The normal function of the<unk>  is to help the body regulate responses to environmental stimuli. These stimuli travel through the sympathetic nervous system by means of preganglionic nerve fibers that emerge from the thoracic spinal cord. Electrical impulses carried by the sympathetic nervous system are converted to a chemical response in the adrenal gland. Chromaffin cells contained in the adrenal medulla act as postganglionic nerve fibers that release this chemical response into the blood as a circulating messenger. The<unk>  can activate and discharge chemical messengers as a single unit to activate an organism's “fight or flight” response. This<unk>  discharge” causes an increase in heart rate, cardiac output, blood pressure, and glucose levels. These<unk>  functions show the combined responses of the central nervous system on a multitude of external stimuli. Chemical messengers The two main chemical messengers of the<unk>  are norepinephrine and epinephrine (also called noradrenaline and adrenaline respectively). These chemicals are created by the adrenal glands after receiving neuronal signals from the sympathetic nervous system. The different physiological effects of these chemicals depend on the particular tissue that it innervates. As part of the<unk> , these chemicals act rapidly and dispel quickly as opposed to the longer-lasting effect of hormones.
Sympathoadrenal system
Hypothalamic–pituitary–adrenal axis
Hypothalamic–pituitary–somatotropic axis
Hypothalamic–pituitary–thyroid axis
Parasympathetic nervous system
Hypothalamic–pituitary–gonadal axis
Autonomic nervous system
Hypothalamic–pituitary–prolactin axis