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The nervous system is the part of an animal's body that coordinates its actions and transmits signals to and from different parts of its body.
Nervous tissue first arose in wormlike organisms about 550 to 600 million years ago.
It controls the mouthparts, the salivary glands and certain muscles.
Many arthropods have well-developed sensory organs, including compound eyes for vision and antennae for olfaction and pheromone sensation.
Both autonomic and enteric nervous systems function involuntarily.
In the human brain, it is estimated that the total number of glia roughly equals the number of neurons, although the proportions vary in different brain areas.Most nerves serve both functions and are called mixed nerves.The PNS is divided into a) somatic and b) autonomic nervous system, and c) the enteric nervous system. The autonomic nervous system is further subdivided into the sympathetic and the parasympathetic nervous systems.The sympathetic nervous system is activated in cases of emergencies to mobilize energy, while the parasympathetic nervous system is activated when organisms are in a relaxed state.The enteric nervous system functions to control the gastrointestinal system.