Innervation of Serratus posterior inferior.

• A. Long thoracic nerve
• B. Dorsal rami of C4-C8
• C. Suboccipital nerve
• D. Intercostal nerves T9-T12

Answer: (D)

The important idea is that serratus posterior inferior is supplied by nerves that course with the intercostal spaces of the lower thorax, reflecting its position and rib-depressing role. Specifically, the motor supply comes from the anterior rami of the lower thoracic nerves, T9 through T12, which become the intercostal nerves as they run along the ribs. These nerves deliver the innervation needed for the serratus posterior inferior to help move the lower ribs during respiration.

This fits the anatomy because the muscle attaches to the spinous processes of the lower thoracic vertebrae and inserts on the inferior borders of ribs 9–12, so its nerve supply aligns with the nerves that innervate the thoracic wall. The long thoracic nerve targets serratus anterior, the suboccipital nerve serves muscles at the base of the skull, and the dorsal rami (C4–C8) supply deep back muscles more than this thoracic wall muscle, so they don’t fit the innervation pattern of serratus posterior inferior.