Brain

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A human brain
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A human brain

The brain is a part of the body that allows us to make sense of the world around us and to change our behavior in response to it. In most animals, the brain is inside the head. The brain is kept safe by the skull and by layers of tissue under it called the meninges.

The brain does the thinking, learning, and feeling for the body. It is the source of consciousness, but it also controls basic body functions, like breathing and beating of the heart, that happen without you realizing it. All the information about the world gathered by our five senses is sent through nerves into the brain, allowing us to see, hear, smell, taste and feel things. The brain processes this information so that we can experience it as pictures, sounds, and so on. The brain also uses nerves to tell the body what to do, for example by telling muscles to move or our heart to beat faster.

In human beings, the brain is made of three main parts: the cerebrum, the cerebellum and the brainstem. A human brain accounts for about 2% of the body's weight, but it uses about 20% of its energy. It has about 50-100 billion (one billion = 1,000,000,000) nerve cells (also called neurons), along with at least 10 times as many support cells, called glia. The job of neurons is to receive and send information to and from the rest of the body, while glia provide nutrients and divert blood flow to the neurons, allowing them to do their job. Each nerve cell has contact with as many as 10,000 other nerve cells through connections called synapses.

The volume of the human brain (relative to the size of the whole body) is very large, compared to that of most other animals. The human brain also has a very large surface (called cortex) for its size, which is possible because it is very wrinkled. If the human cortex were flattened and spread out, it would be close to a square meter in area. Some other animals also have very wrinkled brains, such as dolphins and elephants. Some people believe that what makes humans so smart is the large volume or surface area of their brains, but there is no evidence for a relationship among humans between size of the brain and intelligence.

See also: Spinal cord

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