The autonomic nervous system (ANS) regulates involuntary physiological functions including heart rate, blood pressure, digestion, respiration, and temperature. It operates largely below conscious awareness.
Overview
Divisions
| Division | Primary Function | General Effect |
|---|---|---|
| Sympathetic | Fight or flight | Catabolic, energy expenditure |
| Parasympathetic | Rest and digest | Anabolic, energy conservation |
| Enteric | Gut function | Local control of GI tract |
Two-Neuron Chain
The ANS uses a two-neuron chain to reach its targets:
Preganglionic neuron: Cell body in CNS (CNS → ganglion) Postganglionic neuron: Cell body in ganglion (ganglion → target)
Sympathetic Division (Thoracolumbar)
Central Origin
Preganglionic neurons: Intermediolateral cell column (IML), lateral horn of spinal cord, T1-L2 segments
Preganglionic fibers:
- Short (myelinated)
- Exit via ventral root → white ramus communicans → sympathetic chain
Organization
Sympathetic chain (paravertebral ganglia):
- 22-23 pairs of ganglia
- Located along the vertebral column
- Connected by interganglionic rami
- Extends from C1 to coccyx
Prevertebral (collateral) ganglia:
- Located anterior to the vertebral column
- Associated with major arteries
- Include: Celiac, superior mesenteric, inferior mesenteric, aorticorenal
Pathways
Three possible routes for preganglionic fibers:
- Synapse at same level: Synapse in ganglion at same spinal level
- Ascend/descend: Travel up or down the sympathetic chain before synapsing
- Pass through without synapsing: Continue as splanchnic nerves to prevertebral ganglia
Splanchnic Nerves
| Nerve | Origin | Ganglion | Target |
|---|---|---|---|
| Greater splanchnic | T5-T9 | Celiac | Foregut (stomach, liver, pancreas, small intestine) |
| Lesser splanchnic | T10-T11 | Superior mesenteric | Midgut (large intestine proximal to splenic flexure) |
| Least splanchnic | T12 | Aorticorenal | Kidneys |
| Lumbar splanchnic | L1-L2 | Inferior mesenteric, superior hypogastric | Hindgut, pelvic organs |
| Sacral splanchnic | S1-S4 | Inferior hypogastric | Pelvic organs |
Distribution
To spinal nerves (via gray rami communicantes):
- Every spinal nerve receives sympathetic fibers
- Postganglionic fibers enter the spinal nerve via gray ramus communicans
- Distributed to: Blood vessels, sweat glands, arrector pili muscles
To viscera (via splanchnic nerves and periarterial plexuses):
- Periarterial plexuses accompany arteries to target organs
- Examples: Cardiac plexus, pulmonary plexus, celiac plexus, hypogastric plexus
Neurotransmitters and Receptors
| Structure | Neurotransmitter | Receptor |
|---|---|---|
| Preganglionic neuron | Acetylcholine (ACh) | Nicotinic (Nn) |
| Postganglionic neuron (most) | Norepinephrine (NE) | Alpha-1, alpha-2, beta-1, beta-2 |
| Postganglionic (sweat glands) | Acetylcholine (ACh) | Muscarinic (M3) |
| Adrenal medulla | ACh (from preganglionic) | Nicotinic (Nn) → epinephrine release |
Parasympathetic Division (Craniosacral)
Central Origin
Cranial outflow: Brainstem nuclei (CN III, VII, IX, X) Sacral outflow: Intermediolateral cell column S2-S4
Cranial Parasympathetic
| Nerve | Nucleus | Ganglion | Target |
|---|---|---|---|
| CN III (oculomotor) | Edinger-Westphal | Ciliary | Pupillary sphincter, ciliary muscle |
| CN VII (facial) | Superior salivatory | Pterygopalatine | Lacrimal gland, nasal glands |
| CN VII (facial) | Superior salivatory | Submandibular | Submandibular, sublingual glands |
| CN IX (glossopharyngeal) | Inferior salivatory | Otic | Parotid gland |
| CN X (vagus) | Dorsal motor nucleus | Intramural (in organ wall) | Thoracic and abdominal viscera |
Sacral Parasympathetic
Pelvic splanchnic nerves (S2-S4):
- Preganglionic fibers
- Form pelvic plexus (inferior hypogastric)
- Distribute to: Descending colon, rectum, bladder, genitalia
Neurotransmitters and Receptors
| Structure | Neurotransmitter | Receptor |
|---|---|---|
| Preganglionic neuron | Acetylcholine (ACh) | Nicotinic (Nn) |
| Postganglionic neuron | Acetylcholine (ACh) | Muscarinic (M1-M5) |
Enteric Division
Organization
- Myenteric plexus (Auerbach): Between circular and longitudinal muscle layers; regulates motility
- Submucosal plexus (Meissner): Within the submucosa; regulates secretion, absorption, blood flow
Neurotransmitters
| Neurotransmitter | Function |
|---|---|
| Acetylcholine | Excitatory (motility, secretion) |
| Nitric oxide | Inhibitory (relaxation) |
| Substance P | Excitatory |
| Vasoactive intestinal peptide (VIP) | Inhibitory, vasodilation |
| Serotonin (5-HT) | Modulation of motility |
Autonomic Innervation of Organs
| Organ | Sympathetic Effect | Parasympathetic Effect |
|---|---|---|
| Eye | ||
| Iris (radial muscle) | Mydriasis (dilation, α1) | — |
| Iris (sphincter) | — | Miosis (constriction, M3) |
| Ciliary muscle | Relaxation (far vision) | Contraction (near vision, M3) |
| Heart | ||
| SA node | Tachycardia (β1) | Bradycardia (M2) |
| Atria | Increased contractility (β1) | Decreased contractility (M2) |
| Ventricles | Increased contractility (β1) | Minor effect |
| AV node | Increased conduction (β1) | Decreased conduction (M2) |
| Lungs | ||
| Bronchioles | Dilation (β2) | Constriction (M3) |
| Secretions | Decreased | Increased |
| GI tract | ||
| Motility | Decreased (α2, β2) | Increased (M3) |
| Sphincters | Contraction (α1) | Relaxation |
| Secretions | Decreased | Increased |
| Gallbladder | Relaxation | Contraction |
| Pancreas | ||
| Exocrine | Decreased secretion | Increased secretion |
| Endocrine (insulin) | Decreased (α2) | Increased (M3) |
| Urinary system | ||
| Kidney | Renin release (β1) | — |
| Detrusor | Relaxation (β2) | Contraction (M3) |
| Internal sphincter | Contraction (α1) | Relaxation |
| Reproductive system | ||
| Male | Ejaculation (α1) | Erection (M3, NO) |
| Female | Uterine contraction (α1) | Uterine relaxation, lubrication |
| Skin | ||
| Sweat glands | Secretion (ACh, M3) | — |
| Arrector pili | Contraction (α1) | — |
| Blood vessels (skin) | Constriction (α1) | — |
| Blood vessels | ||
| Most | Constriction (α1) | — |
| Skeletal muscle | Dilation (β2) | — |
| Coronary | Constriction (α1), dilation (β2) | Dilation |
| Salivary glands | Thick secretion | Profuse watery secretion |
| Liver | Glycogenolysis, gluconeogenesis (α1, β2) | Glycogen synthesis |
| Adipose tissue | Lipolysis (β2, β3) | — |
| Adrenal medulla | ACh → Epinephrine release | — |
| Pineal gland | Melatonin synthesis (β1) | — |
Autonomic Reflexes
| Reflex | Afferent | Integrating Center | Efferent | Response |
|---|---|---|---|---|
| Baroreceptor | CN IX (carotid sinus), X (aortic arch) | NTS (medulla) | Parasympathetic, sympathetic | ↓ HR, ↓ BP |
| Chemoreceptor | CN IX (carotid body), X (aortic body) | NTS (medulla) | Sympathetic | ↑ Ventilation |
| Pupillary light | CN II | Pretectal nucleus (midbrain) | CN III (parasympathetic) | Pupillary constriction |
| Ciliospinal | Pain afferents | Spinal cord (T1-T2) | Sympathetic | Pupillary dilation |
| Micturition | Stretch receptors (bladder) | Pontine micturition center (Barrington), sacral cord | Parasympathetic, somatic | Bladder contraction, sphincter relaxation |
| Defecation | Stretch receptors (rectum) | Sacral cord | Parasympathetic, somatic | Rectal contraction, sphincter relaxation |
| Sexual (erection) | Sensory (genital) | Sacral cord | Parasympathetic (NO) | Vasodilation |
| Sexual (ejaculation) | Sensory (genital) | Lumbar cord (sympathetic) | Sympathetic | Semen expulsion |
Autonomic Disorders
| Disorder | Pathology | Features |
|---|---|---|
| Horner syndrome | Sympathetic chain lesion | Ptosis, miosis, anhidrosis, enophthalmos |
| Autonomic dysreflexia | Spinal cord injury above T6 | Hypertensive crisis from below-level stimuli |
| Orthostatic hypotension | Sympathetic dysfunction | BP drop on standing |
| Vasovagal syncope | Excessive vagal tone | Bradycardia, hypotension, fainting |
| Diabetic autonomic neuropathy | Peripheral nerve damage | Postural hypotension, gastroparesis, erectile dysfunction |
| Multiple system atrophy (Shy-Drager) | Central autonomic degeneration | Severe orthostatic hypotension |
| Holmes-Adie syndrome | Ciliary ganglion damage | Tonic pupil, areflexia |
| Familial dysautonomia (Riley-Day) | Autonomic nerve development | Altered pain sensation, BP instability |