Autonomic Nervous System I: Introduction

List the two general types of adrenergic receptors

1) Alpha (generally excitatory, except in the GI tract) 2) Beta (generally inhibitory, except in the heart)

List the three major neurotransmitters of the autonomic nervous system and their sites of action

1) Epinephrine: released by cells of the adrenal medulla into the blood; neurohormone. Has sites of action all over the body. 2) Norepinephrine is released by postganglionic neurons of the sympathetic nervous system and acts at alpha and beta adrenergic receptors. - Sites of action: sympathetic postganglionic neuro-effector junctions 3) Acetylcholine is released by preganglionic neurons (both sympathetic and parasympathetic) and by postganglionic neurons of the parasympathetic nervous system. The preganglionic release acts at nicotinic cholinergic receptors, and the postganglionic release acts at muscarinic cholinergic receptors. - Sites of action: sympathetic ganglia, parasympathetic ganglia, adrenal medulla, parasympathetic postganglionic nerve terminal, neuromuscular junction, sweat glands.

Describe the major mechanism by which acetylcholine is inactivated, by which norepinephrine is inactivated

Acetylcholine is inactivated by acetylcholinesterase, which breaks it down into choline and acetate. Norepinephrine is inactivated by 1) reuptake; 2) breakdown by COMT; 3) breakdown by MAO; 4) diffusion out of the synaptic terminal. Drugs can act at any point in these steps.

Describe the mechanisms by which adrenergic agonists and muscarinic antagonists both produce mydriasis

Adrenergic agonists produce mydriasis (dilation) by stimulating the alpha receptors of the radial muscles to contract the muscle. Mimic sympathetic activity. Muscarinic antagonists block the muscarinic cholinergic receptors of the pupillary sphincter muscles, thus preventing constriction and causing dilation. Prevents parasympathetic activity (ex: fattening lens no longer possible).

Describe the origin of epinephrine and its site of release

Epinephrine is produced in the adrenal medulla and is released into the bloodstream where it then travels to its site of action.

Describe the effect of parasympathetic nervous system activation on heart, respiratory tract, eye and gastrointestinal tract

Heart rate and contractile strength is decreased Bronchoconstriction Pupil constriction (miosis) The muscle of the GI tract is stimulated to contract "Rest and digest"

Describe the effect of sympathetic nervous system activation on heart, respiratory tract, eye, and adrenal gland

Heart rate and contractile strength is increased Bronchodilation Pupils dilate (mydriasis) Cells of the adrenal medulla are stimulated to release epinephrine (by preganglionic sympathetic fibers) "Fight or flight"

Describe the effects of the brainstem sympathetic centers on heart, veins, kidneys and arterioles and changes in blood pressure

Heart: increased rate of contraction, increased force of contraction Veins: venoconstriction to increase venous return Kidneys: increased renin secretion (increases blood pressure and causes vasoconstriction) Arterioles: vasoconstriction (in areas where blood is not needed) and vasodilation (in the skeletal muscle) Blood pressure: increases

Describe the sympathetic/parasympathetic neurotransmitters and their receptors for increasing or decreasing heart rate.

Norepinephrine is a sympathetic neurotransmitter that acts at beta adrenergic receptors in the heart to increase heart rate. Acetylcholine is a parasympathetic neurotransmitter that acts at muscarinic cholinergic receptors in the heart to decrease heart rate.

Describe the CNS origins and anatomical organization of the sympathetic and parasympathetic divisions of the autonomic nervous system (ANS)

Some effectors are enervated by both; others solely by one. Sympathetic preganglionic neurons are located in the intermediolateral column of the thoracic and lumbar spinal cord. They travel out through the ventral root and then split off via the white rami communicantes to the paravertebral ganglion. The fiber synapses on the post-synaptic neuron, which then rejoins the peripheral nerve via grey rami communicantes then travels to the organ it innervates. Ex: constricts blood vessels Parasympathetic preganglionic neurons are located in cranial nerves III, VII, IX, and X, as well as in the sacral spinal cord. These fibers travel to the organ they innervate before synapsing on second order neurons (ganglia are near or on the organ). Ex: lacrimal glands

Describe how single innervation of the arterioles by the sympathetic division can lead to both constriction and dilation of these blood vessels

The arterioles have tone, i.e. a base level of sympathetic activation, so that the blood pressure is under control even at rest. If constriction is needed, the sympathetic nervous system activates the receptors. If dilation is needed, the sympathetic division is turned off, allowing the vessels to passively dilate under the pressure of the blood.