The human brain is an amazing organ. It controls memory and learning, the senses (hearing, sight, smell, taste, and touch), and emotion. It also controls other parts of the body, including muscles, organs, and blood vessels.

The brain also is a very complex structure. It contains billions of nerve cells -- called neurons -- that must communicate and work together for the body to function normally. The neurons communicate through electrical signals. Special chemicals, called neurotransmitters, help move these electrical messages from neuron to neuron.

Information is fed into the brain through the senses. What is heard, felt, tasted, seen, or smelled is detected by receptors in or on the body and sent to the brain through sensory neurons. The brain decides what to do with the information from the senses and tells the body how to respond by sending out messages via motor neurons. For example, if a person puts his or her hand near something hot, the sense of touch tells the brain about the heat, and the brain sends a message to the muscles of the arm to move the hand away. Another type of neuron -- called interneurons -- connects various neurons within the brain and spinal cord, which together make up the central nervous system.

Just as there are different types of neurons, there are also different types of chemical neurotransmitters. Researchers studying mental illness believe an imbalance of these chemicals -- in particular, brain circuits -- contributes to the development of many mental illnesses. This chemical imbalance makes it difficult for messages to move from neuron to neuron and for brain circuits to function normally. As a result, the brain may "misunderstand" what the senses are telling it and/or may not send the right response messages back to the body. In turn, a person may develop symptoms of mental illness.

Researchers also believe that changes in size or shape of different parts of the brain may be responsible for causing some mental illnesses.