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.