Autonomic Nervous System Series  ·  Module 1

ANS Organization and Functional Anatomy

The two-neuron arc, three divisions, tonic tone, and anatomical drug targets at a glance

The Two-Neuron Efferent Arc — Both Divisions

Central Nervous System Preganglionic Neuron
→ ACh → Nicotinic N-N
Peripheral Ganglion Postganglionic Neuron
→ Symp: NE • Para: ACh
Target Organ Effector Cell

Ganglionic synapse uses acetylcholine on nicotinic N-N receptors in both divisions. Ganglionic blockers (trimethaphan) disrupt both simultaneously — orthostatic hypotension, ileus, urinary retention, anhidrosis.

Division 1

Sympathetic

OriginT1–L2 (thoracolumbar)
GangliaNear spine (paravertebral chain)
Pre/PostShort pre / long post
Post TXNorepinephrine
AdrenalModified ganglion; epinephrine + norepinephrine into blood

Division 2

Parasympathetic

OriginCN III, VII, IX, X; S2–S4 (craniosacral)
GangliaTerminal (at or within target organ)
Pre/PostLong pre / short post
Post TXAcetylcholine (muscarinic)
VagusHeart, lungs, most abdominal viscera

Division 3

Enteric

LocationGut wall (esophagus to anus)
PlexusesMyenteric (motility) + Submucosal (secretion)
Key TXSerotonin, acetylcholine, nitric oxide
AutonomyFunctions without extrinsic input
Drug noteOpioids suppress motility via mu receptors
Organ Dominant Resting Tone Drug Consequence
Heart (sinoatrial node) Parasympathetic Atropine → tachycardia; vagotomy → rate rises to intrinsic ~100 bpm
Blood vessels Sympathetic Alpha-1 blocker (prazosin) → vasodilation, orthostatic hypotension
Pupil Parasympathetic Atropine → mydriasis; muscarinic agonist → miosis
Bronchi Parasympathetic Ipratropium (muscarinic blocker) → bronchodilation in chronic obstructive pulmonary disease
Gastrointestinal motility Parasympathetic Opioids suppress enteric nervous system → constipation

Target Level 1

Ganglionic Blockers

Block nicotinic N-N receptors. Disrupt both divisions simultaneously. Example: trimethaphan. Indiscriminate profile — orthostatic hypotension, ileus, urinary retention.

Target Level 2

Muscarinic Drugs

Act at parasympathetic neuroeffector junctions. Agonists (pilocarpine) mimic parasympathetic activation. Antagonists (atropine) block it. Organ specificity from muscarinic receptor subtype distribution.

Target Level 3

Adrenergic Drugs

Act at sympathetic neuroeffector junctions and on adrenal-medullary catecholamine targets. Selectivity determined by receptor subtype: alpha-1, alpha-2, beta-1, beta-2.