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The Autonomic Nervous System The peripheral nervous system consists of the somatic nervous system(SNS) and the autonomic nervous system (ANS). The SNS consists of motor neurons that stimulate skeletal muscles. In contrast, the ANS con-sists of motor neurons that control smooth muscles, cardiac muscles, andglands. In addition, the ANS monitors visceral organs and blood vesselswith sensory neurons, which provide input information for the CNS. The ANS is further divided into the sympathetic nervous system and theparasympathetic nervous system. Both of these systems can stimulate andinhibit effectors. However, the two systems work in opposition—where onesystem stimulates an organ, the other inhibits. Working in this fashion, eachsystem prepares the body for a different kind of situation, as follows. The sympathetic nervous system prepares the body for situationsrequiring alertness or strength or situations that arouse fear, anger,excitement, or embarrassment (“fight-or-flight” situations). In thesekinds of situations, the sympathetic nervous system stimulates cardiacmuscles to increase the heart rate, causes dilation of the bronchiolesof the lungs (increasing oxygen intake), and causes dilation of bloodvessels that supply the heart and skeletal muscles (increasing bloodsupply). The adrenal medulla is stimulated to release epinephrine(adrenalin) and norepinephrine (noradrenalin), which in turn increasesthe metabolic rate of cells and stimulate the liver to release glucose intothe blood. Sweat glands are stimulated to produce sweat. In addition,the sympathetic nervous system reduces the activity of various “tran-quil” body functions, such as digestion and kidney functioning.