Which nerve innervates the motor supply to the diaphragm, and from which spinal segments does it originate?

Explanation

## Innervation of the Diaphragm **Key Point:** The **phrenic nerve** is the sole motor nerve supply to the diaphragm, originating from the cervical spinal segments C3, C4, and C5. This is a fundamental anatomical fact with major clinical implications. ### Embryological and Anatomical Basis The diaphragm develops from the **septum transversum**, which originates from cervical mesoderm. As the diaphragm descends during embryonic development, it carries its nerve supply (phrenic nerve) with it, explaining why a cervical nerve innervates a thoracic/abdominal structure. ### Phrenic Nerve: Key Features | Feature | Details | | --- | --- | | **Spinal origin** | C3, C4, C5 | | **Course** | Descends through the thorax on the mediastinal surface of the pericardium | | **Motor supply** | Entire diaphragm (all three portions: sternal, costal, lumbar) | | **Sensory supply** | Parietal pleura, pericardium, peritoneum covering diaphragm | | **Accessory motor supply** | Intercostal nerves (T5–T12) provide minor motor innervation to peripheral diaphragm | **High-Yield:** The mnemonic **"C3, 4, 5 keeps the diaphragm alive"** is universally used in medical education to recall the spinal origin of the phrenic nerve. **Mnemonic:** **C3, 4, 5 keeps the diaphragm alive** - C3 = Cervical segment 3 - C4 = Cervical segment 4 - C5 = Cervical segment 5 ### Clinical Pearls **Phrenic nerve injury** results in: - Paralysis of the ipsilateral diaphragm - Elevated hemidiaphragm on chest X-ray - Reduced ventilation on the affected side - Common causes: birth trauma, surgical injury (cardiac surgery, thyroid surgery), tumors, trauma **Referred pain:** Irritation of the diaphragm (e.g., subphrenic abscess, peritonitis) causes **shoulder tip pain** via phrenic nerve sensory fibers (C3–C5 dermatome). **Bilateral phrenic nerve injury** → respiratory failure requiring mechanical ventilation. ### Why Other Options Are Wrong - **Vagus nerve** innervates laryngeal and pharyngeal muscles, not the diaphragm - **Intercostal nerves** provide only minor accessory motor supply; they are not the primary motor supply - **Lumbar plexus** innervates abdominal wall and lower limb muscles, not the diaphragm