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Physiology Quiz

The Autonomic Nervous System

The autonomic nervous system (ANS) is the division of the peripheral nervous system that controls involuntary functions: heart rate, blood pressure, digestion, glandular secretion, pupil size and bladder emptying. It works through two mainly opposing divisions, sympathetic and parasympathetic, plus the enteric nervous system in the gut wall. These notes cover its anatomy, its two-neuron efferent pathway, the transmitters and receptors at each synapse, the organ-by-organ effects of each division, autonomic reflexes such as the baroreceptor reflex, and the drugs and clinical conditions most often examined. A 20-question practice set follows.

Organisation and General Plan

The autonomic nervous system is the efferent (motor) supply to smooth muscle, cardiac muscle and glands. Unlike the somatic motor system, which uses a single motor neuron running from the central nervous system straight to skeletal muscle, every autonomic pathway uses two neurons in series with a synapse in a peripheral ganglion.

  • Preganglionic neuron: cell body inside the central nervous system, myelinated axon, ends in an autonomic ganglion.
  • Postganglionic neuron: cell body in the ganglion, unmyelinated axon, ends on the target organ.
  • Three divisions: sympathetic, parasympathetic and enteric.
  • Sympathetic outflow is thoracolumbar, arising from spinal segments T1 to L2 (lateral horn, intermediolateral cell column).
  • Parasympathetic outflow is craniosacral, arising from cranial nerves III, VII, IX and X and from sacral segments S2 to S4.
  • Most organs receive dual innervation, and the two divisions usually oppose each other. Exceptions such as sweat glands, most blood vessels, the adrenal medulla, piloerector muscles and the spleen receive sympathetic supply only.
  • The hypothalamus is the highest integrating centre for autonomic control; the medulla oblongata houses the cardiovascular, respiratory and vomiting centres.

Sympathetic Division: Fight or Flight

Sympathetic preganglionic fibres are short and synapse close to the spinal cord in the paravertebral sympathetic chain or in prevertebral ganglia such as the coeliac and superior mesenteric ganglia. Postganglionic fibres are long. This anatomy allows one preganglionic fibre to diverge onto many postganglionic neurons, so sympathetic discharge tends to be widespread rather than local.

  • Preganglionic fibre short, postganglionic fibre long; divergence ratio can exceed one to twenty.
  • The adrenal medulla is a modified sympathetic ganglion: preganglionic fibres synapse directly on chromaffin cells, which release adrenaline (about 80 percent) and noradrenaline (about 20 percent) into the blood.
  • Overall effects: increased heart rate and force, bronchodilation, pupil dilation, glycogenolysis and lipolysis, reduced gut motility, sweating and blood flow redirected to skeletal muscle.
  • Noradrenaline released at nerve terminals is cleared mostly by reuptake into the nerve terminal (uptake 1) and then broken down by monoamine oxidase and catechol-O-methyl transferase.

Parasympathetic Division: Rest and Digest

Parasympathetic preganglionic fibres are long and travel almost the whole way to the target organ, synapsing in ganglia in or near the organ wall. Postganglionic fibres are therefore very short, and the divergence ratio is low, so parasympathetic actions are discrete and organ-specific.

  • The vagus nerve (cranial nerve X) carries roughly 75 percent of all parasympathetic fibres and supplies the heart, lungs and the gut as far as the distal third of the transverse colon.
  • Cranial nerve III supplies the ciliary ganglion for pupil constriction and accommodation; cranial nerve VII supplies the pterygopalatine and submandibular ganglia for lacrimation and salivation; cranial nerve IX supplies the otic ganglion for the parotid gland.
  • Sacral outflow S2 to S4 (pelvic splanchnic nerves) supplies the descending and sigmoid colon, rectum, bladder and genitalia.
  • Overall effects: slowing of the heart, bronchoconstriction, pupil constriction, increased gut motility and secretion, bladder emptying, and erection.

Transmitters and Receptors

Knowing which transmitter acts on which receptor at each synapse is the single most examined part of this topic. All preganglionic fibres, in both divisions, release acetylcholine onto nicotinic receptors in the ganglion. The divisions differ only at the second synapse, on the organ itself.

  • All preganglionic fibres: acetylcholine acting on nicotinic (ganglionic, Nn) receptors.
  • All parasympathetic postganglionic fibres: acetylcholine acting on muscarinic receptors (M1 to M5).
  • Most sympathetic postganglionic fibres: noradrenaline acting on adrenergic receptors (alpha 1, alpha 2, beta 1, beta 2, beta 3).
  • Exception: sympathetic fibres to eccrine sweat glands are cholinergic and act on muscarinic receptors, which is why atropine causes dry skin.
  • Exception: sympathetic fibres to renal blood vessels release dopamine acting on D1 receptors.
  • Nicotinic receptors are ligand-gated ion channels (fast); muscarinic and adrenergic receptors are G-protein-coupled (slower, second messengers).
  • Alpha 1: vasoconstriction, pupil dilation (radial muscle), bladder sphincter contraction, works through the Gq pathway.
  • Alpha 2: presynaptic autoreceptor that reduces further noradrenaline release, plus reduced insulin secretion, works through Gi.
  • Beta 1: increased heart rate, contractility and renin release, works through Gs.
  • Beta 2: bronchodilation, vasodilation in skeletal muscle, uterine relaxation, glycogenolysis, works through Gs.
  • M2: found on the heart, slows the sinoatrial node through Gi and potassium efflux.
  • M3: found on smooth muscle and glands, causes contraction and secretion through Gq.

Autonomic Reflexes, Drugs and Clinical Points

The baroreceptor reflex is the classic autonomic reflex. Stretch receptors in the carotid sinus (glossopharyngeal nerve) and aortic arch (vagus nerve) send signals to the nucleus tractus solitarius in the medulla. A rise in blood pressure increases their firing, which increases vagal outflow and reduces sympathetic outflow, so heart rate and blood pressure fall. A fall in blood pressure does the reverse.

  • Muscarinic agonists such as pilocarpine constrict the pupil and are used in glaucoma; muscarinic antagonists such as atropine cause tachycardia, dry mouth, blurred vision, urinary retention and hot dry skin.
  • Anticholinesterases such as neostigmine and physostigmine raise acetylcholine at all cholinergic synapses; organophosphate poisoning is treated with atropine plus pralidoxime.
  • Beta 1 blockers such as metoprolol lower heart rate and blood pressure; beta 2 agonists such as salbutamol relieve bronchospasm.
  • Alpha 1 blockers such as prazosin and tamsulosin lower blood pressure and relieve prostatic outflow obstruction; the alpha 2 agonist clonidine lowers central sympathetic outflow.
  • Horner syndrome (loss of sympathetic supply to the head) gives ptosis, miosis and anhidrosis on the affected side.
  • Autonomic dysfunction is a common early feature of diabetes mellitus, producing postural hypotension, gastroparesis and a fixed heart rate.
  • Cutting the vagus (vagotomy) reduces gastric acid secretion, which is why it was once used for peptic ulcer disease.

Key Terms

Preganglionic neuron
The first neuron of an autonomic pathway; its cell body lies in the central nervous system and it always releases acetylcholine onto nicotinic receptors in a ganglion.
Thoracolumbar outflow
The sympathetic outflow, arising from the lateral horn of spinal segments T1 to L2.
Craniosacral outflow
The parasympathetic outflow, arising from cranial nerves III, VII, IX and X and from sacral segments S2 to S4.
Baroreceptor reflex
A rapid negative-feedback reflex in which carotid sinus and aortic arch stretch receptors adjust vagal and sympathetic outflow to keep arterial blood pressure stable.

Practice Quiz — 20 Questions

correct out of 20

  1. The sympathetic outflow arises from which spinal segments?

    • A.C1 to C8
    • B.T1 to L2
    • C.S2 to S4
    • D.L3 to S1
    B. T1 to L2 — Sympathetic preganglionic cell bodies lie in the lateral horn from T1 to L2, which is why the division is called thoracolumbar.

  2. Which transmitter is released by ALL autonomic preganglionic fibres?

    • A.Noradrenaline
    • B.Acetylcholine
    • C.Dopamine
    • D.Adrenaline
    B. Acetylcholine — Every preganglionic fibre, sympathetic or parasympathetic, is cholinergic and acts on nicotinic receptors in the ganglion.

  3. Receptors on the postsynaptic membrane of an autonomic ganglion are:

    • A.Muscarinic
    • B.Nicotinic
    • C.Alpha 1 adrenergic
    • D.Beta 2 adrenergic
    B. Nicotinic — Ganglionic transmission uses nicotinic (Nn) receptors, which are ligand-gated ion channels.

  4. Which nerve carries about three quarters of all parasympathetic fibres?

    • A.Trigeminal
    • B.Facial
    • C.Vagus
    • D.Phrenic
    C. Vagus — The vagus nerve supplies the heart, lungs and most of the gut and accounts for roughly 75 percent of parasympathetic outflow.

  5. Compared with the sympathetic division, parasympathetic preganglionic fibres are:

    • A.Short, with long postganglionic fibres
    • B.Long, with short postganglionic fibres
    • C.Absent
    • D.Unmyelinated throughout
    B. Long, with short postganglionic fibres — Parasympathetic ganglia lie in or near the target organ, so preganglionic fibres are long and postganglionic fibres very short.

  6. The adrenal medulla is best described as:

    • A.A modified sympathetic ganglion
    • B.A parasympathetic ganglion
    • C.Part of the enteric nervous system
    • D.A somatic motor nucleus
    A. A modified sympathetic ganglion — Preganglionic sympathetic fibres synapse directly on chromaffin cells, which act as modified postganglionic neurons and secrete catecholamines into the blood.

  7. Roughly what proportion of adrenal medullary secretion is adrenaline?

    • A.20 percent
    • B.50 percent
    • C.80 percent
    • D.100 percent
    C. 80 percent — About 80 percent is adrenaline and about 20 percent noradrenaline.

  8. Sympathetic fibres to eccrine sweat glands are unusual because they release:

    • A.Noradrenaline onto alpha 1 receptors
    • B.Acetylcholine onto muscarinic receptors
    • C.Dopamine onto D1 receptors
    • D.Adrenaline onto beta 2 receptors
    B. Acetylcholine onto muscarinic receptors — These sympathetic postganglionic fibres are cholinergic and muscarinic, which is why atropine blocks sweating.

  9. Stimulation of beta 1 receptors on the heart causes:

    • A.Bradycardia
    • B.Increased heart rate and contractility
    • C.Bronchoconstriction
    • D.Pupil constriction
    B. Increased heart rate and contractility — Beta 1 receptors act through Gs and cyclic AMP to raise rate, conduction velocity and force of contraction.

  10. Bronchodilation is produced by stimulation of which receptor?

    • A.Alpha 1
    • B.Beta 2
    • C.M3
    • D.Nicotinic
    B. Beta 2 — Beta 2 receptors relax bronchial smooth muscle, which is why salbutamol relieves asthma.

  11. Presynaptic alpha 2 receptors on a noradrenergic nerve terminal:

    • A.Increase noradrenaline release
    • B.Reduce further noradrenaline release
    • C.Destroy noradrenaline
    • D.Have no known function
    B. Reduce further noradrenaline release — Alpha 2 autoreceptors work through Gi to provide negative feedback on transmitter release.

  12. The muscarinic receptor subtype that slows the heart is:

    • A.M1
    • B.M2
    • C.M3
    • D.M5
    B. M2 — Cardiac M2 receptors couple to Gi and increase potassium efflux, slowing sinoatrial node depolarisation.

  13. The pupil is dilated (mydriasis) by contraction of the:

    • A.Circular muscle via M3 receptors
    • B.Radial muscle via alpha 1 receptors
    • C.Ciliary muscle via M3 receptors
    • D.Levator palpebrae via beta 1 receptors
    B. Radial muscle via alpha 1 receptors — Sympathetic alpha 1 stimulation contracts the radial (dilator pupillae) muscle; parasympathetic M3 stimulation constricts the pupil.

  14. Which structure supplies the parasympathetic fibres for pupil constriction?

    • A.Otic ganglion
    • B.Ciliary ganglion
    • C.Superior cervical ganglion
    • D.Coeliac ganglion
    B. Ciliary ganglion — Cranial nerve III synapses in the ciliary ganglion, whose short ciliary nerves supply the sphincter pupillae and ciliary muscle.

  15. Bladder emptying (micturition) requires:

    • A.Detrusor contraction and internal sphincter relaxation
    • B.Detrusor relaxation and internal sphincter contraction
    • C.Sympathetic stimulation only
    • D.Somatic stimulation only
    A. Detrusor contraction and internal sphincter relaxation — Parasympathetic S2 to S4 fibres contract the detrusor through M3 receptors while the alpha 1 mediated internal sphincter tone falls.

  16. Which triad describes Horner syndrome?

    • A.Ptosis, miosis, anhidrosis
    • B.Mydriasis, exophthalmos, sweating
    • C.Ptosis, mydriasis, dry mouth
    • D.Diplopia, miosis, flushing
    A. Ptosis, miosis, anhidrosis — Loss of the sympathetic supply to one side of the head gives drooping eyelid, small pupil and absent sweating on that side.

  17. A sudden rise in arterial blood pressure will, through the baroreceptor reflex:

    • A.Increase sympathetic and decrease vagal outflow
    • B.Increase vagal and decrease sympathetic outflow
    • C.Increase both outflows
    • D.Have no effect on heart rate
    B. Increase vagal and decrease sympathetic outflow — Increased baroreceptor firing raises vagal tone and reduces sympathetic tone, so heart rate and blood pressure fall.

  18. Baroreceptors of the carotid sinus send afferents through which nerve?

    • A.Vagus
    • B.Glossopharyngeal
    • C.Facial
    • D.Trigeminal
    B. Glossopharyngeal — Carotid sinus afferents run in the glossopharyngeal nerve; aortic arch afferents run in the vagus.

  19. Atropine poisoning produces all of the following EXCEPT:

    • A.Dry mouth
    • B.Tachycardia
    • C.Excessive sweating
    • D.Blurred vision
    C. Excessive sweating — Atropine blocks muscarinic receptors, including those on sweat glands, so the skin is hot and dry rather than sweaty.

  20. The main route by which released noradrenaline is removed from the synaptic cleft is:

    • A.Enzymatic breakdown by acetylcholinesterase
    • B.Reuptake into the nerve terminal (uptake 1)
    • C.Diffusion into red blood cells
    • D.Filtration by the kidney
    B. Reuptake into the nerve terminal (uptake 1) — Most noradrenaline is recaptured by the noradrenaline transporter; monoamine oxidase and catechol-O-methyl transferase then metabolise it.

References