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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

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Action potential overview
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 gatedNa+ and voltage gatedK+ channels open in response to changes in the membrane potential

  • Voltage gatedK+ channels take longer to open than voltage gatedNa+ 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
Action Potential: Preview

  • GP’s are summed on the dendrites and soma

  • Sum of GP’s exceeds threshold

  • Na+ channels open, Na+ rushes in

  • Na+ channels close, K+ channels open

  • K+ rushes out

  • So much that membrane hyperpolarizes

  • Na+/K+/ATPase restores concentration and electricalgradients

Action potential resting state
Action Potential: Resting State

  • In a resting membrane, inactivation gate of Na+is

    open & activation gate is

    closed (Na+cannot get in)

  • Voltage gatedK+channels

    are closed

  • K+ leakage channels are open

  • The electrogenicsodium-potassium pump maintains the concentration gradient and the electrical gradient

  • The resting membrane potential is at -70 mV

Action potential depolarization phase
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
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 phase1
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
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
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 propagation
The Action Potential: Propagation

  • The APpropagates 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

Comparison of graded action potentials
Comparison of Graded & Action Potentials

  • 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.