Action Potential
ACTION POTENTIAL(AP)
- Nerve signals are transmitted by action potential (nerve impulse)
Action potential –
- Rapid changes in membrane potential that spread along nerve fiber membrane.
- Each AP begins with sudden change from negative resting membrane potential (- 70 mV) to a positive potential (+ 35 mV).
- Ends with equally rapid change back to negative potential.
- AP conducts nerve signal, until nerve fiber’s end.
PHASES OF ACTION POTENTIAL
- Action potential, recorded using an intracellular electrode, has following phases:
1. Resting stage:
- Resting membrane potential before action potential.
- At rest, membrane is “Polarized“.
- Since RMP is negative (-70 mV).
- (This stage is not a part of action potential).
2. Depolarization phase:
- Potential rises to +35 mV in less than a millisecond.
- Due to “Voltage-gated Na2+ channels” opening.
- Causing sodium influx,
- Changes negative to positive charge.
- From -70 mV to -55 mV (“Firing level/Threshold point”)
- Partial opening of voltage-gated sodium channels.
- This AP portion (-70 mV to -55 mV) also called “Pre-Potential”/”Foot of AP”.
- On threshold level (-55 mV),
- Complete opening of voltage-gated Na2+channels occurs.
- Leading to Na2+ influx
- Produced by sudden & brief increasing Na2+ permeability.
- Thus, increases potential suddenly.
- Representing loss of original polarity of cell membrane.
- Membrane depolarization occurs.
3. Repolarization phase:
- When, potential drops to near resting level repolarization occurs.
- Ie., Reversal to original polarity.
Events during start repolarization/depolarization end:
- Voltage-gated Na+ channels close.
- Voltage-gated K+ channels open.
- Increases K+ permeability.
- Causing K+efflux out of cell
- Causing inside positive charge again turning negative.
4. After depolarization:
- Until 70%, repolarization maintains faster pace.
- Point after is slower repolarization phase.
- Referred as “After Depolarization”.
5. After-hyperpolarization :
- During repolarization phase,
- K+ channels open with K+ efflux.
- Membrane voltage falls back/repolarizes to resting potential (Ie, -70 mV).
- At this point, K+ channels close slowly.
- Hence, more K+ ions leak out.
- Causes “Hyperpolarization”.
- Brief period of voltage fall below -70 mV
- K+/Na2+ pump works to re-establish resting potential.
- By pumping Na2+ back out & K+ back into cells.
Final events involved:
- At end of AP, neuron have more sodium & potassium ions.
- Na2+-K+ ATPase activation restores their concentration.
- By pumping 3Na2+out of cell & 2K+ into cell.
- Yet, Number of ions involved in AP is infinitely small as compared to total ions present.
- Phase of rapid rise (in depolarization) and of rapid fall (in repolarization).
- Spike potential rather than entire action potential.
1. Channels involved in RMP:
- K+ “Leak” channels.
2. Channels involved in AP:
- Voltage-gated Na2+ channels:
- During Depolarization
- Voltage-gated K+ channels:
- During Repolarization
FEATURES OF ACTION POTENTIAL:
1. Self-propagation:
- AP is propagated without decrement.
- Conducted along nerve fiber, without any reduction in amplitude or speed until fiber’s end,
- Conduction is rapid yet, much slower than electricity.
2. All or none phenomenon:
- Action potential is not graded.
- It is an all or none change.
- Under given set of conditions, fixed values are obtained.
Condition 1:
- If stimulus of threshold strength is applied –
- Generated AP does not increase further its amplitude/duration.
Condition 2:
- Stimulus with subthreshold intensity:
- Fails to generate AP.
3. Membrane excitability during AP:
- “Threshold Stimulus”-
- Minimum stimulus strength triggering AP.
- Firing level above – 55 mV.
- During AP, neuron excitability is reduced.
- “Absolute Refractory Period” –
- Period where any stimulus strength fails to produce response.
- Because only about one-third response completes from onset of depolarization until early repolarization.
3b. Relative Refractory Period:
- Membrane is in “Relative Refractory Period”-
- Later part of repolarization until onset of after depolarization.
- Only sufficiently high stimulus can elicit response.
- Once initiated, AP does not depolarize membrane below firing level.
- Instead membrane is hyperpolarized.
- Ie, increased RMP magnitude.
- RMP changes from – 70 mV to – 85mV.
- This results in less excitable membrane.
- As potential reduced below firing level.
ACTION POTENTIAL (AP)
4 phases:
1. Resting stage:
- At rest, membrane is “Polarized”.
- (This stage is not a part of action potential).
2. Depolarization phase:
- There is “Voltage-gated Na2+ channels” opening.
- Causing sodium influx.
“Firing level/Threshold point”:
- -70 mV to -55 mV
- Partially opened voltage-gated Na2+ channels.
- This AP portion also called “Pre-Potential”/”Foot of AP”.
- At threshold level -55 mV:
- Complete opening of voltage-gated Na2+ channels.
3. Repolarization phase:
- Closure of voltage-gated Na+channels.
- Opening of voltage-gated K+channels.
- Causing efflux of K+ out of cell.
4. After depolarization:
- Slower phase of repolarization.
5. After-hyperpolarization :
- Increased K+ ions leak out.
- Causing brief period of voltage fall below -70 mV.
- Ie., “Hyperpolarization”.
1. Channels involved in RMP:
- K+ – “Leak” channels.
2. Channels involved in AP:
- Voltage-gated Na2+ channels:
- During Depolarization
- Voltage-gated K+ channels:
- During Repolarization
FEATURES OF ACTION POTENTIAL:
1. Self-propagation:
- AP is propagated without decrement.
- Nerve impulse conduction is rapid yet, much slower than electricity.
4. “Absolute Refractory Period” –
- Period where any stimulus strength fails to produce response.
- Because only about one-third response completes from onset of depolarization until early repolarization.
- Only sufficiently high stimulus can elicit a response.
5. Relative Refractory Period:
- Period between later part of repolarization till onset of after depolarization.
- Where once initiated, AP does not depolarize membrane below firing level.
- There is increased RMP magnitude.
- RMP changes from – 70 mV to – 85mV.
- Results in less excitable membrane.