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Sensory nerve cells feed information to the brain from every part of the body, external and internal. The brain evaluates the data, then sends directives through the motor nerve cells to muscles and glands, causing them to take suitable action. Alternatively, the brain may inhibit action, as when a person tries not to laugh or cry, or it may simply store the information for later use. Both incoming information and outgoing commands traverse the brain and the rest of the nervous system in the form of electrochemical impulses.
The human brain consists of some 10 billion interconnected nerve cells with innumerable extensions. This interlacing of nerve fibers and their junctions allows a nerve impulse to follow any of a virtually unlimited number of pathways. The effect is to give humans a seemingly infinite variety of responses to sensory input, which may depend upon experience, mood, or any of numerous other factors. During both sleep and consciousness, the ceaseless electrochemical activity in the brain generates brain waves that can be electronically detected and recorded (see electroencephalography).
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