Lecture Outline #14: Catecholamines and Neurohormones
I. ANATOMY OF THE AUTONOMIC NERVOUS SYSTEM (Fig. 14.2)
1. Catecholamines (i.e., epinephrine and norepinephrine) are the principal effector hormones of the sympathetic portion of the autonomic nervous system.
2. The sympathetic is thoraco-lumbar and the parasympathetic is cranio-sacral.
3. These nerves supply the visceral organs, the skin, and other tissues.
4. Their actions are mainly vasomotor and secretomotor in function. That is to say, they innervate the vascular and visceral smooth muscle and cardiac muscle cells, and they innervate salivary, adrenal, pancreatic and other glandular tissues.
5. Point out the preganglionic and postganglionic neurons of the two autonomic subsystems.
6. Acetylcholine [acetylcholine] is the neurohormone secreted by all neurons in the autonomic system, except for the postganglionic neurons of the sympathetic system.
7. Norepinephrine [norepinephrine] (and some epinephrine) is secreted by the sympathetic postganglionic neurons.
8. In general, the parasympathetic system is associated with resting processes, while the sympathetic system is associated with exciting processes.
II. THE SYNTHESIS OF CATECHOLAMINES
1. The synthesis of catecholamines starts with the amino acids phenylalanine and/or tyrosine in the post-ganglionic neurons, the central nervous system (CNS), adrenal medulla (mainly chromaffin cells) [chromaffin], and other sites where they are produced. (Fig. 14.3)
2. The main metabolic step that limits their synthesis is the conversion of tyrosine to dopa, which is regulated by tyrosine hydroxylase.
3. However, in the chromaffin cells (which are actually sympathetic postganglionic neurons) of the adrenal medulla, catecholamine synthesis is also effected by phenylethanolamine N-methyltransferase (PNMT), which converts norepinephrine into epinephrine.
4. The chromaffin cells that are anatomically situated closer to the adrenal cortex (surrounding the medulla) produce more epinephrine because the glucocorticoid hormones from the cortex promote the transcription and translation of PNMT in the outer medullary cells.