Introduction

The human body can respond to changes in its environment within fractions of a second. Whether you pull your hand away from a hot object, catch a falling ball, or hear someone call your name, these rapid responses are made possible by neurons, the specialised cells of the nervous system.

Neurons communicate using electrical nerve impulses that travel along their length and chemical signals that pass between neurons at tiny junctions called synapses. Together, billions of neurons form an extensive communication network that links the brain, spinal cord, muscles, glands, and sense organs.

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What Is a Neuron?

A neuron is a specialised cell that carries information throughout the nervous system.

Neurons:

• Receive information.
• Process signals.
• Transmit nerve impulses.
• Communicate with other neurons, muscles, and glands.

The human brain contains approximately 86 billion neurons, each connected to many others.

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Figure 1. A neuron is specially adapted to receive, conduct, and transmit nerve impulses.

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Parts of a Neuron

A typical neuron has several important parts.

Dendrites

• Receive signals from other neurons or receptors.
• Carry information toward the cell body.

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Cell Body (Soma)

• Contains the nucleus.
• Controls the activities of the neuron.
• Processes incoming information.

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Axon

• A long fibre that carries nerve impulses away from the cell body.
• Some axons are over one metre long.

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Myelin Sheath

• A fatty insulating layer surrounding many axons.
• Speeds up the transmission of nerve impulses.
• Protects the axon.

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Axon Terminals

• Branches at the end of the axon.
• Release chemical messengers to communicate with other cells.

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What Is a Nerve Impulse?

A nerve impulse is an electrical signal that travels along a neuron.

Nerve impulses travel:

• From the dendrites.
• Through the cell body.
• Along the axon.
• To the axon terminals.

The impulse travels in one direction only.

This allows information to move quickly through the nervous system.

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Figure 2. A nerve impulse travels from the dendrites to the axon terminals in one direction.

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How Nerve Impulses Travel

When a neuron is stimulated:

1. A receptor or another neuron generates an electrical impulse.
2. The impulse travels along the axon.
3. The myelin sheath helps the impulse move more rapidly.
4. The impulse reaches the axon terminals.

Electrical impulses travel extremely quickly—some at speeds of over 100 metres per second.

This allows the body to respond rapidly to changes.

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Synapses

Neurons do not usually touch one another directly.

Instead, they are separated by tiny gaps called synapses.

When a nerve impulse reaches the axon terminal:

• Chemicals called neurotransmitters are released.
• Neurotransmitters diffuse across the synapse.
• They bind to receptors on the next neuron.
• A new electrical impulse is generated in the next neuron.

This allows information to pass from one neuron to another.

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Figure 3. Neurotransmitters carry signals across synapses between neurons.

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Types of Neurons

There are three main types of neurons.

Sensory Neurons

Carry information:

• From receptors.
• To the central nervous system.

Examples:

• Detecting light.
• Detecting sound.
• Detecting temperature.
• Detecting pain.

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Interneurons

Found only in the:

• Brain.
• Spinal cord.

Functions:

• Process information.
• Connect sensory neurons to motor neurons.
• Coordinate responses.

Most neurons in the human nervous system are interneurons.

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Motor Neurons

Carry information:

• From the central nervous system.
• To muscles or glands.

Motor neurons produce the body's responses.

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Figure 4. Sensory neurons, interneurons, and motor neurons work together to process information and coordinate responses.

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Communication Throughout the Body

Neurons allow information to travel rapidly throughout the body.

Example:

Touching a hot surface.

1. Heat receptors detect the stimulus.
2. Sensory neurons carry the signal to the spinal cord.
3. Interneurons process the information.
4. Motor neurons carry commands to arm muscles.
5. Muscles contract.
6. The hand moves away from the heat.

The entire process can occur in less than one second.

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Why Neurons Are Important

Neurons allow the body to:

• Sense the environment.
• Think and learn.
• Remember information.
• Control movement.
• Coordinate organs.
• Maintain homeostasis.

Without neurons, communication between body systems would not be possible.

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Figure 5. Neurons form communication pathways that allow the body to respond rapidly to stimuli.

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Worked Example

Question

Identify the correct order of structures through which a nerve impulse travels within a neuron.

• Axon
• Dendrites
• Cell body
• Axon terminals

Solution

Dendrites → Cell body → Axon → Axon terminals

At the axon terminals, neurotransmitters carry the signal across the synapse to the next cell.

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Real-World Connection

When you touch a sharp object, sensory neurons immediately send nerve impulses to your spinal cord and brain. In many cases, your spinal cord coordinates a rapid withdrawal reflex before your brain has fully processed the pain. This quick communication helps protect your body from injury while your brain becomes aware of what happened a fraction of a second later.

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Did You Know?

Some of the fastest nerve impulses in the human body travel at speeds of over 100 metres per second—more than 360 kilometres per hour. Myelin, the insulating layer around many axons, allows these electrical signals to travel much faster than they would in unmyelinated neurons.

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Key Terms

Axon – The long extension of a neuron that carries nerve impulses away from the cell body.

Axon terminal – The end of an axon where neurotransmitters are released.

Cell body (soma) – The part of a neuron containing the nucleus and most organelles.

Dendrite – A branch of a neuron that receives incoming signals.

Interneuron – A neuron within the central nervous system that processes information and connects other neurons.

Motor neuron – A neuron that carries impulses from the central nervous system to muscles or glands.

Myelin sheath – A fatty insulating layer that speeds the transmission of nerve impulses.

Neurotransmitter – A chemical messenger released at a synapse that transmits signals to another cell.

Neuron – A specialised nerve cell that transmits electrical impulses.

Nerve impulse – An electrical signal that travels along a neuron.

Sensory neuron – A neuron that carries information from receptors to the central nervous system.

Synapse – The tiny gap between neurons where neurotransmitters transmit signals.

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Key Takeaways

• Neurons are specialised cells that transmit information throughout the nervous system.
• A typical neuron consists of dendrites, a cell body, an axon, a myelin sheath, and axon terminals.
• Nerve impulses are electrical signals that travel along neurons from the dendrites to the axon terminals.
• Signals pass between neurons at synapses using chemical messengers called neurotransmitters.
• Sensory neurons, interneurons, and motor neurons work together to detect stimuli, process information, and coordinate responses.
• The rapid communication provided by neurons allows the body to respond quickly to its environment and maintain homeostasis.