Semispinalis thoracis muscle
Semispinalis are a group of back muscles that together with the rotatores and multifidus comprise the spinotransverse group of deep back muscles. Semispinalis group is found lying on either side of the vertebral column spanning from the skull as far as to the vertebra T10. These muscles are divided regionally into the semispinalis capitis, cervicis and thoracis.
Semispinalis thoracis consists of five fascicles bridging over five to six vertebral levels between the transverse and spinous processes of certain cervical and thoracic vertebrae. With these attachments, semispinalis thoracis aids several movements of the vertebral column; specifically extension, lateral flexion and rotation of the thoracic spine. Acting together with semispinalis capitis and cervicis, it supports the same movements occuring at the head and neck.
|Origin||Transverse processes of vertebrae T6-T10|
|Insertion||Spinous processes of vertebrae C6-T4|
|Action||Bilateral contraction - Extension of head, cervical and thoracic spine Unilateral contraction - Lateral flexion of head, cervical and thoracic spine (ipsilateral), rotation of head, cervical and thoracic spine (contralateral)|
|Innervation||Medial branches of posterior rami of spinal nerves|
|Blood supply||Dorsal branches of posterior intercostal arteries|
This article will discuss the anatomy and function of the semispinalis thoracis muscle.
Semispinalis thoracis consists of five muscle fascicles that respectively originate from the transverse processes of five consecutive vertebrae; T6-T10 levels. After a short superior course, these thin fibers come together, comprising a single muscle belly that continues its way toward the higher thoracic levels.
At the C6 vertebral level, it starts giving off small muscle slips that successively insert to the spinous processes of vertebrae C6-T4. This type of laminar arrangement is often seen in the muscles of the back and it means that the fibers with the lowest vertebral level of origin (T10) have the lowest vertebral level of insertion (T4).
This muscle lies medial to the erector spinae muscle group and lateral to the cervical and thoracic vertebrae. Sitting deep to semispinalis thoracis are the rotatores and levatores costarum, while superficially it is covered by the spinalis thoracis (erector spinae group). Just like all the spinotransverse muscles, semispinalis thoracis is innervated by the medial branches of the posterior rami of adjacent spinal nerves. Dorsal branches of the posterior intercostal arteries provide its blood supply.
Semispinalis thoracis muscle rarely acts alone but rather in synergy with its capitis and cervicis counterparts, and with other extensors of the neck and spine. Depending on whether it contracts unilaterally or bilaterally, the functions of this muscle are;
- Extension of the head at the neck and extension of the cervical and thoracic spine at intervertebral joints; when contracting bilaterally
- Lateral flexion of the head, cervical and thoracic spine on the same side and rotation of head, cervical and thoracic spine on the opposite side; unilateral contraction