Action Potential

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”.
2a. Depolarization:
  • 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 Kpermeability.
  • 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.
Spike potential:
  • Phase of rapid rise (in depolarization) and of rapid fall (in repolarization).
Nerve potential/Nerve Impulse:
  • Spike potential rather than entire action potential.
CHANNELS INVOLVED IN AP & RMP:

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.
3a. Refractory period:
  • 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.
Exam Question
 

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”.
2a. Depolarization:
  • 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.
2. All or none phenomenon: 
3. “Threshold Stimulus” is “Minimum stimulus strength for triggering AP.

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.
Don’t Forget to Solve all the previous Year Question asked on Action Potential

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