Semispinalis Thoracis

What is the Semispinalis Thoracis

The semispinalis thoracis is a narrow muscle located in the upper to mid-back region. It is the lowest of the three semispinalis muscles, belonging to the transversospinalis or deep intrinsic back muscle group. Along with the other semispinalis muscles, it helps with the extension and rotation of the head and neck.

Anatomy

Location and Attachments

Origin	Transverse processes of the T6 to T10 vertebrae
Insertion	Spinous processes of the C6 to T4 vertebrae

Origin

Like other muscles attached to the vertebrae, the semispinalis thoracis begins as several thin fascicles, each originating from the transverse processes of the 6th to 10th thoracic vertebrae. In some cases, this origin may extend to the 11th and 12th thoracic vertebrae. These tendons then merge as they move upward, forming a single muscle belly that continues toward the cervical spine.

Insertion

As the muscle reaches the upper thoracic region, it splits into smaller sections that attach to the spinous processes of the 6th to 7th cervical vertebra, down to the 1st to 4th thoracic vertebra. The muscle fibers follow a laminated arrangement, meaning those originating from the lower vertebrae, like T10, are inserted lower at T4, while fibers from higher points, such as T6, are inserted higher at C6.

Relations With Surrounding Muscles and Structures

The erector spinae muscles lie lateral to the semispinalis thoracis, while it has the spinous processes of the thoracic and cervical vertebrae lying medially. The spinalis thoracis covers the muscle superficially. On the deep side, it has the other transversospinalis muscles, rotatores, and multifidus, as well as the levatores costarum.

Function

Action

Extending, laterally flexing, and rotating the head and body trunk

The semispinalis muscle works in synergy with other muscles in its group. When it contracts bilaterally (on both sides), it extends both the cervical and thoracic spine, allowing you to tilt your head backward to look overhead and to arch your back, bending the entire trunk backward. Notably, the semispinalis thoracis is the only muscle in this group that significantly contributes to thoracic spine movement and, consequently, the movement of the body trunk. This is due to its greater attachment to the thoracic vertebrae, extending down to T10.

When the semispinalis muscle contracts unilaterally (on one side), it aids in the ipsilateral lateral flexion of the head and trunk, allowing you to tilt your head and upper body toward the same side as the contracting muscle. Additionally, it plays a role in rotating the head and trunk toward the opposite side of the contracting muscle (contralateral rotation).

Innervation

Nerve

Dorsal rami of cervical and thoracic nerves

The medial branches of the posterior or dorsal rami of the adjacent thoracic and cervical spinal nerves innervate the muscle.

Blood Supply

Artery

The dorsal rami of the posterior intercostal arteries

The primary blood supply to the Semispinalis Thoracis muscle comes from the posterior intercostal arteries, which originate from the thoracic aorta.