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.

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.

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.