5.5 Nerve Impulses

Introduction

The nervous system works by sending electrical signals called nerve impulses. These impulses travel along neurons and allow rapid communication between different parts of the body. This topic explains what nerve impulses are, how they are generated, and how they travel.

Definition

Nerve impulses are electrical signals that travel along neurons. They result from a difference in electrical charge across the plasma membrane of a neuron.

Main Idea

Nerve impulses are electrical in nature. They result from a difference in electrical charge across the plasma membrane of a neuron. How does this difference in electrical charge come about? The answer involves ions, which are electrically charged atoms or molecules.

Two key states are involved:

• Resting potential — when a neuron is not actively transmitting a nerve impulse.
• Action potential — a sudden reversal of electrical charge that travels along the neuron.

Core Concept

Resting Potential vs. Action Potential

Detailed Explanation

1. Resting Potential

When a neuron is not actively transmitting a nerve impulse, it is in a resting state, ready to transmit a nerve impulse.

What happens:

• During the resting state, the sodium-potassium pump maintains a difference in charge across the cell membrane.
• It uses energy in ATP to pump positive sodium ions (Na⁺) out of the cell and potassium ions (K⁺) into the cell.
• As a result, the inside of the neuron is negatively charged compared to the extracellular fluid surrounding the neuron.
• This is due to many more positively charged ions outside the cell compared to inside the cell.
• This difference in electrical charge is called the resting potential.

Simple explanation: When a neuron is resting, it is like a charged battery. The inside is negative, and the outside is positive. This difference in charge is maintained by the sodium-potassium pump.

Why this is important:

• The resting potential prepares the neuron to transmit a nerve impulse.
• It is the baseline state of the neuron.

2. Action Potential

A nerve impulse is a sudden reversal of the electrical charge across the membrane of a resting neuron.

What happens:

• The reversal of charge is called an action potential.
• It begins when the neuron receives a chemical signal from another cell.
• The signal causes gates in sodium ion channels to open, allowing positive sodium ions to flow back into the cell.
• As a result, the inside of the cell becomes positively charged compared to the outside of the cell.
• This reversal of charge ripples down the axon very rapidly as an electric current.

Simple explanation: When a neuron is stimulated, sodium channels open, and sodium ions rush in. This reverses the charge — the inside becomes positive. This reversal travels down the axon as a nerve impulse.

Why this is important:

• The action potential is the nerve impulse.
• It allows rapid communication along the neuron.

3. Do You Know?

An action potential speeds along an axon in milliseconds.

Simple explanation: Nerve impulses are very fast. They travel along the axon in thousandths of a second.

4. For Your Information

In neurons with myelin sheaths, ions flow across the membrane only at the nodes between sections of myelin.

• As a result, the action potential jumps along the axon membrane from node to node, rather than spreading smoothly along the entire membrane.
• This increases the speed at which it travels.

Simple explanation: In myelinated neurons, the impulse jumps from node to node (saltatory conduction). This makes the impulse travel much faster.

Why this is important:

• Myelinated neurons conduct impulses faster than unmyelinated neurons.
• This is important for rapid responses.

Intermediate Level

Nerve impulses are electrical signals that travel along neurons.

Key points:

• Nerve impulses result from a difference in electrical charge across the neuron membrane.
• The resting potential is maintained by the sodium-potassium pump.
• The action potential begins when sodium channels open.
• Sodium ions rush into the cell, reversing the charge.
• The action potential travels along the axon.
• In myelinated neurons, the impulse jumps from node to node (saltatory conduction).

Important Points

• - Nerve impulses are electrical in nature.
• - They result from a difference in electrical charge across the plasma membrane of a neuron.
• - The difference in charge involves ions, which are electrically charged atoms or molecules.
• - When a neuron is not actively transmitting a nerve impulse, it is in a resting state.
• - During the resting state, the sodium-potassium pump maintains a difference in charge across the cell membrane.
• - It uses energy in ATP to pump positive sodium ions out of the cell and potassium ions into the cell.
• - As a result, the inside of the neuron is negatively charged compared to the extracellular fluid.
• - This difference in electrical charge is called the resting potential.
• - A nerve impulse is a sudden reversal of the electrical charge across the membrane of a resting neuron.
• - The reversal of charge is called an action potential.
• - It begins when the neuron receives a chemical signal from another cell.
• - The signal causes gates in sodium ion channels to open, allowing positive sodium ions to flow back into the cell.
• - As a result, the inside of the cell becomes positively charged compared to the outside.
• - This reversal of charge ripples down the axon very rapidly as an electric current.
• - An action potential speeds along an axon in milliseconds.
• - In neurons with myelin sheaths, ions flow across the membrane only at the nodes between sections of myelin.
• - As a result, the action potential jumps along the axon membrane from node to node.
• - This increases the speed at which it travels.

Remember

• - Nerve impulse = electrical signal
• - Resting potential = neuron at rest; inside negative, outside positive
• - Sodium-potassium pump = maintains resting potential
• - Action potential = sudden reversal of charge; inside positive
• - Sodium channels open → Na⁺ rushes in → depolarization
• - Potassium channels open → K⁺ rushes out → repolarization
• - Myelin sheath = saltatory conduction; faster impulse
• - Action potential = all-or-none principle
• - An action potential speeds along an axon in milliseconds

Common Mistakes

• 1. Confusing resting potential and action potential.
• - Resting potential = neuron at rest; inside negative.
• - Action potential = neuron transmitting; inside positive.
• 2. Thinking the sodium-potassium pump uses no energy.
• - The sodium-potassium pump uses ATP.
• 3. Mixing up sodium and potassium ions.
• - Sodium (Na⁺) rushes in during action potential.
• - Potassium (K⁺) rushes out during repolarization.
• 4. Forgetting the role of myelin sheath.
• - Myelin sheath increases the speed of nerve impulse conduction.
• 5. Believing the action potential gets weaker as it travels.
• - The action potential is all-or-none. It does not weaken.
• 6. Thinking nerve impulses travel in both directions.
• - Nerve impulses travel in one direction only (from dendrites to axon).
• 7. Confusing the nodes of Ranvier with Schwann cells.
• - Nodes of Ranvier = gaps between Schwann cells.
• - Schwann cells = form myelin sheath.

Quick Revision

60-Second Revision

• Nerve impulse = electrical signal
• Resting potential = neuron at rest; inside negative, outside positive; maintained by sodium-potassium pump
• Action potential = sudden reversal of charge; inside positive; caused by Na⁺ rushing in
• **Depolarization** = Na⁺ rushes in
• **Repolarization** = K⁺ rushes out
• Myelin sheath = saltatory conduction; faster impulse
• All-or-none principle = action potential fires completely or not at all
• An action potential speeds along an axon in milliseconds
• Warning: Do not confuse resting potential (negative inside) with action potential (positive inside).