Thorax

The diaphragm, and why the phrenic nerve comes from the neck

5 steps · 3 anomalies · Week 4 to Weeks 9 to 12

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One story that answers three separate exam questions: why the phrenic nerve is C3, 4 and 5; why diaphragmatic irritation is felt at the shoulder tip; and why a congenital diaphragmatic hernia is nearly always on the left and posterolateral. It is all in the fact that the diaphragm is built in the neck and then migrates.

Week 4

Step 1 of 5

What happens: The septum transversum appears as a mass of mesoderm opposite the third, fourth and fifth cervical segments.
What it forms: The future central tendon.
What remains in the adult: It takes its nerve supply from those cervical segments and never gives it up.

Nothing in this set goes wrong at this step — it is here because the story does not make sense without it.

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Senior’s 30-second takeaway

The diaphragm, and why the phrenic nerve comes from the neck — the whole bench in four lines. Read this on the way in.

If you see this: A newborn with respiratory distress and a scaphoid abdomen · A question about the root value of the phrenic nerve · Shoulder-tip pain with an abdominal problem
If asked to identify it: The diaphragm is built at cervical levels and descends, which is why its nerve is C3, C4 and C5.
If asked one more: Four parts: central tendon, pleuroperitoneal membranes, crura from the oesophageal mesentery, and body wall muscle.
If asked why it matters: The left canal closes last, so a congenital diaphragmatic hernia is usually left and posterolateral.

Then stop. Let the examiner ask the next question.

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Senior’s 30-second takeaway

Read this first