Medical School · Year 2 · Neuroscience · includes a quiz and discussion video

Lecture 3: Autonomic Nervous System

Unit 2.5: Neuroscience

Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the organization and anatomy of the sympathetic and parasympathetic nervous systems
2. Compare and contrast the two-neuron chains, ganglia locations, and fiber lengths of both divisions
3. Explain the neurotransmitters, receptor subtypes, and signal transduction of autonomic pathways
4. Describe the physiological effects of autonomic activation on each organ system
5. Explain the enteric nervous system organization and function
6. Describe autonomic disorders including orthostatic hypotension, Horner syndrome, and autonomic neuropathy

1. Autonomic Nervous System Organization

The autonomic nervous system governs involuntary visceral functions essential for homeostasis, regulating cardiovascular, respiratory, gastrointestinal, genitourinary, and thermoregulatory activities without conscious input. Its three divisions, sympathetic, parasympathetic, and enteric, work in concert to maintain internal stability while enabling appropriate responses to environmental demands and physiological stressors.

A fundamental organizational principle distinguishes the autonomic from somatic motor systems: the two-neuron chain. Rather than a single motor neuron projecting from CNS to effector as in skeletal muscle control, autonomic pathways involve preganglionic neurons with cell bodies in the CNS that synapse on postganglionic neurons in peripheral ganglia, which then innervate target organs. This arrangement allows for signal amplification, divergence to multiple targets, and integration of inputs at the ganglionic level.

The sympathetic division, often characterized as mediating "fight or flight" responses, prepares the body for stressful or emergency situations. Activation increases heart rate and contractility, diverts blood flow from viscera to skeletal muscle, dilates bronchi to enhance ventilation, mobilizes glucose from hepatic stores, and dilates pupils. The parasympathetic division, conversely described as governing "rest and digest" functions, predominates during relaxed states, slowing heart rate, promoting digestion and absorption, and supporting restorative processes.

Most visceral organs receive dual innervation from both divisions, with opposing effects that allow fine-tuned regulation. The heart exemplifies this balance: sympathetic activation accelerates heart rate while parasympathetic activation slows it. However, this dual innervation is not universal; blood vessels receive primarily sympathetic innervation, with vasodilation achieved by decreased sympathetic tone rather than parasympathetic activation. Sweat glands represent another exception, receiving sympathetic innervation exclusively, though using acetylcholine rather than the typical norepinephrine.

<image>Panel A: A human figure with internal organs visible showing sympathetic outflow from the thoracolumbar cord (T1-L2), with short preganglionic fibers (solid lines) projecting to the paravertebral sympathetic chain (depicted as connected ganglia alongside the spine) and prevertebral ganglia (celiac, superior and inferior mesenteric), then long postganglionic fibers (dashed lines) to target organs. Panel B: Parasympathetic outflow from craniosacral regions (CN III, VII, IX, X superiorly and S2-S4 inferiorly), with long preganglionic fibers to ganglia near or within target organs, then short postganglionic fibers. Panel C: Organs color-coded showing dual innervation including heart (sympathetic increases rate, parasympathetic decreases), lungs (sympathetic dilates, parasympathetic constricts), GI tract (sympathetic inhibits, parasympathetic stimulates), and pupil (sympathetic dilates, parasympathetic constricts). Panel D: The adrenal medulla shown with direct preganglionic innervation releasing epinephrine and norepinephrine into blood.</image>

2. Sympathetic Nervous System Anatomy

The sympathetic division originates from the thoracolumbar spinal cord, with preganglionic neuron cell bodies located in the intermediolateral cell column spanning segments T1 through L2. This discrete anatomical origin creates the characteristic pattern of thoracolumbar outflow, with fibers reaching all body regions through specific routing pathways.

Preganglionic fibers exit the spinal cord through ventral roots, enter spinal nerves, then leave via white rami communicantes to reach the sympathetic chain ganglia. These paravertebral ganglia form bilateral columns of interconnected ganglia running alongside the vertebral column from cervical to sacral levels. Within the chain, preganglionic fibers may synapse at the level of entry, ascend or descend to synapse at different levels, or pass through without synapsing to reach prevertebral ganglia.

Postganglionic fibers from chain ganglia return to spinal nerves via gray rami communicantes to distribute throughout somatic structures, providing sympathetic innervation to sweat glands, blood vessels, and arrector pili muscles of the skin. Superior cervical ganglion neurons innervate the head, including pupil dilator muscles, blood vessels, and salivary glands. Middle and inferior cervical ganglia (the latter often fused with T1 as the stellate ganglion) supply the heart and upper extremities.

The prevertebral ganglia, including the celiac, superior mesenteric, and inferior mesenteric ganglia, lie anterior to the aorta near their respective arterial trunks. Preganglionic fibers reach these ganglia via splanchnic nerves that pass through the sympathetic chain without synapsing. The greater splanchnic nerve from T5-T9 supplies the celiac ganglion, innervating foregut derivatives. The lesser splanchnic nerve from T10-T11 supplies the superior mesenteric ganglion for midgut structures. The least splanchnic nerve from T12 and lumbar splanchnic nerves supply the inferior mesenteric ganglion for hindgut and pelvic organs.

<image>Panel A: A cross-section of the thoracic spinal cord with the intermediolateral cell column highlighted (T1-L2 origin), showing preganglionic fibers exiting via ventral root, traveling in spinal nerve, then exiting through white ramus communicans (myelinated, shown in white) to the sympathetic chain ganglion, with three pathways illustrated showing synapse at same level with postganglionic returning via gray ramus, ascend/descend in chain to synapse at different level, and pass through to prevertebral ganglion via splanchnic nerve. Panel B: The full sympathetic chain from superior cervical ganglion (at C2-3) through stellate ganglion to sacral levels, with splanchnic nerves (greater T5-9, lesser T10-11, least T12) projecting to celiac, superior mesenteric, and inferior mesenteric ganglia anterior to the aorta, with target organ innervation patterns indicated. Panel C: Gray ramus (unmyelinated, shown in gray) returning to spinal nerve and the pattern of postganglionic distribution to somatic structures. Panel D: The adrenal medulla receiving preganglionic fibers directly (no postganglionic synapse), with chromaffin cells releasing epinephrine (80%) and norepinephrine (20%) into blood.</image>

3. Parasympathetic Nervous System Anatomy

The parasympathetic division exhibits craniosacral outflow, with preganglionic neurons located in brainstem nuclei associated with cranial nerves III, VII, IX, and X, plus sacral spinal cord segments S2 through S4. This anatomical distribution restricts parasympathetic effects to head structures, thoracic and abdominal viscera, and pelvic organs, without direct innervation of the body wall or limbs.

Cranial nerve III (oculomotor) carries parasympathetic fibers from the Edinger-Westphal nucleus in the midbrain to the ciliary ganglion, from which postganglionic fibers innervate the pupillary constrictor muscle (producing miosis) and the ciliary muscle (enabling accommodation for near vision). Cranial nerve VII (facial) conveys fibers from the superior salivatory nucleus to the pterygopalatine ganglion (supplying lacrimal gland and nasal mucosa) and the submandibular ganglion (supplying submandibular and sublingual salivary glands).

Cranial nerve IX (glossopharyngeal) carries fibers from the inferior salivatory nucleus to the otic ganglion, which innervates the parotid salivary gland. Cranial nerve X (vagus) represents the most extensive parasympathetic nerve, with preganglionic fibers from the dorsal motor nucleus and nucleus ambiguus in the medulla traveling to terminal ganglia located in or near target organs throughout the thorax and abdomen. Vagal parasympathetic innervation includes the heart (decreasing rate and conduction velocity), lungs (bronchoconstriction, mucus secretion), and gastrointestinal tract from esophagus through the transverse colon (increasing motility and secretion).

The sacral parasympathetic outflow arises from the lateral gray matter of S2-S4 segments. Preganglionic fibers travel in the pelvic splanchnic nerves to terminal ganglia near or within pelvic organs including the descending colon, sigmoid colon, rectum (promoting motility and relaxing internal anal sphincter), bladder (contracting detrusor for voiding), and reproductive organs (mediating erection through vasodilation). The transition between vagal and sacral parasympathetic territories occurs near the splenic flexure of the colon.

<image>Panel A: Cranial outflow in a sagittal head/brainstem view showing CN III from Edinger-Westphal nucleus to ciliary ganglion to pupil (constriction) and ciliary muscle (accommodation), and CN VII from superior salivatory nucleus to pterygopalatine ganglion (lacrimal gland, nasal mucosa) and submandibular ganglion (submandibular and sublingual glands). Panel B: CN IX from inferior salivatory nucleus to otic ganglion to parotid gland, and CN X from dorsal motor nucleus and nucleus ambiguus with its extensive course shown on a torso diagram innervating heart (SA and AV nodes), lungs (bronchial smooth muscle and glands), and GI tract (esophagus through splenic flexure indicated with color gradient). Panel C: Sacral outflow from S2-S4 lateral gray, with pelvic splanchnic nerves to pelvic ganglia near organs including descending/sigmoid colon, rectum (motility, internal sphincter relaxation), bladder (detrusor contraction), and reproductive organs (erection pathway highlighted). Panel D: A key distinguishing long preganglionic fibers (craniosacral) versus short postganglionic fibers (near target).</image>

4. Autonomic Neurotransmitters and Receptors

Neurotransmitter identity follows predictable patterns based on fiber type and location within autonomic pathways. All preganglionic neurons, both sympathetic and parasympathetic, release acetylcholine acting on nicotinic receptors at ganglia. All parasympathetic postganglionic neurons release acetylcholine acting on muscarinic receptors at effector organs. Most sympathetic postganglionic neurons release norepinephrine acting on adrenergic receptors, with the notable exception of sweat gland innervation, which uses acetylcholine on muscarinic receptors. The adrenal medulla, a modified sympathetic ganglion, releases epinephrine (80%) and norepinephrine (20%) directly into the bloodstream.

Nicotinic receptors at autonomic ganglia (Nn subtype) differ from those at the neuromuscular junction (Nm subtype) in subunit composition and pharmacology. Both are ligand-gated cation channels producing fast excitatory responses. Ganglionic blockers such as hexamethonium and mecamylamine were historically used for hypertension but produced intolerable side effects by blocking both divisions.

Muscarinic receptors comprise five subtypes (M1-M5) coupled to G proteins. M1 receptors in the CNS and gastric parietal cells couple to Gq, activating phospholipase C to increase IP3 and DAG. M2 receptors in the heart couple to Gi, decreasing cAMP and activating potassium channels to slow heart rate. M3 receptors on smooth muscle and glands couple to Gq, causing smooth muscle contraction and glandular secretion. The clinical utility of muscarinic agonists (bethanechol for urinary retention, pilocarpine for glaucoma) and antagonists (atropine for bradycardia, oxybutynin for overactive bladder) derives from understanding these receptor distributions.

Adrenergic receptors divide into alpha and beta subtypes with distinct second messenger coupling. Alpha-1 receptors on vascular smooth muscle couple to Gq, causing vasoconstriction through IP3-mediated calcium release. Alpha-2 receptors on presynaptic terminals and central neurons couple to Gi, inhibiting norepinephrine release and providing negative feedback. Beta-1 receptors predominantly in the heart couple to Gs, increasing cAMP to enhance rate and contractility. Beta-2 receptors in bronchial smooth muscle and blood vessels couple to Gs, causing relaxation and dilation. Beta-3 receptors in adipose tissue couple to Gs, promoting lipolysis.

<image>Panel A: The two-neuron pathway for each division showing sympathetic with ACh at ganglionic nicotinic receptor (Nn) and NE at effector adrenergic receptors (or ACh at sweat gland muscarinic), parasympathetic with ACh at nicotinic then ACh at muscarinic receptors, and the adrenal medulla receiving preganglionic ACh and releasing Epi/NE into blood. Panel B: Muscarinic receptor subtypes showing M1 (Gq, CNS, parietal cells), M2 (Gi, heart - decreases rate, opens K+ channels), M3 (Gq, smooth muscle, glands - contracts, secretes), with drug examples (pilocarpine M3 agonist for glaucoma, atropine antagonist for bradycardia, oxybutynin M3 antagonist for bladder). Panel C: Adrenergic receptors showing alpha-1 (Gq, vascular smooth muscle - vasoconstriction, with phenylephrine agonist), alpha-2 (Gi, presynaptic - decreases NE release, with clonidine agonist for hypertension), and beta-1 (Gs, heart - increases rate/contractility, with metoprolol antagonist). Panel D: Beta-2 (Gs, bronchi/vessels - relaxation, with albuterol agonist for asthma) and beta-3 (Gs, adipose - lipolysis), with second messenger pathways (cAMP up/down, IP3/DAG) illustrated for each receptor type.</image>

5. Organ System Effects

Cardiovascular effects illustrate the complementary actions of autonomic divisions. Sympathetic activation increases heart rate through beta-1 receptor effects on the sinoatrial node pacemaker cells, accelerating diastolic depolarization. Contractility increases through enhanced calcium influx and myofilament sensitivity. Conduction velocity through the AV node increases, shortening PR interval. Blood vessels constrict through alpha-1 receptor activation on arteriolar smooth muscle, increasing peripheral resistance and blood pressure. Conversely, parasympathetic (vagal) activation through M2 receptors decreases heart rate by hyperpolarizing SA node cells and slowing AV conduction. Parasympathetic effects on contractility are minimal and limited to atria.

Respiratory effects primarily involve bronchial smooth muscle and secretions. Sympathetic activation through circulating epinephrine acting on beta-2 receptors produces bronchodilation, enhancing airflow during stress or exercise. Parasympathetic activation through vagal M3 receptors causes bronchoconstriction and increased mucus secretion. The therapeutic relevance is evident in asthma treatment, where beta-2 agonists (albuterol) relieve bronchospasm while muscarinic antagonists (ipratropium) reduce vagal-mediated constriction.

Gastrointestinal effects demonstrate parasympathetic dominance during digestion. Parasympathetic activation increases motility through enhanced smooth muscle contraction, relaxes sphincters to permit bolus transit, and stimulates secretion from salivary glands, gastric glands, pancreas, and bile ducts. Sympathetic activation produces opposite effects, decreasing motility, constricting sphincters, and reducing secretion, essentially putting digestion on hold during stress. Salivation shows qualitative differences: parasympathetic produces watery, enzyme-rich secretion suitable for digestion, while sympathetic produces thick, mucous secretion.

Genitourinary effects govern bladder function and sexual responses. Bladder filling occurs under sympathetic influence, with beta-2 receptors relaxing the detrusor muscle and alpha-1 receptors contracting the internal urethral sphincter, promoting continence. Voiding requires parasympathetic activation: M3 receptors contract the detrusor while relaxation of the internal sphincter (through inhibition of sympathetic tone) and voluntary relaxation of the external sphincter allow micturition. Sexual function involves coordinated autonomic responses: erection requires parasympathetic vasodilation (via nitric oxide release), while ejaculation requires sympathetic contraction of reproductive structures.

<image>Panel A: Heart autonomic responses showing sympathetic (beta-1) increasing rate (faster SA node slope shown on pacemaker potential graph), contractility (stronger contraction curve), and conduction velocity, while parasympathetic (M2) decreases rate with minimal contractility effect (atria only). Panel B: Lungs showing sympathetic (beta-2) causing bronchodilation (cross-section showing dilated airway) and parasympathetic (M3) causing bronchoconstriction and mucus secretion, and GI tract showing sympathetic decreasing motility (slow wave amplitude reduced) and constricting sphincters while parasympathetic increases motility, relaxes sphincters, and increases secretions (salivary gland comparison showing watery vs thick secretion). Panel C: Bladder cross-section showing detrusor muscle and sphincters with sympathetic (beta-2 detrusor relaxation, alpha-1 sphincter contraction) promoting storage and parasympathetic (M3 detrusor contraction) promoting voiding. Panel D: Eye showing sympathetic causing mydriasis (dilated pupil shown) via dilator muscle and parasympathetic causing miosis (constricted pupil) via constrictor muscle plus accommodation via ciliary muscle, and male reproductive showing parasympathetic for erection (point) and sympathetic for ejaculation (shoot).</image>

6. Enteric Nervous System

The enteric nervous system constitutes a remarkably autonomous neural network embedded within the gastrointestinal tract wall, containing approximately 500 million neurons, comparable to the entire spinal cord. This "little brain" of the gut can coordinate digestive functions independently of central input, though it normally operates under modulatory influence from sympathetic and parasympathetic systems.

Two major plexuses organize enteric neurons. The myenteric plexus (Auerbach's plexus) lies between the circular and longitudinal muscle layers throughout the GI tract, primarily controlling motility patterns including peristalsis, segmentation, and tonic contractions. The submucosal plexus (Meissner's plexus) lies within the submucosa, mainly in the small and large intestine, regulating mucosal secretion, absorption, and local blood flow. These plexuses contain sensory neurons detecting mechanical and chemical stimuli, interneurons integrating information, and motor neurons controlling muscle and glands.

Enteric neurotransmitters extend well beyond acetylcholine and norepinephrine to include numerous peptides and small molecules enabling nuanced local control. Acetylcholine provides the primary excitatory signal, stimulating smooth muscle contraction and glandular secretion. Nitric oxide serves as the primary inhibitory transmitter, relaxing smooth muscle including lower esophageal sphincter and pylorus. Serotonin (5-HT) from enterochromaffin cells initiates peristaltic and secretory reflexes. Vasoactive intestinal peptide (VIP) promotes relaxation and stimulates secretion. Substance P provides excitatory input. The diversity of enteric transmitters explains why many GI disorders involve complex dysregulation and why targeted therapies remain challenging.

Intrinsic reflexes enable the gut to function even when disconnected from the CNS. The peristaltic reflex illustrates this capability: mucosal stimulation activates sensory neurons that trigger ascending contraction (through acetylcholine and substance P) proximal to the stimulus and descending relaxation (through nitric oxide and VIP) distal to it, propelling contents forward. The secretory reflex responds to luminal contents by stimulating appropriate enzyme and fluid release. Extrinsic parasympathetic input generally enhances these intrinsic activities, while sympathetic input inhibits them, providing central override capability during stress.

<image>Panel A: A cross-section of the intestinal wall with layers labeled showing mucosa, submucosa, circular muscle, myenteric plexus (Auerbach's) between muscle layers (shown as interconnected neurons), longitudinal muscle, and serosa, with the submucosal plexus (Meissner's) shown within the submucosa, and neuron types color-coded including sensory neurons (detecting stretch, chemistry), interneurons (integrating), and motor neurons (to muscle and glands). Panel B: The peristaltic reflex showing a bolus in the lumen with sensory neurons detecting it, ascending pathway (ACh, Substance P) causing contraction proximal to bolus, and descending pathway (NO, VIP) causing relaxation distal to bolus, with arrows showing propulsion direction. Panel C: Enteric neurotransmitters with their functions including ACh (excitatory, contraction), NO (inhibitory, relaxation), 5-HT (reflex initiation, from enterochromaffin cells shown in mucosa), VIP (relaxation, secretion), and Substance P (excitation). Panel D: Extrinsic modulation showing parasympathetic (vagus) enhancing activity and sympathetic (splanchnic) inhibiting activity.</image>

7. Central Autonomic Control

The hypothalamus serves as the master coordinator of autonomic function, integrating visceral, endocrine, and behavioral responses to maintain homeostasis. Different hypothalamic regions preferentially activate different autonomic divisions. The anterior and preoptic regions activate parasympathetic responses and cooling mechanisms including sweating and cutaneous vasodilation. The posterior and lateral hypothalamus activates sympathetic responses and heat conservation, as well as the coordinated "fight or flight" response to perceived threats. The paraventricular nucleus integrates autonomic control with endocrine function through its projections to both autonomic preganglionic neurons and the pituitary.

Brainstem centers execute specific autonomic functions under hypothalamic direction and in response to local afferent input. The nucleus tractus solitarius (NTS) in the dorsomedial medulla serves as the primary integration center for visceral afferents, receiving input from baroreceptors, chemoreceptors, and stretch receptors conveyed by cranial nerves IX and X. From the NTS, projections reach other autonomic nuclei to coordinate appropriate responses. The rostral ventrolateral medulla (RVLM) provides tonic excitatory drive to sympathetic preganglionic neurons; its activity sets the baseline level of sympathetic tone. The caudal ventrolateral medulla (CVLM) inhibits RVLM, allowing baroreceptor-mediated decreases in sympathetic activity.

The baroreceptor reflex exemplifies central autonomic regulation, maintaining blood pressure within a narrow range through negative feedback. Baroreceptors in the carotid sinus and aortic arch detect arterial stretch proportional to pressure. Increased pressure increases baroreceptor firing, transmitted via cranial nerves IX and X to the NTS. The NTS then activates the nucleus ambiguus (increasing vagal output to slow heart rate), activates the CVLM (inhibiting RVLM to reduce sympathetic vasoconstriction), and inhibits sympathetic preganglionic neurons directly. The net effect returns blood pressure toward normal. This reflex operates continuously, buffering moment-to-moment pressure variations during activities like standing.

Additional brainstem centers coordinate other autonomic functions. The Pontine micturition center (Barrington's nucleus) coordinates bladder voiding through simultaneous detrusor contraction and sphincter relaxation. Respiratory centers in the medulla and pons regulate breathing rhythm and depth. Pupillary centers in the midbrain coordinate light reflex and accommodation through parasympathetic projections via cranial nerve III.

<image>Panel A: A sagittal brain section with labeled structures showing hypothalamus (anterior/preoptic region colored for parasympathetic/cooling, posterior/lateral colored for sympathetic/heat conservation, paraventricular nucleus marked for integration). Panel B: Brainstem centers showing NTS in dorsomedial medulla receiving CN IX and X afferents, RVLM for sympathetic excitation, CVLM for sympathetic inhibition, nucleus ambiguus for vagal motor output, and Barrington's nucleus in pons for micturition. Panel C: The baroreceptor reflex pathway showing carotid sinus baroreceptors (shown in neck at carotid bifurcation) detecting increased BP, afferents via CN IX to NTS, with outputs to nucleus ambiguus (increasing vagal tone, decreasing HR) and to CVLM (inhibiting RVLM, decreasing sympathetic tone), resulting in vasodilation and decreased HR returning BP toward normal with arrows showing the complete reflex arc. Panel D: A summary diagram showing the negative feedback loop with increased BP leading to increased baroreceptor firing leading to decreased sympathetic plus increased parasympathetic leading to decreased BP.</image>

8. Autonomic Pharmacology

Cholinergic agonists enhance parasympathetic effects by activating muscarinic receptors. Bethanechol, a selective M3 agonist resistant to cholinesterase, contracts the bladder detrusor to treat postoperative or neurogenic urinary retention. Pilocarpine activates M3 receptors in the eye, causing pupillary constriction and ciliary muscle contraction that opens trabecular meshwork drainage in glaucoma. It also stimulates salivation, useful in Sjögren syndrome or radiation-induced xerostomia. Carbachol provides similar ocular effects. These agents produce predictable side effects reflecting generalized parasympathetic activation: bradycardia, increased secretions, bronchoconstriction, and gastrointestinal hypermotility.

Cholinergic antagonists (antimuscarinics) block parasympathetic effects. Atropine blocks all muscarinic subtypes, useful for treating symptomatic bradycardia by removing vagal tone on the heart, and as an antidote for organophosphate poisoning by blocking accumulated acetylcholine effects. Ipratropium, delivered by inhalation, blocks M3 receptors in bronchial smooth muscle to treat COPD and asthma without significant systemic effects. Oxybutynin and tolterodine block M3 receptors in the bladder to reduce urgency and frequency in overactive bladder syndrome. Scopolamine provides vestibular suppression for motion sickness. Tropicamide dilates the pupil for ophthalmoscopic examination.

Adrenergic agonists enhance sympathetic effects at specific receptor subtypes. Phenylephrine, an alpha-1 selective agonist, causes vasoconstriction useful for nasal decongestion or to raise blood pressure in hypotensive states. Clonidine, an alpha-2 agonist, activates presynaptic and central receptors to reduce sympathetic outflow, treating hypertension, withdrawal syndromes, and ADHD. Epinephrine activates all adrenergic receptors, representing the first-line treatment for anaphylaxis where its beta-2 bronchodilation, alpha-1 vasoconstriction, and beta-1 cardiac effects all contribute benefit. Albuterol selectively activates beta-2 receptors, providing bronchodilation without significant cardiac effects.

Adrenergic antagonists block sympathetic effects. Prazosin and other alpha-1 blockers cause vasodilation, treating hypertension and benign prostatic hyperplasia (by relaxing prostatic smooth muscle). Propranolol non-selectively blocks beta-1 and beta-2 receptors, useful for hypertension, angina, arrhythmias, and migraine prophylaxis, but contraindicated in asthma due to beta-2 blockade. Metoprolol and other beta-1 selective blockers provide cardiac benefits with less bronchospasm risk, though selectivity is not absolute.

<image>Panel A: Cholinergic agonists showing bethanechol (M3 bladder, shown as bladder contracting), pilocarpine (M3 eye/salivary, pupil constricting and trabecular flow), with side effects listed, and cholinergic antagonists showing atropine (all M, heart with increased rate), ipratropium (M3 bronchi via inhaler), oxybutynin (M3 bladder relaxation), and scopolamine (vestibular). Panel B: Adrenergic agonists showing phenylephrine (alpha-1 with vessel constricting), clonidine (alpha-2 with brain sympathetic outflow decreasing), epinephrine (all receptors, shown with anaphylaxis indication), and albuterol (beta-2 with bronchi dilating). Panel C: Adrenergic antagonists showing prazosin (alpha-1 with vessel dilating), propranolol (beta non-selective, heart slowing, note asthma contraindication), and metoprolol (beta-1 selective, heart with preserved airway), with each drug showing mechanism of action and primary clinical use. Panel D: A summary pathway diagram showing all drug targets on the two-neuron chain from preganglionic to effector organ.</image>

9. Autonomic Disorders

Orthostatic hypotension represents failure of the sympathetic vasoconstriction necessary to maintain blood pressure during postural change. When standing from supine position, gravity causes blood pooling in lower extremities and decreased venous return. Normally, baroreceptors detect the resulting pressure drop and trigger compensatory sympathetic activation increasing heart rate and vascular tone. In orthostatic hypotension, this compensation fails, producing a sustained drop in systolic pressure of 20 mmHg or more, or diastolic pressure of 10 mmHg or more, within three minutes of standing. Patients experience lightheadedness, visual dimming, or syncope upon standing.

Causes of orthostatic hypotension include primary autonomic failure (pure autonomic failure, multiple system atrophy), secondary autonomic neuropathy (diabetes mellitus, amyloidosis, Parkinson disease), medications (antihypertensives, antidepressants, alpha-blockers), and hypovolemia. Diabetic autonomic neuropathy deserves emphasis given diabetes prevalence; it produces orthostatic hypotension along with resting tachycardia (loss of vagal tone), gastroparesis, constipation alternating with diarrhea, neurogenic bladder, erectile dysfunction, and sweating abnormalities. Treatment includes increasing fluid and salt intake, compression stockings, sleeping with head elevated, and medications such as fludrocortisone (expanding volume) or midodrine (alpha-1 agonist vasoconstriction).

Horner syndrome results from interruption of the sympathetic pathway to the face and eye at any level. Loss of sympathetic innervation produces the classic triad: miosis (pupillary constriction from unopposed parasympathetic tone), ptosis (drooping of the upper eyelid from loss of Mueller's muscle tone), and anhidrosis (absence of sweating from loss of sympathetic sweat gland innervation). Central lesions (first-order neurons from hypothalamus to T1 cord) may result from stroke, tumor, or demyelination. Preganglionic lesions (second-order from T1 to superior cervical ganglion) occur with Pancoast tumors, trauma, or surgery. Postganglionic lesions (third-order from superior cervical ganglion to eye) result from carotid dissection, cavernous sinus pathology, or cluster headache. Pharmacologic testing helps localize: cocaine eye drops dilate normal pupils but not Horner pupils (at any level), while hydroxyamphetamine dilates preganglionic but not postganglionic Horner pupils.

Neurogenic bladder results from lesions affecting bladder innervation at various levels. Upper motor neuron lesions above the sacral cord (S2-S4) produce spastic bladder with reflex voiding but loss of voluntary control, urgency, and incontinence. Lower motor neuron lesions at S2-S4 or affecting peripheral nerves produce flaccid bladder with urinary retention and overflow incontinence, as the detrusor cannot contract. Management depends on type and includes catheterization, pharmacotherapy, and behavioral approaches.

<image>Panel A: Orthostatic hypotension showing a patient standing from supine with blood pressure readings (lying normal, standing dropped), blood pooling in legs indicated by arrows, pathophysiology diagram showing baroreceptor pathway disrupted, causes listed (primary autonomic failure, diabetic neuropathy with diabetes complications illustrated, medications), and treatment (compression stockings, fludrocortisone, midodrine shown). Panel B: Horner syndrome showing face with miosis (constricted pupil vs normal), ptosis (drooped eyelid), anhidrosis (dry half of forehead indicated), and the three-neuron pathway showing first-order (hypothalamus to T1), second-order (T1 through apex of lung to superior cervical ganglion), third-order (along carotid to eye) with lesion locations and causes at each level indicated. Panel C: Pharmacologic testing box showing cocaine and hydroxyamphetamine responses for Horner syndrome localization. Panel D: Neurogenic bladder showing diagram of bladder with sacral cord and peripheral nerves, with UMN lesion (above S2-S4) producing spastic bladder (reflex voiding, incontinence) and LMN lesion (at or below S2-S4) producing flaccid bladder (retention, overflow).</image>

10. Clinical Assessment of Autonomic Function

History taking provides crucial diagnostic information for autonomic disorders. Specific questions target each autonomic domain. Orthostatic symptoms include lightheadedness, visual changes, or syncope upon standing, improved by sitting or lying down. Sudomotor dysfunction manifests as reduced sweating in expected situations or compensatory hyperhidrosis in unaffected areas. Gastrointestinal symptoms include early satiety, bloating, nausea (gastroparesis), or alternating constipation and diarrhea. Urinary symptoms encompass urgency, frequency, incomplete emptying, or incontinence. Sexual dysfunction includes erectile dysfunction in men and vaginal dryness in women. Recognizing patterns across multiple domains suggests generalized autonomic failure.

Physical examination includes specific autonomic assessments. Orthostatic vital signs, measured after five minutes supine then at one and three minutes standing, quantify blood pressure and heart rate changes. A drop exceeding 20/10 mmHg without appropriate heart rate increase suggests neurogenic orthostatic hypotension. Pupillary examination assesses size, symmetry, and reactivity to light and accommodation. Resting heart rate variability with respiration (sinus arrhythmia) reflects parasympathetic tone; its absence suggests vagal dysfunction. Skin examination notes moisture, temperature, and color changes reflecting sudomotor and vasomotor function.

Specialized autonomic function testing quantifies specific pathways. The Valsalva maneuver (sustained expiratory effort against resistance) produces a characteristic four-phase blood pressure and heart rate response; abnormal patterns indicate baroreflex failure. Heart rate variability analysis, particularly with deep breathing, quantifies vagal function. Tilt table testing evaluates cardiovascular responses to prolonged orthostatic stress, helping diagnose neurogenic orthostatic hypotension versus vasovagal syncope. Quantitative sudomotor axon reflex testing (QSART) measures sweat output in response to iontophoresed acetylcholine, assessing postganglionic sympathetic sudomotor function. Thermoregulatory sweat testing maps body-wide sweating patterns, identifying central versus peripheral and preganglionic versus postganglionic sudomotor lesions.

Treatment approaches address specific deficits. Orthostatic hypotension management includes non-pharmacologic measures (increased fluid and salt intake, compression garments, sleeping with head elevated, rising slowly) and medications (fludrocortisone for volume expansion, midodrine or droxidopa for vasoconstriction). Gastroparesis responds to dietary modification (small frequent meals, low fat, low fiber) and prokinetics (metoclopramide, domperidone, erythromycin). Neurogenic bladder management depends on type: spastic bladder may respond to anticholinergics; flaccid bladder requires intermittent catheterization. Erectile dysfunction treatment includes PDE5 inhibitors (sildenafil).

<image>Panel A: History questions organized by domain including orthostatic (lightheadedness on standing), sudomotor (sweating changes), GI (early satiety, bloating, bowel changes), GU (urgency, retention, incontinence), and sexual (erectile dysfunction). Panel B: Physical examination showing orthostatic vitals technique (supine 5 min then standing with BP cuff shown, normal vs abnormal response graphed), pupillary exam (anisocoria, light reflex), and skin examination (checking moisture, color). Panel C: Autonomic function tests showing Valsalva maneuver with normal four-phase BP response graphed (phases I-IV labeled), heart rate variability tracing showing respiratory variation, tilt table testing setup with patient on tilting table and continuous monitoring, and QSART electrode placement on limb with sweat response graph. Panel D: Treatments summarized and organized by condition including orthostatic hypotension (compression stockings, fludrocortisone pill, midodrine), gastroparesis (small meals icon, metoclopramide), neurogenic bladder (catheter for flaccid, oxybutynin for spastic), and erectile dysfunction (sildenafil pill).</image>

Summary

The autonomic nervous system controls involuntary visceral functions through sympathetic ("fight or flight"), parasympathetic ("rest and digest"), and enteric divisions. A two-neuron chain characterizes both sympathetic and parasympathetic pathways: preganglionic neurons in the CNS synapse on postganglionic neurons in peripheral ganglia.

The sympathetic division originates from T1-L2 (thoracolumbar outflow), with short preganglionic fibers to chain ganglia near the cord and long postganglionic fibers to targets. The parasympathetic division has craniosacral outflow (CN III, VII, IX, X; S2-S4), with long preganglionic fibers to ganglia near or within target organs and short postganglionic fibers.

All preganglionic neurons release acetylcholine acting on nicotinic receptors. Parasympathetic postganglionic neurons release acetylcholine on muscarinic receptors (M1-M5). Most sympathetic postganglionic neurons release norepinephrine on adrenergic receptors (alpha-1, alpha-2, beta-1, beta-2, beta-3), except sweat glands which receive cholinergic sympathetic innervation.

Organ effects demonstrate dual innervation: heart (sympathetic increases rate via beta-1, parasympathetic decreases via M2), lungs (sympathetic dilates via beta-2, parasympathetic constricts via M3), GI tract (sympathetic inhibits, parasympathetic stimulates), bladder (sympathetic promotes storage, parasympathetic promotes voiding).

The enteric nervous system contains myenteric (motility) and submucosal (secretion) plexuses using diverse transmitters (ACh, NO, 5-HT, VIP, Substance P) to coordinate digestive functions.

Central control involves the hypothalamus (integration) and brainstem centers (NTS for afferent integration, RVLM for sympathetic tone). The baroreceptor reflex maintains blood pressure homeostasis.

Autonomic disorders include orthostatic hypotension (failed sympathetic vasoconstriction), Horner syndrome (miosis, ptosis, anhidrosis from sympathetic pathway lesion), and autonomic neuropathy (diabetic).

Key Terms

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