Which nerves innervate serratus posterior inferior?

• A. Intercostal nerves T2-T5
• B. Dorsal ramus of C8
• C. Subcostal nerve
• D. Intercostal nerves T9-T12

Answer: (D)

Serratus posterior inferior is supplied by the ventral rami of the lower thoracic nerves, specifically the intercostal nerves T9 through T12. This fits its location and function, as the muscle lies in the posterior thorax and depresses the lower ribs during expiration, receiving motor fibers from the thoracic spinal nerves that correspond to the ribs it acts upon. The intercostal nerves T9–T12 course in the intercostal spaces and provide the primary motor supply to this muscle. The intercostal nerves T2–T5 innervate serratus posterior superior, not the inferior part. The dorsal ramus of C8 serves mainly intrinsic back muscles rather than serratus posterior inferior, and the subcostal nerve (T12) alone is not the full supply for this muscle.