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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