Semispinalis capitis muscle

Anatomy

Semispinalis capitis spans the neck and upper back regions as it runs from the cervical and thoracic vertebrae to the occipital bone. Its muscle tendons arise from the articular processes of vertebrae C4-C7 and transverse processes of vertebrae T1-T6. From these attaching tendons, the muscle fibers of semispinalis capitis converge into a single muscle belly that runs superiorly across the lateral side of the vertebral column. The muscle inserts onto the occipital bone, between superior and inferior nuchal lines and posterior to the insertions of obliquus superior, rectus capitis posterior major and rectus capitis posterior minor muscles. 

The outer surface of semispinalis capitis is covered by splenius capitis muscle, while it’s inner surface overlies semispinalis cervicis muscle. Semispinalis muscles are located medial to longissimus muscle, which comprises the central cord of erector spinae muscle.

Semispinalis capitis muscle receives its innervation through the greater occipital nerve (posterior ramus of C2 spinal nerve) and spinal nerve C3. Blood supply comes from the occipital artery that arises from the external carotid artery. 

Regarding function, semispinalis capitis assists obliquus capitis superior, splenius, trapezius, rectus capitis posterior major and rectus capitis posterior minor muscles to extend the head, cervical and thoracic spines. Unilateral contraction however results in ipsilateral lateral flexion of the head, cervical and thoracic spine, and contralateral rotation of head, cervical and thoracic spine.