Sternocleidomastoid muscle is supplied by:

The spinal accessory nerve provides the motor supply. It passes through the muscle. The spinal accessory nerve supplies: The sternocleidomastoid, the chin turning The trapezius, the shrugging muscle. Cervical nerves provide a proprioceptive sensations to these muscles. Reference: B D Chaurasia’s Head and Neck anatomy, 6th ed page no 384