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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.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 is difficult to believe that until approximately year 1900 it was not known that neurons are the basic units of the brain (Santiago Ram? Equally surprising is the fact that the concept of chemical transmission in the brain was not known until around 1930 (Henry Hallett Dale) and (Otto Loewi).We began to understand the basic electrical phenomenon that neurons use in order to communicate among themselves, the action potential, in the decade of 1950 (Alan Lloyd Hodgkin, Andrew Huxley and John Eccles).A protoplasmic fiber runs from the cell body and branches profusely, with some parts transmitting signals and other parts receiving signals.
Immediately behind the brain is the subesophageal ganglion, which is composed of three pairs of fused ganglia.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.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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The nervous system derives its name from nerves, which are cylindrical bundles of fibers (the axons of neurons), that emanate from the brain and spinal cord, and branch repeatedly to innervate every part of the body.