5.5 Nerve Impulse

A nerve impulse is a wave of electrochemical changes that travels along the length of a neuron. It is the signal that carries information from receptors to the central nervous system and from the central nervous system to effectors. The nerve impulse is based on changes in the electrical potential across the neuron membrane.

Definition

Nerve Impulse: Nerve impulse is information or signal about a stimulus that is transmitted from receptors to the CNS and from CNS to the effectors. In technical terms, a nerve impulse can be defined as a wave of electrochemical changes that travel along the length of neuron, from one end to the other.

Membrane Potential: In the case of a neuron, the electrical potential is termed as membrane potential, which is exhibited in two different forms: Resting Membrane Potential (RMP) and Active Membrane Potential (AMP).

Main Idea

• Nerve impulse is information or signal about a stimulus.
• It is transmitted from receptors to the CNS and from CNS to effectors.
• It is a wave of electrochemical changes.
• It travels along the length of a neuron.
• The electrical potential exists on the neuron membrane.
• This potential is called membrane potential.
• Two forms of membrane potential: Resting Membrane Potential (RMP) and Active Membrane Potential (AMP).

Core Concept

• 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 = 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

Detailed Explanation

Nerve Impulse

Nerve impulse is information or signal about a stimulus that is transmitted from receptors to the CNS and from CNS to the effectors. In technical terms, a nerve impulse can be defined as a wave of electrochemical changes that travel along the length of neuron, from one end to the other.

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:

• 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:

Summary Table of Membrane Potentials

Summary of Nerve Impulse Transmission

• Resting state — neuron is polarized (−70 mV).
• Threshold stimulus — stimulus is strong enough.
• Depolarization — Na⁺ gates open; Na⁺ influx; potential reaches +50 mV.
• Repolarization — K⁺ gates open; K⁺ efflux; potential returns toward −70 mV.
• Hyperpolarization — slight overshoot; potential becomes more negative than −70 mV.
• Resting state restored — sodium-potassium pump restores original potential.

Intermediate Level

Resting Membrane Potential (RMP)

Active Membrane Potential (AMP)

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

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

A nerve impulse is information or signal about a stimulus that is transmitted from receptors to the CNS and from CNS to the effectors. In technical terms, a nerve impulse can be defined as a wave of electrochemical changes that travel along the length of neuron, from one end to the other. The electrical potential that exists on neuron membrane 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 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 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. 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. 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:
• 5.5.1 GENERATION AND TRANSMISSION OF NERVE IMPULSE

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