The semispinalis muscles are the longest and most superficial set of the transversospinalis group. They are subdivided based on their location into semispinalis capitis, cervicis, and thoracis.
The semispinalis capitis (Latin: musculus semispinalis capitis) is a long thin muscle lying deep in the back of the neck, overlying the semispinalis cervicis. It is involved in head and neck movements.
The semispinalis capitis inserts into the spinous processes of the second to fifth cervical vertebrae (C2 - C5).
By contracting on both sides (bilaterally), the semispinalis capitis provides extension of the head and neck. By contracting on one side (unilaterally), it rotates the head and neck to the opposite side and laterally flexes the head and neck to the same side of activation.
The semispinalis capitis is innervated by descending branches of the greater occipital nerve (C2) and the third cervical nerve (C3).
The semispinalis cervicis (also known as the semispinalis colli, Latin: musculus semispinalis cervicis) is a deep back muscle that lies over the cervical and thoracic parts of the multifidus muscle.
The semispinalis cervicis arises from the transverse processes of the first to sixth thoracic vertebrae (T1 - T6).
The muscle fibers of the semispinalis cervicis insert into the spinous processes of the second to fifth cervical vertebrae (C2 - C5).
By contracting on one side, the semispinalis cervicis rotates the neck to the contralateral (opposite) side of activation and provides lateral flexion to the ipsilateral (same) side. By contracting on both sides, this muscle extends the spine.
The semispinalis cervicis is innervated by the medial branches of the posterior rami of the spinal nerves.
The semispinalis thoracis (Latin: musculus semispinalis thoracis) is a deep muscle of the back that overlies the thoracic part of the multifidus.
The semispinalis thoracis has thin fascicles with long tendons extending from both ends. They arise from the transverse processes of the sixth to tenth thoracic vertebrae (T6 - T10).
The tendons of the semispinalis insert into the spinous processes of the sixth cervical to fourth thoracic vertebrae (C6 - T4).
When it contracts on both sides, the semispinalis thoracis provides extension of the thoracic spine. When it contracts on one side, it causes lateral flexion of the spine to the same side, but rotation to the opposite side.
The semispinalis thoracis is innervated by the medial branches of the posterior rami of the spinal nerves.