Which nerves innervate iliocostalis lumborum?

• A. Spinal accessory nerve.
• B. Dorsal rami of thoracic and lumbar nerves.
• C. Ventral rami of cervical nerves.
• D. Dorsal rami of cervical nerves.

Answer: (B)

Understanding innervation of intrinsic back muscles is essential here. The iliocostalis lumborum is part of the erector spinae group in the lower back, and its motor supply comes from the dorsal rami of the thoracic and lumbar spinal nerves at the corresponding levels. These dorsal rami provide segmental innervation to the deep back muscles and carry sensory information from the paraspinal region. The spinal accessory nerve does not supply back muscles; it innervates trapezius and sternocleidomastoid. The dorsal rami of cervical nerves target muscles in the neck, not the lumbar region, and the ventral rami of cervical nerves primarily serve the anterior trunk and limbs. So, the nerves innervating iliocostalis lumborum are the dorsal rami of thoracic and lumbar nerves.