Innervation of serratus posterior inferior is which of the following?

• A. Thoracic spinal nerves T9-T12 and ventral rami
• B. Dorsal rami of thoracic nerves
• C. Cervical spinal nerves C1-C4
• D. Intercostal nerves T1-T4

Answer: (A)

The key idea is that serratus posterior inferior gets its nerve supply from the ventral rami of the lower thoracic nerves, specifically T9 through T12. These ventral rami form the intercostal nerves that run along the ribs and provide motor to the serratus posterior inferior, enabling it to help depress the lower ribs during expiration. Dorsal rami supply the intrinsic back muscles and skin, not this muscle. Cervical nerves innervate neck and upper back structures, and intercostal nerves from higher levels (T1-T4) wouldn’t match the location and function of serratus posterior inferior. So the correct innervation is the ventral rami of thoracic nerves T9-T12.