1. The Nervous System

2. Neurons and Nerve Impulses

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.

3. The Brain and Spinal Cord

Introduction

The brain and spinal cord form the Central Nervous System (CNS), the body's main control and communication centre. Together, they receive information from the senses, process that information, make decisions, and coordinate responses. Every movement you make, every thought you have, and every heartbeat you experience depends on the coordinated activity of the brain and spinal cord.

The brain contains billions of neurons that communicate using electrical and chemical signals. The spinal cord acts as the main communication pathway between the brain and the rest of the body, allowing information to travel rapidly in both directions. Together, these structures control both voluntary actions, such as walking, and involuntary processes, such as breathing and heart rate.

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The Central Nervous System

The Central Nervous System (CNS) consists of:

• The brain
• The spinal cord

Its main functions are to:

• Receive sensory information.
• Process information.
• Make decisions.
• Coordinate body responses.
• Maintain homeostasis.

The CNS communicates with the rest of the body through the Peripheral Nervous System (PNS).

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Figure 1. The central nervous system consists of the brain and spinal cord.

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Major Regions of the Brain

The brain has three major regions commonly studied at this level:

• Cerebrum
• Cerebellum
• Brainstem

Each region performs specialised functions.

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The Cerebrum

The cerebrum is the largest part of the brain.

It is responsible for higher mental functions, including:

• Thinking.
• Learning.
• Memory.
• Intelligence.
• Decision-making.
• Language.
• Emotions.

It also:

• Interprets information from the senses.
• Controls voluntary muscle movements.

The cerebrum is divided into two halves called the left and right cerebral hemispheres, which communicate through bundles of nerve fibres.

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The Cerebellum

The cerebellum is located beneath the cerebrum at the back of the brain.

Its main functions are to:

• Coordinate muscle movements.
• Maintain balance.
• Maintain posture.
• Control fine motor skills.

Activities such as writing, cycling, playing musical instruments, and catching a ball all depend on the cerebellum.

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The Brainstem

The brainstem connects the brain to the spinal cord.

It controls many involuntary functions that occur automatically, including:

• Breathing.
• Heart rate.
• Blood pressure.
• Swallowing.
• Digestion.

Because these functions are essential for life, damage to the brainstem can be extremely serious.

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Figure 2. The cerebrum, cerebellum, and brainstem each have specialised roles in controlling the body.

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The Spinal Cord

The spinal cord is a long bundle of nervous tissue that extends from the brainstem down the backbone.

It is protected by the vertebrae of the spinal column.

The spinal cord acts as the main communication pathway between the brain and the rest of the body.

Its functions include:

• Carrying sensory information to the brain.
• Carrying motor commands from the brain.
• Coordinating some reflex actions.

Without the spinal cord, communication between the brain and body would not be possible.

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Processing Sensory Information

The brain receives information from receptors throughout the body.

Examples include:

• Eyes detecting light.
• Ears detecting sound.
• Skin detecting temperature and pressure.
• Nose detecting smells.
• Tongue detecting taste.

The process follows these steps:

1. A receptor detects a stimulus.
2. Sensory neurons carry information to the spinal cord.
3. The spinal cord relays the information to the brain.
4. The brain interprets the information.
5. The brain decides on an appropriate response.
6. Motor neurons carry commands to muscles or glands.

This process often occurs in fractions of a second.

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Figure 3. Sensory information travels to the brain, where it is processed before responses are coordinated.

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How the Brain and Spinal Cord Work Together

The brain and spinal cord communicate continuously.

The Brain

• Processes information.
• Makes decisions.
• Coordinates responses.

The Spinal Cord

• Carries messages between the brain and the body.
• Coordinates some rapid reflex actions.

Example:

When walking:

• The brain plans and controls movement.
• The spinal cord carries instructions to the leg muscles.
• Sensory information from the feet returns through the spinal cord to the brain.
• The brain adjusts movement to maintain balance.

Together, they allow smooth, coordinated movement.

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Reflex Actions

Some responses happen so quickly that the spinal cord coordinates them before the brain becomes fully aware.

Example:

Touching a hot object.

The pathway is:

1. Heat receptors detect the stimulus.
2. Sensory neuron sends an impulse to the spinal cord.
3. An interneuron in the spinal cord processes the information.
4. Motor neuron sends an impulse to the muscles.
5. The hand is withdrawn rapidly.

The brain is informed immediately afterwards.

This rapid response helps prevent injury.

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Figure 4. The spinal cord can coordinate simple reflexes to protect the body from harm.

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Maintaining Homeostasis

The brain, particularly the hypothalamus, helps maintain homeostasis by monitoring internal conditions.

It helps regulate:

• Body temperature.
• Hunger.
• Thirst.
• Blood pressure.
• Breathing rate.
• Heart rate.
• Sleep-wake cycles.

The brain coordinates responses through the nervous and endocrine systems to keep the body's internal environment stable.

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Why the Brain and Spinal Cord Are Important

Together, the brain and spinal cord allow the body to:

• Sense the environment.
• Think and learn.
• Control movement.
• Coordinate organs.
• Maintain homeostasis.
• Protect itself through reflexes.

They act as the body's central communication and control system.

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Figure 5. The brain and spinal cord work together to coordinate body functions and maintain homeostasis.

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Worked Example

Question

Match each part of the central nervous system to its main function.

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Real-World Connection

Learning to ride a bicycle shows how different parts of the brain work together. The cerebrum helps you think about what to do, the cerebellum coordinates your balance and precise muscle movements, and the brainstem automatically controls breathing and heart rate while you ride. At the same time, the spinal cord carries messages between your brain and your muscles, allowing you to steer, pedal, and stay balanced.

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Did You Know?

The human brain contains about 86 billion neurons, connected by hundreds of trillions of synapses. These connections constantly change as you learn new skills, form memories, and adapt to new experiences—a remarkable ability known as neuroplasticity.

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Key Terms

Brainstem – The part of the brain that connects to the spinal cord and controls many involuntary functions.

Central nervous system (CNS) – The brain and spinal cord.

Cerebellum – The part of the brain responsible for balance, coordination, and fine motor control.

Cerebrum – The largest part of the brain, responsible for thinking, memory, learning, and voluntary movement.

Hypothalamus – A region of the brain that regulates many homeostatic processes.

Peripheral nervous system (PNS) – All nerves outside the brain and spinal cord.

Reflex – A rapid, automatic response to a stimulus.

Spinal cord – A bundle of nervous tissue that carries messages between the brain and the rest of the body.

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Key Takeaways

• The brain and spinal cord together form the Central Nervous System (CNS).
• The cerebrum controls thinking, memory, learning, and voluntary movement.
• The cerebellum coordinates balance, posture, and fine muscle movements.
• The brainstem controls vital involuntary functions such as breathing and heart rate.
• The spinal cord carries information between the brain and the rest of the body and coordinates some reflexes.
• The brain processes sensory information, coordinates responses, and works with the spinal cord to control body functions and maintain homeostasis.

4. The Endocrine System

5. Hormones and Homeostasis