ABSTRACT
There are two components to the sympathetic nervous system: neural and hormonal. The most rapid response is neural. Centers in the diencephalon (the hypothalamus, primarily) are stimulated by the frightening event, and therefore, the sympathetic pathways in the spinal cord and then the nerves of the sympathetic chain are stimulated. The neurotransmitters released by the sympathetic nerves are norepinephrine and epinephrine (adrenaline and noradrenaline are alternative names). The structures innervated by the sympathetic nerves are the blood vessels, the hair follicles, the heart and lungs, and the gastrointestinal tract. The action on the gastrointestinal tract is primarily negative: Secretion and motility are inhibited. The actions on the heart are to increase the frequency and strength of contraction and to dilate the bronchioles of the lungs. The pupils of the eyes dilate. The hair stands on end (piloerection).
