Spinalis muscle

Origin and insertion

Spinalis muscles are slender, flat fascicles of varying lengths that occupy the medial aspect of the erector spinae column of muscles. The fascicles vary in length, with the medial fascicles tending to run the shortest distance (having lower origins and higher insertions) while the more lateral fascicles run longer distances (they have higher origins and lower insertion points).

Spinalis is comprised of three divisions, each named according to the region of the vertebrae that the fascicles are attached to; capitis, cervicis and thoracis. Spinalis cervicis and capitis are often poorly developed.

The inconsistent spinalis capitis fibers represent extensions from the adjacent semispinalis capitis that extend down to the spinous processes of the seventh cervical (C7) and first thoracic (T1) vertebrae. It is referred to as “capitis” because it has cranial attachments to the base of the skull.

The fibers of spinalis cervicis are also irregular and poorly developed. However, when they are present they originate from the spinous processes C7 – T1 as well as the caudal part of the nearby nuchal ligament. The fibers travel cranially (sometimes blending with those of semispinalis cervicis) to insert on the spinous processes of the C2 – C4 vertebrae.

The most prominent and well-organized part of this muscle group is the spinalis thoracis component. These muscle fibers originate from the spinous processes of T11 – L2 vertebrae and travel superiorly to attach along the spinous processes of T2 – T8 vertebrae.

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Relations

The insertion point of spinalis cervicis on C2 is inferior to that of both rectus capitis posterior major and obliquus capitis inferior muscles. The cervical portion is also medial to the semispinalis capitis muscle.

Spinalis thoracis lies medial to the longissimus muscle (and often blends indistinguishably with it) and deep to the serratus posterior inferior, latissimus dorsi and the inferior aspect of trapezius muscles.

Innervation

Spinalis is innervated along its course by the lateral branches of the dorsal rami of the adjacent spinal nerves (cervical, thoracic and lumbar).

Blood supply

The blood supply of the spinalis component of erector spinae varies depending on the segment of the muscle being discussed.

• Spinalis cervicis and capitis are supplied by the muscular branches of vertebral and deep cervical arteries as well as descending branches of occipital artery.
• Spinalis thoracis is supplied by dorsal branches of superior and posterior intercostal arteries.
• The portion of spinalis thoracis that extends to attach in the upper lumbar spine is supplied by branches of lumbar arteries.

The venous drainage of spinalis mirrors the arterial supply of the muscle.

Functions

Spinalis works synergistically with the other members of the erector spinae group to execute two major functions. Unilateral contraction of erector spinae results in lateral flexion (ipsilateral) and rotation of the cervical, thoracic spine and lumbar spines.

On the other hand, bilateral contraction of erector spinae causes extension of the head, neck and cervical, thoracic and lumbar vertebrae.Erector spinae muscle group also helps to stabilize the pelvis while balancing on one leg. During this activity, the contralateral muscle group contracts and prevents the pelvis from dipping.