The Nervous system receives sensory input from the internal and external environment. It then integrates the input in the sensory centers of the brain and responds to the various stimuli with a motor output, which can be converted by the organs to some form of energy ie. movement, release of hormones, changes in heart rate. The response is not always conscious, for instance if you were climbing a mountain and the air became thinner, you would start breathing deeper and quicker. Was this response voluntary? Of course not, the nervous system assessed the situation and responded accordingly.
The Endocrine System
Some animals have a second response system, the endocrine system. The nervous system controls fast responses to external stimuli, while the endocrine system controls longer lasting responses to internal stimuli; for instance a virus. The endocrine system would fight the viral cells until they're destroyed.
Divisions of the Nervous System
The nervous system controls almost every organ in the body through a series of positive and negative feedback loops.
The central nervous system consists of the brain and the spinal cord. The peripheral nervous system connects the central nervous system to the rest of the organs through networks of nerves. Nerves are made up of bundles of neurons.
Not all animals have a highly specialized nervous system. Those with more simplified systems are usually small and very mobile or large and immobile. The evolution of nervous systems must have been an important adaptation in the evolution of body size and mobility in different creatures.
Some creatures have neurons that are woven into a nerve "net". This net substitutes a brain. These creatures have radial symmetry, and do not have a defined head or tail region. Even though they have no brain, creatures with a nerve net...