Norepinephrine action, synthesis, and pathways:
It should be noted here that norepinephrine and noradrenaline are different names for the same neurotransmitter. The British prefer the term "noradrenaline."Action: As we discuss in ANS—the autonomic nervous system, norepinephrine has multiple roles. First, it relays messages in the sympathetic nervous system, as part of the autonomic nervous system's fight-or-flight response. Secondly, norepinephrine prepares the brain to encounter and respond to stimuli from the environment, thereby facilitating vigilance. So in both roles, norepinephrine mediates arousal.
Synthesis: Neurons in the loci coerulei, a pair of structures located within the pons of the brain stem (see Brain stem structures and the reticular formation), synthesize norepinephrine. You can see the pons in the image to the right, labeled 4. John A. Beal of Louisiana State University provides this image. The term locus coeruleus is derived from the Latin words "caeruleus" and "locus" meaning, literally, "the blue spot" due to the blue appearance of each nucleus. The blue color is the result of melanin, a class of pigments that are derivatives of the amino acid tyrosine. Melanin is also responsible for the dark color of the substantia nigrae.
Pathways: The axons of neurons in the loci coerulei project to both sides of the brain where they release norepinephrine. A single neuron in the locus coeruleus can innervate tissue in wide-ranging areas. The branching axons of norepinephrine-producing neurons in the loci coerulei innervate the brain stem, spinal cord, and cerebellum, as well as the hypothalami, thalamic relay nuclei, amygdalae, and neocortex. Marianne Fillenz provides details of research in her book, Noradrenergic Neurons, suggesting "that terminals in the cerebellum and cortex could be derived from collaterals of the same neuron." The term collaterals here refers to branches of a single neuron's axon.
Epinephrine action, synthesis, and pathways:
The term epinephrine is derived from the Greek roots epi- and nephros, and literally means "on the kidney," in reference to the anatomical location of the adrenal gland. The Latin roots ad- and renes have similar meanings, and give rise to the word "adrenaline," which is the British term for epinephrine. As mentioned earlier, epinephrine is a catecholamine. Many kinds of reactions convert tyrosine to dopamine, to norepinephrine, and eventually to epinephrine.
Action: Epinephrine drives the autonomic nervous system's fight-or-flight response (see ANS—the autonomic nervous system). Epinephrine is synthesized in the adrenal glands (described below) and released into the bloodstream when dangerous circumstances occur, in an emergency requiring immediate action, and in stressful situations or environments. When in the bloodstream, epinephrine rapidly prepares the body for action. It boosts the supply of oxygen and glucose to the brain and muscles while suppressing other non-emergency bodily processes (digestion in particular).
Epinephrine increases heart rate and stroke volume, dilates the pupils, and constricts arterioles in the skin and gastrointestinal tract while dilating arterioles in skeletal muscles. It increases catabolism of glycogen to glucose in the liver, thereby elevating the blood sugar level. At the same time, epinephrine begins the breakdown of lipids in fat cells. Like some other stress hormones, epinephrine has a suppressive effect on the immune system.
Synthesis: The hypothalamus prompts the anterior lobe of the pituitary gland in the brain to release a protein hormone called adrenocorticotropic hormone (ACTH) into the bloodstream. ACTH stimulates the adrenal cortex to release cortisol, which increases the expression of phenylethanolamine N-methyltransferase (PNMT), an enzyme found primarily in what are called chromaffin cells, deep within the adrenal medulla. Within the adrenal medulla, PNMT uses S-adenosylmethionine (SAMe) as a cofactor to convert norepinephrine (noradrenaline) to epinephrine (adrenaline).
It is important to note that ACTH and cortisol are more commonly referred to as "stress hormones." Adverse conditions prompt release of these chemicals that in turn exacerbate many illnesses and negatively affect health in general.