The nervous system is the body system that
gathers and stores information. It is in control of the body. The nervous
system detects and interprets changes in conditions inside and outside the body
and responds to them. The central nervous system analyses information and
initiates responses; the peripheral nervous system gathers information and
carries the response signals. Some responses are involuntary; others are
dictated by conscious thought. All nervous system activity consists of signals
passed through pathways of interconnected neurons (nerve cells).  


The brain and spinal cord form the central
nervous system (CNS), which consists of billions of interconnected neurons (nerve
cells). The input of information to the CNS comes from the sense organs. Motor
instructions are sent out to skeletal muscles, the muscles controlling speech,
internal organs and glands, and the sweat glands in the skin. The information
is carried along nerves that fan out from the CNS to the entire body, together
making up what is known as the peripheral nervous system (PNS). Each nerve is a
bundle consisting of the axons (filamentous projections) of many individual


There are also functional divisions. Two of the
most important are the autonomic nervous system, which is concerned with the
automatic regulation of internal body functions, and the somatic nervous system,
controlling the muscles responsible for voluntary movement. The overall
function of the nervous system is to gather and analyse information about the
external environment and the body’s internal state and to initiate appropriate
responses aimed at satisfying certain drives. The most powerful drive is for
survival. Many survival responses, which range from avoiding physical pain and
danger to shivering in response to the cold, are initiated unconsciously and
automatically by the nervous system. Other drives are more complex, revolving
around a need to experience positive emotions (such as pleasure and
excitements) 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 on
memory. Voluntary actions can be initiated via activity in conscious areas of
the brain. Certain functions (such as visual perception, memory, thought, and
speech production) are extremely complex and still not fully understood.


The major organ of the nervous system, located in
the cranium, is the brain. The brain receives, sorts, and interprets sensations
from the nerves that extend from the central nervous system (brain and spinal
cord) to the rest of the body; it initiates and coordinates nerve signals
involved in activities such as speech, movement, thought, and emotion. An adult
brain weighs about 1.4kg and has three main structures: the brain-stem; the
cerebellum; and the largest part, the cerebrum, which consists of left and right