Autonomic Nervous System 2 & 3: Physiology & Molecular Mechanisms Margaret C. Biber, D. Phil. OBJECTIVES: Please note that these objectives pertain to ANS lectures I-IV. At the end of these lectures you should know and understand the following material: 1. The relationship between the organization of the sympathetic and parasympathetic divisions of the ANS to their overall physiological effects. 2. Anatomical and functional differences between the skeletal neuromuscular junction and autonomic neuroeffector junctions. 3. Transmitters used at ganglionic and neuroeffector junctions and highlights of the transmitter life cycle: Storage, release, biological inactivation, metabolism and de novo synthesis for acetylcholine (Ach), norepinephrine (NE) and the hormone, epinephrine (EPI). 4. Receptor types for Ach and the catecholamines, NE and EPI and their effects. 5. The mechanism of action of other transmitters/mediators including ATP, NO and peptides. 6. The organization of autonomic reflexes. 7. The overall physiological effects of the parasympathetic and sympatho-adrenal systems and the receptor types that mediate the responses. Reading: Berne, Levy, Koeppen and Stanton: Physiology, 5th edition. 2004; Ch. 11, Pages 206-215 –Table 11-1 is too detailed. Use Table in handout. Costanzo: Physiology, 2006 Ch. 2, Pages 45-64 Note: Please follow the version in the handout wherever discrepancies exist between the textbooks and the handout. LECTURES II & III OUTLINE COMPARISON OF SYMPATHO-ADRENAL & PARASYMPATHETIC Sympatho-adrenal: diffuse targets expenditure of energy Parasympathetic: discrete targets conservation of energy MOLECULAR BASIS FOR ACTIONS OF ANS: TRANSMITTERS: Identity & sites of release Life cycle of ACh Life cycle of NE in sympathetic varicosity Evoked release of small transmitters and peptides Adrenal medulla: storage, release, synthesis and metabolism Pheochromocytoma RECEPTORS Cholinergic Nicotinic & Muscarinic Parasympathetic effects Adrenergic Alpha & Beta Sympatho-adrenal effects Summary of effects of sympathoadrenal and parasympathetic on target tissues (Table 1) COMPARISON OF SYMPATHETIC AND PARASYMPATHETIC The ANS maintains man and adapts him to his environment but the sympathetic and parasympathetic nervous systems have rather different roles: • The sympathetic nervous system mobilizes the body for activity and, in the extreme case, in conjunction with the adrenal gland, allows the body to handle threatening situations (FIGHT or FLIGHT). • The parasympathetic plays a restorative role and conserves energy. • Differences exist in the targets of these two systems: The sympathetic innervates widely distributed systems. The parasympathetic exerts a more discrete control. It is easier to remember the actions of these two systems at the level of individual organs and tissues if one first has an understanding of their overall function and can then relate this to their organization. SYMPATHETIC NERVOUS SYSTEM Diffuse Target Tissues • sweat glands • smooth muscle of blood vessels supplying skeletal muscle, skin • smooth muscle of hair follicles In man, these target tissues do not have any parasympathetic innervation. The sympathetic has excitatory actions on these tissues. Consequently, it regulates: • blood pressure (blood vessels supplying the skeletal muscle are especially important; the ANS innervation to heart also contributes) • the distribution of blood among tissues and within tissues • body temperature (cutaneous blood vessels; sweat glands) Figure 1. AUTONOMIC NERVOUS SYSTEM STRESS RESPONSE Together with the adrenal gland, the sympathetic NS mobilizes the body to handle threatening situations -- the fight or flight reaction. The effects of the sympatho- adrenal enable the body to undertake severe physical exertion. The activity of organs that are nonessential or counterproductive is inhibited. Targets of sympatho-adrenal action include: • Cardiovascular system: o redistribution of blood, e.g., flow of blood to the skin and mesentery is dramatically reduced, flow to skeletal muscle is enhanced. o increase in cardiac output • Respiratory system: o relaxation of muscles of trachea and bronchi • Digestive system: o inhibition of motility and secretions • Metabolism: o mobilization of glucose, o increased lipolysis, o increase in basal metabolic rate PARASYMPATHETIC NERVOUS SYSTEM Conservation/Replenishment of Energy Supplies, Maintenance of the Organism. Discrete Control of Individual Target Tissues Examples of parasympathetic regulation include: • excitatory effects on the gastrointestinal tract (increases motility and secretory activity in sequential fashion as digestion proceeds) • stimulation of glandular secretions (but sympathetic controls sweat glands) • slowing of the heart • control of pupil diameter by the pupillary light reflex (regulates the amount of light falling on the retina) • accommodation of the lens for near vision • voiding the urinary bladder (micturition) SUMMARY OF SYMPATHO-ADRENAL AND PARASYMPATHETIC EFFECTS BY TARGET TISSUE HEART : sympatho-adrenal has excitatory effects: it increases the rate of beating and the force of contraction parasympathetic is inhibitory, slowing heart rate SMOOTH sympatho-adrenal can excite or inhibit smooth muscle MUSCLE: (adrenal medulla relaxes bronchial smooth muscle; sympathetic constricts vascular smooth muscle). parasympathetic excites most of the smooth muscle it innervates (e.g GI tract; urinary bladder) GLANDS: parasympathetic stimulates glandular secretions (sympathetic stimulates sweat glands) METABOLIC EFFECTS: mediated by the sympatho-adrenal system To understand the basis for the different actions of the sympathetic and parasympathetic NS on their target organs requires an understanding of the molecular mechanisms that mediate these effects, including the transmitters released at the neuroeffector junction, and the receptors with which they interact. Figure 2. AUTONOMIC NERVOUS SYSTEM: TRANSMITTERS MOLECULAR BASIS FOR PHYSIOLOGICAL ACTIONS OF ANS TRANSMITTERS AND SITES OF RELEASE (FIG. 2) Acetylcholine (Ach) • Ganglion: Terminals of both parasympathetic and sympathetic preganglionic nerves release ACh • Neuro-Effector Junction: Terminals of parasympathetic postganglionic nerves release ACh onto the target effector tissues Terminals of sympathetic postganglionic nerves that supply the sweat glands that cover the body (generally distributed sweat glands) also release ACh. Norepinephrine (NE) • Neuro-Effector Junction: Terminals of sympathetic postganglionic nerves release the catecholamine, NE onto target effector tissues, (except in the case of generally distributed sweat glands). HORMONE RELEASE from adrenal medulla Epinephrine (EPI), a catecholamine (CA) closely related to NE, is the principal CA released into the blood stream along with small amounts of NE. The methyl substituent on the amine group accounts for EPI’s characteristic properties that distinguish it from NE. LIFE CYCLE OF TRANSMITTERS/HORMONES: ACETYLCHOLINE • ACh is stored in small clear (agranular) vesicles that also contain high concentrations of ATP (FIG 3). • A few large dense cored vesicles that store peptides such as vasoactive intestinal peptide (VIP) are also present. Peptide is released with high frequency stimulation and augments the effects of Ach on the target organ. • Released ACh interacts with receptors and is rapidly destroyed within msecs of its release through metabolic breakdown by acetylcholinesterase (FIG 3), one of the most rapidly acting enzymes in the body. • Inhibition of acetylcholinesterase potentiates and prolongs the effects of ACh in the ANS. • ACh supplies are replenished by choline acetyltransferase from choline and acetylCo A. There is a sodium dependent choline uptake mechanism into the nerve terminal. Figure 3. CHOLINERGIC NERVE VARICOSITY NOREPINEPRINE (FIG 4) • NE in sympathetic postganglionic nerve terminal varicosities is stored mostly in small dense cored vesicles that contain NE, ATP and dopamine beta hydroxylase (converts dopamine to NE)(FIG 4). Some large dense cored vesicles contain enkephalin as well as all the other components. • A separate population of large dense cored vesicles stores peptides such as neuropeptide Y that is released with high frequency stimulation and enhances the effect of NE. • Released NE is biologically inactivated by reuptake by a sodium dependent transporter molecule present in the membrane of postganglionic sympathetic nerve terminal varicosities. • Once inside the sympathetic nerve terminal, NE is either taken up into a storage vesicle for subsequent release or metabolized. • Inhibition of reuptake (e.g. with cocaine) potentiates the effects of NE. • Supplies of NE are maintained by reuptake and by synthesis of new transmitter (see below). Figure 4. POSTGANGLIONIC SYMPATHETIC NERVE VARICOSITY Figure 5. EVOKED RELEASE OF TRANSMITTER EVOKED RELEASE OF TRANSMITTER • Requires calcium entry into the nerve terminal and occurs by exocytosis. • Involves the same quantal release mechanism described for the motor nerve at the skeletal NMJ (see Dr DeSimone’s notes), BUT the probability of release of quanta is much lower than for somatic motor nerves and hence the amount of transmitter released by a single action potential is less. • Substances stored in the same vesicle are released together. For example, ATP and NE are coreleased from NE storing vesicles of postganglionic sympathetic nerves. • Peptides stored in separate large dense cored vesicles (e.g. VIP) are only released in response to high frequency stimulation.