Autonomic Nervous System — Pharmacology and Physiology

Anatomic Organization

• Sympathetic nervous system (SNS): Thoracolumbar outflow (T1-L2) via paravertebral and collateral ganglia; preganglionic fibers are short, postganglionic fibers are long
• Parasympathetic nervous system (PNS): Craniosacral outflow via CN III, VII, IX, X and S2-S4 spinal nerves; preganglionic fibers are long, postganglionic fibers are short
• Enteric nervous system: Intrinsic innervation of GI tract with 500 million neurons; functions semi-independently but receives sympathetic and parasympathetic input

Neurotransmission Mechanisms

• Adrenergic signaling: Sympathetic postganglionic neurons and adrenal medulla release norepinephrine (NE) and epinephrine (Epi)
• NE and Epi bind to α-adrenergic receptors (α1, α2) and β-adrenergic receptors (β1, β2, β3)
• Receptors are G-protein coupled receptors (GPCRs) linked to distinct intracellular cascades
• α1-adrenergic (Gq-coupled): ↑IP3/DAG → ↑[Ca²⁺]i → vasoconstriction, mydriasis, bronchial smooth muscle constriction
• α2-adrenergic (Gi-coupled): ↓cAMP, ↑K⁺ channels → presynaptic inhibition, vasodilation, sedation
• β1-adrenergic (Gs-coupled): ↑cAMP → ↑cardiac contractility, ↑heart rate, ↑renin release, ↑lipolysis
• β2-adrenergic (Gs-coupled): ↑cAMP → bronchial smooth muscle relaxation, vasodilation, skeletal muscle tremor
• β3-adrenergic (Gs-coupled): thermogenesis, lipolysis in adipose tissue
• Reuptake mechanism: NE recycled by norepinephrine transporter (NET), blocked by tricyclic antidepressants (TCAs) and SNRIs
• Metabolism: Enzymatic degradation via monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT)

• Cholinergic signaling: Parasympathetic postganglionic and sympathetic preganglionic neurons release acetylcholine (ACh)
• ACh binds to muscarinic receptors (M1-M5) and nicotinic receptors
• Muscarinic receptors are metabotropic GPCRs:
• M1, M3, M5 (Gq-coupled): ↑IP3/DAG → ↑[Ca²⁺]i → pupil constriction, accommodation, salivation, bronchial constriction, GI smooth muscle contraction, urinary bladder contraction
• M2, M4 (Gi-coupled): ↓cAMP → negative chronotropic/inotropic effects, ↑AV node refractory period
• Nicotinic receptors are ionotropic (ligand-gated ion channels), predominantly on skeletal muscle and at autonomic ganglia
• Reuptake/Inactivation: ACh is rapidly degraded by acetylcholinesterase (AChE) in the synaptic cleft; this is the target for anticholinesterase agents

Signal Transduction Integration

• cAMP cascade: β-agonist binding → Gs activation → ↑adenylyl cyclase → ↑cAMP → ↑PKA → phosphorylation of ion channels, contractile proteins, metabolic enzymes
• IP3/DAG cascade: α1-agonist or M1/M3 binding → Gq activation → ↑phospholipase C → cleavage of PIP2 → IP3 (↑[Ca²⁺]i) and DAG (↑PKC)
• These cascades are regulated by phosphodiesterases (PDEs), GRK (G-receptor kinases), and beta-arrestins

Organ-System Physiology

Autonomic Tone and Balance

• Baseline autonomic activity reflects tonic inhibition and facilitation
• Sympathetic tone sets baseline vascular resistance, heart rate, and metabolic rate
• Parasympathetic tone dominates at rest (vagal brake), limiting heart rate via M2 receptors
• Baroreceptor reflex: Increased blood pressure → ↑baroreceptor firing → ↑PNS activity and ↓SNS activity → decreased heart rate and contractility
• Homeostatic integration: Central nervous system coordinated responses via hypothalamus, brainstem, and spinal autonomic centers

Primary Autonomic Dysfunction

• Primary Autonomic Failure (PAF): Neurodegenerative conditions including Pure Autonomic Failure (PAF), Multiple System Atrophy (MSA), and Parkinson's Disease with autonomic dysfunction; pathology includes neuronal loss in dorsal motor nucleus of vagus, intermediolateral column, and sympathetic ganglia
• Familial Dysautonomia (Riley-Day syndrome): Autosomal recessive, loss-of-function mutation in IKBKAP gene, affects neural crest-derived neurons
• Hereditary Sensory and Autonomic Neuropathy (HSAN): Multiple genetic subtypes (HSAN I-V); progressive loss of small-fiber sensory and autonomic neurons

Secondary Autonomic Neuropathy

• Diabetes mellitus: Most common cause of autonomic neuropathy in developed countries; hyperglycemia → microvascular injury, oxidative stress, AGE formation, and neuronal loss
• Infections: Viral (HIV, hepatitis C), bacterial (Lyme disease, leprosy), parasitic; viral infections may trigger autoimmune mechanisms
• Autoimmune/Inflammatory: Guillain-Barré Syndrome (GBS), Autoimmune Autonomic Ganglionopathy (AAG), Sjögren syndrome, Systemic Lupus Erythematosus (SLE), Celiac disease
• Malignancy: Paraneoplastic syndromes via anti-HU, anti-CRMP5, anti-VGCC antibodies; direct infiltration by tumor
• Amyloidosis: Primary (AL) and hereditary (transthyretin—TTR); amyloid deposition in sympathetic ganglia and postganglionic neurons
• Medications: Anticholinergics (atropine, antihistamines), chemotherapy (vincristine, cisplatin), antiretrovirals (didanosine), antipsychotics (clozapine), sympathomimetics causing tachyphylaxis
• Toxins: Alcohol, organophosphates, heavy metals (thallium, arsenic)
• Metabolic/Endocrine: Hypothyroidism, hypogonadism, vitamin B12 deficiency, uremia
• Dysautonomia syndromes: Postural Orthostatic Tachycardia Syndrome (POTS), Neurocardiogenic Syncope, Vasovagal Syncope, Inappropriate Sinus Tachycardia (IST)

Risk Factors for Autonomic Dysfunction

• Aging (physiologic decline in baroreceptor sensitivity, blood pressure regulation)
• Poor glycemic control in diabetes
• Smoking and hypertension
• Prolonged immobility or deconditioning
• Psychological stress
• Female sex (POTS more common in women)

Cardiovascular Manifestations

• Orthostatic hypotension: Symptomatic blood pressure drop (≥20 mmHg systolic or ≥10 mmHg diastolic) upon standing; presents with presyncope, syncope, lightheadedness, blurred vision, cognitive fog
• Supine hypertension: Elevated blood pressure when recumbent (paradoxical in some PAF/MSA patients)
• Syncope: Loss of consciousness from cerebral hypoperfusion; often preceded by prodrome (palpitations, chest discomfort, dyspnea)
• Arrhythmias: Fixed heart rate (denervation), exercise-induced tachycardia, sudden nocturnal death in some amyloidosis patients
• Palpitations: Sensation of irregular or forceful heartbeats

Respiratory Manifestations

• Sleep-disordered breathing: Central sleep apnea, obstructive sleep apnea (from loss of upper airway tone)
• Stridor: From laryngeal dysfunction (especially in MSA)
• Dyspnea on exertion: From blunted cardiac output increase with exercise
• Sudden unexplained death in sleep (SUDS): Catastrophic in some PAF/MSA patients

Gastrointestinal Manifestations

• Dysphagia: Especially MSA with involvement of brainstem swallowing centers
• Gastroparesis: Delayed gastric emptying causing early satiety, bloating, nausea, vomiting
• Constipation: From decreased GI motility; severe enough to cause pseudo-obstruction
• Diarrhea/fecal incontinence: From loss of parasympathetic coordination or small intestinal bacterial overgrowth (SIBO)
• Abdominal pain: From dysmotility or pseudo-obstruction

Genitourinary Manifestations

• Urinary retention: Inability to empty bladder, postvoid residuals; risk of recurrent UTIs
• Urinary incontinence: From detrusor overactivity or sphincter insufficiency
• Erectile dysfunction: Loss of parasympathetic-mediated vasodilation; often early sign
• Retrograde ejaculation: From sympathetic denervation

Thermoregulatory Manifestations

• Anhidrosis: Reduced or absent sweating, particularly in upper trunk/axilla (distal sweating often preserved in small-fiber neuropathy)
• Hyperhidrosis: Paradoxical excessive sweating, often nocturnal ("sweating spells")
• Intolerance to heat/cold: Inability to maintain temperature homeostasis

Ocular/Pupillary Manifestations

• Pupillary abnormalities: Miosis in Horner syndrome (preganglionic or postganglionic sympathetic lesion), mydriasis with light-near dissociation in Adie tonic pupil
• Accommodation dysfunction: Blurred vision, difficulty focusing

Sudomotor/Skin Manifestations

• Loss of skin turgor: From autonomic-mediated vasodilation loss and dehydration
• Skin color changes: Mottling, cyanosis from vascular dysregulation
• Trophic changes: Skin atrophy, nail dystrophy in chronic neuropathy

Metabolic/Endocrine Manifestations

• Impaired hypoglycemic awareness: Inability to sense low blood glucose in diabetes (loss of adrenergic warning symptoms)
• Inability to increase heart rate with exercise: Blunted cardiac response; exercise intolerance

Neuropsychiatric Manifestations

• Cognitive dysfunction: "Brain fog," attention deficits
• Depression/anxiety: Common in chronic dysautonomia
• Altered pain perception: Loss of small-fiber nociceptors

Physical Examination Findings

• Orthostatic vital sign changes: Repeat BP/HR supine, sitting, standing (after 3-5 minutes upright)
• Diminished or absent reflexes: Deep tendon reflex loss in distal extremities (sign of small-fiber neuropathy)
• Stocking-glove sensory loss: Decreased temperature and pain sensation distally
• Tachycardia at rest: Resting HR >100 bpm
• Skin signs: Mottled skin, loss of hair, trophic changes
• Pupillary examination: Anisocoria, tonic pupils, segmental iris palsy
• Gait abnormality: Parkinsonian features in MSA, wide-based gait from vestibular involvement
• Tremor: Parkinsonian tremor in MSA/Parkinson disease

Clinical History and Physical Examination

• Detailed symptom inventory: Orthostatic symptoms (timing, severity), syncope description, GI symptoms (nausea, vomiting, constipation, diarrhea), GU symptoms (retention, incontinence, erectile dysfunction), thermoregulatory complaints (excessive sweating, heat intolerance), exercise tolerance
• Medication review: Identify autonomic-active drugs (anticholinergics, sympathomimetics, vasodilators)
• Orthostatic vital signs: Gold standard initial screening test
• Measure supine BP/HR after 5 minutes recumbency
• Measure BP/HR after 1, 3, 5 minutes standing
• Positive result: ≥20 mmHg decrease in systolic BP or ≥10 mmHg decrease in diastolic BP from baseline

Laboratory Testing

• Complete Metabolic Panel: Assess kidney function, glucose, electrolytes (hyponatremia may occur in SIADH from autonomic dysfunction)
• Glucose and HbA1c: Screen for diabetes as most common cause of autonomic neuropathy
• Vitamin B12 and folate levels: Deficiency causes reversible autonomic neuropathy
• Thyroid function tests (TSH, free T4): Hypothyroidism impairs autonomic function
• Plasma norepinephrine levels:

Cholinergic excess (muscarinic overstimulation)

• Acetylcholinesterase inhibitors / organophosphates: ACh accumulates at muscarinic and nicotinic synapses → SLUDGE (salivation, lacrimation, urination, defecation, GI distress, emesis) or DUMBBELLS; bradycardia (M2), bronchospasm and bronchorrhea (M3) cause death. Antidote is an antimuscarinic — atropine, titrated to drying of secretions, not to heart rate — plus pralidoxime to reactivate phosphorylated AChE before "aging" renders the bond irreversible. Benzodiazepines treat seizures. Monitor RBC/plasma cholinesterase activity.
• Direct muscarinic agonists (bethanechol, pilocarpine): contraindicated in asthma/COPD, peptic ulcer disease, and bowel or bladder outlet obstruction.

Antimuscarinic toxicity

• Atropine, antihistamines, TCAs, antipsychotics: hot as a hare, dry as a bone, red as a beet, blind as a bat, mad as a hatter. Physostigmine (tertiary amine, crosses the blood–brain barrier) reverses delirium, but is contraindicated in TCA overdose — sodium-channel blockade with QRS widening is treated with sodium bicarbonate. Anticholinergics precipitate acute angle-closure glaucoma and urinary retention in BPH.

Adrenergic agents

• Nonselective beta blockers: bronchospasm (β2 blockade) and blunted adrenergic warning symptoms of hypoglycemia — ADA Standards of Care flag this in insulin-treated patients; cardioselective agents are preferred and, per GOLD, are not contraindicated in COPD. Abrupt withdrawal causes rebound tachycardia/hypertension from receptor upregulation; taper.
• Overdose: beta blocker toxicity bypasses the receptor with glucagon (Gs-coupled, raises cAMP); calcium channel blocker toxicity is treated with IV calcium and high-dose insulin euglycemic therapy with close glucose and potassium monitoring.
• Alpha-1 blockers: first-dose orthostatic syncope; tamsulosin causes intraoperative floppy iris syndrome.
• Clonidine withdrawal produces rebound hypertension; unopposed alpha stimulation follows beta blockade in pheochromocytoma or cocaine toxicity — alpha blockade (phenoxybenzamine) always precedes beta blockade, and AHA guidance favors benzodiazepines first in cocaine-associated chest pain.
• Vasopressor extravasation: local phentolamine prevents ischemic necrosis. MAOI plus tyramine → hypertensive crisis, also reversed with phentolamine.

• Organophosphate poisoning: farm worker with miosis, bronchorrhea, bradycardia, and fasciculations. Single best next step is atropine titrated to secretions, then pralidoxime before enzyme aging. The classic distractor is giving pralidoxime alone — it does not treat muscarinic symptoms.
• Pheochromocytoma: the tested association is alpha blockade before beta blockade. Beta blocker first leaves unopposed alpha vasoconstriction and hypertensive crisis. The same logic explains avoiding beta blockers in acute cocaine toxicity (AHA statement favors benzodiazepines).
• Physostigmine vs. neostigmine: only physostigmine is a tertiary amine that crosses the blood–brain barrier and treats central antimuscarinic delirium. Neostigmine (quaternary, charged) stays peripheral — used for myasthenia gravis and reversal of nondepolarizing blockade. Physostigmine is contraindicated in TCA overdose; give sodium bicarbonate for a widened QRS.
• Beta blocker overdose: glucagon works because it raises cardiac cAMP through its own Gs-coupled receptor, bypassing the blocked β1 receptor.
• Horner syndrome pupil pharmacology: cocaine (blocks NET) fails to dilate any Horner pupil; apraclonidine reverses the anisocoria (denervation supersensitivity); hydroxyamphetamine dilates only preganglionic/central lesions. Dilute pilocarpine constricting a tonic Adie pupil is the parasympathetic analogue of denervation supersensitivity.
• Sweat glands are the exception: sympathetic postganglionic fibers to eccrine glands release acetylcholine onto muscarinic receptors — why anticholinergics cause anhidrosis and hyperthermia.
• Clonidine (central α2 agonist) lowers sympathetic outflow; abrupt stop causes rebound hypertension. Do not treat that rebound with a nonselective beta blocker alone.
• Neurogenic orthostatic hypotension: nonpharmacologic measures first (salt, fluids, compression, head-up sleeping); midodrine (α1 agonist) or droxidopa are added, and both require monitoring for supine hypertension — the reason the last dose is avoided near bedtime.