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Grey matter (which is only grey in preserved tissue, and is better described as pink or light brown in living tissue) contains a high proportion of cell bodies of neurons.White matter is composed mainly of myelinated axons, and takes its color from the myelin.Typically, each body segment has one ganglion on each side, though some ganglia are fused to form the brain and other large ganglia.The head segment contains the brain, also known as the supraesophageal ganglion.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.

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Nervous tissue first arose in wormlike organisms about 550 to 600 million years ago.Both autonomic and enteric nervous systems function involuntarily.Nerves that exit from the cranium are called cranial nerves while those exiting from the spinal cord are called spinal nerves.In the insect nervous system, the brain is anatomically divided into the protocerebrum, deutocerebrum, and tritocerebrum.Immediately behind the brain is the subesophageal ganglion, which is composed of three pairs of fused ganglia.

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