5.3 Neuron

Introduction

The nervous system is made up of specialized cells called neurons. Neurons are the structural and functional units of the nervous system. They carry electrical signals (nerve impulses) throughout the body, enabling communication between different parts. This topic explains the structure and function of a neuron.

Definition

The neuron is the structural and functional unit of the nervous system. Neurons are long, thin, delicate cells that carry electrical signals (nerve impulses).

Main Idea

• The structural and functional unit of the nervous system is the neuron.
• Neurons are long, thin, delicate cells.
• A single neuron may extend from the spinal cord of man to the foot.
• Bundles of neuron fibres are called nerves.
• The portion of the neuron most resembling other cells is the cell body.
• The cell body contains a nucleus and cytoplasm.
• The long fibre of a neuron leading away from the cell body is the axon.
• An axon transmits impulses away from the cell body.
• The other end of the neuron is a highly branched structure called dendrites.
• Impulses move toward the cell body from the dendrites and from cell body to axon.

Core Concept

Parts of a Neuron

Detailed Explanation

1. What is a Neuron?

The structural and functional unit of the nervous system is the neuron. Neurons are long, thin, delicate cells.

Simple explanation: Neurons are the cells that carry electrical messages in your body. They are like tiny wires.

Why neurons are special:

• They can carry electrical signals.
• They can transmit signals over long distances.
• They can connect to other neurons to form networks.

2. Cell Body

The portion of the neuron most resembling other cells is the cell body.

What it contains:

• Nucleus
• Cytoplasm

Function:

• Contains the genetic material (DNA).
• Carries out metabolic activities.
• Receives impulses from dendrites.

Simple explanation: The cell body is the control center of the neuron. It contains the nucleus and keeps the neuron alive.

3. Dendrites

The other end of the neuron is a highly branched structure called dendrites.

Function:

• Receive impulses from other neurons or sensory receptors.
• Carry impulses toward the cell body.

Simple explanation: Dendrites are like branches that receive messages and bring them to the cell body.

Why they are branched:

- More branches = more surface area = more connections with other neurons.

4. Axon

The long fibre of a neuron leading away from the cell body is the axon.

Function:

• Transmits impulses away from the cell body.
• Carries impulses to other neurons, muscles, or glands.

Simple explanation: The axon is like a long wire that carries messages away from the cell body.

Length:

• A single neuron may extend from the spinal cord of man to the foot.
• Axons can be very long.

5. Myelin Sheath

Long neuron fibres are covered by a fatty myelin sheath.

What it is:

• A fatty covering around the axon.
• Formed by Schwann cells.

Function:

• Insulates the axon.
• Increases the speed of nerve impulse conduction.

Simple explanation: The myelin sheath is like the plastic coating around an electrical wire. It insulates the axon and helps the signal travel faster.

Important point:

• Myelin sheath is an insulator, so the membrane coated with this sheath does not conduct nerve impulse.
• In such a neuron, impulse jumps over the myelin going from node to node.
• These jumping or saltatory impulses increase the speed of nerve impulse.

6. Schwann Cells

Cells of myelin sheath are called Schwann cells.

Function:

• Form the myelin sheath.
• Support and insulate the axon.

Simple explanation: Schwann cells are the cells that wrap around the axon to form the myelin sheath.

7. Nodes of Ranvier

Small gaps between consecutive Schwann cells are called nodes of Ranvier.

Function:

• Both myelin sheath and the nodes of Ranvier are important in determining the speed with which impulses are conducted.
• Impulse jumps from node to node (saltatory conduction).

Simple explanation: Nodes of Ranvier are like gaps in the myelin sheath. The nerve impulse jumps from one gap to the next, which makes it travel faster.

Intermediate Level

The neuron is the structural and functional unit of the nervous system.

Key points:

• Neurons are long, thin, delicate cells.
• A single neuron may extend from the spinal cord to the foot.
• Bundles of neuron fibres are called nerves.
• Impulses move from dendrites → cell body → axon.
• Myelin sheath is an insulator.
• Impulse jumps from node to node (saltatory conduction).
• Saltatory conduction increases the speed of nerve impulse.

Important Points

• - The structural and functional unit of the nervous system is the neuron.
• - Neurons are long, thin, delicate cells.
• - A single neuron may extend from the spinal cord of man to the foot.
• - Bundles of neuron fibres are called nerves.
• - The portion of the neuron most resembling other cells is the cell body.
• - The cell body contains a nucleus and cytoplasm.
• - The long fibre of a neuron leading away from the cell body is the axon.
• - An axon transmits impulses away from the cell body.
• - The other end of the neuron is a highly branched structure called dendrites.
• - Impulses move toward the cell body from the dendrites and from cell body to axon.
• - Long neuron fibres are covered by a fatty myelin sheath.
• - Cells of myelin sheath are called Schwann cells.
• - Small gaps between consecutive Schwann cells are called nodes of Ranvier.
• - Both myelin sheath and the nodes of Ranvier are important in determining the speed with which impulses are conducted.
• - Myelin sheath is an insulator so the membrane coated with this sheath does not conduct nerve impulse.
• - In such a neuron, impulse jumps over the myelin going from node to node.
• - These jumping or saltatory impulses increase the speed of nerve impulse.

Remember

• - Neuron = structural and functional unit of nervous system
• - Cell body = contains nucleus and cytoplasm
• - Dendrites = receive impulses, carry toward cell body
• - Axon = transmits impulses away from cell body
• - Myelin sheath = fatty covering, insulates axon
• - Schwann cells = form myelin sheath
• - Nodes of Ranvier = gaps between Schwann cells
• - Saltatory conduction = impulse jumps from node to node
• - Myelin sheath increases speed of impulse

Common Mistakes

• 1. Confusing dendrites and axons.
• - Dendrites = receive impulses, carry toward cell body.
• - Axon = transmits impulses away from cell body.
• 2. Thinking all neurons have myelin sheath.
• - Some neurons are myelinated; others are unmyelinated.
• 3. Mixing up Schwann cells and nodes of Ranvier.
• - Schwann cells = form myelin sheath.
• - Nodes of Ranvier = gaps between Schwann cells.
• 4. Forgetting the direction of impulse.
• - Impulse moves from dendrites → cell body → axon.
• 5. Thinking the myelin sheath conducts impulses.
• - Myelin sheath is an insulator. Impulse jumps from node to node (saltatory conduction).
• 6. Believing neurons cannot be long.
• - A single neuron may extend from the spinal cord to the foot.
• 7. Confusing nerves and neurons.
• - Neuron = single nerve cell.
• - Nerve = bundle of neuron fibres.

Quick Revision

60-Second Revision

• Neuron = structural and functional unit of nervous system
• Cell body = contains nucleus and cytoplasm
• Dendrites = receive impulses, carry toward cell body
• Axon = transmits impulses away from cell body
• Myelin sheath = fatty covering, insulates axon
• Schwann cells = form myelin sheath
• Nodes of Ranvier = gaps between Schwann cells
• Saltatory conduction = impulse jumps from node to node
• Myelin sheath increases speed of impulse
• Warning: Do not confuse dendrites (receive impulses) with axons (transmit impulses away). Myelin sheath insulates but does not conduct impulses.