett, Moffett and Schauf, Human Physiology Electrochemical Gradients I = g x V

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 Fig. 3.8 Ion Channel Alberts et al., Molecular Biology of the Cell

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