Which structure is NOT innervated by the posterior rami of spinal nerves?

Which structure is NOT innervated by the posterior rami of spinal nerves?

• A. Serratus posterior inferior
• B. Serratus posterior superior
• C. Levator scapulae
• D. Latissimus dorsi

Answer: (A)

Posterior rami supply the deep intrinsic back muscles and the skin over the back. The serratus posterior inferior is an extrinsic back muscle whose nerve supply comes from the ventral rami via the lower intercostal nerves (T9–T12), not from the posterior rami. That makes it not innervated by the posterior rami, which is the distinguishing point here. By contrast, the other muscles listed are innervated by nerves that arise from the ventral rami (for example, the levator scapulae by the dorsal scapular nerve from the ventral ramus, and the latissimus dorsi by the thoracodorsal nerve from the posterior cord formed by ventral rami). Serratus posterior superior also receives innervation from intercostal nerves (anterior rami). So serratus posterior inferior is the clearest example of a structure not innervated by the posterior rami.

Posterior rami supply the deep intrinsic back muscles and the skin over the back. The serratus posterior inferior is an extrinsic back muscle whose nerve supply comes from the ventral rami via the lower intercostal nerves (T9–T12), not from the posterior rami. That makes it not innervated by the posterior rami, which is the distinguishing point here. By contrast, the other muscles listed are innervated by nerves that arise from the ventral rami (for example, the levator scapulae by the dorsal scapular nerve from the ventral ramus, and the latissimus dorsi by the thoracodorsal nerve from the posterior cord formed by ventral rami). Serratus posterior superior also receives innervation from intercostal nerves (anterior rami). So serratus posterior inferior is the clearest example of a structure not innervated by the posterior rami.