18.05.04 · organismal-bio / nervous-system

The autonomic nervous system: sympathetic versus parasympathetic, neurotransmitters, and homeostasis

stub3 tiersLean: nonepending prereqs

Anchor (Master): Kandel, E. R. et al. — Principles of Neural Science, 6th ed. (2021), Ch. 45-47

Intuition Beginner

The autonomic nervous system runs your body's automatic functions — heart rate, digestion, breathing — without conscious control. It operates through two opposing divisions that act like a gas pedal and a brake.

The sympathetic division prepares you for action. When you are startled or threatened, it triggers the "fight or flight" response: your heart races, your pupils dilate, your airways open wide, blood is redirected to your skeletal muscles, and your liver dumps glucose into the bloodstream. Everything that helps you fight or run gets turned up. Everything else — digestion, urination, salivation — gets turned down.

The parasympathetic division does the opposite. It promotes "rest and digest": your heart rate slows, your pupils constrict, your digestive system becomes active, and your body conserves energy. The parasympathetic division is dominant when you are calm, safe, and well-fed.

Most internal organs receive signals from both divisions simultaneously. Their opposing influences are balanced at every moment, and your body shifts the balance as conditions change. After a meal, parasympathetic activity increases to drive digestion. During exercise, sympathetic activity surges to meet the metabolic demand. This continuous rebalancing is how the autonomic nervous system maintains homeostasis — keeping internal conditions stable despite a constantly changing environment.

Visual Beginner

The sympathetic outflow originates from the thoracic and upper lumbar spinal cord (T1-L2). Preganglionic fibres are short and synapse in ganglia close to the spinal cord (the paravertebral chain running along either side of the vertebral column). Postganglionic fibres are long and travel to distant target organs.

The parasympathetic outflow originates from the brainstem (via cranial nerves III, VII, IX, and X) and the sacral spinal cord (S2-S4). Preganglionic fibres are long and travel to terminal ganglia located very close to or within the target organ. Postganglionic fibres are short.

This anatomical difference has a functional consequence: sympathetic responses tend to be widespread (a single preganglionic neuron can diverge to many postganglionic neurons across multiple ganglia), while parasympathetic responses tend to be more localised and specific.

Worked example Beginner

You are walking alone at night and hear footsteps behind you. Before you consciously decide what to do, your sympathetic nervous system has already activated:

Heart rate increases (more blood to muscles).
Pupils dilate (more light enters, better peripheral vision).
Bronchioles dilate (more air to the lungs).
Digestion shuts down (blood redirected away from the gut).
Liver releases stored glucose into the blood (fuel for muscles).
Sweat glands activate (prepare for heat generation).
Adrenal medulla releases epinephrine (adrenaline) into the bloodstream, amplifying and prolonging all of the above.

Once you realise the footsteps belong to a jogger who passes you harmlessly, your parasympathetic system gradually restores the balance: heart rate slows, digestion resumes, pupils constrict. The return to baseline is slower than the initial sympathetic surge because epinephrine in the bloodstream must be metabolised away.

Check your understanding Beginner

Formal definition Intermediate+

Sympathetic anatomy: the thoracolumbar outflow

The sympathetic division originates from the intermediolateral cell column (IML) of the spinal cord, segments T1 through L2. Sympathetic preganglionic neurons have their cell bodies in the IML and project myelinated axons (white rami communicantes) to the paravertebral sympathetic chain — a bilateral column of ganglia running from the base of the skull to the coccyx, one ganglion approximately per spinal segment.

At the paravertebral chain, each preganglionic fibre has four possible fates:

Synapse at the same segmental level in the chain ganglion.
Ascend through the chain to synapse in a higher ganglion (e.g., cervical ganglia receiving input from upper thoracic segments).
Descend through the chain to synapse in a lower ganglion.
Pass through the chain without synapsing, via the splanchnic nerves, to synapse in prevertebral (collateral) ganglia located on the abdominal aorta: the coeliac ganglion, the superior mesenteric ganglion, and the inferior mesenteric ganglion.

This divergent architecture means a single preganglionic neuron can synapse onto 20 or more postganglionic neurons across multiple ganglia, producing the widespread, mass-discharge pattern characteristic of sympathetic activation.

The three cervical sympathetic ganglia (superior, middle, and stellate) receive sympathetic input from upper thoracic segments and send postganglionic fibres to the head, neck, and heart. The superior cervical ganglion is the largest: its postganglionic fibres innervate the dilator pupillae muscle (pupil dilation), the lacrimal and salivary glands, and the blood vessels of the head and neck.

The adrenal medulla is a specialised sympathetic ganglion. Its chromaffin cells are modified postganglionic neurons (embryologically derived from neural crest) innervated directly by sympathetic preganglionic fibres. Rather than releasing neurotransmitter at a synapse, chromaffin cells secrete epinephrine (~~80%) and norepinephrine (~~20%) directly into the bloodstream as hormones, producing a prolonged, whole-body sympathetic effect.

Parasympathetic anatomy: the craniosacral outflow

The parasympathetic division originates from two widely separated regions — the brainstem (cranial outflow) and the sacral spinal cord (sacral outflow), giving it the alternative name "craniosacral division."

Cranial parasympathetic outflow travels in four cranial nerves:

CN III (oculomotor) — Edinger-Westphal nucleus projects via the ciliary ganglion to the sphincter pupillae (pupillary constriction) and the ciliary muscle (lens accommodation for near vision).
CN VII (facial) — Superior salivatory nucleus projects via the pterygopalatine ganglion (lacrimal glands, nasal mucosa) and the submandibular ganglion (submandibular and sublingual salivary glands).
CN IX (glossopharyngeal) — Inferior salivatory nucleus projects via the otic ganglion to the parotid salivary gland.
CN X (vagus) — Dorsal motor nucleus of the vagus and the nucleus ambiguus provide the dominant parasympathetic outflow to the thoracic and abdominal viscera: heart (slowing rate), lungs (bronchoconstriction, secretion), stomach and intestines (motility and secretion), liver and pancreas. The vagus carries roughly 75% of all parasympathetic fibres.

Sacral parasympathetic outflow originates from the sacral parasympathetic nucleus at S2-S4 and travels via the pelvic splanchnic nerves to terminal ganglia in the walls of the descending colon, rectum, bladder, and reproductive organs. It controls defecation, urination, and genital erection.

Parasympathetic ganglia are located close to or within the target organ (terminal ganglia or intramural ganglia), so preganglionic fibres are long and postganglionic fibres are very short. This arrangement produces localised, organ-specific responses with minimal divergence.

Neurotransmitters and receptors

The autonomic nervous system uses only two neurotransmitters at its synapses, but the diversity of receptor subtypes creates highly specific effects.

At all autonomic ganglia (both sympathetic and parasympathetic): Preganglionic neurons release acetylcholine (ACh), which binds to nicotinic ( $N_{N}$ ) receptors on the postganglionic neuron. These are ligand-gated ion channels that produce rapid depolarisation. The neurotransmitter is the same at both sympathetic and parasympathetic ganglia — the ganglionic synapse cannot distinguish the two divisions.

At sympathetic target organs: Postganglionic sympathetic neurons release norepinephrine (NE), which binds to adrenergic receptors on the target tissue. The adrenergic receptor subtypes determine the response:

Receptor	Location	Effect	G-protein
$α_{1}$	Vascular smooth muscle, iris dilator, sphincters	Contraction (vasoconstriction, pupil dilation)	$G_{q}$
$α_{2}$	Presynaptic sympathetic terminals, CNS	Inhibits further NE release (negative feedback)	$G_{i}$
$β_{1}$	Heart (SA node, AV node, myocardium)	Increases heart rate, contractility, conduction velocity	$G_{s}$
$β_{2}$	Bronchial smooth muscle, skeletal muscle vessels	Relaxation (bronchodilation, vasodilation)	$G_{s}$
$β_{3}$	Adipose tissue	Lipolysis	$G_{s}$

At parasympathetic target organs: Postganglionic parasympathetic neurons release acetylcholine (ACh), which binds to muscarinic ( $M$ ) receptors on the target tissue. These are G-protein-coupled receptors (metabotropic), producing slower, more sustained responses than nicotinic receptors:

Receptor	Location	Effect
M1	Gastric parietal cells, CNS	Stimulates gastric acid secretion
M2	Heart (SA node, AV node, atria)	Decreases heart rate, slows conduction
M3	Smooth muscle, glands, endothelium	Contraction (bronchoconstriction, gut motility), secretion, vasodilation via NO release
M4/M5	CNS, salivary glands	Modulatory roles

Cholinergic receptor summary:

Type	Subtype	Location	Mechanism
Nicotinic	$N_{N}$ (neuronal)	Autonomic ganglia, CNS	Ligand-gated ion channel (Na $^{+}$ /K $^{+}$ )
Nicotinic	$N_{M}$ (muscle)	Neuromuscular junction	Ligand-gated ion channel (Na $^{+}$ /K $^{+}$ )
Muscarinic	M1-M5	Parasympathetic target organs, CNS	GPCR ( $G_{q}$ or $G_{i}$ )

Organ effects: sympathetic versus parasympathetic

Organ	Sympathetic effect	Adrenergic receptor	Parasympathetic effect	Muscarinic receptor
Heart (rate)	Increases	$β_{1}$	Decreases	M2
Heart (contractility)	Increases	$β_{1}$	Decreases (atria only)	M2
Bronchioles	Dilates	$β_{2}$	Constricts	M3
Pupil	Dilates	$α_{1}$	Constricts	M3
GI motility	Decreases	$α_{2}$ , $β_{2}$	Increases	M3
GI secretion	Decreases	$α_{2}$	Increases	M1, M3
Salivary glands	Thick, mucoid secretion	$α_{1}$	Watery, enzyme-rich secretion	M3
Liver	Glycogenolysis, gluconeogenesis	$α_{1}$ , $β_{2}$	Glycogen synthesis	—
Adipose tissue	Lipolysis	$β_{3}$	—	—
Bladder (detrusor)	Relaxes	$β_{2}$ , $β_{3}$	Contracts	M3
Bladder (sphincter)	Contracts	$α_{1}$	Relaxes	M3
Blood vessels (skin, gut)	Constricts	$α_{1}$	— (no parasympathetic innervation)	—
Blood vessels (skeletal muscle)	Dilates	$β_{2}$	—	—
Sweat glands	Increases (cholinergic sympathetic)	ACh on M3	—	—

Notable exceptions: (1) Sweat glands are innervated by sympathetic neurons that release ACh (not NE) onto muscarinic receptors — a unique cholinergic sympathetic pathway. (2) Most blood vessels receive only sympathetic innervation; parasympathetic vasodilation occurs in a few specialised beds (genital erectile tissue via pelvic splanchnic nerves, cerebral vessels). (3) The adrenal medulla releases epinephrine into the bloodstream rather than synapsing on a target organ.

Enteric nervous system

The enteric nervous system (ENS) is now recognised as a third autonomic division. It consists of approximately 500 million neurons organised into two plexuses embedded in the wall of the gastrointestinal tract:

Myenteric plexus (Auerbach's plexus) — between the longitudinal and circular muscle layers. Primarily controls gut motility (peristalsis).
Submucosal plexus (Meissner's plexus) — in the submucosa. Primarily controls secretion and local blood flow.

The ENS can function autonomously: the basic peristaltic reflex (stretch $\to$ contraction proximal, relaxation distal) operates entirely within the gut wall via intrinsic sensory neurons, interneurons, and motor neurons. The sympathetic and parasympathetic systems modulate rather than initiate gut activity: sympathetic input inhibits motility and secretion; parasympathetic (vagal) input enhances them.

Key mechanism Intermediate+

The baroreflex: autonomic homeostasis in action

The baroreceptor reflex is the canonical example of autonomic homeostatic control. It maintains arterial blood pressure within a narrow range despite rapid changes in posture, blood volume, and cardiac output.

Sensors: Stretch receptors (baroreceptors) in the carotid sinus (at the bifurcation of the common carotid artery) and the aortic arch fire action potentials at a rate proportional to arterial pressure. Higher pressure stretches the vessel wall more, increasing firing rate.

Afferent pathway: Carotid sinus baroreceptors transmit via the carotid sinus nerve (Hering's nerve), a branch of CN IX (glossopharyngeal), to the nucleus tractus solitarius (NTS) in the medulla. Aortic arch baroreceptors transmit via CN X (vagus) to the same target.

Central integration: The NTS is the primary relay for all visceral sensory information. It projects to two medullary centres:

Caudal ventrolateral medulla (CVLM) — inhibitory. Activated by NTS, the CVLM sends GABAergic projections to the rostral ventrolateral medulla.
Rostral ventrolateral medulla (RVLM) — excitatory. The RVLM is the primary source of tonic sympathetic outflow to the IML cell column of the spinal cord.

The NTS also projects to the nucleus ambiguus and the dorsal motor nucleus of the vagus, controlling parasympathetic (vagal) output to the heart.

Response to increased pressure (e.g., standing up quickly):

Blood pressure drops $\to$ baroreceptor firing decreases.
NTS receives less input $\to$ CVLM is less activated $\to$ CVLM inhibition of RVLM decreases.
RVLM output increases $\to$ sympathetic outflow to heart ( $β_{1}$ : increases rate and contractility), blood vessels ( $α_{1}$ : vasoconstriction), and adrenal medulla (epinephrine release) increases.
Simultaneously, decreased NTS input reduces vagal output to the heart $\to$ parasympathetic brake is released.
Combined effect: heart rate increases, contractility increases, total peripheral resistance increases $\to$ blood pressure is restored.

Response to decreased pressure (e.g., lying down, fluid overload):

Blood pressure rises $\to$ baroreceptor firing increases.
NTS activates CVLM $\to$ CVLM inhibits RVLM $\to$ sympathetic outflow decreases.
NTS activates vagal nuclei $\to$ parasympathetic output increases $\to$ heart rate decreases.
Combined effect: heart rate decreases, vasodilation, blood pressure falls back toward set point.

The baroreflex demonstrates the core principle of autonomic homeostasis: a sensed variable (blood pressure) is compared to a set point, and the two autonomic divisions are adjusted in opposite directions to correct the error. The sympathetic and parasympathetic systems are never both increased or both decreased together — they always move in opposition, like two people pulling on opposite ends of a rope to hold a weight at a precise height.

Exercises Intermediate+

Exercise 3 (medium, short answer).

Explain why sympathetic activation produces widespread effects throughout the body while parasympathetic activation tends to produce more localised, organ-specific effects. Relate this to the anatomical organisation of each division.

Hint

Consider the ratio of preganglionic to postganglionic neurons in each division, and the location of the ganglia.

Answer

In the sympathetic division, preganglionic fibres are short and synapse in the paravertebral chain or prevertebral ganglia, where each preganglionic neuron diverges onto many postganglionic neurons (ratios of 1:20 or higher). The postganglionic fibres then travel widely throughout the body. Additionally, the adrenal medulla releases epinephrine into the bloodstream, reaching every organ simultaneously. This produces a coordinated, whole-body "mass discharge" response.

In the parasympathetic division, preganglionic fibres are long and synapse in terminal ganglia located very close to or within the target organ. Divergence is minimal (ratios closer to 1:1 to 1:3), so each preganglionic neuron affects only a small region of one organ. This anatomical arrangement allows precise, localised control — for example, the vagus can increase gastric secretion without simultaneously constricting the pupil.

Exercise 4 (medium, short answer).

A drug that blocks muscarinic acetylcholine receptors (an anticholinergic such as atropine) causes dry mouth, dilated pupils, increased heart rate, and decreased sweating. Explain each effect by identifying which parasympathetic or sympathetic pathway is affected.

Hint

Muscarinic receptors are found at parasympathetic target organs AND at sweat glands (which are sympathetically innervated but use ACh).

Answer

Dry mouth — Salivary glands are stimulated parasympathetically via M3 receptors. Blocking M3 receptors removes parasympathetic drive, reducing saliva production.
Dilated pupils — The sphincter pupillae (constrictor) is driven parasympathetically via CN III and M3 receptors. Blocking M3 receptors removes pupillary constriction, and the unopposed sympathetic $α_{1}$ -mediated dilation dominates.
Increased heart rate — The vagus nerve tonically slows the heart via M2 receptors. Blocking M2 removes the parasympathetic brake, allowing heart rate to rise toward the intrinsic SA node rate (~100 bpm).
Decreased sweating — Sweat glands are uniquely innervated by sympathetic neurons that release ACh onto M3 receptors. Blocking M3 receptors eliminates sweat production despite this being a sympathetic pathway. This is the one exception where anticholinergic blockade affects a sympathetic, not parasympathetic, target.

Exercise 5 (medium, short answer).

Trace the baroreflex pathway that restores blood pressure when you stand up quickly and feel momentarily light-headed. Identify the sensor, afferent pathway, central integration, efferent pathways, and effectors.

Hint

Standing causes blood to pool in the legs, reducing venous return and arterial pressure. The baroreflex must detect this drop and correct it.

Answer

Sensor: Carotid sinus and aortic arch baroreceptors detect decreased arterial stretch (blood pressure has fallen).
Afferent pathway: Reduced firing in CN IX (carotid sinus) and CN X (aortic arch) $\to$ decreased input to the nucleus tractus solitarius (NTS) in the medulla.
Central integration: NTS reduces activation of the CVLM $\to$ CVLM decreases its inhibition of the RVLM $\to$ RVLM output increases (more sympathetic drive). Simultaneously, reduced NTS input decreases activation of vagal nuclei (less parasympathetic drive).
Efferent pathways: Increased sympathetic outflow via the IML cell column to heart, blood vessels, and adrenal medulla. Decreased vagal outflow to the heart.
Effectors: $β_{1}$ receptors on the heart increase rate and contractility. $α_{1}$ receptors on vascular smooth muscle cause vasoconstriction (increasing total peripheral resistance). Adrenal medulla releases epinephrine. Blood pressure is restored within seconds.

Exercise 6 (hard, short answer).

A patient takes a non-selective beta-blocker (blocks both $β_{1}$ and $β_{2}$ receptors) for hypertension. Explain why this drug could be dangerous for a patient with asthma, and why a cardioselective ( $β_{1}$ -selective) beta-blocker would be safer.

Hint

$β_{2}$ receptors are found on bronchial smooth muscle. What do they do there? What happens when they are blocked?

Answer

$β_{2}$ adrenergic receptors on bronchial smooth muscle mediate bronchodilation. In asthma, the airways are already narrowed due to inflammation and smooth muscle contraction, and endogenous epinephrine acting on $β_{2}$ receptors helps keep them open. A non-selective beta-blocker blocks these $β_{2}$ receptors, removing the bronchodilatory tone and potentially causing severe bronchoconstriction — a life-threatening exacerbation of asthma.

A cardioselective $β_{1}$ -selective blocker (e.g., metoprolol, atenolol) preferentially blocks cardiac $β_{1}$ receptors (reducing heart rate and contractility for the antihypertensive effect) while sparing $β_{2}$ receptors in the bronchioles. The selectivity is dose-dependent (at high doses even "selective" blockers lose selectivity), but at therapeutic doses the risk of bronchospasm is substantially lower.

This question illustrates why understanding receptor subtypes is clinically essential: the therapeutic target ( $β_{1}$ in the heart) and the dangerous off-target ( $β_{2}$ in the lungs) share the same neurotransmitter (NE/epinephrine) but are distinguished only by their receptor pharmacology.

Exercise 7 (hard, short answer).

The sweat glands are innervated by sympathetic neurons, yet they secrete in response to acetylcholine acting on muscarinic receptors. Explain the embryological and pharmacological significance of this exception.

Hint

Most sympathetic postganglionic neurons switch from ACh to NE during development. Sweat gland neurons are the exception. What does this imply for drug effects?

Answer

During development, sympathetic postganglionic neurons undergo a neurotransmitter phenotype switch from cholinergic to noradrenergic, induced by target-derived factors. Sweat gland neurons are the exception: they retain their cholinergic phenotype because the sweat glands themselves release factors (likely members of the neuropoietic cytokine family such as LIF or CNTF) that maintain ACh synthesis and suppress NE synthesis in the innervating neurons.

Pharmacological significance: anticholinergic drugs (which block muscarinic receptors) inhibit sweating despite targeting a "parasympathetic" receptor, because sweat glands are driven by the sympathetic cholinergic pathway. This is why anticholinergic overdose produces both parasympathetic blockade (dry mouth, dilated pupils, tachycardia, urinary retention) AND loss of sympathetic sweating (anhidrosis), together forming the "hot as a hare, dry as a bone, red as a beet, blind as a bat, mad as a hatter" toxidrome.

Autonomic disorders and clinical pharmacology Master

Autonomic dysreflexia

Autonomic dysreflexia is a life-threatening condition occurring in patients with spinal cord injury above T6. Below the lesion, the sympathetic outflow is disconnected from supraspinal inhibitory control. A noxious stimulus below the lesion (bladder distension, bowel impaction, pressure sore, or even a tight shoelace) triggers a massive, uncontrolled sympathetic reflex: extreme vasoconstriction below the lesion produces systolic blood pressures exceeding 250 mmHg. The baroreflex detects the hypertension and activates vagal output above the lesion, producing bradycardia and vasodilation above the lesion (flushed skin, pounding headache above the level; pale, diaphoretic skin below). The sympathetic outflow below the lesion cannot be inhibited because the descending inhibitory pathways from the medulla have been interrupted by the spinal cord injury.

Emergency management requires identifying and removing the trigger (typically bladder catheterisation) and administering rapid-acting antihypertensives (nifedipine, nitroglycerin ointment). Autonomic dysreflexia is a medical emergency: untreated, the extreme hypertension can cause intracranial haemorrhage, seizures, myocardial infarction, or death.

Horner syndrome

Horner syndrome results from interruption of the sympathetic pathway to the head, producing a classic triad: ptosis (drooping eyelid, from loss of sympathetic tone to the superior tarsal muscle), miosis (constricted pupil, from loss of sympathetic $α_{1}$ -mediated pupillary dilation with unopposed parasympathetic constriction), and anhidrosis (loss of sweating on the ipsilateral face).

The sympathetic pathway to the eye is long and therefore vulnerable at multiple levels: a first-order neuron lesion (hypothalamus to IML, coursing through the brainstem and cervical cord) can be caused by stroke or tumour; a second-order neuron lesion (preganglionic, from IML across the lung apex to the superior cervical ganglion) classically results from a Pancoast tumour (apical lung cancer) or trauma; a third-order neuron lesion (postganglionic, from the superior cervical ganglion along the internal carotid artery to the eye) can be caused by carotid dissection or cavernous sinus pathology. Pharmacological testing with cocaine (blocks reuptake of NE at the sympathetic synapse — no dilation in Horner's because there is no NE to accumulate) and hydroxyamphetamine (releases stored NE from postganglionic terminals — distinguishes pre- from postganglionic lesions) can localise the level of interruption.

Diabetic autonomic neuropathy

Chronic hyperglycaemia in diabetes mellitus damages autonomic nerves through multiple mechanisms: accumulation of sorbitol and fructose (polyol pathway), formation of advanced glycation end-products (AGEs), reduced blood flow to nerves (microvascular disease), and oxidative stress. Autonomic neuropathy affects multiple systems:

Cardiovascular — resting tachycardia (early: damage to vagal fibres removing the parasympathetic brake), orthostatic hypotension (damage to sympathetic vasoconstrictor fibres, causing blood pressure to fall on standing), loss of heart rate variability, silent myocardial infarction (loss of pain fibres).
Gastrointestinal — gastroparesis (delayed gastric emptying due to vagal damage, causing nausea, bloating, erratic glucose absorption), diabetic diarrhoea, constipation.
Genitourinary — erectile dysfunction (parasympathetic damage), neurogenic bladder (impaired sensation of bladder fullness, incomplete emptying, recurrent infections).
Sudomotor — anhidrosis or inappropriate sweating (thermoregulatory failure).

Cardiovascular autonomic neuropathy is an independent predictor of mortality in diabetes, primarily because of the risk of silent cardiac ischaemia and sudden cardiac death from arrhythmia.

Orthostatic hypotension

Orthostatic (postural) hypotension is defined as a drop in systolic blood pressure of $\geq 20$ mmHg or diastolic blood pressure of $\geq 10$ mmHg within 3 minutes of standing. The normal response to standing is a baroreflex-mediated sympathetic surge (vasoconstriction, tachycardia) that maintains cerebral perfusion. Orthostatic hypotension occurs when this compensatory mechanism fails.

Causes include: autonomic neuropathy (diabetes, amyloidosis, Parkinson's disease, multiple system atrophy), volume depletion (dehydration, haemorrhage), medications (antihypertensives, diuretics, alpha-blockers, tricyclic antidepressants with alpha-blocking properties), and bed rest/deconditioning. In multiple system atrophy (MSA), degeneration of the central autonomic pathways (including the IML cell column and the RVLM) produces severe orthostatic hypotension with little compensatory tachycardia — the sympathetic output is simply absent.

Management includes removing offending medications, increasing salt and fluid intake, compression garments, fludrocortisone (mineralocorticoid to expand plasma volume), and midodrine (an $α_{1}$ agonist that increases vascular tone).

Autonomic pharmacology: major drug classes

Beta-blockers ( $β$ -adrenergic antagonists):

Non-selective (propranolol): blocks $β_{1}$ and $β_{2}$ . Reduces heart rate, contractility, and cardiac output. Also blocks $β_{2}$ -mediated bronchodilation (contraindicated in asthma). Blocks $β_{2}$ -mediated vasodilation in skeletal muscle.
Cardioselective (metoprolol, atenolol): preferentially block $β_{1}$ . Safer in asthma at low doses.
Uses: hypertension, angina, heart failure (reduces mortality by blocking chronic sympathetic overdrive), rate control in atrial fibrillation, migraine prophylaxis, secondary prevention after myocardial infarction.

Alpha-blockers ( $α$ -adrenergic antagonists):

$α_{1}$ -selective (prazosin, doxazosin, tamsulosin): block vascular $α_{1}$ receptors, causing vasodilation. Used for hypertension and benign prostatic hyperplasia (tamsulosin selectively targets $α_{1 A}$ in prostatic smooth muscle).
Non-selective (phenoxybenzamine): used in phaeochromocytoma (a tumour of the adrenal medulla secreting massive amounts of catecholamines) to block the $α_{1}$ -mediated vasoconstriction before surgery.

ACE inhibitors and angiotensin receptor blockers (ARBs):

Reduce angiotensin II formation (ACE inhibitors) or action (ARBs). Angiotensin II is a potent vasoconstrictor ( $α_{1}$ -like effect via the renin-angiotensin system) and stimulates aldosterone release (sodium retention). Blocking it reduces blood pressure by decreasing both vasoconstriction and fluid volume.
First-line for hypertension, heart failure, and diabetic nephropathy.

Anticholinergics (muscarinic antagonists):

Atropine, hyoscine (scopolamine), ipratropium, oxybutynin.
Block parasympathetic effects: increase heart rate (atropine in bradycardia), reduce secretions (pre-operative), reduce gut motility (antispasmodics), reduce bladder contraction (urinary incontinence), bronchodilate (ipratropium in COPD/asthma).
Side effects: dry mouth, blurred vision, constipation, urinary retention, tachycardia, confusion (especially in the elderly — anticholinergic burden).

Cholinesterase inhibitors (acetylcholinesterase inhibitors):

Neostigmine, pyridostigmine, physostigmine, donepezil.
Increase ACh availability at all cholinergic synapses (nicotinic and muscarinic). Used in myasthenia gravis (increase ACh at the neuromuscular junction, $N_{M}$ receptors) and Alzheimer's disease (increase ACh in the CNS, muscarinic and nicotinic receptors).
Side effects: all parasympathetic excess — salivation, lacrimation, urination, defecation, GI distress, emesis (the "SLUDGE" syndrome), plus bradycardia and bronchoconstriction.

Sympathomimetics:

Direct agonists: epinephrine ( $α$ , $β$ ), norepinephrine ( $α$ , $β_{1}$ ), phenylephrine ( $α_{1}$ ), albuterol ( $β_{2}$ ), clonidine ( $α_{2}$ ).
Indirect agonists: amphetamine, cocaine (block NE reuptake), ephedrine (releases NE).
Uses: anaphylaxis (epinephrine), asthma ( $β_{2}$ agonists), shock (NE, phenylephrine), nasal decongestion ( $α_{1}$ agonists), ADHD/abuse potential (amphetamines), hypertension (clonidine reduces central sympathetic outflow via $α_{2}$ agonism).

Heart rate variability, vagal tone, and the stress response Master

Heart rate variability (HRV)

Heart rate variability is the beat-to-beat variation in cardiac inter-beat intervals (RR intervals on ECG). Far from being noise, HRV is a window into autonomic function: the variation reflects the continuous interplay of sympathetic and parasympathetic inputs to the SA node.

HRV is analysed in both the time domain (standard deviation of NN intervals [SDNN], root mean square of successive differences [RMSSD], percentage of intervals differing by >50 ms [pNN50]) and the frequency domain (spectral analysis of RR intervals):

High-frequency (HF) band (0.15-0.40 Hz): primarily reflects parasympathetic (vagal) activity. Respiratory sinus arrhythmia — the acceleration of heart rate during inspiration and deceleration during expiration, mediated by vagal modulation of the SA node — dominates this band.
Low-frequency (LF) band (0.04-0.15 Hz): reflects a mix of sympathetic and parasympathetic activity. The LF/HF ratio is sometimes used as an index of sympathovagal balance, though this interpretation is contested.
Very low-frequency (VLF) and ultra-low-frequency (ULF) bands: reflect thermoregulatory, endocrine, and renin-angiotensin system influences on heart rate.

Higher HRV indicates better autonomic flexibility and cardiovascular health. Low HRV (reduced variability) is a predictor of increased mortality after myocardial infarction, progression of heart failure, and sudden cardiac death. The mechanism: low HRV reflects reduced vagal tone and/or elevated sympathetic tone, indicating that the autonomic nervous system has lost its capacity for adaptive, flexible regulation and is stuck in a relatively fixed, less responsive state.

Vagal tone and baroreflex sensitivity

Vagal tone refers to the tonic level of parasympathetic (vagal) activity to the heart. At rest, vagal tone dominates: the resting heart rate of ~70 bpm is well below the intrinsic SA node rate of ~100 bpm because the vagus is continuously releasing ACh onto M2 receptors, hyperpolarising SA node cells and slowing depolarisation. Athletes have high vagal tone (resting heart rates of 40-60 bpm), reflecting enhanced parasympathetic control.

Baroreflex sensitivity (BRS) measures the change in RR interval per unit change in systolic blood pressure (ms/mmHg). Higher BRS indicates a more responsive baroreflex and greater capacity for rapid autonomic adjustment. BRS declines with age, hypertension, heart failure, and diabetes (autonomic neuropathy). Pharmacological interventions that improve BRS (beta-blockers, ACE inhibitors, exercise training) are associated with better cardiovascular outcomes.

The stress response: HPA axis and autonomic integration

The physiological stress response integrates two major neuroendocrine systems:

Sympathomedullary pathway (SAM): Stress activates the sympathetic nervous system and the adrenal medulla within seconds. The locus coeruleus (noradrenergic) in the pons fires, sympathetic outflow surges, and the adrenal medulla releases epinephrine into the bloodstream. This produces the immediate "alarm" phase: tachycardia, hypertension, hyperglycaemia, bronchodilation, and heightened alertness.

Hypothalamic-pituitary-adrenal axis (HPA axis): Stress (detected by the amygdala and relayed to the hypothalamus) triggers the paraventricular nucleus of the hypothalamus to release corticotropin-releasing hormone (CRH) into the hypophyseal portal system. CRH stimulates the anterior pituitary to release adrenocorticotropic hormone (ACTH), which travels via the blood to the adrenal cortex and stimulates the release of cortisol. Cortisol acts over minutes to hours: it promotes gluconeogenesis, lipolysis, and proteolysis (mobilising energy substrates), suppresses the immune system (anti-inflammatory), and provides negative feedback to the hypothalamus and pituitary to limit the stress response.

The SAM and HPA systems are synergistic: SAM provides the immediate, short-lived response (seconds to minutes), while the HPA axis provides the sustained response (minutes to hours). Both are necessary for survival in acute stress. Chronic activation, however, is pathological: sustained sympathetic tone (chronic stress, modern lifestyle) produces hypertension, atherosclerosis, insulin resistance, immunosuppression, and central effects (hippocampal atrophy, anxiety, depression). Chronic HPA activation produces Cushingoid features, visceral fat accumulation, and metabolic syndrome.

Biofeedback and vagal manoeuvres

Biofeedback is a technique in which a patient receives real-time information about a physiological variable (heart rate, HRV, skin conductance, muscle tension) and learns to voluntarily modulate it. HRV biofeedback trains patients to maximise respiratory sinus arrhythmia by breathing at their resonance frequency (~5-6 breaths per minute in most adults), at which the respiratory modulation of heart rate and the baroreflex feedback loop reinforce each other, producing large-amplitude oscillations in heart rate and blood pressure. This trains vagal tone and baroreflex sensitivity.

Vagal manoeuvres exploit the parasympathetic reflex arc to terminate supraventricular tachycardias: the Valsalva manoeuvre (forced expiration against a closed glottis) initially increases intrathoracic pressure, reducing venous return and blood pressure (activating the sympathetic limb of the baroreflex); when the strain is released, the sudden increase in blood pressure triggers a powerful vagal reflex that can convert AV-nodal re-entry tachycardia back to sinus rhythm. Carotid sinus massage directly stimulates baroreceptors for the same purpose.

Connections Master

This unit extends the autonomic nervous system overview introduced in 18.05.01 into the full anatomical, pharmacological, and clinical detail of the sympathetic and parasympathetic divisions. The baroreflex mechanism connects directly to cardiovascular physiology 18.02.01 (cardiac cycle and heart rate regulation) and 18.02.04 pending (cardiac cycle determinants). The HPA axis integration links to endocrine regulation 18.07.01, where the hypothalamic-pituitary axis is treated in full.

The neurotransmitter-receptor pharmacology connects to cell signalling 17.07.01: adrenergic receptors are GPCRs using $G_{s}$ , $G_{i}$ , and $G_{q}$ pathways identical to those in non-neural cell types. The nicotinic receptor is a ligand-gated ion channel, a class treated in molecular biology's membrane protein units.

Respiratory physiology 18.03.01 depends on autonomic control of bronchial smooth muscle ( $β_{2}$ -mediated dilation, M3-mediated constriction), and the autonomic modulation of respiration by the vagus. Renal physiology 18.08.01 receives sympathetic input regulating renin secretion and renal blood flow. Digestive physiology 18.06.01 is driven by the enteric nervous system modulated by autonomic input.

The autonomic neuropathy sections connect to the clinical consequences of diabetes (metabolic dysregulation damaging peripheral nerves), and the pharmacology sections connect to the receptor-level mechanisms of cardiovascular, respiratory, and neurological therapeutics.

Historical & philosophical context Master

John Newport Langley coined the term "autonomic nervous system" in his 1898 Croonian Lecture and developed the sympathetic-parasympathetic classification in his 1903 paper and 1921 monograph The Autonomic Nervous System. Langley's key insight was that the two divisions could be distinguished not only by anatomy (thoracolumbar vs craniosacral origin) but by pharmacology: he showed that nicotine blocked transmission at autonomic ganglia while pilocarpine (a muscarinic agonist) stimulated parasympathetic target organs. This pharmacological dissection of the autonomic divisions was the beginning of modern receptor pharmacology.

Otto Loewi's 1921 experiment — stimulating the vagus nerve of a frog heart, collecting the perfusate, and showing that it slowed a second heart — provided the first direct evidence for chemical neurotransmission. The substance he identified ("Vagusstoff") was acetylcholine. Henry Dale subsequently classified receptors as "nicotinic" or "muscarinic" based on the effects of these alkaloids, establishing the receptor-subtype framework still in use. Loewi and Dale shared the 1936 Nobel Prize.

Raymond Ahlquist's 1948 paper proposing two types of adrenergic receptors (alpha and beta) was initially rejected by multiple journals because the two-receptor model seemed unnecessarily complex. Published finally in the American Journal of Physiology, it became one of the most influential papers in pharmacology. James Black's development of the first beta-blocker (propranolol, 1964) — based directly on Ahlquist's alpha/beta classification — earned Black the 1988 Nobel Prize and created the multi-billion-dollar beta-blocker drug class.

The concept of homeostasis itself was introduced by Walter Cannon in The Wisdom of the Body (1932), building on Claude Bernard's earlier concept of the milieu interieur. Cannon coined the term "fight or flight" to describe the sympathetic response and was the first to describe the sympathetic-adrenal medullary system as an integrated emergency response. His work established the autonomic nervous system as the primary effector of homeostatic regulation.

Bibliography Master

Langley, J. N. — The Autonomic Nervous System. Brain 26, 1-26 (1903).
Loewi, O. — Uber humorale Ubertragbarkeit der Herznervenwirkung. Pflugers Archiv 189, 239-242 (1921).
Cannon, W. B. — The Wisdom of the Body. W. W. Norton (1932).
Ahlquist, R. P. — A study of the adrenotropic receptors. Am. J. Physiol. 153, 586-600 (1948).
Black, J. W., Duncan, W. A. M. & Shanks, R. G. — Comparison of some properties of pronethalol and propranolol. Br. J. Pharmacol. 25, 577-591 (1965).
Sherwood, L. — Human Physiology: From Cells to Systems, 9th ed. Cengage (2016).
Silverthorn, D. U. — Human Physiology: An Integrated Approach, 8th ed. Pearson (2019).
Kandel, E. R., Koester, J. D., Mack, S. H. & Siegelbaum, S. A. — Principles of Neural Science, 6th ed. McGraw-Hill (2021).
Goldberger, J. J. et al. — Heart rate variability: a measure of cardiac autonomic tone. Am. Heart J. 127, 1377-1381 (1994).
Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology — Heart rate variability: standards of measurement, physiological interpretation and clinical use. Circulation 93, 1043-1065 (1996).
Benarroch, E. E. — The central autonomic network: functional organization, dysfunction, and perspective. Mayo Clin. Proc. 68, 988-1001 (1993).