MRCS Part A — Autonomic Nervous System
16 MCQs · Autonomic Anatomy, Physiology & Applied Surgery
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Autonomic Nervous System — Revision Notes
Start with the rapid distinctions, then open a topic for focused revision. Key pathways, mechanisms and operative risks are grouped together.
- Horner: small pupil + mild ptosis; complete CN III palsy may cause a dilated pupil + ptosis.
- Pelvic splanchnics: erection. Hypogastric sympathetic fibres: emission. Pudendal motor fibres: expulsion.
- Thoracic abdominopelvic splanchnics: preganglionic sympathetic. Cardiopulmonary splanchnics: predominantly postganglionic sympathetic.
- Beta-2: bronchodilation. M3: bronchoconstriction. Alpha-1: vasoconstriction.
01Autonomic foundations & control
Adrenal medulla & transmitters
Chromaffin cells of the adrenal medulla secrete adrenaline (epinephrine) into the bloodstream following stimulation by preganglionic sympathetic cholinergic fibres. The adrenal medulla functions as a modified sympathetic ganglion, permitting a widespread fight-or-flight response.
- Preganglionic sympathetic fibres release acetylcholine onto nicotinic receptors on chromaffin cells.
- Cortisol exposure facilitates conversion of noradrenaline into adrenaline through PNMT.
- The adrenal medulla is a modified sympathetic ganglion without a conventional postganglionic axon.
Remember: Adrenal medulla = circulating adrenaline; most sympathetic nerve endings = noradrenaline.
Sensory roots vs motor outflow
The dorsal root ganglion contains the cell bodies of primary sensory (pseudounipolar) neurons. Injury interrupts afferent signals from the corresponding dermatome and other sensory territories, whereas somatic motor neuron cell bodies are located in the anterior horn.
- Dorsal root = sensory afferent; ventral root = motor efferent.
- Autonomic visceral afferents can also have their cell bodies in dorsal root ganglia.
Remember: Dorsal root ganglia are sensory, not autonomic efferent ganglia.
Higher autonomic integration
The hypothalamus integrates autonomic, endocrine and behavioural responses to maintain homeostasis. It coordinates temperature, appetite, stress, osmolality and cardiovascular regulation via its links to brainstem autonomic nuclei and spinal autonomic outflow.
- Hypothalamic outputs influence sympathetic thoracolumbar and parasympathetic brainstem/sacral pathways.
- The nucleus tractus solitarius integrates key visceral afferent input in the medulla.
- The hypothalamus also links the nervous and endocrine systems via the pituitary.
Remember: Higher autonomic integration = hypothalamus; vital reflex centres = brainstem.
02Horner syndrome, pupils & stellate ganglion
Stellate ganglion: surgical relations
The stellate (cervicothoracic) ganglion lies anterior to the neck of the first rib and close to the subclavian vessels and pleural dome. Injury may interrupt the sympathetic pathway ascending to the head, producing ipsilateral ptosis, miosis and variable anhidrosis.
- Stellate ganglion is usually fusion of inferior cervical and first thoracic sympathetic ganglia.
- The superior cervical ganglion is the usual synapse for sympathetic fibres destined for ocular structures.
- Thoracic sympathectomy and stellate blocks can cause Horner syndrome.
- The anatomic proximity of pleura makes pneumothorax a relevant procedural risk.
Remember: Ganglion anterior to the first-rib neck + Horner = stellate ganglion.
Horner syndrome: lesion effects
Sympathetic disruption denervates the superior tarsal (Müller) muscle, producing mild upper-lid ptosis. Loss of sympathetic input to the iris dilator additionally causes miosis; loss of facial sudomotor fibres may produce anhidrosis. This combination is Horner syndrome.
- Horner triad: ptosis, miosis and ipsilateral anhidrosis.
- Ptosis in Horner is usually mild because levator palpebrae superioris retains CN III supply.
- Internal carotid sympathetic lesions may spare much of facial sweating because sudomotor fibres predominantly follow the external carotid artery.
Remember: Cut sympathetic chain = small pupil + mild ptosis, never a dilated pupil.
Pancoast tumour
Apical lung tumours can invade the preganglionic cervical sympathetic pathway and cause Horner syndrome: ipsilateral ptosis, miosis and anhidrosis. They may also affect the lower brachial plexus, causing shoulder/arm pain and intrinsic hand muscle weakness.
- Pancoast tumours occur in the superior pulmonary sulcus.
- The C8–T1/lower-trunk brachial plexus region can be involved.
- Not every Pancoast tumour causes Horner syndrome.
Remember: Pancoast + ptosis + miosis + anhidrosis = sympathetic chain involvement.
Pupil pathways
The sympathetic radial dilator muscle enlarges the pupil. When parasympathetic supply to sphincter pupillae is lost, intact sympathetic dilator activity becomes unopposed, resulting in mydriasis.
- Pupil constriction: CN III → ciliary ganglion → short ciliary nerves → sphincter pupillae (muscarinic).
- Pupil dilation: sympathetic fibres → superior cervical ganglion → dilator pupillae (alpha-1).
- A complete oculomotor palsy can produce ptosis with a dilated pupil; Horner produces ptosis with miosis.
Remember: Dilated pupil suggests parasympathetic failure or sympathetic activation; Horner produces miosis.
Stellate ganglion: formation
The stellate (cervicothoracic) ganglion commonly forms through fusion of the inferior cervical ganglion with T1 sympathetic ganglion. It lies in the root of the neck near C7 and the neck of the first rib.
- This fusion is present in approximately 80% of people; anatomy varies.
- Do not confuse the vertebral levels C7/T1 with the spinal cord sympathetic centre C8–T2.
- Stellate intervention may risk vascular injury, pneumothorax and Horner syndrome.
Remember: Stellate = inferior cervical ganglion + T1 sympathetic ganglion.
03Organ effects & receptors
Parasympathetic effects & bowel territories
Parasympathetic vagal and pelvic outflow generally increases gastrointestinal motility and secretion by activating enteric circuits and smooth muscle. This is a classic rest-and-digest response.
- Vagus supplies parasympathetic innervation to foregut and midgut derivatives and proximal hindgut up to approximately the proximal two-thirds of transverse colon.
- Pelvic splanchnic S2–S4 supplies distal hindgut and pelvic viscera.
- Parasympathetic neurotransmission uses acetylcholine at ganglia and target organs.
Remember: Parasympathetic activation increases bowel peristalsis and produces miosis.
Airway receptors
Activation of beta-2 adrenergic receptors relaxes bronchial smooth muscle through Gs-mediated increases in cAMP.
- Beta-2 agonists such as salbutamol are standard bronchodilators.
- Airway parasympathetic fibres run principally with the vagus nerve.
- Sympathetic bronchodilation is strongly driven by circulating adrenaline acting on beta-2 receptors.
Remember: Beta-2 opens airways; M3 narrows them.
Vascular resistance
Sympathetic noradrenaline stimulates alpha-1 receptors on resistance-vessel smooth muscle, reducing vessel radius and increasing total peripheral resistance (TPR). This usually contributes to an increase in arterial pressure when other haemodynamic factors are unchanged.
- Mean arterial pressure is approximately cardiac output × total peripheral resistance (plus a small CVP term).
- Alpha-1 mediates arteriolar and venous smooth-muscle constriction.
Remember: Sympathetic alpha-1 activation constricts arterioles and raises TPR.
04Pelvic autonomic pathways & rectal surgery
Erection & pelvic splanchnics
Pelvic splanchnic nerves (S2–S4), also called nervi erigentes, carry parasympathetic fibres to the inferior hypogastric plexus and ultimately the cavernous nerves. Parasympathetic activity promotes nitric oxide–mediated cavernosal smooth-muscle relaxation and penile erection. They can be damaged during total mesorectal excision or abdominoperineal resection.
- Parasympathetic erection: S2–S4 (nervi erigentes); sympathetic emission/ejaculation: predominantly T11–L2 via hypogastric pathways.
- The inferior hypogastric plexus contains both sympathetic and parasympathetic fibres.
- Pelvic autonomic nerve preservation is a key principle during rectal cancer dissection.
Remember: Point and shoot: parasympathetic erection; sympathetic emission; pudendal-mediated expulsion.
Emission vs expulsion
The sympathetic superior hypogastric plexus and hypogastric nerves carry thoracolumbar fibres essential for emission and bladder-neck closure during ejaculation. Injury during rectal surgery may cause absent or retrograde ejaculation even when S2–S4 parasympathetic erectile pathways remain intact.
- Sympathetic emission and bladder-neck closure: principally T11–L2 via hypogastric plexuses.
- Pudendal S2–S4 motor fibres contribute to the expulsion phase by activating bulbospongiosus and related muscles.
- High anterior dissection near the aortic bifurcation and sacral promontory can injure superior hypogastric sympathetic fibres.
Remember: Erection = pelvic parasympathetic; emission/bladder-neck closure = hypogastric sympathetic.
05Cord injury & splanchnic surgical anatomy
Autonomic dysreflexia
Spinal cord injury at or above T6 can disconnect supraspinal regulation of the splanchnic sympathetic vascular bed. A stimulus below the lesion, commonly bladder distension, triggers severe reflex vasoconstriction and hypertension; reflex bradycardia, headache and flushing may follow.
- Autonomic dysreflexia is a potential emergency: sit the patient upright, loosen restrictive clothing, monitor BP and search promptly for bladder/bowel triggers.
- Common trigger: blocked urinary catheter or distended bladder.
- Clinical risk rises with higher and more complete cord lesions.
Remember: Autonomic dysreflexia = spinal cord injury at or above T6.
Thoracic splanchnic nerves
The lesser thoracic splanchnic nerves generally arise from T10–T11 sympathetic trunk levels and carry preganglionic sympathetic fibres through the thorax to prevertebral ganglia, particularly the aorticorenal and related abdominal ganglia.
- Greater thoracic splanchnic: typically T5–T9; lesser: T10–T11; least: T12.
- Thoracic abdominopelvic splanchnics usually bypass sympathetic-chain ganglia without synapsing.
- Cardiopulmonary splanchnic nerves are predominantly postganglionic sympathetic: a common MRCS trap.
Remember: Lesser splanchnic (T10–T11) is preganglionic sympathetic, not postganglionic.
Lumbar sympathetic chain
The left lumbar sympathetic chain runs on the anterolateral lumbar vertebral bodies along the medial margin of psoas, lateral/posterolateral to the abdominal aorta.
- On the right, the lumbar sympathetic chain lies posterior to the inferior vena cava.
- Lumbar sympathetic ganglia are usually described on the anterolateral vertebral bodies, deep to medial psoas.
- Aortic and caval relations change with side, so a question that omits side/level is under-specified.
Remember: Left lumbar sympathetic trunk is lateral to the aorta; right is posterior to the IVC.
During an acute stress response, which structure secretes most circulating adrenaline?
Review explanation
Correct Answer: B. Adrenal medulla.
Why the correct answer is correct
Chromaffin cells of the adrenal medulla secrete adrenaline (epinephrine) into the bloodstream following stimulation by preganglionic sympathetic cholinergic fibres. The adrenal medulla functions as a modified sympathetic ganglion, permitting a widespread fight-or-flight response.
Why the other options are incorrect
Applied Revision — High-Yield Notes
- Preganglionic sympathetic fibres release acetylcholine onto nicotinic receptors on chromaffin cells.
- Cortisol exposure facilitates conversion of noradrenaline into adrenaline through PNMT.
- The adrenal medulla is a modified sympathetic ganglion without a conventional postganglionic axon.
Exam Pearl: Adrenal medulla = circulating adrenaline; most sympathetic nerve endings = noradrenaline.
Destruction of a dorsal root ganglion primarily causes loss of which function?
Review explanation
Correct Answer: D. Somatic and visceral sensory input.
Why the correct answer is correct
The dorsal root ganglion contains the cell bodies of primary sensory (pseudounipolar) neurons. Injury interrupts afferent signals from the corresponding dermatome and other sensory territories, whereas somatic motor neuron cell bodies are located in the anterior horn.
Why the other options are incorrect
Applied Revision — High-Yield Notes
- Dorsal root = sensory afferent; ventral root = motor efferent.
- Autonomic visceral afferents can also have their cell bodies in dorsal root ganglia.
Exam Pearl: Dorsal root ganglia are sensory, not autonomic efferent ganglia.
A man develops erectile dysfunction after rectal cancer resection. Which nerves were most likely injured?
Review explanation
Correct Answer: A. Pelvic splanchnic nerves.
Why the correct answer is correct
Pelvic splanchnic nerves (S2–S4), also called nervi erigentes, carry parasympathetic fibres to the inferior hypogastric plexus and ultimately the cavernous nerves. Parasympathetic activity promotes nitric oxide–mediated cavernosal smooth-muscle relaxation and penile erection. They can be damaged during total mesorectal excision or abdominoperineal resection.
Why the other options are incorrect
Applied Revision — High-Yield Notes
- Parasympathetic erection: S2–S4 (nervi erigentes); sympathetic emission/ejaculation: predominantly T11–L2 via hypogastric pathways.
- The inferior hypogastric plexus contains both sympathetic and parasympathetic fibres.
- Pelvic autonomic nerve preservation is a key principle during rectal cancer dissection.
Exam Pearl: Point and shoot: parasympathetic erection; sympathetic emission; pudendal-mediated expulsion.
Which part of the brain is the principal higher integrating centre for autonomic function?
Review explanation
Correct Answer: C. Hypothalamus.
Why the correct answer is correct
The hypothalamus integrates autonomic, endocrine and behavioural responses to maintain homeostasis. It coordinates temperature, appetite, stress, osmolality and cardiovascular regulation via its links to brainstem autonomic nuclei and spinal autonomic outflow.
Why the other options are incorrect
Applied Revision — High-Yield Notes
- Hypothalamic outputs influence sympathetic thoracolumbar and parasympathetic brainstem/sacral pathways.
- The nucleus tractus solitarius integrates key visceral afferent input in the medulla.
- The hypothalamus also links the nervous and endocrine systems via the pituitary.
Exam Pearl: Higher autonomic integration = hypothalamus; vital reflex centres = brainstem.
A patient develops ipsilateral Horner syndrome after surgery near the neck of the first rib. Which ganglion is most likely affected?
Review explanation
Correct Answer: B. Stellate ganglion.
Why the correct answer is correct
The stellate (cervicothoracic) ganglion lies anterior to the neck of the first rib and close to the subclavian vessels and pleural dome. Injury may interrupt the sympathetic pathway ascending to the head, producing ipsilateral ptosis, miosis and variable anhidrosis.
Why the other options are incorrect
Applied Revision — High-Yield Notes
- Stellate ganglion is usually fusion of inferior cervical and first thoracic sympathetic ganglia.
- The superior cervical ganglion is the usual synapse for sympathetic fibres destined for ocular structures.
- Thoracic sympathectomy and stellate blocks can cause Horner syndrome.
- The anatomic proximity of pleura makes pneumothorax a relevant procedural risk.
Exam Pearl: Ganglion anterior to the first-rib neck + Horner = stellate ganglion.
A cervical sympathetic chain is divided during an operation. Which ipsilateral clinical finding is expected?
Review explanation
Correct Answer: D. Partial ptosis.
Why the correct answer is correct
Sympathetic disruption denervates the superior tarsal (Müller) muscle, producing mild upper-lid ptosis. Loss of sympathetic input to the iris dilator additionally causes miosis; loss of facial sudomotor fibres may produce anhidrosis. This combination is Horner syndrome.
Why the other options are incorrect
Applied Revision — High-Yield Notes
- Horner triad: ptosis, miosis and ipsilateral anhidrosis.
- Ptosis in Horner is usually mild because levator palpebrae superioris retains CN III supply.
- Internal carotid sympathetic lesions may spare much of facial sweating because sudomotor fibres predominantly follow the external carotid artery.
Exam Pearl: Cut sympathetic chain = small pupil + mild ptosis, never a dilated pupil.
A patient with an apical lung (Pancoast) tumour develops ipsilateral ptosis. What additional findings are characteristic?
Review explanation
Correct Answer: A. Miosis and facial anhidrosis.
Why the correct answer is correct
Apical lung tumours can invade the preganglionic cervical sympathetic pathway and cause Horner syndrome: ipsilateral ptosis, miosis and anhidrosis. They may also affect the lower brachial plexus, causing shoulder/arm pain and intrinsic hand muscle weakness.
Why the other options are incorrect
Applied Revision — High-Yield Notes
- Pancoast tumours occur in the superior pulmonary sulcus.
- The C8–T1/lower-trunk brachial plexus region can be involved.
- Not every Pancoast tumour causes Horner syndrome.
Exam Pearl: Pancoast + ptosis + miosis + anhidrosis = sympathetic chain involvement.
Stimulation of the parasympathetic nervous system most directly increases which activity?
Review explanation
Correct Answer: C. Intestinal peristalsis.
Why the correct answer is correct
Parasympathetic vagal and pelvic outflow generally increases gastrointestinal motility and secretion by activating enteric circuits and smooth muscle. This is a classic rest-and-digest response.
Why the other options are incorrect
Applied Revision — High-Yield Notes
- Vagus supplies parasympathetic innervation to foregut and midgut derivatives and proximal hindgut up to approximately the proximal two-thirds of transverse colon.
- Pelvic splanchnic S2–S4 supplies distal hindgut and pelvic viscera.
- Parasympathetic neurotransmission uses acetylcholine at ganglia and target organs.
Exam Pearl: Parasympathetic activation increases bowel peristalsis and produces miosis.
Following paralysis of sphincter pupillae, which action accounts for an enlarged pupil?
Review explanation
Correct Answer: D. Unopposed sympathetic action on dilator pupillae.
Why the correct answer is correct
The sympathetic radial dilator muscle enlarges the pupil. When parasympathetic supply to sphincter pupillae is lost, intact sympathetic dilator activity becomes unopposed, resulting in mydriasis.
Why the other options are incorrect
Applied Revision — High-Yield Notes
- Pupil constriction: CN III → ciliary ganglion → short ciliary nerves → sphincter pupillae (muscarinic).
- Pupil dilation: sympathetic fibres → superior cervical ganglion → dilator pupillae (alpha-1).
- A complete oculomotor palsy can produce ptosis with a dilated pupil; Horner produces ptosis with miosis.
Exam Pearl: Dilated pupil suggests parasympathetic failure or sympathetic activation; Horner produces miosis.
Which autonomic receptor is chiefly responsible for catecholamine-mediated bronchodilation?
Review explanation
Correct Answer: B. Beta-2 adrenergic.
Why the correct answer is correct
Activation of beta-2 adrenergic receptors relaxes bronchial smooth muscle through Gs-mediated increases in cAMP.
Why the other options are incorrect
Applied Revision — High-Yield Notes
- Beta-2 agonists such as salbutamol are standard bronchodilators.
- Airway parasympathetic fibres run principally with the vagus nerve.
- Sympathetic bronchodilation is strongly driven by circulating adrenaline acting on beta-2 receptors.
Exam Pearl: Beta-2 opens airways; M3 narrows them.
The stellate ganglion is usually formed by fusion of which two sympathetic ganglia?
Review explanation
Correct Answer: A. Inferior cervical and first thoracic.
Why the correct answer is correct
The stellate (cervicothoracic) ganglion commonly forms through fusion of the inferior cervical ganglion with T1 sympathetic ganglion. It lies in the root of the neck near C7 and the neck of the first rib.
Why the other options are incorrect
Applied Revision — High-Yield Notes
- This fusion is present in approximately 80% of people; anatomy varies.
- Do not confuse the vertebral levels C7/T1 with the spinal cord sympathetic centre C8–T2.
- Stellate intervention may risk vascular injury, pneumothorax and Horner syndrome.
Exam Pearl: Stellate = inferior cervical ganglion + T1 sympathetic ganglion.
Increased sympathetic discharge raises total peripheral resistance predominantly through which mechanism?
Review explanation
Correct Answer: C. Alpha-1-mediated arteriolar vasoconstriction.
Why the correct answer is correct
Sympathetic noradrenaline stimulates alpha-1 receptors on resistance-vessel smooth muscle, reducing vessel radius and increasing total peripheral resistance (TPR). This usually contributes to an increase in arterial pressure when other haemodynamic factors are unchanged.
Why the other options are incorrect
Applied Revision — High-Yield Notes
- Mean arterial pressure is approximately cardiac output × total peripheral resistance (plus a small CVP term).
- Alpha-1 mediates arteriolar and venous smooth-muscle constriction.
Exam Pearl: Sympathetic alpha-1 activation constricts arterioles and raises TPR.
Autonomic dysreflexia is classically associated with spinal cord injury at or above which level?
Review explanation
Correct Answer: B. T6.
Why the correct answer is correct
Spinal cord injury at or above T6 can disconnect supraspinal regulation of the splanchnic sympathetic vascular bed. A stimulus below the lesion, commonly bladder distension, triggers severe reflex vasoconstriction and hypertension; reflex bradycardia, headache and flushing may follow.
Why the other options are incorrect
Applied Revision — High-Yield Notes
- Autonomic dysreflexia is a potential emergency: sit the patient upright, loosen restrictive clothing, monitor BP and search promptly for bladder/bowel triggers.
- Common trigger: blocked urinary catheter or distended bladder.
- Clinical risk rises with higher and more complete cord lesions.
Exam Pearl: Autonomic dysreflexia = spinal cord injury at or above T6.
After high anterior resection, a man has impaired semen emission despite preserved erections. Which neural pathway was most likely damaged?
Review explanation
Correct Answer: D. Sympathetic hypogastric pathway.
Why the correct answer is correct
The sympathetic superior hypogastric plexus and hypogastric nerves carry thoracolumbar fibres essential for emission and bladder-neck closure during ejaculation. Injury during rectal surgery may cause absent or retrograde ejaculation even when S2–S4 parasympathetic erectile pathways remain intact.
Why the other options are incorrect
Applied Revision — High-Yield Notes
- Sympathetic emission and bladder-neck closure: principally T11–L2 via hypogastric plexuses.
- Pudendal S2–S4 motor fibres contribute to the expulsion phase by activating bulbospongiosus and related muscles.
- High anterior dissection near the aortic bifurcation and sacral promontory can injure superior hypogastric sympathetic fibres.
Exam Pearl: Erection = pelvic parasympathetic; emission/bladder-neck closure = hypogastric sympathetic.
The lesser thoracic splanchnic nerve primarily carries which type of efferent fibres?
Review explanation
Correct Answer: C. Preganglionic sympathetic.
Why the correct answer is correct
The lesser thoracic splanchnic nerves generally arise from T10–T11 sympathetic trunk levels and carry preganglionic sympathetic fibres through the thorax to prevertebral ganglia, particularly the aorticorenal and related abdominal ganglia.
Why the other options are incorrect
Applied Revision — High-Yield Notes
- Greater thoracic splanchnic: typically T5–T9; lesser: T10–T11; least: T12.
- Thoracic abdominopelvic splanchnics usually bypass sympathetic-chain ganglia without synapsing.
- Cardiopulmonary splanchnic nerves are predominantly postganglionic sympathetic: a common MRCS trap.
Exam Pearl: Lesser splanchnic (T10–T11) is preganglionic sympathetic, not postganglionic.
During left lumbar sympathectomy, the lumbar sympathetic chain lies in what relation to the abdominal aorta?
Review explanation
Correct Answer: A. Lateral to the aorta.
Why the correct answer is correct
The left lumbar sympathetic chain runs on the anterolateral lumbar vertebral bodies along the medial margin of psoas, lateral/posterolateral to the abdominal aorta.
Why the other options are incorrect
Applied Revision — High-Yield Notes
- On the right, the lumbar sympathetic chain lies posterior to the inferior vena cava.
- Lumbar sympathetic ganglia are usually described on the anterolateral vertebral bodies, deep to medial psoas.
- Aortic and caval relations change with side, so a question that omits side/level is under-specified.
Exam Pearl: Left lumbar sympathetic trunk is lateral to the aorta; right is posterior to the IVC.
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