Multicellularity in animal’s necessiated the development of some system for the control and coordination of various cellular activities of the body. All living organisms have characteristic ability to respond to the external and the internal stimuli. The maintenance of a steady state in animals (homoeostasis) is achieved by two mechanisms; the nervous coordination and the hormonal coordination. The nervous coordination is immediate and fast however the hormonal coordination is low but long lasting.
The bodily activities, whether voluntary or involuntary are controlled and coordinated by the nervous system. It is like a computer having an input, a processing unit and output elements.
The nervous system is derived from the embryonic ectoderm. Basically it is similar in all vertebrates. The human nervous system is mostly advanced, complex and also most studied. Although integrated; for study purpose, it is divided in three division:
Central Nervous System (CNS): It is mid-dorsal and hollow system consisting of brain and spinal cord well protected in the skull and vertebral column.
Peripheral Nervous System (PNS): It is formed by the nerves arising from or ending into the brain and the nerves arising from the spinal cord. The nerves may be myelinated or non-myelinated.
Autonomous Nervous System (ANS): This nervous system controls internal environment or working of visceral organs. It gives response to internal receptors and it is involuntary except micturition.