Action Potential: Overview The action potential (AP) is a series of rapidly occurring events that change and then restore the membrane potential of a cell to its resting state During the AP, voltage gated Na + and voltage gated K + channels open in response to changes in the membrane potential Voltage gated K + channels take longer to open than voltage gated Na + channels Na + rushes in (depolarization), then K + rushes out (repolarization) Following the AP is a refractory period, during which another AP cannot occur, or can occur only with a larger stimulus

Action Potential: Preview 1)GP’s are summed on the dendrites and soma 2)Sum of GP’s exceeds threshold 3)Na + channels open, Na + rushes in 4)Na+ channels close, K + channels open 5)K + rushes out 6)So much that membrane hyperpolarizes 7)Na + /K + /ATPase restores concentration and electrical gradients

Action Potential: Resting State In a resting membrane, inactivation gate of Na + is open & activation gate is closed (Na + cannot get in) Voltage gated K + channels are closed K + leakage channels are open The electrogenic sodium-potassium pump maintains the concentration gradient and the electrical gradient The resting membrane potential is at -70 mV

Action Potential: Depolarization Phase Chemical or mechanical stimulus causes a graded potential to reach threshold Voltage-gated Na + channels open & Na + rushes into cell  When threshold (-55mV) is reached, Na + activation gates open and Na + enters  Na + inactivation gate closes again in few ten-thousandths of second  Only a total of ~20,000 Na + actually enter the cell, but they change the membrane potential considerably (up to +30mV) Positive feedback process

Action Potential: Repolarization Phase When threshold potential of -55mV is reached, voltage-gated K + channels also open K + channel opening is much slower than Na + channel opening The outflow of K + starts the repolarization of the membrane

Action Potential: Repolarization Phase When K + channels open, the Na+ inactivation gates have already closed (Na+ inflow stops) K + flows out and returns membrane potential to -70mV So much K + leaves the cell that it reaches a -90mV membrane potential and enters the after-hyperpolarizing phase K + channels close and the membrane potential returns to the resting potential of -70mV

Action Potential: Refractory Period Period of time during which neuron can not generate another AP Absolute refractory period  Even a very strong stimulus will not begin another AP  Inactivated Na + channels must return to the resting state before they can be reopened  Large fibers have absolute refractory period of 0.4 msec and up to 1000 impulses per second are possible Relative refractory period  A suprathreshold stimulus will be able to start an AP  K + channels are still open, but Na + channels have closed

The Action Potential: Summarized Resting membrane potential is -70mV Depolarization is the change from -70mV to +30 mV Repolarization is the reversal from +30 mV back to -70 mV

The Action Potential: Summarized

The Action Potential: Propagation The AP propagates along the axon As the wave of depolarization moves along the axon, Na + and K + channels open in sequence Eventually the AP reaches the synapse and neurotransmitters are released

The Action Potential: Propagation

Origin  GPs arise on dendrites and cell bodies  APs arise only at the trigger zone on the axon hillock Types of Channels  AP is produced by voltage-gated ion channels  GP is produced by ligand or mechanically-gated channels Conduction  GPs are localized (not propagated)  APs conduct (propagate) over the surface of the axon Amplitude  amplitude of the AP is constant (all-or-none)  graded potentials vary depending upon stimulus strength Duration  The AP is always the same  The duration of the GP is as long as the stimulus lasts Refractory period  The AP has a refractory period due to the nature of the voltage-gated channels, and the GP has none. Comparison of Graded & Action Potentials