Semispinalis

The semispinalis is a group of intrinsic back muscles extending through the thoracic and cervical regions to the back of the skull. It forms the most superficial component of the transversospinalis group, which also includes multifidus and rotatores. Like the other transversospinalis muscles, its fibers generally pass superiorly and medially from transverse processes toward spinous processes or, in the case of semispinalis capitis, the occipital bone.

Semispinalis is divided into three regional parts: semispinalis thoracis, semispinalis cervicis, and semispinalis capitis. Together they extend the thoracic and cervical vertebral column and the head. Unilateral contraction contributes to rotation toward the opposite side.

Location

Semispinalis lies on either side of the vertebral column in the upper thoracic and cervical regions. Within the transversospinalis group, it lies superficial to multifidus and rotatores. The erector spinae muscles occupy a more superficial and generally more lateral position in much of the back.

The three parts form a superiorly directed muscular system. Semispinalis thoracis occupies the upper thoracic back, semispinalis cervicis extends into the cervical region, and semispinalis capitis forms a substantial muscle of the posterior neck that reaches the occipital bone.

Unlike multifidus and rotatores, semispinalis does not extend prominently into the lumbar region. Its development is greatest in the upper thoracic and cervical regions, particularly in the neck where semispinalis capitis forms a thick muscular layer.

Structure

The semispinalis muscles follow the characteristic architecture of the transversospinalis group. Their fascicles arise laterally from transverse processes and pass superomedially toward more medial attachments several vertebral levels above.

Semispinalis generally spans more vertebral segments than multifidus and rotatores. Individual fascicles commonly cross approximately four to six vertebral levels, creating relatively long connections within the transversospinalis system.

Semispinalis thoracis

Semispinalis thoracis is the most inferior part of the group. It arises from transverse processes in the lower thoracic region and passes superiorly and medially to attach to spinous processes of upper thoracic and lower cervical vertebrae.

Its oblique fascicles connect widely separated thoracic vertebrae and contribute to extension and contralateral rotation of the thoracic vertebral column.

Semispinalis cervicis

Semispinalis cervicis lies primarily in the posterior neck. Its fibers arise from upper thoracic transverse processes and ascend to the spinous processes of cervical vertebrae.

The muscle is particularly well developed around the lower cervical spine. Its attachment to the axis (C2) is substantial, reflecting the importance of the upper cervical region in movements of the head and neck.

Semispinalis capitis

Semispinalis capitis is the largest and most superficial of the three parts. It forms a broad vertical muscle mass in the posterior neck and extends from the upper thoracic and lower cervical vertebrae to the occipital bone.

Its fibers are arranged more vertically than those of semispinalis thoracis and cervicis. The muscle is interrupted by a tendinous intersection in many individuals, giving portions of it a digastric appearance.

Semispinalis capitis lies deep to splenius capitis and the upper part of trapezius. Longissimus capitis lies more laterally, while the deeper suboccipital and short cervical muscles occupy the region beneath its superior portion.

Origin and Insertion

Each division of semispinalis has distinct attachments, although the three parts share the same general superomedial fiber direction. The thoracic and cervical portions connect transverse processes to spinous processes, while semispinalis capitis continues superiorly to the occipital bone.

Exact vertebral levels vary slightly among anatomical descriptions and between individual fascicles. The consistent pattern is more important: semispinalis thoracis and cervicis pass from transverse processes to spinous processes several levels above, while semispinalis capitis continues from the upper thoracic and cervical region to the occipital bone.^[1]

Innervation

Semispinalis is innervated segmentally by the posterior rami of spinal nerves. The branches supplying the muscle arise at the cervical and thoracic levels corresponding to its different parts.

This pattern is characteristic of intrinsic back muscles. Semispinalis does not depend on a single named peripheral nerve; instead, successive posterior rami supply different portions of the muscle along its length.^[1][2]

Blood Supply

The semispinalis receives blood from regional arteries supplying the deep posterior trunk and neck. In the thoracic region, dorsal branches of the posterior intercostal arteries contribute to semispinalis thoracis. The cervical and cranial portions receive branches from vessels of the posterior neck, particularly the deep cervical and occipital arteries.

These vessels provide a regional supply to the different parts of the muscle rather than forming a single arterial trunk along the entire semispinalis column.

Relations

The relationship between semispinalis and the other intrinsic back muscles changes somewhat between the thorax and neck, but its position within the transversospinalis group remains consistent.

• Superficially: Erector spinae in the thoracic region and splenius muscles in the posterior neck; trapezius forms a still more superficial layer.
• Deeply: Multifidus and rotatores, together with the posterior elements of the vertebral column.
• Laterally in the neck: Longissimus capitis lies lateral to semispinalis capitis.
• Medially: The fibers approach the spinous processes and midline structures of the vertebral column.

Semispinalis capitis is particularly useful for understanding the layered anatomy of the posterior neck. After trapezius and splenius capitis are reflected, semispinalis capitis becomes a prominent muscular structure extending toward the occipital bone.

Function

The semispinalis muscles act on the vertebral column and, through semispinalis capitis, directly on the head. Their long, superomedially directed fascicles allow them to produce extension when the muscles on both sides act together and rotation when one side contracts independently.

• Bilateral contraction: Extends the thoracic and cervical vertebral column.
• Unilateral contraction: Rotates the vertebral column toward the opposite side.
• Semispinalis capitis: Extends the head when acting bilaterally and rotates the head toward the opposite side when acting unilaterally.
• Postural control: Works with other intrinsic back muscles to stabilize and control the vertebral column and head during movement.

The mechanical effect of semispinalis reflects its fiber direction. Because its fascicles pass from a lateral inferior attachment to a medial superior attachment, unilateral shortening contributes to contralateral rotation, a characteristic action of the transversospinalis muscles.^[2]

Anatomical Variations

The exact attachment levels and development of individual semispinalis fascicles vary. Adjacent fascicles may blend with one another, and the boundaries between semispinalis and neighboring transversospinalis muscles are not always sharply defined.

Semispinalis capitis commonly contains a tendinous intersection, although its position and extent can vary. Differences may also occur in the number of fascicles attaching to individual cervical or thoracic vertebrae.

These variations reflect the overlapping architecture of the intrinsic back musculature and do not alter the basic organization of semispinalis into thoracis, cervicis, and capitis portions.

Clinical Significance

Semispinalis is an important component of the deep posterior cervical and thoracic musculature. Its clinical anatomy is particularly relevant in imaging, posterior surgical approaches, and procedures involving structures of the posterior neck.

Imaging anatomy

Semispinalis can be identified on CT and MRI as part of the paraspinal muscle mass. Semispinalis capitis is especially prominent on cross-sectional imaging of the posterior neck, where its relationship to splenius capitis superficially and the deeper cervical musculature helps distinguish it from neighboring muscles.

Posterior cervical approaches

Semispinalis cervicis and capitis form important muscular layers in posterior approaches to the cervical vertebral column. Their attachments to cervical spinous processes and the occipital region make them relevant landmarks during exposure of the posterior cervical spine.

The strong attachment of semispinalis cervicis to the spinous process of C2 is particularly important because disruption of the posterior cervical extensor musculature around the axis can affect the normal muscular support of the upper cervical region.

Greater occipital nerve relationship

The greater occipital nerve, the medial branch of the posterior ramus of C2, has an important relationship with semispinalis capitis. After emerging from the upper cervical region, the nerve passes through semispinalis capitis before continuing toward the posterior scalp.

This relationship provides an important anatomical landmark when tracing the nerve during dissection and when performing procedures directed toward the greater occipital nerve.

Semispinalis therefore links the segmental architecture of the transversospinalis muscles with the more specialized anatomy of the posterior neck. Its progression from thoracic transverse processes toward cervical spinous processes and ultimately the occipital bone allows the group to act across the upper vertebral column while remaining closely integrated with the deeper stabilizing muscles of the back.