Semispinalis thoracis muscle

Anatomy

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 transversospinalis  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.

Function

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