The nervous and endocrine systems are the two major regulatory systems of the body, and together they regulate and coordinate the activity of essentially all other body structures. The nervous system functions something like telephone messages sent along telephone wires to their destination. It transmits information in the form of action potentials along the axons of nerve cells. Chemical signals in the form of neurotransmitters are released at synapses between neurons and the cells they control. The endocrine system is more like radio signals broadcast widely that everyone with radios tuned to the proper channel can receive. It sends information to the cells it controls in the form of chemical signals released from endocrine glands. The chemical signals are carried to all parts of the body by the circulatory system. Cells that are able to recognize the chemical signals respond to them and other cells do not.
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