BiologyChapter 5: Nervous System of Man

5.5.3 Active Membrane Potential

Active Membrane Potential:

Active membrane potential, also called action potential, is the electrical potential across the neuron membrane when the neuron is stimulated. It is characterized by a more positive inside and a more negative outside, which is called the depolarized state. This change occurs briefly and then the neuron recovers to its polarized state.

Definition

Active Membrane Potential: Active membrane potential (also called action potential) is characterized by more positive inside of neuron than outside (depolarized state). This happens when positive charges tend to move inside of neuron on receiving a particular stimulus.

Main Idea

Active membrane potential is also called action potential.
It is characterized by more positive inside and more negative outside.
This state is called the depolarized state.
It happens when positive charges move inside the neuron on receiving a stimulus.
The electrochemical change appears on a short region of the neuron for a brief period.
Then the neuron recovers to its polarized state.
This creates a wave of action potential that moves toward the other end of the neuron.

Core Concept

Active Membrane Potential (AMP) = depolarized state
**Inside** = positive
**Outside** = negative
**Value** = +50 mV
Threshold stimulus = stimulus capable of producing action potential
Subthreshold stimulus = stimulus not capable of producing action potential
Influx of Na⁺ = causes depolarization
**Repolarization** = K⁺ efflux
Hyperpolarization = slight overshoot
Sodium-potassium pump = restores resting potential

Detailed Explanation

Active Membrane Potential

Active membrane potential (also called action potential) is characterized by more positive inside of neuron than outside (depolarized state).
This happens when positive charges tend to move inside of neuron on receiving a particular stimulus.
This electrochemical change appears on a short region of neuron for a brief period of time followed by the recovery of polarized state.
In this way a wave of action potential begins to move towards other end of neuron.
Action potential is established by the following factors.

Threshold Stimulus

If a stimulus is capable to produce action potential in neuron, it is called threshold stimulus.
If stimulus is not capable to excite or fails to arise any response, it is called subthreshold stimulus.

Influx of Na⁺ Ions

When a neuron fibre is stimulated by threshold stimulus, it causes the opening of voltage regulated Na⁺ gates.
As a result Na⁺ gates permit the influx of Na⁺ ions by diffusion.
Since there are more Na⁺ ions entering than leaving, the electrical potential of the membrane changes from −70 mV towards zero and then reaches to the +50 mV.
This reversal of polarity across two sides of membrane is called depolarization.
This electropositivity inside and electronegativity outside lasts for about one millisecond till the Na⁺ gates are not closed.

Repolarization of Neuron Fibre

A fraction of second after the sodium gates open, depolarization of the axon membrane causes potassium gates to open.
Potassium therefore diffuses out of the cell.
Since the potassium is positively charged, this makes the inside of the cell more negative and starts the process of repolarization.

Hyperpolarization

At the peak of the action potential, the sodium gates start to close again.
Sodium permeability therefore declines.
The sodium-potassium pump continues to work during this time, so it gradually begins to restore the original resting potential.
This repolarization is shown by the falling phase of the action potential spike and results in the membrane potential returning to its original level.
In fact, there is a slight overshoot into a more negative potential than the original resting potential.
This is called hyperpolarization.
It is due to the slight delay in closing all the potassium gates compared with the sodium gates.
As potassium ions continue to enter the axon their positive charge restores the normal resting potential.

Summary Table of Action Potential Phases

Phase	Event	Ion Movement	Membrane Potential
Resting state	Polarized	—	−70 mV
Threshold stimulus	Stimulus applied	—	−70 mV
Depolarization	Na⁺ gates open	Na⁺ influx	+50 mV
Repolarization	K⁺ gates open	K⁺ efflux	Toward −70 mV
Hyperpolarization	K⁺ continues to leave	K⁺ efflux	More than −70 mV
Resting state restored	Sodium-potassium pump	Na⁺ out; K⁺ in	−70 mV

Intermediate Level

Active Membrane Potential

Feature	Description
State	Depolarized
Inside	Positive
Outside	Negative
Value	+50 mV
Definition	Electrical potential when stimulated

Phases of Action Potential

Phase	Ion Movement	Membrane Potential
Resting state	—	−70 mV
Depolarization	Na⁺ influx	+50 mV
Repolarization	K⁺ efflux	Toward −70 mV
Hyperpolarization	K⁺ continues to leave	More than −70 mV
Resting state restored	Sodium-potassium pump	−70 mV

Threshold and Subthreshold Stimulus

Stimulus	Description
Threshold stimulus	Capable of producing action potential
Subthreshold stimulus	Not capable of producing action potential

Ion Movements During Action Potential

Ion	Movement	Effect
Na⁺	Influx	Depolarization
K⁺	Efflux	Repolarization
Na⁺/K⁺ pump	3 Na⁺ out; 2 K⁺ in	Restores resting potential

Formula / Mathematical Form

Not applicable to this topic.

Solved Example

Not applicable to this topic (descriptive topic).

Important Points

1. Active membrane potential is also called action potential.
2. It is characterized by more positive inside and more negative outside.
3. This state is called the depolarized state.
4. It happens when positive charges move inside the neuron.
5. The change appears briefly.
6. The neuron recovers to its polarized state.
7. A wave of action potential moves toward the other end of the neuron.
8. Threshold stimulus produces action potential.
9. Subthreshold stimulus does not produce action potential.
10. Na⁺ gates open during depolarization.
11. Na⁺ influx causes depolarization.
12. Membrane potential changes from −70 mV to +50 mV.
13. Depolarization lasts about one millisecond.
14. K⁺ gates open during repolarization.
15. K⁺ efflux causes repolarization.
16. Hyperpolarization is a slight overshoot.
17. Sodium-potassium pump restores resting potential.
18. Action potential is essential for nerve impulse transmission.
19. It allows communication between neurons.
20. It is the basis of nervous coordination.

Remember

- Active Membrane Potential (AMP) = depolarized state
- **Inside** = positive
- **Outside** = negative
- **Value** = +50 mV
- Threshold stimulus = produces action potential
- Subthreshold stimulus = does not produce action potential
- **Depolarization** = Na⁺ influx
- **Repolarization** = K⁺ efflux
- Hyperpolarization = slight overshoot
- Sodium-potassium pump = restores resting potential
- Depolarization duration = about 1 millisecond

Common Mistakes

1. Confusing resting and active membrane potential — RMP = polarized; AMP = depolarized.
2. **Mixing up the values** — RMP = −70 mV; AMP = +50 mV.
3. Confusing depolarization and repolarization — Depolarization = Na⁺ influx; repolarization = K⁺ efflux.
4. Thinking the inside is negative during action potential — Inside is positive during depolarization.
5. Forgetting the role of the sodium-potassium pump — It restores resting potential.
6. Mixing up the ion movements — Na⁺ in; K⁺ out.
7. Confusing threshold and subthreshold stimulus — Threshold produces action potential; subthreshold does not.
8. Forgetting hyperpolarization — It is a slight overshoot.
9. Thinking the action potential is constant — It changes during the impulse.
10. Believing the neuron is always at rest — Neurons can be stimulated.

Quick Revision

Active membrane potential (also called action potential) is characterized by a more positive inside of the neuron than outside (depolarized state). This happens when positive charges tend to move inside the neuron on receiving a particular stimulus. This electrochemical change appears on a short region of the neuron for a brief period of time followed by the recovery of the polarized state. In this way, a wave of action potential begins to move towards the other end of the neuron. Action potential is established by the following factors: threshold stimulus, influx of Na⁺ ions, repolarization of neuron fibre, and hyperpolarization. If a stimulus is capable of producing action potential in a neuron, it is called a threshold stimulus. If a stimulus is not capable of exciting or fails to arise any response, it is called a subthreshold stimulus. When a neuron fibre is stimulated by a threshold stimulus, it causes the opening of voltage-regulated Na⁺ gates. As a result, Na⁺ gates permit the influx of Na⁺ ions by diffusion. The electrical potential of the membrane changes from −70 mV towards zero and then reaches +50 mV. This reversal of polarity is called depolarization. A fraction of a second after the sodium gates open, depolarization of the axon membrane causes potassium gates to open. Potassium therefore diffuses out of the cell, making the inside more negative and starting repolarization. Hyperpolarization is a slight overshoot into a more negative potential than the original resting potential. The sodium-potassium pump restores the original resting potential.

60-Second Revision

Active Membrane Potential (AMP) = depolarized state
Inside = positive
Outside = negative
Value = +50 mV
Threshold stimulus = produces action potential
Subthreshold stimulus = does not produce action potential
Depolarization = Na⁺ influx
Repolarization = K⁺ efflux
Hyperpolarization = slight overshoot
Sodium-potassium pump = restores resting potential
Depolarization duration = about 1 millisecond
Warning: AMP is +50 mV; RMP is −70 mV

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Chapter:
5. Nervous System of Man

Topic:
5.5.4 VELOCITIES OF NERVE IMPULSE

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