Diaphragm: Anatomy, Phrenic Nerve, Action & Clinical Relevance

The Diaphragm is the primary muscle of respiration. It is a large, dome-shaped musculotendinous sheet that separates the thoracic cavity (heart and lungs) from the abdominal cavity. It functions like a piston: when it contracts, it descends, drawing air into the lungs.

[Image of Diaphragm muscle anatomy]

Quick Anatomy Snapshot

Origin (Proximal)	1. Sternal Part: Xiphoid Process. 2. Costal Part: Inner surfaces of lower 6 ribs (Ribs 7-12). 3. Lumbar Part: L1-L3 vertebrae (Crura) and Arcuate Ligaments.
Insertion (Distal)	Central Tendon (A strong aponeurosis at the summit of the dome; has no bony attachment).
Nerve Supply	Phrenic Nerve (C3, C4, C5) - "C3, 4, 5 keeps the diaphragm alive."
Blood Supply	Musculophrenic, Pericardiacophrenic, and Phrenic Arteries (Superior/Inferior).
Primary Actions	Inspiration: Contracts and flattens, increasing thoracic volume. Expulsive Efforts: Increases intra-abdominal pressure (Valsalva maneuver) for vomiting, defecation, and childbirth.

Deep Dive: The Three Holes (Hiatuses)

Because major vessels must travel between the chest and abdomen, the diaphragm has three major openings. Their levels are easy to remember with the mnemonic: "I Ate 10 Eggs At 12."

• T8 (I Ate): Inferior Vena Cava passes through the Caval Opening.
• T10 (10 Eggs): Esophagus passes through the Esophageal Hiatus.
• T12 (At 12): Aorta passes through the Aortic Hiatus.

The Crura

The diaphragm anchors to the lumbar spine via two tendinous legs called Crura (singular: Crus).
• Right Crus: Larger/Longer (L1-L3). Loops around the esophagus to form a sphincter preventing reflux.
• Left Crus: Smaller (L1-L2).

Physio Corner: Clinical Relevance

💪 Functional Fact: The diaphragm is also a postural stabilizer. Before you lift a heavy object, you naturally hold your breath. This increases intra-abdominal pressure, creating a rigid cylinder that supports the lumbar spine.

Assessment

We cannot MMT the diaphragm like a limb muscle. Instead, we assess:
1. Excursion: Measuring chest expansion vs. abdominal rise.
2. Fluoroscopy: Watching the diaphragm move via X-ray (Sniff Test).
3. Manual Resistance: Place hand on the epigastric region and ask patient to inhale against resistance (checking for strength and symmetry).

⚠️ Clinical Pathology: Referred Pain (Kehr's Sign)
The Phrenic nerve arises from C3-C5. The supraclavicular nerves (which supply sensation to the shoulder skin) also arise from C3-C4. Therefore, irritation of the diaphragm (e.g., from blood in the abdomen or gall bladder issues) is often felt as sharp pain in the tip of the shoulder.

Functional Assessment

While a standard Oxford Scale 0-5 is difficult to apply, we assess function in supine.

Testing Tip: Look for "Paradoxical Breathing." Normally, the abdomen rises during inhalation. If the diaphragm is paralyzed, the abdomen will suck in during inhalation because the negative chest pressure pulls the flaccid muscle upward.

Assessment Procedure

Assessment	Patient Action & Observation
Functional Strength	Position: Supine. Resistance: Therapist places hand gently over the epigastrium (just below xiphoid). Action: "Take a deep breath in and push my hand up." Normal: Patient can lift the hand against moderate resistance (approx 10-15 lbs of force).
Clinical Sign (Sniff Test)	Action: Patient performs a sharp, quick sniff. Observation: A sharp descent of the diaphragm is the mechanism of a sniff. If one side is paralyzed, it will not descend (or may move paradoxically upwards).

Frequently Asked Questions

What keeps the diaphragm alive?

The classic mnemonic is "C3, 4, 5 keeps the diaphragm alive." This refers to the nerve roots of the Phrenic Nerve. A spinal cord injury above C3 causes respiratory arrest.

Why do we get hiccups?

A hiccup (singultus) is an involuntary spasm of the diaphragm followed by the sudden closure of the vocal cords (glottis), producing the "hic" sound. Causes range from eating too fast to phrenic nerve irritation.

Does the diaphragm work during exhalation?

Generally, no. Quiet exhalation is passive (elastic recoil of lungs). The diaphragm simply relaxes and rises. However, during forced exhalation (blowing out candles), abdominal muscles contract to push the diaphragm up forcefully.

Test Your Knowledge: Diaphragm Quiz

1. Which nerve supplies the Diaphragm?

2. At what vertebral level does the Inferior Vena Cava pass through the diaphragm?

3. What is the insertion of the Diaphragm?

4. Which crus of the diaphragm loops around the esophagus?

5. During inspiration, the diaphragm:

6. Referred pain from the diaphragm is typically felt where?

7. Paradoxical breathing involves the abdomen moving ______ during inhalation.

8. The Esophageal Hiatus is located at which level?

9. The diaphragm creates increased intra-abdominal pressure during:

10. Which spinal cord injury level would result in complete loss of diaphragm function?

References

• Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2018). Clinically Oriented Anatomy. 8th ed. Philadelphia: Wolters Kluwer.
• Standring, S. (2016). Gray's Anatomy: The Anatomical Basis of Clinical Practice. 41st ed. Elsevier.
• West, J. B. (2012). Respiratory Physiology: The Essentials. 9th ed. Lippincott Williams & Wilkins.