Nervous system is the system consisting of the cells, tissues and organ and controls the adjustment of the body to its environment. All the organisms possess nervous system with different degrees of complexity. Complexity of nervous system in invertebrates is varying whereas in case of vertebrates the complexity of nervous system reaches the peak.

Autonomic and peripheral are the two types of nervous system found in vertebrates comprising brain, nerves, spinal cord, ganglia and different parts of effector and receptor organs. Brain is the centre of the nervous system and there are 200billion neurons in the human nervous system half of which are found in the brain.

Nervous systems monitor and retain the steady external and internal environment in animals in coordination with the endocrine system of the body in multicellular organisms. The nervous is system is to receive sensory stimuli from the external and internal environments; integrating the input and responding to the stimuli and thus controlling and maintaining he body’s adjustment to the environment.