Autonomic Nervous System: Introduction

Cholinergic agonists mimic or amplify the effects of acetylcholine. They fall into two mechanistic categories: direct-acting agents that bind and activate cholinergic receptors themselves, and indirect-acting agents that inhibit acetylcholinesterase, allowing endogenous acetylcholine to accumulate and exert prolonged effects at all cholinergic synapses.

Direct-acting muscarinic agonists include bethanechol, which selectively activates muscarinic receptors without nicotinic activity and resists acetylcholinesterase hydrolysis. It is used to stimulate bladder and gastrointestinal smooth muscle in postoperative or neurogenic urinary retention and ileus. Pilocarpine, a naturally occurring alkaloid, activates muscarinic receptors in the eye (producing miosis and ciliary muscle contraction that opens the trabecular meshwork) and is a first-line treatment for open-angle glaucoma. Carbachol has both muscarinic and nicotinic activity and is used ophthalmically. Muscarine itself has primarily historical interest as the mushroom toxin that defined the receptor class.

Indirect-acting cholinergic agents — the anticholinesterases — inhibit acetylcholinesterase and thereby amplify acetylcholine at every cholinergic synapse: autonomic ganglia (nicotinic), parasympathetic targets (muscarinic), the neuromuscular junction (nicotinic), and central nervous system synapses. The reversible carbamate agents (neostigmine, pyridostigmine) do not cross the blood-brain barrier and are used for myasthenia gravis and reversal of non-depolarizing neuromuscular blockade. Physostigmine is a tertiary amine that does cross the blood-brain barrier and is used for anticholinergic toxidrome. Donepezil, rivastigmine, and galantamine are centrally acting agents used in Alzheimer disease. Irreversible organophosphate inhibitors (nerve agents: sarin, soman, VX; insecticides: malathion, parathion) bind covalently and cause life-threatening cholinergic crisis — treated with atropine (muscarinic antagonist) and pralidoxime (reactivates the enzyme if given early).

Adrenergic agonists activate adrenergic receptors either directly (by binding the receptor themselves) or indirectly (by increasing norepinephrine availability at the synapse through release or reuptake inhibition). Most clinically important agents are direct-acting; a few are indirect or mixed.

Epinephrine activates all adrenergic receptor subtypes — alpha-1, alpha-2, beta-1, and beta-2. At low doses the beta effects predominate (increased heart rate and contractility, bronchodilation, vasodilation in skeletal muscle), while at higher doses alpha-1 vasoconstriction becomes dominant, raising diastolic pressure. Epinephrine is the drug of first choice for anaphylaxis (reverses bronchospasm via beta-2, and vasoconstriction via alpha-1), and is included in local anesthetic formulations to cause local vasoconstriction that prolongs anesthetic duration.

Norepinephrine has potent alpha-1 and beta-1 activity but minimal beta-2 activity. The result is vasoconstriction (alpha-1) and increased cardiac contractility (beta-1), with reflex bradycardia masking the direct chronotropic effect. It is used as a vasopressor in septic shock. Dopamine at low infusion rates activates D1 receptors (renal vasodilation); at intermediate rates it activates beta-1 receptors (increased cardiac output); at high rates it activates alpha-1 receptors (vasoconstriction).

Selective agonists target specific subtypes for therapeutic precision. Phenylephrine is a pure alpha-1 agonist — used as a nasal decongestant and to raise blood pressure in hypotension without increasing heart rate. Dobutamine is primarily a beta-1 agonist used in acute heart failure and stress echocardiography. Albuterol and salmeterol are beta-2 selective bronchodilators. Clonidine is an alpha-2 agonist used as an antihypertensive (central sympatholysis) and in opioid and alcohol withdrawal. Indirect sympathomimetics — amphetamine and ephedrine — release norepinephrine from presynaptic terminals and also block its reuptake.

The framework built across this chapter — two-neuron arc, neurotransmitter lifecycle, receptor subtype distribution, tonic tone, drug class mechanisms — converges here into a practical decision tool: given a drug and a patient, what will happen at each organ? The answer follows a consistent three-step reasoning sequence.

Step one: identify the anatomical level of drug action (ganglionic, postganglionic neuroeffector, central). Step two: identify the receptor subtype targeted and the organ-level distribution of that subtype. Step three: determine the drug's action (agonist or antagonist) and apply it to the resting tone at that organ. An antagonist removes tonic input; an agonist adds to it or activates receptors beyond their resting state.

The most common point of confusion is predicting heart rate responses. Atropine (muscarinic antagonist) accelerates the heart because resting parasympathetic tone via M2 receptors slows it — removing the brake accelerates. Propranolol (beta-1 antagonist) slows the heart because resting sympathetic tone via beta-1 receptors supports the rate — removing that support slows it. A drug that does both (blocking both muscarinic and adrenergic input to the heart simultaneously, as in a ganglionic blocker) produces a heart rate near the intrinsic pacemaker rate of approximately 100 beats per minute — the rate the sinoatrial node generates without any autonomic input.

Drug interactions in the autonomic system also follow from receptor logic. A patient on a beta-blocker who develops anaphylaxis may not respond adequately to epinephrine — the beta-2 bronchodilating effect is blocked, and the alpha-1 vasoconstriction may be exaggerated (paradoxical hypertension). Glucagon, which raises cyclic adenosine monophosphate independently of beta receptors, is the treatment for beta-blocker-complicated anaphylaxis. Understanding receptor pharmacology at this level allows prediction of atypical drug responses before they occur clinically.