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The Brain


The Manual's Editorial Staff

Reviewed/Revised May 2022 | Modified Sep 2022
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What is the brain?

  • Your brain is like the central processing unit in a computer

The brain receives information from your eyes, ears, nose, and other sense organs. It processes information, generates thoughts and ideas, and sends messages to your body. For example, it tells your muscles how to move so you can walk, talk, and do the things you want your body to do. Your brain also controls a lot of what your body does without you thinking about it. For example, your brain automatically adjusts your breathing, heart rate, and blood pressure.

  • You need your brain to be able to move, speak, taste, smell, hear, and see

  • The brain needs a lot of blood and oxygen to work—at every moment, about 20% of the blood pumped by your heart goes to the brain

You'll pass out if blood flow to your brain stops for more than 10 seconds.

Viewing the Brain

The brain consists of the cerebrum, brain stem, and cerebellum. Each half (hemisphere) of the cerebrum is divided into lobes.

Viewing the Brain

What are the parts of the brain?

The brain sits inside your skull surrounded by fluid that cushions and supports it. The fluid is called cerebrospinal fluid. It is held around your brain inside 3 layers of membranes called the meninges.

The brain has 3 main parts:

  • Cerebrum: The main upper part of your brain that is divided into parts called lobes—these lobes control thought, movement, speech, memory, emotions, and all your senses

  • Brain stem: The very bottom part of your brain that connects with the spinal cord—the brain stem controls critical body functions, such as consciousness, breathing, blood pressure, and heartbeat

  • Cerebellum: A separate part of your brain just above your brain stem that controls balance and coordination

Your cerebrum has two halves, one on the left and one on the right. The left half controls the right side of your body and vice versa. That's why when someone has a stroke in the left side of their brain, they can't move the right side of their body.

Tissues Covering the Brain

Within the skull, the brain is covered by three layers of tissue called the meninges.

Tissues Covering the Brain

How does the brain work?

Your brain is made of:

  • Nerve cells and their fibers

There are billions and billions of nerve cells in your brain.

Each nerve cell has a microscopic cell body:

  • The body of the nerve cell is responsible for processing nutrients and keeping the cell alive

Each nerve cell body has fibers going to and from it:

  • Input fibers receive signals from other nerve cells or from receptors in your sense organs

  • Output fibers send signals to other nerves in your brain or spinal cord

  • Signals travel only one way in a nerve cell

Although nerve fibers and their signals act a lot like a wire carrying electrical signals, that's not exactly right. Nerve cells really send their signals using chemicals.

  • Chemical changes take place progressively along the length of a nerve fiber

  • When the chemical changes reach the end of the nerve fiber, they release other chemicals called neurotransmitters

  • The neurotransmitters drift across a microscopic gap where they hit the chemical receptors of another cell

  • The neurotransmitters trigger chemical changes in that other cell

  • If that cell is a nerve cell, then the progressive chemical changes continue down the fibers of that cell to pass the signal along

One nerve cell sends just one kind of signal that can't carry a lot of information. However, when billions of nerve cells are interconnected like they are in your brain, they form a very powerful information processor.

What can go wrong with the brain?

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