involuntary nervous system

(in-VAH-lun-TAYR-ee NER-vus SIS-tem) The part of the nervous system that controls muscles of internal organs (such as the heart, blood vessels, lungs, stomach, and intestines) and glands (such as salivary glands and sweat glands). One part of the involuntary nervous system helps the body rest, relax, and digest food and another part helps a person fight or take flight in an emergency. Also called ANS and autonomic nervous system.