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 whichneuron can not generateanother 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