The Brain and Nervous System

The body's master control system

Think about everything happening in your body right now. Your heart is beating, your lungs are breathing, your eyes are reading these words, and your brain is turning them into ideas. Something has to coordinate all of this, faster than you can blink. That something is your nervous system — the body's master control and communication network.

The nervous system does two huge jobs. First, it gathers information about what is happening inside and outside your body, using your senses. Second, it sends out instructions to make your body respond. It does both with astonishing speed, using signals that travel as fast as 100 metres every second. To understand how, we need to start with the remarkable cell that makes it all possible.

Neurons: the messengers

The nervous system is built from billions of special cells called neurons (nerve cells). A neuron is shaped for one job: carrying messages. It has:

• a cell body containing the nucleus,
• branching dendrites that receive signals from other neurons,
• a long, thin fibre called an axon that carries the signal away to the next cell.

A neuron passes its message as a tiny electrical signal that races along the axon. But neurons do not actually touch each other. Between the end of one neuron and the start of the next is a microscopic gap called a synapse. When the electrical signal reaches the gap, the neuron releases chemicals called neurotransmitters that float across and trigger the next neuron to fire. So a message hops along a chain of neurons in a mix of electrical and chemical steps.

Your brain alone has around 86 billion neurons, and each one can connect to thousands of others. This creates trillions of connections — a network more complicated than any computer ever built. Learning a new skill actually changes these connections, a property scientists call neuroplasticity.

Two main parts of the nervous system

The whole nervous system is organised into two parts that work together.

1. The central nervous system (CNS). This is the control centre: your brain and your spinal cord. The brain makes decisions and stores memories; the spinal cord is the main cable that connects the brain to the rest of the body. Both are so important that they are protected by bone — the skull guards the brain and the vertebrae of your skeleton guard the spinal cord — and cushioned by fluid.

2. The peripheral nervous system (PNS). This is the vast web of nerves that branches out from the spinal cord to every part of the body. These nerves are like the body's wiring. Sensory nerves carry information in to the CNS (for example, "this kettle is hot"), and motor nerves carry instructions out to the muscles and glands (for example, "move your hand now").

Together, these two systems let your brain stay in constant two-way conversation with your whole body.

Inside the brain

Your brain is not a single blob doing one job. It is divided into regions, each specialising in different tasks. The three you should know are:

• The cerebrum. This is the large, wrinkled top part, and it is the centre of thinking, memory, language, the senses and voluntary movement. The folds and wrinkles let a huge surface area pack into your skull. The cerebrum is split into left and right halves, and different areas handle different jobs, such as seeing, hearing or deciding.

• The cerebellum. Tucked underneath the back of the cerebrum, the cerebellum coordinates movement and balance. It does not decide to move your arm, but it makes the movement smooth, accurate and steady. When you ride a bike or write neatly, you are using your cerebellum.

• The brainstem. Sitting at the base, connecting to the spinal cord, the brainstem controls the automatic jobs that keep you alive without thinking — your heartbeat, breathing, and digestion. It works day and night, even while you sleep. Notice that this is why deep sleep does not stop your breathing or heartbeat.

This division of labour means the brain can think deeply and keep you alive and move you smoothly, all at once.

Voluntary and automatic control

Some actions you control on purpose. Deciding to pick up a pen is a voluntary action: your cerebrum sends a signal through motor nerves to the right muscles. You are in charge.

Other actions are automatic (or involuntary). You do not decide to digest your lunch, sweat when hot, or beat your heart faster when you run. The nervous system handles these in the background through a branch called the autonomic nervous system. This is why, when you exercise, your breathing and heartbeat speed up without you choosing to — the system adjusts automatically to give your muscles more oxygen, showing how all the body's systems cooperate.

Reflexes: the body's emergency shortcut

Some situations are too dangerous to wait for the brain to think. If your hand touches something burning hot, you pull it back before you even feel the pain. How? Through a reflex.

In a reflex, the signal does not make the full trip up to the brain and back. Instead, it takes a shortcut through the spinal cord, which sends the "pull away!" command straight back to your muscles. This path is called a reflex arc:

1. A sensor in your skin detects danger (heat or pain).
2. A sensory neuron carries the signal to the spinal cord.
3. The spinal cord instantly sends a command out through a motor neuron.
4. Your muscle contracts and pulls your hand away.
5. Only then does the message reach your brain, so you become aware of what happened.

By skipping the brain, a reflex saves precious fractions of a second — enough to protect you from serious harm. The knee-jerk reflex a doctor tests with a small hammer works the same way.

Looking after your nervous system

Because the nervous system controls everything, protecting it matters. Wearing a helmet when cycling or skating guards your brain. Plenty of sleep lets the brain repair itself and lock in memories. Good food, especially with healthy fats and water, keeps neurons working well. And learning and practising new skills builds stronger connections, keeping your brain sharp.

Try it: measure your reaction time

This activity lets you measure how fast signals travel through your nervous system.

1. Work with a partner. Have them hold a 30 cm ruler vertically, with the zero end pointing down, just above your open hand.
2. Hold your thumb and finger ready on either side of the zero mark, without touching the ruler.
3. Without warning, your partner lets the ruler drop. Catch it as fast as you can by pinching your fingers shut.
4. Read the centimetre mark where you caught it, just above your thumb. The smaller the number, the faster you reacted. Record it.
5. Repeat several times and take an average. Then test other people, or try it when tired versus alert.

Why it works: The distance the ruler falls before you catch it depends on your reaction time — the time for your eyes to see it drop, for the signal to reach your brain, for your brain to decide, and for the command to reach your hand muscles. A shorter catch distance means signals zipped through your nervous system more quickly. You will likely find that practice improves your time, and that tiredness slows it down — real evidence of your nervous system at work.