5.5.1 Generation and Transmission of Nerve Impulse

The generation and transmission of a nerve impulse involve a series of electrochemical changes across the neuron membrane. These changes include resting membrane potential, depolarization, repolarization, and hyperpolarization. The impulse travels along the neuron as a wave of these changes.

Definition

Generation and Transmission of Nerve Impulse: The generation and transmission of a nerve impulse refer to the sequence of electrochemical changes that occur across the neuron membrane when a stimulus is applied, and the propagation of these changes along the length of the neuron.

Main Idea

• The word "electrochemical" refers to the electrical potential that exists on the neuron membrane.
• In the case of a neuron, the electrical potential is termed membrane potential.
• Membrane potential is exhibited in two different forms:
• The resting membrane potential is the polarized state.
• The active membrane potential is the depolarized state.
• The nerve impulse is generated and transmitted through changes in membrane potential.

Core Concept

• Resting Membrane Potential (RMP) = polarized state; −70 mV
• Active Membrane Potential (AMP) = depolarized state; +50 mV
• Threshold stimulus = stimulus capable of producing action potential
• Subthreshold stimulus = stimulus not capable of producing action potential
• Depolarization = influx of Na⁺ ions
• Repolarization = efflux of K⁺ ions
• Hyperpolarization = slight overshoot; more negative than resting potential
• Sodium-potassium pump = restores resting potential

Detailed Explanation

Generation and Transmission of Nerve Impulse

Here, the word "electrochemical" refers to the electrical potential (a capacity to do electrical work) that exists on neuron membrane. In case of neuron the electrical potential is termed as membrane potential which is exhibited in two different forms i.e., Resting Membrane Potential (RMP) and Active Membrane Potential (AMP).

Resting Membrane Potential

• It is characterized by more positive outer surface of neuron membrane than inner surface.
• This state is also referred as polarized state and the neuron is supposed to be at rest.
• This means that there is an unequal distribution of ions on the two sides of the nerve cell membrane.
• This potential generally measures about 70 mV (with the inside of the membrane negative with respect to the outside).
• So, the resting membrane potential is expressed as −70 mV.
• The minus sign indicates that the inside is negative relative to (or compared to) the outside.
• It is called a resting membrane potential because it occurs when a membrane is not being stimulated or conducting impulses.

Factors Establishing Resting Membrane Potential:

Distribution and Active Movement of Na⁺ and K⁺ Ions

• The concentration of potassium (K⁺) is 30 times greater in the fluid inside the neuron cell than outside.
• The concentration of sodium ions (Na⁺) is nearly 10 times greater in the fluid outside the cell than inside.
• These ions are continuously moved against their concentration gradient through sodium-potassium pumps by the expenditure of energy.
• For every two K⁺ that are actively transported inward, three Na⁺ are pumped out.
• So inside becomes more negative than outside of the neuron membrane.

Negative Organic Ions

• There are many types of organic compounds in the neuron cytoplasm that also have negative charges.
• These ions include some amino acids, many proteins, and RNA.
• Presence of these ions in the neuron cytoplasm makes inside of neuron more negative than outside.

Leakage of K⁺ Ions

• Cell membrane of neuron also has many channel proteins called gates.
• K⁺ ions leak continuously through leaky K⁺ gates.
• This also makes more positive outside of neuron than inside.

• Overall there are more positive charges on the outside than on the inside.
• This is known as resting membrane potential.
• This potential will be maintained until the membrane is disturbed or stimulated by a sufficiently strong stimulus (threshold), then action potential will be produced.

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.

Factors Establishing Action Potential:

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 (More K⁺ Ions Are on the Outside Than Na⁺ Ions on the Inside)

• 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 Membrane Potentials

Summary of the Sequence

Intermediate Level

Resting Membrane Potential

Active Membrane Potential

Phases of Action Potential

Threshold and Subthreshold Stimulus

Formula / Mathematical Form

Not applicable to this topic.

Solved Example

Not applicable to this topic (descriptive topic).

Important Points

• 1. Nerve impulse is a wave of electrochemical changes.
• 2. It travels along the length of a neuron.
• 3. Membrane potential is the electrical potential on neuron membrane.
• 4. Two forms: Resting Membrane Potential (RMP) and Active Membrane Potential (AMP).
• 5. RMP is the polarized state.
• 6. RMP is −70 mV.
• 7. Inside is negative; outside is positive in RMP.
• 8. AMP is the depolarized state.
• 9. AMP is +50 mV.
• 10. Inside is positive; outside is negative in AMP.
• 11. Threshold stimulus produces action potential.
• 12. Subthreshold stimulus does not produce action potential.
• 13. Depolarization is caused by Na⁺ influx.
• 14. Repolarization is caused by K⁺ efflux.
• 15. Hyperpolarization is a slight overshoot.
• 16. Sodium-potassium pump restores resting potential.
• 17. For every 2 K⁺ transported inward, 3 Na⁺ are pumped out.
• 18. K⁺ concentration is 30 times greater inside.
• 19. Na⁺ concentration is nearly 10 times greater outside.
• 20. Depolarization lasts about one millisecond.

Remember

• - Nerve impulse = wave of electrochemical changes
• - Membrane potential = electrical potential on neuron membrane
• - Resting Membrane Potential (RMP) = polarized state; −70 mV
• - Active Membrane Potential (AMP) = depolarized state; +50 mV
• - Polarized = more positive outside; more negative inside
• - Depolarized = more positive inside; more negative outside
• - Threshold stimulus = produces action potential
• - Subthreshold stimulus = does not produce action potential
• - **Depolarization** = Na⁺ influx
• - **Repolarization** = K⁺ efflux
• - Hyperpolarization = slight overshoot; more negative than −70 mV
• - Sodium-potassium pump = restores resting potential
• - 2 K⁺ in; 3 Na⁺ out = sodium-potassium pump ratio

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 positive at rest — Inside is negative at rest.
• 5. Forgetting the role of the sodium-potassium pump — It restores resting potential.
• 6. Mixing up the ion concentrations — K⁺ is higher inside; Na⁺ is higher outside.
• 7. Confusing threshold and subthreshold stimulus — Threshold produces action potential; subthreshold does not.
• 8. Forgetting hyperpolarization — It is a slight overshoot.
• 9. Thinking the membrane potential is constant — It changes during action potential.
• 10. Believing the neuron is always at rest — Neurons can be stimulated.

Quick Revision

The generation and transmission of a nerve impulse involve a series of electrochemical changes across the neuron membrane. The word "electrochemical" refers to the electrical potential that exists on the neuron membrane. In the case of a neuron, the electrical potential is termed membrane potential, which is exhibited in two different forms: Resting Membrane Potential (RMP) and Active Membrane Potential (AMP). Resting membrane potential is characterized by a more positive outer surface of the neuron membrane than the inner surface. This state is also referred to as the polarized state, and the neuron is supposed to be at rest. The resting membrane potential is expressed as −70 mV. Active membrane potential (also called action potential) is characterized by a more positive inside of the neuron than outside (depolarized state). During depolarization, Na⁺ ions rush in, changing the potential from −70 mV to +50 mV. During repolarization, K⁺ ions rush out, restoring the negative potential. Hyperpolarization is a slight overshoot. The sodium-potassium pump restores the original resting potential.

60-Second Revision

• Nerve impulse = wave of electrochemical changes
• Membrane potential = electrical potential on neuron membrane
• Resting Membrane Potential (RMP) = polarized state; −70 mV
• Active Membrane Potential (AMP) = depolarized state; +50 mV
• Polarized = more positive outside; more negative inside
• Depolarized = more positive inside; more negative outside
• Threshold stimulus = produces action potential
• Subthreshold stimulus = does not produce action potential
• Depolarization = Na⁺ influx
• Repolarization = K⁺ efflux
• Hyperpolarization = slight overshoot; more negative than −70 mV
• Sodium-potassium pump = restores resting potential
• Warning: RMP is −70 mV; AMP is +50 mV

NEXT TOPIC

• Chapter:
• 5. Nervous System of Man

• Topic:
• 5.5.2 RESTING MEMBRANE POTENTIAL

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