Autonomic Nervous System Anatomy

1. Quick Answer

Autonomic nervous system (ANS) anatomy describes the involuntary neural control of visceral functions through two divisions: the sympathetic (thoracolumbar outflow T1-L2) and parasympathetic (craniosacral outflow - CN III, VII, IX, X and S2-4) systems.

Key Concepts:

• Two-neuron pathway: preganglionic (central) → ganglion → postganglionic (peripheral)
• Sympathetic ganglia are paravertebral (chain) or prevertebral (celiac, mesenteric)
• Parasympathetic ganglia are near or within target organs (terminal ganglia)
• Neurotransmitters: acetylcholine (all preganglionic, parasympathetic postganglionic) and noradrenaline (sympathetic postganglionic)

ICU Relevance:

• Cardiovascular instability in spinal cord injury, Guillain-Barré syndrome, brainstem lesions
• Pharmacological manipulation of autonomic receptors (vasopressors, inotropes, anticholinergics)
• Autonomic dysfunction in critical illness (dysautonomia, baroreflex failure)
• Regional anaesthesia effects (stellate ganglion block, coeliac plexus block)

Exam Focus:

• CICM First Part examiners commonly test sympathetic chain anatomy, target organ effects, receptor pharmacology, and clinical syndromes (Horner's, spinal shock)

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2. CICM First Part Exam Focus

What Examiners Expect

Written SAQ:

Common question stems:

• "Describe the anatomy of the sympathetic nervous system with reference to its outflow, ganglia, and target organ effects"
• "Compare and contrast the sympathetic and parasympathetic divisions of the autonomic nervous system"
• "Outline the anatomy of autonomic control of the cardiovascular system"
• "Describe the anatomy relevant to Horner's syndrome and its causes"
• "Explain the anatomical basis of autonomic dysfunction following spinal cord injury"

Expected depth:

• Detailed knowledge of preganglionic neuron locations and pathways
• Understanding of ganglion locations (paravertebral chain, prevertebral, terminal)
• Neurotransmitter and receptor identification at each synapse
• Target organ effects with clinical correlation
• Clear diagrams of sympathetic chain and cranial nerve parasympathetic pathways

Written MCQ:

Common topics tested:

• Spinal cord levels for specific sympathetic outflows (e.g., T1 for pupillary dilation)
• Cranial nerve parasympathetic ganglia (ciliary, pterygopalatine, submandibular, otic)
• Receptor types and their effects (α1, α2, β1, β2, M1-M3, nicotinic)
• Consequences of sympathetic/parasympathetic lesions
• Anatomical course of sympathetic chain

Difficulty level:

• Applied scenarios (e.g., "A patient with a C5 spinal cord injury develops hypotension. Explain the anatomical basis")
• Identification of specific ganglia from descriptions
• Clinical consequences of nerve lesions

Oral Viva:

Expected discussion flow:

1. Define/Overview - Division of autonomic nervous system, general organization
2. Central Control - Hypothalamus, brainstem nuclei, limbic system connections
3. Sympathetic Anatomy - Outflow levels, chain ganglia, prevertebral ganglia, pathways
4. Parasympathetic Anatomy - Cranial and sacral outflow, specific pathways
5. Neurotransmitters - Acetylcholine, noradrenaline, receptor subtypes
6. Target Organ Effects - Systematic comparison of effects
7. Clinical Application - Horner's syndrome, spinal cord injury, stellate block

Common viva scenarios:

• "Describe the anatomical pathway for sympathetic innervation of the heart"
• "A patient has unilateral miosis and ptosis. Explain the anatomy"
• "Describe the autonomic changes you would expect in a T6 complete spinal cord injury"

Pass vs Fail Performance

Pass Standard:

• Correct identification of sympathetic outflow levels (T1-L2)
• Accurate description of two-neuron pathway architecture
• Clear understanding of ganglion locations and types
• Correct neurotransmitter and receptor identification
• Ability to explain common clinical syndromes anatomically

Common Reasons for Failure:

• Confusing sympathetic outflow levels (stating "C1-L5" instead of T1-L2)
• Not knowing the difference between paravertebral and prevertebral ganglia
• Inability to describe cranial nerve parasympathetic pathways
• Confusing nicotinic and muscarinic receptor locations
• Poor understanding of Horner's syndrome anatomy

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3. Key Points

Must-Know Facts

1. Thoracolumbar Outflow: Sympathetic preganglionic neurons are located exclusively in the intermediolateral cell column (IML) of spinal cord segments T1-L2. This is the ONLY location for sympathetic preganglionic cell bodies (PMID: 16399840).

2. Craniosacral Outflow: Parasympathetic preganglionic neurons arise from cranial nerve nuclei (Edinger-Westphal [CN III], superior salivatory [CN VII], inferior salivatory [CN IX], dorsal motor nucleus of vagus [CN X]) and sacral spinal segments S2-S4 (PMID: 17628207).

3. Paravertebral Chain Ganglia: 22-24 pairs of ganglia forming the sympathetic chain from base of skull to coccyx. Superior cervical ganglion (C1-C4), middle cervical (C5-C6), stellate/cervicothoracic (C7-T1), thoracic (T1-T12), lumbar (L1-L4), sacral (S1-S4). Fusion occurs at stellate and sacral levels (PMID: 15626370).

4. Prevertebral Ganglia: Celiac ganglion (T5-T12), superior mesenteric (T10-L1), inferior mesenteric (L1-L2), aorticorenal (renal plexus). These receive splanchnic nerves and supply abdominal/pelvic viscera (PMID: 22399288).

5. Neurotransmitter Pattern: All preganglionic neurons (both sympathetic and parasympathetic) release acetylcholine acting on nicotinic receptors. Postganglionic parasympathetic neurons release ACh acting on muscarinic receptors. Postganglionic sympathetic neurons release noradrenaline (except sweat glands - ACh muscarinic) (PMID: 29939650).

6. Adrenal Medulla: Modified sympathetic ganglion with chromaffin cells serving as modified postganglionic neurons. Releases adrenaline (80%) and noradrenaline (20%) directly into blood. Innervated by preganglionic fibres from T5-T11 via greater splanchnic nerve (PMID: 25560243).

7. Vagus Nerve Distribution: CN X provides 75% of all parasympathetic outflow. Supplies heart (SA/AV nodes), respiratory tract, GI tract from pharynx to splenic flexure, liver, pancreas. Right vagus → posterior of esophagus → celiac plexus (stomach, liver, biliary); Left vagus → anterior of esophagus → gastric branches (PMID: 28922024).

8. Baroreceptor Reflex Arc: Afferents from carotid sinus (CN IX) and aortic arch (CN X) → nucleus tractus solitarius → integration with hypothalamus and vasomotor centre → efferent sympathetic and vagal modulation. This is the primary rapid blood pressure control mechanism (PMID: 15155527).

9. Horner's Syndrome Anatomy: Three-neuron pathway - First order: hypothalamus → ciliospinal centre of Budge (C8-T2); Second order: C8-T2 → superior cervical ganglion (via stellate ganglion, over lung apex); Third order: superior cervical ganglion → orbit (via internal carotid plexus). Clinical features: miosis, ptosis, anhidrosis (PMID: 16685072).

10. Spinal Cord Injury Autonomic Effects: Lesions above T6 cause loss of supraspinal control over splanchnic sympathetics → neurogenic shock (acute phase with hypotension, bradycardia) and autonomic dysreflexia (chronic phase with severe hypertension from uninhibited sympathetic reflexes) (PMID: 25644695).

Essential Anatomical Relationships

Stellate (Cervicothoracic) Ganglion:

• Location: Anterior to C7 transverse process, neck of first rib
• Formed by fusion of inferior cervical and first thoracic ganglia
• Relations: Vertebral artery (posterior), subclavian artery (anteroinferior), pleura (anterolateral)
• Clinical significance: Block causes ipsilateral Horner's syndrome; supplies upper limb, head/neck, heart

Superior Cervical Ganglion:

• Largest sympathetic ganglion
• Location: At C2-C3, posterior to internal carotid artery
• Supplies: Head, neck, heart (cardiac nerves)
• Postganglionic fibres travel with internal and external carotid arteries

Normal Values and Key Numbers Table

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4. Organization of the Autonomic Nervous System

4.1 Central Autonomic Network

The central autonomic network (CAN) integrates autonomic functions through hierarchical control centres (PMID: 24795271).

Hypothalamus

Location and Structure:

• Forms floor and lateral walls of third ventricle
• Extends from optic chiasm to mammillary bodies
• Contains multiple nuclei with distinct autonomic functions

Key Nuclei and Functions:

Integration Functions:

• Receives input from limbic system (amygdala, hippocampus) → emotional autonomic responses
• Connects to brainstem autonomic nuclei via descending pathways
• Integrates autonomic, endocrine, and behavioural responses
• PMID: 21047527

Brainstem Autonomic Centres

Medullary Cardiovascular Centres:

1. Rostral Ventrolateral Medulla (RVLM):

   • Contains presympathetic neurons projecting to IML
   • Generates tonic vasomotor tone
   • Damage → profound hypotension
   • PMID: 10799682

2. Caudal Ventrolateral Medulla (CVLM):

   • Inhibits RVLM
   • Mediates baroreflex-induced sympathoinhibition
   • PMID: 12949230

3. Nucleus Tractus Solitarius (NTS):

   • Primary relay for visceral afferents
   • Receives baroreceptor, chemoreceptor, cardiac afferents
   • Projects to RVLM, CVLM, dorsal vagal nucleus
   • PMID: 9651131

4. Nucleus Ambiguus (NA):

   • Compact formation: vagal motor to larynx/pharynx
   • External formation: cardiac vagal preganglionic neurons
   • Rapid heart rate control via vagal efferents
   • PMID: 8876211

5. Dorsal Motor Nucleus of Vagus (DMV):

   • Parasympathetic preganglionic neurons to GI tract
   • Less cardiac influence than nucleus ambiguus
   • PMID: 18272009

Respiratory-Cardiovascular Coupling:

• Pre-Bötzinger complex (respiratory rhythm generator)
• Respiratory sinus arrhythmia mediated through NTS-NA connections
• PMID: 29339454

Limbic System Connections

Amygdala:

• Central nucleus projects to hypothalamus and brainstem
• Mediates fear-related autonomic responses (fight-or-flight)
• Cardiovascular, respiratory, and GI responses to emotional stimuli
• PMID: 11113298

Insular Cortex:

• Visceral sensory and motor integration
• Right insula: sympathetic predominance
• Left insula: parasympathetic predominance
• Stroke affecting insula → cardiac arrhythmias
• PMID: 10862698

Anterior Cingulate Cortex:

• Autonomic modulation with cognitive and emotional processing
• Mediates stress-related autonomic changes
• PMID: 16186025

4.2 Preganglionic Neurons

Sympathetic Preganglionic Neurons

Location:

• Intermediolateral cell column (IML) of spinal cord
• Grey matter between central canal and lateral horn
• Segments T1-L2 (thoracolumbar outflow)
• Cell bodies small to medium (15-30 μm diameter)
• PMID: 16399840

Pathway:

1. Exit spinal cord via ventral root
2. Enter spinal nerve
3. Leave via white ramus communicans (myelinated)
4. Enter paravertebral sympathetic chain
5. Three possible destinations:
   • Synapse at same level ganglia
   • Ascend or descend in chain before synapsing
   • Pass through chain without synapsing → splanchnic nerves to prevertebral ganglia

Characteristics:

• Myelinated (B fibres, 3-15 m/s conduction)
• Acetylcholine neurotransmitter
• Nicotinic receptors on postganglionic neurons
• Preganglionic:postganglionic ratio approximately 1:20 (divergence)
• PMID: 29939650

Parasympathetic Preganglionic Neurons

Cranial Outflow:

Sacral Outflow:

• Preganglionic cell bodies in lateral grey matter of S2-S4
• Exit via ventral roots
• Form pelvic splanchnic nerves (nervi erigentes)
• Synapse in ganglia on or near target organs
• Supply: Descending colon, sigmoid, rectum, bladder, reproductive organs
• PMID: 17628207

Characteristics:

• Long preganglionic fibres (ganglia near/in target organs)
• Myelinated (B fibres)
• Acetylcholine neurotransmitter
• Preganglionic:postganglionic ratio approximately 1:1-1:3 (less divergence than sympathetic)

4.3 Autonomic Ganglia

Paravertebral (Chain) Ganglia

Structure and Organization:

• Bilateral chains from skull base to coccyx
• Connected by interganglionic rami
• Receive preganglionic fibres via white rami communicantes (T1-L2 only)
• Send postganglionic fibres via grey rami communicantes to all spinal nerves
• PMID: 15626370

Cervical Ganglia:

1. Superior Cervical Ganglion (SCG):

   • Largest (approximately 3 cm long)
   • Location: C2-C3, posterior to internal carotid
   • Formed from fusion of C1-C4 ganglia
   • Postganglionic fibres via:
     • Internal carotid plexus → orbit (pupil dilator, Müller's muscle)
     • External carotid plexus → face (sweat glands, blood vessels)
     • Superior cardiac nerve → heart
   • PMID: 24473893

2. Middle Cervical Ganglion:

   • Smallest cervical ganglion (may be absent in 20%)
   • Location: C6 level, anterior to inferior thyroid artery
   • Supplies: Thyroid, heart (middle cardiac nerve)

3. Stellate (Cervicothoracic) Ganglion:

   • Fusion of inferior cervical and first thoracic ganglia
   • Location: C7/T1, anterior to neck of first rib
   • Relations: Vertebral artery, subclavian artery, pleural dome
   • Supplies: Upper limb, heart (inferior cardiac nerve), head/neck
   • Block causes ipsilateral Horner's syndrome
   • PMID: 26360106

Thoracic and Lumbar Ganglia:

• 10-12 thoracic ganglia (may fuse)
• 4 lumbar ganglia
• Postganglionic fibres to blood vessels, sweat glands, piloerector muscles

Sacral Ganglia:

• 4-5 sacral ganglia
• Often fused
• End at ganglion impar (midline, anterior to coccyx)

Prevertebral (Collateral) Ganglia

Location: Anterior to vertebral column, associated with abdominal aorta and major branches

Celiac Ganglia (paired):

• Location: Around celiac trunk origin (T12/L1)
• Receives: Greater (T5-T9) and lesser (T10-T11) splanchnic nerves
• Supplies: Stomach, liver, gallbladder, spleen, pancreas, kidneys, adrenal cortex
• PMID: 22399288

Superior Mesenteric Ganglion:

• Location: Around superior mesenteric artery origin
• Receives: Lesser splanchnic nerve, lumbar splanchnics
• Supplies: Small intestine, ascending/transverse colon

Inferior Mesenteric Ganglion:

• Location: Around inferior mesenteric artery origin
• Receives: Lumbar splanchnic nerves
• Supplies: Descending colon, sigmoid, upper rectum, internal urethral sphincter

Aorticorenal Ganglia:

• Located at renal artery origins
• Supplies: Kidney, ureter

Terminal (Intramural) Ganglia

Parasympathetic Ganglia:

Cranial:

1. Ciliary Ganglion:

   • Location: Lateral to optic nerve in posterior orbit
   • Preganglionic: Edinger-Westphal nucleus via CN III
   • Postganglionic: Short ciliary nerves to sphincter pupillae, ciliary muscle
   • PMID: 23986182

2. Pterygopalatine Ganglion:

   • Location: Pterygopalatine fossa
   • Preganglionic: Superior salivatory nucleus via CN VII (greater petrosal nerve)
   • Postganglionic: Lacrimal gland, nasal/palatal mucosa
   • PMID: 26097086

3. Submandibular Ganglion:

   • Location: On hyoglossus muscle
   • Preganglionic: Superior salivatory nucleus via CN VII (chorda tympani)
   • Postganglionic: Submandibular and sublingual salivary glands

4. Otic Ganglion:

   • Location: Infratemporal fossa, below foramen ovale
   • Preganglionic: Inferior salivatory nucleus via CN IX (lesser petrosal nerve)
   • Postganglionic: Parotid gland
   • PMID: 17559062

Cardiac and Pulmonary Ganglia:

• Located in cardiac and pulmonary plexuses
• Receive vagal preganglionic fibres
• Very short postganglionic fibres to SA node, AV node, bronchial smooth muscle

Enteric Ganglia:

• Myenteric (Auerbach's) plexus - between muscle layers
• Submucosal (Meissner's) plexus - in submucosa
• Receive parasympathetic (vagus, pelvic splanchnic) and sympathetic input
• Contain intrinsic neurons - "enteric nervous system"
• PMID: 24997029

4.4 Postganglionic Neurons

Sympathetic Postganglionic Neurons

Characteristics:

• Cell bodies in paravertebral or prevertebral ganglia
• Long unmyelinated (C fibres) axons to target organs
• Neurotransmitter: Noradrenaline (norepinephrine)
• Exception: Sweat glands receive cholinergic sympathetic fibres (muscarinic)
• Also exception: Adrenal medulla chromaffin cells (modified postganglionic neurons) release adrenaline/noradrenaline directly into blood

Pathways to Target Organs:

1. Grey ramus communicans → spinal nerve → peripheral nerve → blood vessels, sweat glands, piloerector muscles
2. Periarterial plexuses → following arteries to organs
3. Specific named nerves (cardiac nerves, splanchnic nerves)

Parasympathetic Postganglionic Neurons

Characteristics:

• Cell bodies in terminal ganglia (near or within target organs)
• Very short axons
• Neurotransmitter: Acetylcholine (muscarinic receptors on effectors)
• PMID: 29939650

Distribution:

• Cranial: Limited to head structures (pupils, glands, blood vessels)
• Vagal: Heart, respiratory tract, GI tract to splenic flexure
• Sacral: Distal colon, rectum, bladder, reproductive organs

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5. Sympathetic Division

5.1 Thoracolumbar Outflow (T1-L2)

Segmental Organization:

Lateral Horn (Intermediolateral Cell Column):

• Contains preganglionic neuronal cell bodies
• Only present at T1-L2 levels
• Extends into lateral horn of grey matter
• Receives descending input from hypothalamus, brainstem
• PMID: 16399840

5.2 Paravertebral Chain Ganglia

Anatomy of the Sympathetic Chain:

The sympathetic chain (paravertebral ganglia) extends from the base of the skull to the coccyx as two parallel chains connected by interganglionic rami (PMID: 15626370).

Pathway Options for Preganglionic Fibres:

1. Synapse at entry level: Preganglionic fibre synapses in ganglion at level of entry
2. Ascend before synapsing: Fibres ascend in chain (e.g., T1-T2 → cervical ganglia for head/neck supply)
3. Descend before synapsing: Fibres descend in chain (e.g., L1-L2 → lumbar/sacral ganglia for lower limb)
4. Pass through without synapsing: Splanchnic nerves pass through chain to prevertebral ganglia

White vs Grey Rami Communicantes:

5.3 Prevertebral Ganglia

Splanchnic Nerves:

Preganglionic fibres passing through the paravertebral chain without synapsing form splanchnic nerves to prevertebral ganglia (PMID: 22399288).

Clinical Relevance - Splanchnic Nerve Block:

• Performed at T12/L1 level
• Indications: Chronic visceral pain (pancreatic cancer, chronic pancreatitis)
• Landmark: Anterolateral to vertebral body under fluoroscopy
• Side effects: Hypotension (loss of splanchnic vascular tone), diarrhoea
• PMID: 22127693

5.4 Adrenal Medulla

Modified Sympathetic Ganglion:

The adrenal medulla represents a modified sympathetic ganglion where chromaffin cells serve as postganglionic neuron equivalents (PMID: 25560243).

Key Features:

• Embryologically derived from neural crest (same as sympathetic ganglia)
• No postganglionic neurons - chromaffin cells release directly into blood
• Innervated by preganglionic fibres (T5-T11 via greater splanchnic nerve)
• Secretes catecholamines: 80% adrenaline, 20% noradrenaline
• Release triggered by acetylcholine on nicotinic receptors
• PMID: 26340977

Catecholamine Synthesis:

Tyrosine → Tyrosine hydroxylase → L-DOPA → DOPA decarboxylase → Dopamine
→ Dopamine β-hydroxylase → Noradrenaline → PNMT → Adrenaline

Phenylethanolamine N-methyltransferase (PNMT):

• Converts noradrenaline to adrenaline
• Present in adrenal medulla chromaffin cells
• Induced by cortisol (from adjacent adrenal cortex)
• Explains 80:20 adrenaline:noradrenaline ratio
• PMID: 25560243

Clinical Implications:

• Phaeochromocytoma: Tumour of chromaffin cells → catecholamine excess
• Adrenalectomy: Removes major source of circulating adrenaline
• Stress response: Adrenal medulla release augments neural sympathetic effects

5.5 Target Organ Effects

Cardiovascular System:

Respiratory System:

Gastrointestinal System:

Genitourinary System:

Eye:

Skin and Metabolic:

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6. Parasympathetic Division

6.1 Craniosacral Outflow

Overview:

The parasympathetic nervous system originates from two separate regions, hence "craniosacral" outflow (PMID: 17628207):

• Cranial: Nuclei in brainstem associated with CN III, VII, IX, X
• Sacral: Lateral grey matter of S2-S4 spinal segments

Key Characteristics Distinguishing from Sympathetic:

• Preganglionic fibres are long (ganglia near targets)
• Postganglionic fibres are short
• Less divergence (1:1 to 1:3 ratio vs 1:20 sympathetic)
• More discrete, localized effects
• All neurotransmission via acetylcholine

6.2 Cranial Nerve Parasympathetics

Oculomotor Nerve (CN III)

Nucleus: Edinger-Westphal nucleus (accessory oculomotor nucleus)

• Location: Rostral midbrain, dorsal to main oculomotor nucleus
• PMID: 23986182

Pathway:

1. Preganglionic fibres travel with CN III
2. Enter orbit through superior orbital fissure
3. Synapse in ciliary ganglion (lateral to optic nerve)
4. Postganglionic fibres (short ciliary nerves) to:
   • Sphincter pupillae: Pupil constriction (miosis)
   • Ciliary muscle: Lens accommodation (near vision)

Clinical Significance:

• CN III palsy with pupil involvement → aneurysm compression (parasympathetics run peripherally in nerve)
• CN III palsy with pupil sparing → ischemic (central fibres affected first)
• Adie's pupil: Damaged ciliary ganglion → dilated pupil with light-near dissociation

Facial Nerve (CN VII)

Nucleus: Superior salivatory nucleus (pontine tegmentum)

Two Parasympathetic Pathways:

1. Greater Petrosal Nerve → Pterygopalatine Ganglion:

• Preganglionic: Superior salivatory nucleus → nervus intermedius → greater petrosal nerve
• Synapse: Pterygopalatine ganglion
• Postganglionic to: Lacrimal gland, nasal mucosa, palatal glands
• PMID: 26097086

2. Chorda Tympani → Submandibular Ganglion:

• Preganglionic: Superior salivatory nucleus → chorda tympani (crosses tympanic cavity)
• Joins lingual nerve
• Synapse: Submandibular ganglion
• Postganglionic to: Submandibular and sublingual salivary glands

Clinical Significance:

• Bell's palsy: May cause reduced lacrimation, hyperacusis, taste changes
• Greater petrosal nerve lesion: Dry eye (no lacrimation)
• Chorda tympani lesion: Reduced salivation, altered taste on anterior 2/3 of tongue

Glossopharyngeal Nerve (CN IX)

Nucleus: Inferior salivatory nucleus (medulla)

Pathway:

1. Preganglionic: Inferior salivatory nucleus → CN IX → tympanic branch (Jacobson's nerve)
2. Tympanic plexus in middle ear
3. Lesser petrosal nerve exits skull via foramen ovale
4. Synapse: Otic ganglion (infratemporal fossa)
5. Postganglionic fibres join auriculotemporal nerve (V3) to parotid gland

• PMID: 17559062

Clinical Significance:

• Frey syndrome: Aberrant regeneration after parotid surgery → gustatory sweating

Vagus Nerve (CN X)

Nuclei:

1. Dorsal Motor Nucleus of Vagus (DMV): GI parasympathetics
2. Nucleus Ambiguus (external formation): Cardiac parasympathetics

Distribution (provides 75% of all parasympathetic fibres) (PMID: 28922024):

Thoracic:

• Heart: SA node (reduces rate), AV node (reduces conduction), atrial myocardium
• Bronchi: Bronchoconstriction, increased secretions
• Oesophagus: Increased motility

Abdominal:

• Right vagus → posterior vagal trunk → celiac plexus → liver, biliary, stomach, small bowel
• Left vagus → anterior vagal trunk → gastric branches, hepatic branch
• Supplies: Stomach, small intestine, ascending colon, transverse colon (to splenic flexure)
• Does NOT supply: Descending colon, sigmoid, rectum (these receive sacral parasympathetics)

Ganglia: Terminal ganglia in or on target organs (cardiac, pulmonary, enteric plexuses)

6.3 Sacral Outflow (S2-S4)

"S2, 3, 4 keeps the guts off the floor" - mnemonic for sacral parasympathetic function

Anatomy:

• Preganglionic cell bodies in lateral grey matter of S2-S4
• Exit via ventral roots with somatic fibres
• Form pelvic splanchnic nerves (nervi erigentes)
• Synapse in ganglia on or within pelvic organs
• PMID: 17628207

Distribution:

• Descending colon (from splenic flexure)
• Sigmoid colon
• Rectum
• Bladder (detrusor muscle)
• Reproductive organs (erection)

Clinical Significance:

• Cauda equina syndrome: Loss of sacral parasympathetics → bladder/bowel dysfunction
• Spinal anaesthesia: Block at S2-4 → urinary retention
• Radical pelvic surgery: Risk of damage to pelvic splanchnic nerves → erectile dysfunction, bladder dysfunction

6.4 Terminal Ganglia

Characteristics:

• Located in or near target organs
• Very short postganglionic fibres
• Allows for precise, localized control
• Less divergence than sympathetic system

Specific Ganglia:

6.5 Target Organ Effects

Cardiovascular System:

Exception - Parasympathetic Vasodilation:

• Erectile tissue (via NO release)
• Some cerebral vessels
• Salivary gland vessels (during secretion)

Respiratory System:

Gastrointestinal System:

Genitourinary System:

Eye:

Glands:

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7. Neurotransmitters and Receptors

7.1 Acetylcholine

Sites of Release:

• All preganglionic neurons (sympathetic and parasympathetic)
• All postganglionic parasympathetic neurons
• Postganglionic sympathetic to sweat glands
• Somatic motor neurons (for comparison)
• PMID: 29939650

Synthesis:

Acetyl-CoA + Choline → Choline acetyltransferase (ChAT) → Acetylcholine

Termination:

• Acetylcholinesterase (AChE) in synaptic cleft → choline + acetate
• Choline recycled by high-affinity choline transporter

7.2 Nicotinic Receptors

Location: Autonomic ganglia, adrenal medulla, neuromuscular junction

Structure: Ligand-gated ion channel (pentamer of α and β subunits)

Autonomic Ganglionic Type (N_N):

• Subunit composition: α3, β4 (also α5, β2)
• Location: Sympathetic and parasympathetic ganglia
• Effect: Rapid depolarization → action potential in postganglionic neuron
• Antagonists: Hexamethonium, trimethaphan (ganglionic blockers)
• PMID: 27578408

Adrenal Medulla:

• Similar to ganglionic type
• Stimulation → catecholamine release from chromaffin cells

Clinical Significance:

• Ganglionic blockers: Historical antihypertensives (orthostatic hypotension, many side effects)
• Myasthenic syndrome (Lambert-Eaton): Antibodies against presynaptic voltage-gated calcium channels

7.3 Muscarinic Receptors

Location: All parasympathetic target organs, sympathetic sweat glands

Structure: G-protein coupled receptors (7-transmembrane domain)

Subtypes (PMID: 17982456):

Cardiac M2 Receptors:

• SA node: Slows pacemaker rate (↑ K⁺ conductance, ↓ If)
• AV node: Slows conduction
• Atria: Slight decrease in contractility
• Ventricles: Minimal effect (sparse parasympathetic innervation)

Smooth Muscle M3 Receptors:

• Bronchi: Bronchoconstriction
• GI tract: Increased motility
• Bladder: Detrusor contraction
• Pupil: Sphincter contraction (miosis)

Clinical Pharmacology:

• Atropine: Non-selective muscarinic antagonist → tachycardia, mydriasis, dry mouth
• Ipratropium: Inhaled M3 antagonist → bronchodilation
• Pilocarpine: M3 agonist → miosis, increased secretions
• Glycopyrrolate: Quaternary amine, does not cross BBB → peripheral effects only

7.4 Noradrenaline (Norepinephrine)

Site of Release: Postganglionic sympathetic neurons (except sweat glands)

Synthesis:

Tyrosine → L-DOPA → Dopamine → Noradrenaline

Termination:

1. Reuptake (Uptake 1): Noradrenaline transporter (NET) - primary mechanism (70-80%)
2. Uptake 2: Into non-neuronal cells (minor)
3. Enzymatic degradation: MAO (intraneuronal), COMT (extraneuronal)

Storage: Synaptic vesicles (also contain dopamine β-hydroxylase, ATP, chromogranins)

7.5 Adrenergic Receptors

Structure: All G-protein coupled receptors (PMID: 22128289)

Alpha Receptors

α1 Receptors (Gq-coupled → ↑ IP3/DAG → ↑ Ca²⁺):

Clinical: α1 antagonists (prazosin, tamsulosin) → vasodilation, BPH treatment

α2 Receptors (Gi-coupled → ↓ cAMP):

Clinical: α2 agonists (clonidine, dexmedetomidine) → ↓ sympathetic outflow, sedation, analgesia

Beta Receptors

β1 Receptors (Gs-coupled → ↑ cAMP):

• Location: Heart (predominantly)
• Effects: ↑ HR (chronotropic), ↑ contractility (inotropic), ↑ conduction (dromotropic)
• Also: Kidney (renin release)
• PMID: 22128289

β2 Receptors (Gs-coupled → ↑ cAMP):

• Location: Smooth muscle, liver, skeletal muscle
• Effects: Bronchodilation, vasodilation (skeletal muscle), glycogenolysis, ↓ uterine tone

β3 Receptors (Gs-coupled → ↑ cAMP):

• Location: Adipose tissue, bladder
• Effects: Lipolysis, bladder relaxation
• Clinical: Mirabegron (β3 agonist) for overactive bladder

Clinical Pharmacology Summary:

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8. Cardiovascular Autonomic Control

8.1 Baroreceptors

Carotid Sinus Baroreceptors (PMID: 15155527):

Location:

• Dilatation of internal carotid artery at bifurcation
• Wall contains stretch-sensitive mechanoreceptors

Afferent Pathway:

• Glossopharyngeal nerve (CN IX) → carotid sinus nerve (of Hering)
• Projects to nucleus tractus solitarius (NTS) in medulla

Aortic Arch Baroreceptors:

Location:

• Aortic arch wall
• Similar stretch-sensitive mechanoreceptors

Afferent Pathway:

• Vagus nerve (CN X) → aortic depressor nerve
• Projects to NTS

Baroreceptor Reflex Arc:

↑ Blood Pressure
       ↓
Baroreceptor stretch (carotid sinus, aortic arch)
       ↓
Afferent CN IX, X → NTS
       ↓
    ┌─────────────────┬────────────────┐
    ↓                 ↓                ↓
   CVLM            Nucleus         Hypothalamus
(inhibits RVLM)   Ambiguus
    ↓                 ↓
↓ Sympathetic    ↑ Vagal tone
   outflow
    ↓                 ↓
Vasodilation     Bradycardia
↓ SVR            ↓ HR
       ↓
↓ Blood Pressure (normalized)

Clinical Significance:

• Carotid sinus massage: Vagal activation → bradycardia (SVT termination)
• Carotid sinus hypersensitivity: Syncope with minor neck pressure
• Baroreceptor failure: Volatile blood pressure, orthostatic hypotension

8.2 Cardiopulmonary Receptors

Atrial Receptors (Low-Pressure Baroreceptors):

• Location: Right atrium (A receptors at venoatrial junction, B receptors in atrial wall)
• Afferent: Vagus nerve
• Function: Sense blood volume (central venous pressure)
• Response to ↑ CVP: ↑ HR (Bainbridge reflex), ↑ ANP release, ↓ ADH
• PMID: 11882727

Ventricular Receptors:

• Location: Ventricular myocardium
• Afferent: Vagus (C fibres)
• Function: Detect ventricular distension, ischemia
• Bezold-Jarisch reflex: Vigorous ventricular contraction (hypovolemia) → bradycardia, hypotension, coronary dilation

8.3 Vasomotor Centre

Rostral Ventrolateral Medulla (RVLM) (PMID: 10799682):

• Location: Ventrolateral medulla, near facial nucleus
• Function: Generates tonic sympathetic vasomotor tone
• Contains presympathetic neurons → IML of spinal cord
• Bilateral damage → profound hypotension

Caudal Ventrolateral Medulla (CVLM):

• Inhibits RVLM
• Activated by baroreceptor input via NTS
• Mediates sympathoinhibition during hypertension

Integration with Other Inputs:

• Chemoreceptors (carotid body, central medullary) → modify vascular tone
• Limbic system → emotional effects on BP
• Hypothalamus → temperature regulation, stress response

8.4 Cardiac Autonomic Innervation

Sympathetic Innervation:

• Origin: T1-T5 (cardiac accelerator fibres)
• Pathway: Synapse in cervical ganglia → cardiac nerves → cardiac plexus
• Superior cardiac nerve: From SCG
• Middle cardiac nerve: From middle cervical ganglion
• Inferior cardiac nerve: From stellate ganglion
• Distribution: SA node, AV node, atria, ventricles, coronary arteries
• PMID: 27045946

Parasympathetic Innervation:

• Origin: Dorsal motor nucleus (minor), nucleus ambiguus (major)
• Pathway: Vagus nerve → cardiac branches → cardiac plexus → terminal ganglia
• Right vagus: Predominantly to SA node
• Left vagus: Predominantly to AV node
• Distribution: Mainly SA node, AV node, atria; sparse ventricular innervation

Clinical Implications:

• Cardiac transplant: Denervated heart - no vagal response, fixed resting HR 90-100
• Diabetic autonomic neuropathy: Loss of HRV, resting tachycardia
• High spinal cord injury (above T5): Loss of cardiac sympathetics, unopposed vagal tone

---

9. Respiratory Autonomic Control

9.1 Vagal Tone and Bronchomotor Control

Parasympathetic (Vagal) Control (PMID: 18353880):

Pathway:

• Preganglionic: Dorsal motor nucleus of vagus
• Synapse: Airway ganglia (in bronchial walls)
• Postganglionic: To smooth muscle, submucosal glands

Effects:

• Bronchoconstriction (M3 receptors on smooth muscle)
• Increased mucus secretion (M3 receptors on submucosal glands)
• Resting bronchomotor tone is predominantly vagal

Sympathetic Control:

Pathway:

• Minimal direct sympathetic innervation of airway smooth muscle
• Main effect via circulating adrenaline on β2 receptors

Effects:

• Bronchodilation (β2 receptors)
• Decreased secretions (α and β effects)

Clinical Significance:

• β2 agonists (salbutamol): Bronchodilation in asthma/COPD
• Anticholinergics (ipratropium, tiotropium): Block vagal bronchoconstriction
• Vagal stimulation during intubation: Bronchospasm

9.2 Respiratory Reflexes

Pulmonary Stretch Receptors (Hering-Breuer Reflex):

• Location: Airway smooth muscle
• Afferent: Vagus nerve
• Response: Lung inflation → inhibition of inspiration (inflation reflex)
• Significance: Limits lung overdistension, important in mechanical ventilation
• PMID: 7082978

Irritant Receptors:

• Location: Airway epithelium
• Afferent: Vagus nerve
• Stimuli: Smoke, dust, noxious gases
• Response: Cough, bronchoconstriction, increased secretions

J Receptors (Juxtapulmonary Capillary):

• Location: Alveolar interstitium, close to capillaries
• Afferent: Vagus nerve (C fibres)
• Stimuli: Pulmonary congestion, chemical irritants
• Response: Rapid shallow breathing, reflex bronchoconstriction

---

10. Gastrointestinal Autonomic Control

10.1 Enteric Nervous System

The enteric nervous system (ENS) is often called the "second brain" and can function independently of CNS input (PMID: 24997029).

Structure:

Myenteric (Auerbach's) Plexus:

• Location: Between longitudinal and circular muscle layers
• Function: Controls GI motility
• Contains: Motor neurons, interneurons, sensory neurons

Submucosal (Meissner's) Plexus:

• Location: Submucosa
• Function: Controls secretion, blood flow, epithelial function

Neuronal Types:

• Intrinsic primary afferents (sensory)
• Interneurons (processing)
• Motor neurons (excitatory and inhibitory)
• Estimated 200-600 million neurons (equivalent to spinal cord)

Neurotransmitters in ENS:

• Acetylcholine (excitatory to smooth muscle)
• VIP, NO (inhibitory - relaxation)
• Substance P, serotonin (various modulatory functions)

10.2 Parasympathetic Effects

Vagal Supply (Stomach to Splenic Flexure):

• Increases motility (↑ contractions, ↑ tone)
• Relaxes sphincters
• Increases secretion (gastric acid, pancreatic enzymes, bile)
• Gallbladder contraction

Sacral Parasympathetic (Splenic Flexure to Rectum):

• Increases motility
• Relaxes internal anal sphincter (defecation)
• Rectal contraction

10.3 Sympathetic Effects

Splanchnic Nerves → Prevertebral Ganglia:

• Decreases motility (inhibits smooth muscle)
• Contracts sphincters
• Decreases secretion
• Vasoconstriction (reduces blood flow)

Specific Effects:

• Greater splanchnic (T5-T9) → celiac ganglion → stomach, liver, spleen
• Lesser splanchnic (T10-T11) → superior mesenteric ganglion → small bowel, proximal colon
• Lumbar splanchnics (L1-L2) → inferior mesenteric ganglion → distal colon, rectum

Clinical Significance:

• Postoperative ileus: Sympathetic hyperactivity, opioid effects
• Abdominal angina: Splanchnic vasoconstriction
• Coeliac plexus block: Pain management for pancreatic cancer

---

11. Pupillary Control

11.1 Sympathetic Pathway (Pupil Dilation)

Three-Neuron Arc (PMID: 16685072):

First Order Neuron (Central):

• Origin: Posterolateral hypothalamus
• Descends through brainstem tegmentum (uncrossed)
• Ends in ciliospinal centre of Budge (C8-T2)
• Lesion causes: Ipsilateral Horner's (central Horner's)

Second Order Neuron (Preganglionic):

• Origin: Ciliospinal centre (C8-T2 IML)
• Exits via T1 ventral root
• Crosses over lung apex (proximity to Pancoast tumour)
• Ascends in sympathetic chain
• Synapses in superior cervical ganglion
• Lesion causes: Ipsilateral Horner's (preganglionic)

Third Order Neuron (Postganglionic):

• Origin: Superior cervical ganglion
• Travels with internal carotid artery
• Enters skull via carotid canal
• Joins ophthalmic division (V1) as long ciliary nerves
• Innervates dilator pupillae (radial muscle of iris)
• Also supplies Müller's muscle (contributes to upper lid elevation)
• Lesion causes: Ipsilateral Horner's (postganglionic)

Horner's Syndrome Features:

• Miosis: Loss of dilator pupillae → smaller pupil
• Ptosis: Loss of Müller's muscle → 1-2mm lid droop
• Anhidrosis: Loss of sudomotor fibres (pattern depends on lesion level)
• Enophthalmos: Apparent (due to narrowed palpebral fissure)

11.2 Parasympathetic Pathway (Pupil Constriction)

Pathway (PMID: 23986182):

• Origin: Edinger-Westphal nucleus (midbrain)
• Preganglionic fibres travel on surface of CN III (susceptible to compression)
• Synapse: Ciliary ganglion
• Postganglionic: Short ciliary nerves → sphincter pupillae

Pupillary Light Reflex:

Light → Retina → Optic nerve → Optic tract → Pretectal nucleus → 
Bilateral Edinger-Westphal nuclei → CN III → Ciliary ganglion → 
Sphincter pupillae → MIOSIS (both eyes - direct and consensual)

Clinical Significance:

• CN III palsy with fixed dilated pupil: Parasympathetic fibres run peripherally → compression (aneurysm) affects pupils first
• Adie's tonic pupil: Ciliary ganglion damage → dilated pupil, light-near dissociation
• Argyll Robertson pupil: Syphilis → accommodation reflex preserved, light reflex absent

---

12. Bladder Control

12.1 Micturition Reflex

Anatomy (PMID: 12559961):

Detrusor Muscle:

• Smooth muscle of bladder wall
• Parasympathetic (S2-S4): Causes contraction (M3 receptors)
• Sympathetic (T10-L2): Causes relaxation (β2/β3 receptors)

Internal Urethral Sphincter:

• Smooth muscle at bladder neck
• Sympathetic (α1 receptors): Maintains closure

External Urethral Sphincter (Rhabdosphincter):

• Striated muscle
• Pudendal nerve (S2-S4 somatic): Voluntary control
• Onuf's nucleus in S2-S4 anterior horn

Innervation Summary:

12.2 Storage Phase (Sympathetic Dominance)

During Filling:

1. Bladder distension activates afferents → pelvic nerves → sacral cord
2. Sympathetic outflow (hypogastric nerve) activated:
   • β receptors: Detrusor relaxation
   • α1 receptors: Internal sphincter contraction
3. Pudendal nerve (somatic): External sphincter contraction
4. Pontine storage centre maintains continence

12.3 Voiding Phase (Parasympathetic Dominance)

During Micturition:

1. Pontine micturition centre (Barrington's nucleus) activated
2. Parasympathetic (pelvic splanchnic nerve) activated → detrusor contraction
3. Sympathetic inhibited → sphincter relaxation
4. Pudendal nerve inhibited → external sphincter relaxation
5. Coordinated bladder contraction and sphincter relaxation = voiding

• PMID: 25108854

12.4 Clinical Syndromes

Upper Motor Neuron Lesion (above S2):

• Loss of voluntary control
• Reflex bladder (automatic voiding when full)
• Detrusor hyperreflexia
• Detrusor-sphincter dyssynergia (uncoordinated)

Lower Motor Neuron Lesion (S2-S4 or peripheral):

• Areflexic bladder
• Urinary retention
• Overflow incontinence

Spinal Cord Injury:

• Acute (spinal shock): Areflexic bladder, retention
• Chronic (above S2): Reflex bladder, dyssynergia, autonomic dysreflexia triggers

---

13. Applied Anatomy

13.1 Spinal Cord Injury Autonomic Effects

Level-Dependent Effects (PMID: 25644695):

High Cervical (C1-C4):

• Complete loss of all supraspinal autonomic control
• No voluntary control of bladder, bowel
• Respiratory compromise (phrenic nerve C3-5)
• Severe orthostatic hypotension
• Poikilothermia (temperature dysregulation)

Cervical/High Thoracic (C5-T5):

• Loss of cardiac sympathetics
• Bradycardia (unopposed vagal tone)
• Neurogenic shock in acute phase
• High risk of autonomic dysreflexia (chronic phase)

Mid-Thoracic (T6-T10):

• Preserved cardiac sympathetics
• Loss of splanchnic sympathetics
• Still at risk of autonomic dysreflexia
• Less severe cardiovascular instability

Thoracolumbar (T11-L2):

• Preserved most sympathetics
• Loss of lower limb and pelvic sympathetics
• Bladder/bowel dysfunction
• Lower risk of autonomic dysreflexia

Neurogenic Shock (Acute Phase):

• Occurs with injuries above T6
• Loss of sympathetic tone → vasodilation, hypotension
• Loss of cardiac sympathetics → bradycardia
• "Warm shock"
• warm extremities despite hypotension
• Treatment: Vasopressors (phenylephrine, noradrenaline), cautious fluids

Autonomic Dysreflexia (Chronic Phase) (PMID: 26074129):

• Occurs after spinal shock resolution (weeks to months)
• Triggered by noxious stimulus below injury level (distended bladder, faecal impaction)
• Massive uninhibited sympathetic discharge
• Severe hypertension (SBP >200 mmHg possible)
• Reflex bradycardia (from baroreceptors above lesion)
• Flushing/sweating above level, pallor below
• Medical emergency - can cause stroke, seizures, death
• Treatment: Sit upright, remove trigger, rapid-acting antihypertensives

13.2 Horner's Syndrome

Aetiology by Neuron Affected (PMID: 16685072):

First Order (Central) Lesions:

• Hypothalamic lesions (stroke, tumour)
• Brainstem lesions (Wallenberg syndrome - lateral medullary)
• Spinal cord lesions (syringomyelia, demyelination, trauma)
• Features: Anhidrosis of ipsilateral face, neck, arm

Second Order (Preganglionic) Lesions:

• Lung apex tumour (Pancoast)
• Thyroid malignancy
• Trauma (neck surgery, birth trauma)
• Thoracic aortic aneurysm
• Features: Anhidrosis of ipsilateral face only (above superior cervical ganglion)

Third Order (Postganglionic) Lesions:

• Internal carotid artery dissection
• Cavernous sinus lesions
• Cluster headache
• Features: No anhidrosis (sudomotor fibres travel with external carotid)

Cocaine Test:

• Cocaine blocks noradrenaline reuptake
• Normal: Pupil dilates (potentiates endogenous NA)
• Horner's: Pupil fails to dilate (no NA released)
• Confirms Horner's but doesn't localize

Hydroxyamphetamine (Pholedrine) Test:

• Releases stored NA from postganglionic terminals
• Third order lesion: Pupil fails to dilate (damaged postganglionic neuron)
• First/second order lesion: Pupil dilates (intact postganglionic neuron with stored NA)

13.3 Stellate Ganglion Block

Anatomy (PMID: 26360106):

Location:

• C7 transverse process level
• Anterior to longus colli muscle
• Posterior to carotid sheath
• Medial to vertebral artery

Relations:

• Anterior: Carotid sheath, prevertebral fascia
• Posterior: Neck of first rib, transverse process C7
• Medial: Vertebral body, oesophagus, trachea
• Lateral: Vertebral artery, brachial plexus trunks
• Inferior: Pleural dome (apex of lung)

Indications:

• Complex regional pain syndrome (CRPS) type I (upper limb)
• Vascular insufficiency (Raynaud's, frostbite)
• Hyperhidrosis
• Post-herpetic neuralgia
• Hot flushes (off-label)

Technique:

• Anterior paratracheal approach at C6 (Chassaignac's tubercle)
• Local anaesthetic injection (10-15 mL)
• Ultrasound-guided technique preferred

Signs of Successful Block:

• Ipsilateral Horner's syndrome (confirms sympathetic block)
• Temperature increase in ipsilateral hand (>1-2°C)
• Nasal congestion
• Anhidrosis of ipsilateral face

Complications:

• Horner's syndrome (expected)
• Recurrent laryngeal nerve block (hoarseness)
• Phrenic nerve block (dyspnoea)
• Vertebral artery injection (seizures, stroke)
• Pneumothorax
• Oesophageal perforation
• Haematoma (carotid)

13.4 Splanchnic Nerve Block (Coeliac Plexus Block)

Anatomy (PMID: 22127693):

Coeliac Plexus Location:

• Anterior to aorta at T12-L1
• Surrounds celiac trunk origin
• Contains celiac ganglia and interconnecting fibres

Splanchnic Nerves:

• Greater (T5-T9), lesser (T10-T11), least (T12) splanchnic nerves
• Pierce diaphragmatic crura
• Synapse in celiac ganglia
• Carry sympathetic efferents and visceral afferents

Indications:

• Chronic abdominal pain (pancreatic cancer)
• Chronic pancreatitis
• Visceral pain from upper abdominal malignancy

Approaches:

• Posterior (retrocrural or transcrural)
• Anterior (transgastric, EUS-guided)
• Intraoperative

Neurolytic Block:

• Alcohol (50-100%) or phenol (6-10%)
• For malignant pain
• Destroys nerve tissue for prolonged relief

Complications:

• Hypotension (loss of splanchnic vasoconstriction) - most common
• Diarrhoea (loss of sympathetic GI tone)
• Back pain (local irritation)
• Paraplegia (rare - artery of Adamkiewicz, spinal cord injury)
• Pneumothorax
• Retroperitoneal haematoma

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14. Autonomic Dysfunction in ICU

14.1 Dysautonomia in Critical Illness

ICU-Acquired Autonomic Dysfunction (PMID: 26074129):

Aetiology:

• Sepsis-related autonomic dysfunction
• Medication effects (sedatives, vasopressors, anticholinergics)
• Immobility and deconditioning
• Critical illness polyneuropathy/myopathy (CIP/CIM)
• Post-cardiac surgery vagal dysfunction

Manifestations:

• Heart rate variability reduction (early marker)
• Resting tachycardia
• Orthostatic intolerance
• Temperature dysregulation
• Gastroparesis, ileus
• Urinary retention
• Sweat abnormalities

Assessment:

• Heart rate variability (HRV) analysis
• Tilt table testing (when feasible)
• Ewing battery (autonomic reflex tests)
• Plasma catecholamines

14.2 Guillain-Barré Syndrome

Autonomic Features (PMID: 27045946):

Incidence: 65-70% of GBS patients have autonomic dysfunction

Cardiovascular:

• Tachycardia (most common)
• Bradycardia (can be severe, requiring pacing)
• Blood pressure lability (hypertension and hypotension)
• Cardiac arrhythmias (including asystole)
• Sudden death (autonomic storms)

Gastrointestinal:

• Gastroparesis
• Constipation, ileus
• Diarrhoea (less common)

Genitourinary:

• Urinary retention
• Incontinence

Sudomotor:

• Excessive sweating or anhidrosis
• Temperature dysregulation

Management:

• Continuous cardiac monitoring (essential)
• Cautious use of drugs affecting autonomic function
• Avoid triggering vagal responses (suctioning, position changes)
• Temporary pacing for bradycardia
• Short-acting antihypertensives (BP lability)

14.3 Traumatic Brain Injury

Autonomic Dysfunction in TBI (PMID: 28245770):

Paroxysmal Sympathetic Hyperactivity (PSH):

• Previously: "Sympathetic storming," "dysautonomia"
• Occurs in 15-33% of severe TBI
• Features: Tachycardia, hypertension, tachypnoea, hyperthermia, diaphoresis, dystonic posturing
• Triggered by external stimuli (turning, suctioning)
• Mechanism: Loss of cortical inhibition of sympathetic centres

Diagnostic Criteria (PSH-AM):

• Clinical features: HR, RR, BP, temperature, sweating, posturing
• Probability of PSH: Unlikely, possible, probable

Management:

• Minimize triggers (gentle handling, quiet environment)
• Pharmacological:
  • β-blockers (propranolol, labetalol)
  • α2 agonists (clonidine, dexmedetomidine)
  • Bromocriptine (dopamine agonist)
  • Gabapentin, baclofen (reduce allodynia)
  • Opioids (morphine)
• Avoid hyperthermia (active cooling)

14.4 Brainstem Lesions

Location-Specific Effects (PMID: 22953774):

Medullary Lesions:

• Lateral medullary syndrome (Wallenberg): Ipsilateral Horner's, dysphagia
• Damage to cardiovascular centres: Blood pressure lability
• Damage to respiratory centres: Central apnoea, Cheyne-Stokes

Pontine Lesions:

• Loss of micturition centre control: Bladder dysfunction
• Central hyperthermia (if hypothalamic connections affected)

Midbrain Lesions:

• Pupillary abnormalities (Edinger-Westphal nucleus)
• Decerebrate posturing with autonomic components

Cushing Response (Triad):

• Hypertension
• Bradycardia
• Irregular respiration
• Indicates increased ICP with brainstem herniation
• Last-ditch effort to maintain cerebral perfusion
• PMID: 23820679

14.5 Indigenous Health Considerations

Aboriginal and Torres Strait Islander Populations:

Relevance to Autonomic Dysfunction:

• Higher rates of diabetes mellitus → diabetic autonomic neuropathy
• Higher rates of cardiovascular disease
• Higher rates of chronic kidney disease → uremic neuropathy
• Higher rates of rheumatic heart disease

Cultural Considerations in ICU:

• Family/community involvement in decision-making
• Use of Aboriginal Health Workers (AHW) and Aboriginal Liaison Officers (ALO)
• Understanding of "Sorry Business" and cultural obligations
• Language barriers - use professional interpreters
• Preference for traditional healing alongside Western medicine
• Health literacy considerations

Māori Health Considerations (New Zealand):

• Whānau (extended family) involvement critical
• Te Tiriti o Waitangi obligations for equitable care
• Higher rates of cardiovascular disease and diabetes
• Access to Māori Health Workers
• Tikanga (cultural practices) around serious illness and death
• Manaakitanga (hospitality, care) as core value

Remote and Rural Considerations:

• Limited specialist access for complex autonomic conditions
• Telemedicine for specialist consultation
• RFDS (Royal Flying Doctor Service) for retrieval
• Need for locally upskilled workforce
• Equipment and monitoring limitations

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15. SAQ Practice

Question 1: Sympathetic Nervous System Anatomy (15 marks)

Describe the anatomy of the sympathetic nervous system, including its outflow, ganglia, and pathways to target organs. Include a labelled diagram. (15 marks)

Model Answer

Introduction (1 mark): The sympathetic nervous system is one of two divisions of the autonomic nervous system, providing "fight-or-flight" responses through widespread activation of target organs via a two-neuron pathway.

Spinal Outflow (2 marks):

• Preganglionic neurons located exclusively in intermediolateral cell column (IML)
• Thoracolumbar outflow: T1-L2 (only these levels contain sympathetic cell bodies)
• Exit spinal cord via ventral roots, enter spinal nerves

White Rami Communicantes (1 mark):

• Myelinated preganglionic fibres
• Connect spinal nerve to paravertebral chain
• Only present at T1-L2 levels

Paravertebral Chain Ganglia (3 marks):

• 22-24 pairs of ganglia from skull base to coccyx
• Cervical: Superior (C1-C4), middle (C5-C6), stellate/cervicothoracic (C7-T1)
• Thoracic: 10-12 ganglia (may fuse)
• Lumbar: 4 ganglia
• Sacral: 4-5 ganglia, end at ganglion impar
• Connected by interganglionic rami

Preganglionic Fibre Destinations (2 marks):

1. Synapse at same level in chain
2. Ascend/descend in chain before synapsing
3. Pass through chain without synapsing → splanchnic nerves to prevertebral ganglia

Prevertebral Ganglia (2 marks):

• Celiac ganglion: T5-T9 via greater splanchnic nerve
• Superior mesenteric ganglion: T10-T11 via lesser splanchnic
• Inferior mesenteric ganglion: L1-L2 via lumbar splanchnics
• Supply abdominal and pelvic viscera

Postganglionic Pathways (2 marks):

• Grey rami communicantes (unmyelinated) → all spinal nerves → blood vessels, sweat glands, piloerectors
• Periarterial plexuses to organs
• Named nerves (cardiac nerves, splanchnic nerves)

Diagram (2 marks): [Diagram should show: spinal cord with T1-L2 labelled, IML, ventral root, white ramus, paravertebral chain, grey ramus to spinal nerve, splanchnic nerves to prevertebral ganglia, postganglionic pathways]

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Question 2: Autonomic Control of the Cardiovascular System (15 marks)

Outline the anatomy of autonomic control of the cardiovascular system, including the baroreceptor reflex arc. Discuss the anatomical basis of cardiovascular instability in high spinal cord injury. (15 marks)

Model Answer

Introduction (1 mark): Cardiovascular autonomic control integrates sympathetic and parasympathetic input to regulate heart rate, contractility, and vascular tone through reflex arcs centred in the brainstem.

Cardiac Sympathetic Innervation (2 marks):

• Origin: T1-T5 (cardiac accelerator fibres)
• Preganglionic: IML → paravertebral chain → cervical ganglia
• Cardiac nerves: Superior (SCG), middle, inferior (stellate)
• Postganglionic to: SA node, AV node, atria, ventricles
• Effects: ↑ HR, ↑ contractility, ↑ conduction velocity

Cardiac Parasympathetic Innervation (2 marks):

• Origin: Nucleus ambiguus, dorsal motor nucleus of vagus
• Preganglionic: Vagus nerve → cardiac plexus → terminal ganglia
• Right vagus → SA node (predominantly)
• Left vagus → AV node (predominantly)
• Effects: ↓ HR, ↓ conduction velocity

Vascular Innervation (1 mark):

• Sympathetic only (no parasympathetic to blood vessels except erectile tissue)
• Postganglionic from paravertebral chain via grey rami
• α1 receptors: Vasoconstriction

Baroreceptor Locations (2 marks):

• Carotid sinus: Dilation of internal carotid at bifurcation
  • "Afferent: CN IX (glossopharyngeal) → carotid sinus nerve of Hering"
• Aortic arch: Stretch receptors in aortic wall
  • "Afferent: CN X (vagus) → aortic depressor nerve"

Baroreceptor Reflex Arc (3 marks):

1. ↑ BP → baroreceptor stretch → ↑ afferent firing
2. Afferents → nucleus tractus solitarius (NTS) in medulla
3. NTS → CVLM → inhibits RVLM → ↓ sympathetic outflow → vasodilation, ↓ SVR
4. NTS → nucleus ambiguus → ↑ vagal tone → bradycardia
5. Net effect: ↓ BP (negative feedback)

Spinal Cord Injury Cardiovascular Instability (4 marks):

High Cervical/Thoracic Injury (above T5):

• Loss of all cardiac sympathetics (T1-T5)
• Unopposed vagal tone → bradycardia
• Loss of supraspinal control of all sympathetic outflow

Acute Phase (Neurogenic Shock):

• Loss of vascular sympathetic tone → vasodilation, ↓ SVR
• Hypotension with bradycardia (unlike hypovolemic shock)
• "Warm shock"
• warm extremities due to vasodilation

Chronic Phase (Autonomic Dysreflexia):

• Loss of inhibitory supraspinal control over spinal sympathetic reflexes
• Noxious stimulus below injury → uninhibited massive sympathetic discharge
• Severe hypertension, reflex bradycardia (baroreceptors intact above lesion)
• Triggered by bladder distension, faecal impaction, pressure sores

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16. Viva Scenarios

Scenario 1: Horner's Syndrome and Sympathetic Pathways (10 minutes)

Opening Stem: A 58-year-old man presents with right-sided ptosis, miosis, and facial anhidrosis. Describe the anatomical pathway affected.

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Examiner: "What is the anatomical pathway for sympathetic innervation of the eye?"

Candidate: "The sympathetic pathway to the eye is a three-neuron pathway:

First order neuron originates in the posterolateral hypothalamus, descends uncrossed through the brainstem tegmentum, and synapses in the ciliospinal centre of Budge at C8-T2 in the spinal cord.

Second order neuron - the preganglionic fibre - exits via the T1 ventral root, crosses over the apex of the lung (where it's vulnerable to Pancoast tumours), ascends in the sympathetic chain, and synapses in the superior cervical ganglion at C2-C3.

Third order neuron - the postganglionic fibre - travels with the internal carotid artery, enters the skull via the carotid canal, and eventually reaches the orbit via the long ciliary nerves to innervate the dilator pupillae muscle and Müller's muscle in the upper eyelid."

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Examiner: "Good. What are the features of Horner's syndrome and why do they occur?"

Candidate: "Horner's syndrome has four classical features:

1. Miosis - pupil constriction due to loss of the dilator pupillae, leaving unopposed parasympathetic action on the sphincter pupillae

2. Ptosis - mild, 1-2mm drooping of the upper eyelid due to loss of Müller's muscle (sympathetic), though the main levator palpebrae (CN III) is intact

3. Anhidrosis - reduced sweating on the affected side, but the distribution depends on lesion level:

   • First/second order: Entire ipsilateral face, neck, and upper body
   • Third order: No anhidrosis (sudomotor fibres travel with external carotid, not internal)

4. Apparent enophthalmos - the eye appears sunken due to the narrowed palpebral fissure, but true enophthalmos doesn't occur."

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Examiner: "This patient has anhidrosis only on his face. What does this tell you about the lesion location?"

Candidate: "Facial-only anhidrosis indicates a second order (preganglionic) lesion. This is because the sudomotor fibres to the face separate from the pathway at the superior cervical ganglion. They travel with the external carotid branches rather than continuing with the internal carotid.

A first order lesion would cause anhidrosis of the entire ipsilateral face, neck, arm, and upper trunk because the lesion is above the point where sympathetic fibres diverge.

A third order lesion causes no anhidrosis because the sudomotor fibres have already separated to travel with the external carotid before the internal carotid-associated fibres are affected.

Given this patient has facial anhidrosis only, the lesion is between T1 and the superior cervical ganglion - the second order neuron pathway. Common causes at this level include Pancoast tumour, thyroid malignancy, trauma, or iatrogenic injury from neck surgery."

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Examiner: "What investigations would you request?"

Candidate: "I would approach this systematically:

Confirm the diagnosis:

• Cocaine eye drops 4% - in Horner's, the pupil fails to dilate (cocaine blocks noradrenaline reuptake, but in Horner's there's no NA being released)

Localize the lesion:

• Hydroxyamphetamine (pholedrine) drops - this releases stored NA from postganglionic terminals
  • "If third order lesion: No dilation (no functional postganglionic neuron)"
  • "If first/second order: Dilation (intact postganglionic neuron with stored NA)"

Imaging:

• Given second order lesion is suspected:
  • CT chest (Pancoast tumour - lung apex)
  • CT/MRI of neck (thyroid, lymphadenopathy, vascular abnormality)
  • Consider CT angiography if concerned about subclavian artery aneurysm

Urgent consideration: If painful Horner's with third order features and headache → CT/MR angiography of carotid to exclude internal carotid dissection (stroke risk)."

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Examiner: "What is the stellate ganglion and why is it clinically important?"

Candidate: "The stellate ganglion, also called the cervicothoracic ganglion, is formed by fusion of the inferior cervical ganglion and the first thoracic ganglion of the sympathetic chain.

Location: Anterior to the C7 transverse process and neck of the first rib.

Relations:

• Posterior: Vertebral artery, C7 transverse process
• Anterior: Subclavian artery, carotid sheath
• Lateral: Brachial plexus trunks
• Inferior: Pleural dome (lung apex)

Clinical importance:

1. Stellate ganglion block: Used for complex regional pain syndrome of the upper limb, vascular insufficiency (Raynaud's), hyperhidrosis. Block at C6 level (Chassaignac's tubercle) using ultrasound guidance. Success confirmed by ipsilateral Horner's syndrome and temperature increase in the hand.

2. Complications of block: Include vertebral artery injection (seizures), pneumothorax, recurrent laryngeal nerve block (hoarseness), phrenic nerve block, haematoma.

3. Surgical consideration: At risk during anterior approach to lower cervical spine, thyroid surgery, or thoracic outlet surgery."

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Scenario 2: Spinal Cord Injury and Autonomic Effects (10 minutes)

Opening Stem: A 24-year-old male has a complete C6 spinal cord injury from a diving accident. He is intubated and ventilated in ICU. Describe the autonomic consequences of this injury.

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Examiner: "What autonomic effects would you expect in the acute phase following this injury?"

Candidate: "With a complete C6 spinal cord injury, the patient has lost all supraspinal control of sympathetic outflow below the level of injury. In the acute phase, we see neurogenic shock:

Cardiovascular effects:

• Loss of sympathetic tone to blood vessels → vasodilation → hypotension
• Loss of cardiac sympathetics (T1-T5) → unopposed vagal tone → bradycardia
• This is 'warm shock' - hypotensive with warm peripheries and bradycardia

Other acute effects:

• Paralytic ileus from loss of splanchnic sympathetics and pelvic parasympathetics
• Urinary retention (detrusor areflexia during spinal shock)
• Temperature dysregulation (poikilothermia) - unable to vasoconstrict or shiver below lesion
• Priapism may occur from unopposed parasympathetic tone

Duration: Neurogenic shock typically lasts days to weeks, corresponding to the period of 'spinal shock.'"

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Examiner: "How would you manage the cardiovascular instability acutely?"

Candidate: "Management of neurogenic shock involves:

Exclude other causes: Rule out concurrent haemorrhage, tension pneumothorax, or other causes of hypotension - trauma patients can have both neurogenic and haemorrhagic shock.

Fluid resuscitation: Cautious - they have lost vascular tone so are preload-dependent, but excessive fluid can cause pulmonary oedema (especially with concomitant contusion).

Vasopressors:

• First line: Noradrenaline - provides α1 vasoconstriction to restore vascular tone
• Alternatively: Phenylephrine (pure α1 agonist)
• For persistent bradycardia: Temporary pacing may be required
• Avoid atropine as sole treatment - addresses bradycardia but not vasodilation

Temperature regulation: Active warming blankets, warm IV fluids, control environment temperature

Target: MAP ≥85mmHg is typically targeted in acute spinal cord injury to optimise spinal cord perfusion (based on observational evidence)."

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Examiner: "After three months, this patient develops severe headache, hypertension to 220/130, and sweating on his face. What is happening?"

Candidate: "This is autonomic dysreflexia, a potentially life-threatening condition that occurs in chronic spinal cord injury above T6. It's caused by:

Mechanism:

1. A noxious stimulus below the level of injury (commonly bladder distension, faecal impaction, pressure sores) triggers afferent signals to the spinal cord
2. Below the level of injury, there is uninhibited reflex sympathetic discharge through intact spinal reflex arcs
3. This causes massive vasoconstriction below the lesion
4. The baroreceptors above the lesion (carotid sinus, aortic arch) detect hypertension and trigger vagal-mediated bradycardia
5. However, the descending inhibitory signals cannot pass the spinal cord lesion to terminate the sympathetic reflex
6. Result: Severe hypertension with reflex bradycardia

Symptoms:

• Above lesion: Flushing, sweating, nasal congestion (compensatory vasodilation)
• Below lesion: Pallor, piloerection (vasoconstriction)
• Headache (often pounding)
• Can cause stroke, seizures, retinal haemorrhage, death if untreated"

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Examiner: "How would you manage this emergency?"

Candidate: "This is a medical emergency requiring immediate intervention:

Immediate actions:

1. Sit the patient upright - utilizes orthostatic reduction in BP
2. Loosen any tight clothing - remove compression stockings, abdominal binders
3. Monitor BP every 2-5 minutes

Identify and remove the trigger (most important):

• Bladder: Is the catheter blocked? Irrigate or replace. If no catheter, drain bladder with lidocaine gel
• Bowel: Check for faecal impaction - apply topical anaesthetic (lidocaine gel) before any per-rectum examination or manual evacuation
• Skin: Check for pressure sores, ingrown toenails, DVT, tight clothing

Pharmacological management (if SBP remains >150mmHg after trigger removed):

• Glyceryl trinitrate (GTN) paste/spray - rapid onset, easy to remove if BP drops
• Nifedipine - 10mg sublingual (avoid if at risk of hypotension)
• Phentolamine IV - α-blocker, rapid onset (0.1mg/kg)
• Labetalol IV - if tachycardia present

Avoid: Precipitating bradycardia with excessive β-blockade; avoid excessive BP lowering once trigger removed as BP will normalize."

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Examiner: "Why does autonomic dysreflexia occur only with injuries above T6?"

Candidate: "T6 is the critical level because it represents the upper extent of the splanchnic sympathetic outflow, which controls the largest vascular bed in the body.

The splanchnic circulation (gut, liver, spleen) receives sympathetic innervation from T5-L2, with the greater splanchnic nerve arising from T5-T9. This vascular bed contains approximately 25-30% of total blood volume.

With injuries above T6:

• All splanchnic sympathetics are below the lesion
• The massive vasoconstriction of the splanchnic bed during dysreflexia causes severe hypertension
• The normal descending inhibition from brainstem cardiovascular centres cannot reach these sympathetic neurons

With injuries below T6:

• Some or all splanchnic sympathetics remain under supraspinal control
• The compensatory mechanisms can limit the hypertensive response
• Autonomic dysreflexia is rare or mild

This is why T6 is the key demarcation level for autonomic dysreflexia risk."

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17. References

Primary Sources and Guidelines

1. ANZICS-CORE Guidelines - Spinal Cord Injury Management
2. CICM First Part Curriculum - Neuroanatomy and Neurophysiology
3. Therapeutic Guidelines Australia - Cardiovascular and Neurological

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19. Related Topics

• Cardiovascular Physiology
• Neuroanatomy: Spinal Cord
• Spinal Cord Injury Management
• Vasopressor and Inotrope Pharmacology
• Guillain-Barré Syndrome
• Autonomic Pharmacology
• Regional Anaesthesia: Nerve Blocks
• Upper Airway Anatomy

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Last updated: January 2026 Citation Count: 48 PubMed PMIDs Word Count: ~13,500 Anki Cards: 50