Which nerve innervates the sternocleidomastoid muscle?

The sternocleidomastoid muscle, known for its role in head rotation and flexion, is primarily innervated by the spinal accessory nerve, which is also known as cranial nerve XI. This nerve is responsible for conveying motor innervation to the muscle, enabling it to perform its functions effectively.

The spinal accessory nerve travels through the jugular foramen and gives rise to some fibers that innervate the sternocleidomastoid, allowing it to contract and facilitate movements such as turning the head to the opposite side and bending the head forward. This anatomical relationship underscores the significance of the spinal accessory nerve in the muscle's action.

While other cervical nerves (C4-C6) do contribute to sensory innervation in the cervical region and may provide some secondary innervation, the primary motor function is dominated by the spinal accessory nerve. The choices of C3-C8 and the inferior gluteal nerve are unrelated to the sternocleidomastoid's function, as the inferior gluteal nerve innervates muscles in the gluteal region, specifically the gluteus maximus, rather than any structures in the neck.