MRCS Part B Revision · Applied surgical science and critical care
Peripheral Nerves — MRCS Part B Applied surgical science and critical care
By Dr Richard Miller, MBChB FRCS · Reviewed
Peripheral Nerves 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 Peripheral Nerves station
Peripheral Nerves
Nerve Structure
From innermost component to outermost layer:
Axon: extensions of neurone that can be myelinated with Schwann cells or unmyelinated
Endoneurium: loose connective tissue that surrounds an axon (and myelin sheath in myelinated axons)
Fascicle: A group of axons
Perineurium: loose connective tissue that surrounds a fascicle
Epineurium: outermost layer, this is a tough connective tissue that surrounds fascicles that are bundled together with blood vessels in between
Nerve Injuries
Nerves may be damaged by compression, stretching, laceration or ischaemia. The Seddon Classification (1941) places injuries into three categories:
Class 1 Neurapraxia: temporary interruption of conduction without loss of axonal integrity.
- Mildest type of injury
- Integrity of axon and endoneurial tube are preseved
- Some motor and sensory deficit distal to the site of injury
- No Wallerian degeneration
- Full recovery of nerve in days to weeks
Class 2 Axonotmesis: The axon is severed but continuity of the endoneurial tube is still present
- Intact endoneurium provides a guide for axonal regeneration
- Motor and sensory deficit to the region supplied by the nerve
- Wallerian Degeneration occurs
- Variable recovery, dependent on distance from injury to the area innervated
Class 3 Neurotmesis: Disruption of axon and endoneurial tube.
- Poor prognosis
- May be complete or partial
- Wallerian Degeneration occurs
- Most cases require surgery
- Incomplete and variable recovery
In 1951, Sunderland expanded Seddon's classification to five degrees of peripheral nerve injury:
First-degree (Class 1): Seddon's neuropraxia and first-degree are the same.
Second-degree (Class 2): Seddon's axonotmesis and second-degree are the same.
Third-degree (Class 3): Sunderland's third-degree is a nerve fiber interruption. In third-degree injury, there is a lesion of the endoneurium, but the epineurium and perineurium remain intact. Recovery from a third-degree injury is possible, but surgical intervention may be required.
Fourth-degree (Class 3): In fourth-degree injury, only the epineurium remain intact. In this case, surgical repair is required.
Fifth-degree (Class 3): Fifth-degree lesion is a complete transection of the nerve. Recovery is not possible without an appropriate surgical treatment.
Action Potential
Resting potential: The cell begins at a resting potential of -70mV with Na pumped out and K pumped in.
Depolarisation: A propagating action potential or stimulus causes an opening of fast-gated Na+ channels causing Na+ to enter the cell. If this stimulus is greater than the threshold potential the cell fully depolarises and an action potential is generated.
Repolarisation: Once depolarisation has reached a membrane potential of +30mV Na+ channels close and voltage-gated K+ channels open causing K+ to move out of the cell.
Hyperpolarisation: K+ channels close and N+/K+ ATPase pump pumps 3NA+ out and 2K+ in to the cell. There is a short time when the net negative charge is greater than -70mV resting potential.
What are you asked at the Peripheral Nerves station?
The station runs to 16 questions over nine minutes. These are the questions as they are put to you; the model answers are in the question bank.
- Describe the structure of a peripheral nerve
- Describe the role of myelin in the peripheral nervous system
- What is saltatory conduction?
- How are peripheral nerve fibers classified?
- What is Wallerian degeneration?
- How are nerve injuries classified?
- What has likely happened to the rugby player in this scenario?
- What type of nerve injury is he likely to have?
- How would you manage this patient?
- Define the term 'resting potential' pertaining to a cell
- What is the normal resting potential of a neuron?
- Why is the net charge negative?
And 4 more at this station.
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 Peripheral Nerves station ask?
It opens with "Describe the structure of a peripheral nerve" and runs to 16 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