Autonomic Nervous System and Pain Pathways

AUTONOMIC NERVOUS SYSTEM — ORGANISATION

The autonomic nervous system (ANS) is the involuntary motor system controlling the heart, smooth muscle (blood vessels, gut, bronchi, bladder, pupil), and glands. It is organised into two main divisions — sympathetic and parasympathetic — plus the enteric nervous system (intrinsic neural network of the gut, often called the "second brain").

SYMPATHETIC DIVISION (thoracolumbar, T1–L2): Pre-ganglionic neurons in the lateral horn of the spinal cord send short myelinated axons (white rami communicantes) to the paravertebral sympathetic chain ganglia or to prevertebral ganglia (coeliac, superior/inferior mesenteric). Pre-ganglionic neurotransmitter: ACh acting on nicotinic receptors (Nn subtype, heteropentameric α₃β₄) in the ganglion. Post-ganglionic neurons send long unmyelinated axons to target organs, releasing noradrenaline (norepinephrine) onto α and β-adrenoceptors. Key exceptions: (1) Sweat glands (eccrine) — sympathetic post-ganglionic neurons release ACh acting on muscarinic M3 receptors. (2) Adrenal medulla — innervated by pre-ganglionic sympathetic neurons directly (no post-ganglionic neuron); chromaffin cells are modified post-ganglionic neurons that release adrenaline (80%) and noradrenaline (20%) directly into the bloodstream as hormones.

PARASYMPATHETIC DIVISION (craniosacral: CN III, VII, IX, X + S2–4): Pre-ganglionic neurons send long myelinated axons to ganglia located near or within the effector organ. Post-ganglionic neurons are short. ACh is the neurotransmitter throughout (pre- and post-ganglionic), acting on muscarinic receptors at target organs.

ADRENOCEPTORS AND THEIR SIGNALLING

α1-adrenoceptors: Gq-coupled → PLC → IP3 + DAG → ↑Ca²⁺ → smooth muscle contraction. Locations: vascular smooth muscle (vasoconstriction, ↑BP), iris dilator (mydriasis), urethral sphincter (continence), uterus. Blocked by prazosin, doxazosin (used in BPH, hypertension).

α2-adrenoceptors: Gi-coupled → ↓adenylyl cyclase → ↓cAMP. Locations: presynaptic terminals (negative feedback — reduces noradrenaline release), platelet aggregation promotion, inhibition of insulin secretion, CNS (central sympatholytic). Clonidine and methyldopa are α2-agonists (reduce sympathetic outflow from CNS — antihypertensive mechanism). Yohimbine is an α2-antagonist.

β1-adrenoceptors: Gs-coupled → ↑cAMP → PKA activation. Locations: heart (↑HR = positive chronotropy, ↑contractility = positive inotropy, ↑conduction speed = positive dromotropy), juxtaglomerular apparatus (↑renin secretion → RAAS activation → ↑BP). Blocked by all β-blockers.

β2-adrenoceptors: Gs-coupled → ↑cAMP → PKA activation. Locations: bronchial smooth muscle (bronchodilation), vascular smooth muscle in skeletal muscle (vasodilation), uterus (relaxation — tocolysis), liver and skeletal muscle (glycogenolysis and gluconeogenesis → ↑blood glucose), mast cells (reduced mediator release). Salbutamol is a selective β2-agonist.

β3-adrenoceptors: Gs-coupled. Locations: adipose tissue (lipolysis), bladder detrusor (relaxation — mirabegron, a β3-agonist, used for overactive bladder), brown fat (thermogenesis).

MUSCARINIC RECEPTORS AND THEIR EFFECTS

M1 (Gq): CNS neurotransmission, gastric parietal cells (↑HCl secretion — stimulated by vagal tone; blocked by pirenzepine).

M2 (Gi): Heart — ↓HR (negative chronotropy via IKACh → K⁺ efflux → hyperpolarisation of SA node), ↓AV conduction (negative dromotropy). Smooth muscle presynaptically. Atropine blocks M2 → ↑HR (used for bradycardia, organophosphate poisoning).

M3 (Gq): Smooth muscle contraction (bronchospasm, gut motility, bladder detrusor contraction → voiding); exocrine gland secretion (salivary, lacrimal, pancreatic, bronchial); eye: sphincter pupillae contraction (miosis), ciliary muscle contraction (accommodation for near vision). Blocked by ipratropium (inhaled, for COPD/asthma), oxybutynin (overactive bladder), hyoscine (motion sickness, pre-operative drying).

ANS PHARMACOLOGY SUMMARY

Sympathomimetics: Adrenaline (epinephrine) — α1+α2+β1+β2+β3; first-line for anaphylaxis (IM thigh, 500 mcg — vasopressor via α1, bronchodilator via β2, cardiac stimulant via β1). Noradrenaline — α1 dominant, β1 moderate, minimal β2; used for vasodilatory/vasogenic shock (raises SVR). Salbutamol — selective β2 agonist; asthma.

Sympatholytics: β-blockers — propranolol (non-selective β1+β2), atenolol/bisoprolol (β1-selective, cardioselective). Uses: HTN, angina, heart failure (carvedilol, metoprolol reduce mortality), AF rate control, anxiety (propranolol). Caution: β2-blockade → bronchoconstriction (avoid non-selective in asthma). α1-blockers (doxazosin, tamsulosin) — BPH (relax urethral smooth muscle), hypertension, phaeochromocytoma (use phenoxybenzamine, irreversible non-selective α-blocker, before surgery).

Parasympathomimetics (cholinomimetics): Pilocarpine (M3 agonist, eye drops — reduces IOP in glaucoma via ciliary muscle contraction opening trabecular meshwork); neostigmine (AChE inhibitor — NMJ block reversal, post-operative ileus).

Antimuscarinics: Atropine (non-selective muscarinic antagonist — bradycardia treatment, organophosphate/nerve agent poisoning — given with pralidoxime); ipratropium (inhaled M3 antagonist — bronchodilation in COPD); oxybutynin, solifenacin (overactive bladder); hyoscine (motion sickness — M1 in vestibular system; pre-operative antisialagogue); glycopyrrolate (quaternary, does not cross BBB — preferred perioperatively).

PAIN PATHWAYS

Nociception is the neural process of encoding and processing noxious stimuli. Nociceptors are free nerve endings of primary afferent neurons sensitive to thermal, mechanical, and chemical stimuli. Two fibre types: Aδ fibres (small diameter, thinly myelinated, conduction velocity 5–30 m/s) — detect sharp, well-localised, acute pain (first pain); C fibres (unmyelinated, <1 μm, 0.5–2 m/s) — detect diffuse, burning, aching pain (second pain). C fibres are the most common nociceptor type in humans.

Transduction: noxious stimuli activate transient receptor potential (TRP) channels. TRPV1 (capsaicin receptor, activated by >43°C heat and low pH) and TRPA1 (cold, mechanical, chemical irritants) are the principal nociceptor ion channels.

Spinal cord: primary afferents enter the dorsal horn and synapse in the substantia gelatinosa (Rexed laminae I–II). Glutamate mediates fast transmission (AMPA); substance P (neuropeptide) contributes to slow, sustained transmission (NK1 receptor). Second-order neurons cross to the contralateral anterolateral quadrant and ascend in the spinothalamic tract (STT) to the thalamus (VPL nucleus for the body, VPM for the face). Third-order neurons project to the primary somatosensory cortex (S1 — discriminative component of pain) and also to the anterior cingulate cortex and insular cortex (affective component).

Descending modulation: the periaqueductal grey matter (PAG) of the midbrain activates serotonergic neurons of the raphe nuclei and noradrenergic neurons of the locus coeruleus; these project to the dorsal horn and inhibit nociceptive transmission (releasing serotonin and noradrenaline onto interneurons → inhibition of second-order neurons). Endogenous opioids (enkephalins, endorphins, dynorphins) act at μ, δ, and κ receptors at spinal and supraspinal sites to inhibit pain — the basis of opioid analgesia.

Gate control theory (Melzack & Wall, 1965): Non-nociceptive Aβ fibres (large, myelinated, low-threshold, conveying touch/vibration) activate inhibitory interneurons in the substantia gelatinosa that "gate" incoming C fibre (pain) signals. When Aβ input is high (e.g., rubbing an injured area), the gate partially closes and pain is reduced. This explains the analgesic effect of TENS (transcutaneous electrical nerve stimulation), which selectively activates Aβ fibres. Conversely, when attention focuses on pain (anxious state), descending facilitation opens the gate.

ANALGESIC LADDER (WHO)

Step 1 (mild pain): Paracetamol (mechanism uncertain — possible COX-3 and endocannabinoid involvement) + NSAIDs (COX-1/2 inhibition → ↓prostaglandin synthesis; caution: renal, GI, cardiovascular risks).

Step 2 (moderate pain): Weak opioids — codeine (prodrug, CYP2D6 → morphine; polymorphic response), tramadol (μ-opioid partial agonist + NRI + SRI; serotonin syndrome risk). Add-on to Step 1 agents.

Step 3 (severe pain): Strong opioids — morphine (gold standard; μ-opioid agonist; active metabolite M6G accumulates in renal failure), oxycodone, hydromorphone, fentanyl (transdermal patch or IV), buprenorphine (partial μ-agonist — ceiling effect, useful in addiction medicine).

Adjuvants for neuropathic pain: tricyclic antidepressants (amitriptyline — NRI + SRI + Na⁺ channel blockade → inhibits dorsal horn pain transmission); gabapentinoids (gabapentin, pregabalin — bind α₂δ subunit of VGCCs in dorsal horn → reduce glutamate release; first-line for diabetic neuropathy, post-herpetic neuralgia); duloxetine (SNRI — activates descending noradrenergic inhibition); lidocaine patches; capsaicin cream (TRPV1 desensitisation).