semispinalis

The semispinalis is a muscle of the deep (intrinsic) back that belongs to the transversospinalis group, along with the multifidus and rotatores. It consists of three parts: semispinalis capitis, semispinalis cervicis, and semispinalis thoracis. The muscle lies deep to the erector spinae and superficial to the multifidus, with fibers running obliquely superomedially. Each part spans several vertebral segments, typically four to six.

Semispinalis capitis originates from the transverse processes of the lower cervical and upper thoracic vertebrae (commonly C4 to T6) and from the articular processes of C5 to C7. It inserts on the occipital bone between the superior and inferior nuchal lines. Semispinalis cervicis originates from the transverse processes of T1 to T6 and inserts on the spinous processes of C2 to C5. Semispinalis thoracis arises from the transverse processes of T6 to T11 and inserts on the spinous processes of C6 to T4.

The primary actions of the semispinalis muscles are to extend the head, neck, and upper thoracic spine, with bilateral contraction producing backward bending and stabilization of the spine. Unilateral contraction aids in rotating the head and vertebral column to the opposite side. They work in concert with other back muscles to maintain posture and control vertebral segments during movement.

Nerve supply is via the dorsal rami of spinal nerves at the levels of the muscle segments. Blood supply comes from posterior intercostal and deep cervical arteries, with drainage to their corresponding venous networks.