The suboccipital region is a muscle compartment, located inferior to the external occipital protuberance and the inferior nuchal line. These are anatomical landmarks on the occipital bone of the skull. It is of a pyramidal shape and includes the posterior aspects of the atlas and axis (C1 and C2 vertebrae respectively). This region comprises four pairs of small muscles:
- Rectus capitis posterior major
- Rectus capitis posterior minor
- Obliquus capitis inferior
- Obliquus capitis superior
These muscles mainly function as postural muscles but can also contribute to movements of the head.
Three of the four muscles contribute to the formation of the boundaries of the suboccipital triangle: rectus capitis posterior major, obliquus capitis inferior and obliquus capitis superior. This triangle is a region of the posterior neck containing the vertebral artery and the dorsal ramus of the first cervical nerve (suboccipital nerve). The superomedial and superolateral boundaries are formed by the rectus capitis posterior major and obliquus capitis superior respectively. The obliquus capitis inferior forms the inferolateral boundary. The roof of the triangle is formed by semispinalis capitis, which is one of the three parts of a deep intrinsic back muscle. The floor consists of the posterior atlanto-occipital membrane and the posterior arch of the atlas. Located deep to the semispinalis capitis, is a layer of fibrous adipose tissue.
The rectus capitis posterior major originates from the spinous process of the axis by a pointed tendon. It becomes broader as it ascends and it inserts into the occipital bone at the lateral part of the inferior nuchal line. A triangular space is left between the two muscles resulting in visualisation of the recti capitis posterior minor muscles.
The rectus capitis posterior minor has its origin at the posterior tubercle of the posterior arch of the atlas. Like the rectus capitis posterior major, it is attached to its origin by a narrowed tendon that broadens as it ascends upwards. Insertion of the muscle is also at the inferior nuchal line on the occipital bone but at the lateral part. There may also be a soft tissue attachment to part of the floor of the suboccipital triangle, namely the posterior atlanto-occipital membrane.
The obliquus capitis inferior is the only suboccipital muscle that doesn’t have an attachment to the cranium. It instead inserts into the transverse process of the atlas on the inferoposterior part. Its origin is at the posterior tubercle of the posterior arch of the axis.
The obliquus capitis superior is smaller than the obliquus capitis inferior and originates from the transverse process of the atlas. It ascends dorsally, whilst expanding in width, to insert between the superior and inferior nuchal lines, on the occipital bone, laterally to semispinalis capitis.
All of the suboccipital muscles are supplied by the vertebral artery as well as by the deep descending branches of the occipital artery. The four pairs of muscles are all innervated by the dorsal ramus of the first cervical nerve, the suboccipital nerve.
As mentioned above, the main action of the suboccipital muscles is to maintain posture. However, they can also produce movements of the head. These movements include extension, lateral flexion and rotation at the atlanto-axial joints and are summarised below:
- Rectus capitis posterior major – extension and rotation to the ipsilateral side
- Rectus capitis posterior minor – extension and rotation to the ipsilateral side
- Obliquus capitis inferior – rotation to the ipsilateral side
- Obliquus capitis superior – extension and lateral flexion
Flexion and rotation of the head in the contralateral direction are other movements at the atlanto-occipital joint but are carried out by other muscles in the neck.