The phrenic nerve arises from which spinal cord segments?

The phrenic nerve arises from which spinal cord segments?

• A. C2-C4
• B. C3-C5
• C. C4-C6
• D. C5-C7

Answer: (B)

The essential idea is that the phrenic nerve comes from the cervical plexus, specifically the spinal cord segments C3 through C5. The diaphragm—the main muscle of respiration—receives its motor innervation from these roots, with C4 usually providing the primary contribution and small input sometimes coming from C2. This arrangement is why injuries or procedures that affect these nerve roots can directly impact breathing, since the diaphragm’s activity hinges on signals from C3-C5. A helpful mnemonic is that “C3, C4, and C5 keep the diaphragm alive,” highlighting the standard range. Other segment ranges would miss one or more of these critical roots, which would fail to supply full diaphragmatic innervation.

The essential idea is that the phrenic nerve comes from the cervical plexus, specifically the spinal cord segments C3 through C5. The diaphragm—the main muscle of respiration—receives its motor innervation from these roots, with C4 usually providing the primary contribution and small input sometimes coming from C2. This arrangement is why injuries or procedures that affect these nerve roots can directly impact breathing, since the diaphragm’s activity hinges on signals from C3-C5. A helpful mnemonic is that “C3, C4, and C5 keep the diaphragm alive,” highlighting the standard range. Other segment ranges would miss one or more of these critical roots, which would fail to supply full diaphragmatic innervation.