The nervous system is the body system thatgathers and stores information. It is in control of the body. The nervoussystem detects and interprets changes in conditions inside and outside the bodyand responds to them. The central nervous system analyses information andinitiates responses; the peripheral nervous system gathers information andcarries the response signals.

Some responses are involuntary; others aredictated by conscious thought. All nervous system activity consists of signalspassed through pathways of interconnected neurons (nerve cells).   The brain and spinal cord form the centralnervous system (CNS), which consists of billions of interconnected neurons (nervecells).

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The input of information to the CNS comes from the sense organs. Motorinstructions are sent out to skeletal muscles, the muscles controlling speech,internal organs and glands, and the sweat glands in the skin. The informationis carried along nerves that fan out from the CNS to the entire body, togethermaking up what is known as the peripheral nervous system (PNS). Each nerve is abundle consisting of the axons (filamentous projections) of many individualneurons.  There are also functional divisions.

Two of themost important are the autonomic nervous system, which is concerned with theautomatic regulation of internal body functions, and the somatic nervous system,controlling the muscles responsible for voluntary movement. The overallfunction of the nervous system is to gather and analyse information about theexternal environment and the body’s internal state and to initiate appropriateresponses aimed at satisfying certain drives. The most powerful drive is forsurvival. Many survival responses, which range from avoiding physical pain anddanger to shivering in response to the cold, are initiated unconsciously andautomatically by the nervous system.

Other drives are more complex, revolvingaround a need to experience positive emotions (such as pleasure andexcitements) and to avoid negative emotions (such as pain, anxiety and frustration).The nervous system functions largely through automatic responses to stimuli,but it can also improve its performance through learning, which relies onmemory. Voluntary actions can be initiated via activity in conscious areas ofthe brain. Certain functions (such as visual perception, memory, thought, andspeech production) are extremely complex and still not fully understood.  The major organ of the nervous system, located inthe cranium, is the brain. The brain receives, sorts, and interprets sensationsfrom the nerves that extend from the central nervous system (brain and spinalcord) to the rest of the body; it initiates and coordinates nerve signalsinvolved in activities such as speech, movement, thought, and emotion.

An adultbrain weighs about 1.4kg and has three main structures: the brain-stem; thecerebellum; and the largest part, the cerebrum, which consists of left and righthemispheres.