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The human brain is the center of the Nervous System and controls everything. Through the nervous and endocrine system, the brain controls all the voluntary and involuntary body functions such as homeostasis, metabolism, reactions, etc. Additionally, brain "produces mind" and is associated with all cognitive functions such as thinking, memory, speech, mood, etc (Christidis, 2001).
The Nervous System consists of the Central Nervous System and Peripheral Nervous System. The role of the nervous system is to perceive the changes that occur in the inside or outside environment, process this information as regards to their importance for the survival of the body and to react accordingly.
 The Central Nervous System (CNS) includes the brain and spinal cord
CNS = Brain + Spinal cord  
On the other hand the peripheral nervous system (PNS)   consists of:
 1. Body (Intentional) Nervous System and the
 2. Autonomic Nervous System.

PNS = Body + Autonomic Nervous System
The Somatic Nervous System or the voluntary nervous system controls the senses and drives us.
The Autonomic Nervous System (ANS) controls all the automatic body functions involuntary   such as heart function, respiration, digestion, etc. and includes:
  1. The Sympathetic Nervous System (SNS) and
  2. The Parasympathetic Nervous System. (PNS)
ANS = Sympathetic and Parasympathetic Nervous System
The Autonomic Nervous System (ANS) is involved in the appearence of every model of behavior and every communication with the environment. The ANS is responsible for the maintaining of the internal homeostasis of the body.
The Sympathetic Nervous System (SNS) is responsible for the arousal of all systems of the body (apart from three that stimulate are stimulated by PNS). It also prepares the body for taking strong activity (emergency, "flight or fight" responce), causing increase of the heart rate and the rate of breathing, and reduce of the digestive function.
The Parasympathetic Nervous System (PNS) is responsible for the mobilization and the proper functioning of the reproductive, digestive and immune system. Also the PNS contributes to the relaxation of all other systems.