MRCS Part B Revision · Applied surgical science and critical care
Autonomic Nervous System — MRCS Part B Applied surgical science and critical care
By Dr Richard Miller, MBChB FRCS · Reviewed
Autonomic Nervous System is an applied surgical science and critical care station. Three of the seventeen examined stations in the MRCS Part B OSCE fall in this area. These stations ask you to interpret data and manage a sick surgical patient: a chart, a blood gas, an imaging study or a deteriorating patient on the ward, and the physiology underneath the decision.
What you need to know for the Autonomic Nervous System station
A referral for endoscopic thoracic sympathectomy for hyperhidrosis, which is the excuse for a station on the autonomic nervous system.
Overview
The involuntary motor system for smooth muscle, cardiac muscle and glands, in two divisions that usually oppose each other, with a two-neurone pathway in each: a preganglionic neurone with its cell body in the brainstem or spinal cord synapses in a ganglion with a postganglionic neurone that reaches the organ. Visceral afferents travel back with both.
Sympathetic
Thoracolumbar outflow: preganglionic cell bodies in the lateral horn from T1 to L2. Fibres leave in the ventral root, pass through the white ramus into the sympathetic chain, and either synapse there at their own level or after running up or down it, or pass through as splanchnic nerves to synapse in the prevertebral ganglia (coeliac, superior and inferior mesenteric) in front of the aorta. Postganglionic fibres return to the spinal nerves through grey rami to reach the skin and limbs, or follow the arteries to the viscera. Short preganglionic, long postganglionic.
Parasympathetic
Craniosacral outflow. Cranial: the oculomotor nerve (Edinger-Westphal nucleus, to the ciliary ganglion, for the pupil and lens), the facial nerve (superior salivatory nucleus, to the pterygopalatine and submandibular ganglia, for the lacrimal, nasal, submandibular and sublingual glands), the glossopharyngeal nerve (inferior salivatory nucleus, to the otic ganglion, for the parotid) and the vagus (dorsal nucleus and nucleus ambiguus, to ganglia in the walls of the thoracic and abdominal viscera as far as the splenic flexure). Sacral: S2–4 through the pelvic splanchnic nerves to the hindgut, bladder and genitalia. Long preganglionic, short postganglionic in or near the organ.
Transmitters
Every preganglionic fibre, in both divisions, releases acetylcholine onto nicotinic receptors in the ganglion. Sympathetic postganglionic fibres release noradrenaline onto alpha and beta adrenoceptors. The exceptions are the sweat glands, whose sympathetic postganglionic fibres release acetylcholine onto muscarinic receptors, and the adrenal medulla, whose chromaffin cells are modified postganglionic neurones supplied directly by preganglionic cholinergic fibres and secreting adrenaline (mostly) and noradrenaline into the blood. Parasympathetic postganglionic fibres release acetylcholine onto muscarinic receptors.
Sexual function
Erection is parasympathetic (S2–4, through nitric oxide relaxing the arteriolar smooth muscle of the corpora); emission and ejaculation are sympathetic (L1–2), with the somatic pudendal nerve for the final contractions. “Point and shoot.” It is why a retroperitoneal lymph node dissection or an aorto-iliac operation that damages the hypogastric plexus causes retrograde ejaculation, and why a prostatectomy that injures the cavernous nerves causes impotence.
Endoscopic thoracic sympathectomy
Thoracoscopic division or clipping of the sympathetic chain, usually at the T2–T3 level (over the second and third ribs), which removes the sympathetic supply to the palm and axilla and stops the sweating and blushing. Its price is compensatory sweating elsewhere in about half of patients, and a Horner's syndrome if the stellate ganglion at T1 is damaged. It is reserved for severe hyperhidrosis that has failed aluminium chloride, iontophoresis and botulinum toxin.
What are you asked at the Autonomic Nervous System station?
The station runs to 9 questions over nine minutes. These are the questions as they are put to you; the model answers are in the question bank.
- Applied Sciences: Autonomic NS
- Describe the autonomic nervous system
- Describe the anatomical location of parasympathetic nervous system neurons
- Which cranial nerve nuclei have a parasympathetic ganglia?
- What neurotransmitters does the sympathetic nervous system use
- How do sweat cell and chromaffin cell neurons differ from the rest of the sympathetic nervous system?
- What neurotransmitters does the parasympathetic nervous system use?
- How are the sympathetic and parasympathetic nervous systems involved in male sexual arousal?
- What is Endoscopic thoracic sympathectomy (ETS)?
How is the applied surgical science and critical care station marked in MRCS Part B?
Each of these three stations is marked out of 20, split 12 marks for clinical knowledge and its application, 4 for clinical and technical skill and 4 for professionalism. No communication marks are available. The three cover critical care management, interpretation of clinical data, and interpretation of visual information.
FAQ
What does the Autonomic Nervous System station ask?
It opens with "Applied Sciences: Autonomic NS" and runs to 9 questions over nine minutes. Each of these three stations is marked out of 20, split 12 marks for clinical knowledge and its application, 4 for clinical and technical skill and 4 for professionalism. No communication marks are available. The three cover critical care management, interpretation of clinical data, and interpretation of visual information.
What counts as applied surgical science in MRCS Part B?
Three of the seventeen examined stations: critical care management, interpretation of clinical data such as blood results and charts, and interpretation of visual information such as imaging and traces.
How is an applied science station marked?
Out of 20, with 12 marks for clinical knowledge and its application, 4 for clinical and technical skill and 4 for professionalism. Unlike the communication stations, none of the marks are for how you say it.
How much physiology do I need?
Enough to explain the decision you are making. The station rewards applying physiology to the patient in front of you rather than reciting it, so practise talking through why a number changes your management.
How many stations are in the MRCS Part B OSCE?
Seventeen examined stations of nine minutes each, with a minute to read the task before each one. Two preparation stations and at least one rest station bring the circuit to about twenty, and the exam takes about three and a half hours.
What is the pass mark for MRCS Part B?
There is no published pass mark. The cut score is set separately for Applied Knowledge and Applied Skills, for each circuit, by borderline regression. Published pass rates across the 2024/25 diets ranged from 51% to 66%.
Can I fail a station and still pass?
Yes. There is no rule about how many stations you may fail: the cut score applies to your total mark in each component, so a weak station costs the marks you lost on it and strong stations elsewhere can make them back. Applied Knowledge and Applied Skills are passed separately and must both be passed at the same sitting, so a strong anatomy performance cannot rescue a weak communication one.
Dr Richard Miller, MBChB FRCS
Station summaries are reviewed against the current intercollegiate MRCS syllabus and the published marking blueprint. Guidance changes between diets: check the royal colleges' own pages before relying on a date, a fee or a threshold.
Practise this station
The question bank carries the model answer to every question above, with the rest of the applied surgical science and critical care stations.
More applied surgical science and critical care stations