Spinalis

The spinalis is the most medial of the three longitudinal columns that form the erector spinae. It lies medial to longissimus and close to the spinous processes of the vertebral column. Compared with iliocostalis and longissimus, spinalis is considerably smaller and less consistently developed.

Spinalis is usually described in three regional parts: spinalis thoracis, spinalis cervicis, and spinalis capitis. The thoracic portion is the most distinct. The cervical portion is variable and may blend with neighboring muscles, while spinalis capitis is often poorly separated from semispinalis capitis and may not form an independent muscle.

As part of the erector spinae, spinalis contributes primarily to extension of the vertebral column and maintenance of an upright posture. Its medial position and predominantly longitudinal fibers distinguish it from the more lateral iliocostalis and longissimus columns.

Location

Spinalis lies immediately lateral to the vertebral spinous processes and forms the medial edge of the erector spinae mass. Longissimus lies directly lateral to it, followed by iliocostalis farther laterally.

The muscle is best developed in the thoracic region, particularly around the upper and middle thoracic vertebrae. Superiorly, smaller and more variable fascicles may continue into the cervical region and toward the skull.

Spinalis lies deep to the more superficial intrinsic and extrinsic muscles of the back. The transversospinalis muscles, including semispinalis and multifidus, occupy deeper planes between the transverse and spinous processes.

Structure

Unlike a single continuous muscle belly, spinalis consists of longitudinal fascicles connecting spinous processes across several vertebral levels. Its fibers run predominantly superiorly, remaining close to the midline throughout their course.

The three traditionally described parts differ considerably in how consistently they can be identified. Spinalis thoracis is the clearest component, whereas spinalis cervicis and especially spinalis capitis show greater variation.

Spinalis thoracis

Spinalis thoracis is the principal and most consistently developed part of spinalis. It occupies the medial side of the erector spinae in the thoracic region and consists of slender fascicles extending between spinous processes.

Its fibers generally arise from spinous processes in the lower thoracic and upper lumbar region and ascend to spinous processes in the upper thoracic region. Because the fascicles remain close to the vertebral midline, spinalis thoracis is positioned well to contribute to extension of the thoracic vertebral column.

Spinalis cervicis

Spinalis cervicis is a small and variable continuation into the cervical region. When present as a distinct muscle, it extends from upper thoracic or lower cervical spinous processes to spinous processes of more superior cervical vertebrae.

Its fascicles may blend with semispinalis cervicis, making a sharp boundary between the two muscles difficult to identify in some individuals.

Spinalis capitis

Spinalis capitis is the least distinct of the three traditionally named portions. It is commonly described as blending extensively with semispinalis capitis rather than forming a clearly separable muscle.

When identifiable, its fibers extend superiorly from the cervical region toward the occipital bone. Because of its variable development and close union with semispinalis capitis, many practical anatomical descriptions treat it as part of the semispinalis capitis complex rather than as a prominent independent muscle.

Origin and Insertion

The attachments of spinalis reflect its position along the vertebral midline. Unlike iliocostalis, which has extensive rib attachments, and longissimus, which attaches to transverse processes and ribs, spinalis is associated mainly with spinous processes.

Exact attachment levels vary among anatomical descriptions, particularly for spinalis cervicis and capitis. This variability reflects the small size and frequent blending of these superior portions. The characteristic feature of the group is its predominantly longitudinal course close to the spinous processes.^[1]

Innervation

Spinalis is innervated segmentally by the posterior rami of spinal nerves at the corresponding cervical and thoracic levels. This pattern is shared by the other intrinsic muscles of the back.

Individual fascicles receive local branches at multiple vertebral levels rather than being supplied by a single named peripheral nerve. This segmental innervation parallels the long, repeated organization of the erector spinae along the vertebral column.^[1][2]

Blood Supply

The arterial supply of spinalis is regional and segmental. In the thoracic region, it receives muscular branches from the dorsal branches of the posterior intercostal arteries. Superior portions receive branches from vessels supplying the deep posterior neck, while lumbar vessels contribute to its lowest attachments.

The vessels approach the muscle at successive levels, supplying the spinalis as part of the larger intrinsic paraspinal muscle mass.

Relations

The position of spinalis is easiest to understand in relation to the other two erector spinae columns. From lateral to medial, the three columns are arranged as iliocostalis, longissimus, and spinalis. Spinalis therefore forms the portion closest to the vertebral spinous processes.

• Laterally: Longissimus.
• Medially: Spinous processes, supraspinous ligaments, and structures near the vertebral midline.
• Superficially: More superficial layers of the intrinsic and extrinsic back musculature, depending on the vertebral level.
• Deeply: Semispinalis and other components of the transversospinalis group.

The medial position of spinalis provides a useful landmark when identifying the erector spinae in dissection or cross-sectional anatomy. Moving laterally from the spinous processes, spinalis is encountered before the larger longissimus and iliocostalis columns.

Function

Spinalis acts primarily as an extensor of the vertebral column. Its nearly vertical fibers and position close to the midline make extension more prominent than lateral flexion compared with the more lateral erector spinae columns.

• Bilateral contraction: Extends the thoracic and cervical vertebral column and contributes to maintenance of an upright posture.
• Unilateral contraction: Can assist ipsilateral lateral flexion of the vertebral column, although its contribution is smaller than that of the more laterally positioned erector spinae muscles.
• Postural control: Works with longissimus, iliocostalis, and other intrinsic back muscles to resist forward flexion and stabilize the vertebral column during movement.

Spinalis normally functions as part of the erector spinae system rather than as an isolated muscle. The three columns act together to generate and control extension while their different positions allow them to contribute differently to lateral movements of the trunk and neck.^[2]

Anatomical Variations

Variation is particularly characteristic of spinalis. The muscle may be slender, discontinuous, or partly blended with adjacent intrinsic back muscles, and its precise attachments differ between individuals.

Spinalis thoracis is usually the most recognizable component, although its number of fascicles and vertebral attachments vary. Spinalis cervicis may be poorly developed or absent as a clearly separable muscle and frequently blends with semispinalis cervicis.

Spinalis capitis shows even greater variability. It is commonly inseparable from semispinalis capitis and is therefore not always treated as a distinct muscle in practical anatomical descriptions. These variations explain why descriptions of the superior spinalis differ more among anatomical references than descriptions of the thoracic portion.

Clinical Significance

Because spinalis is small and lies within a larger mass of intrinsic back muscles, it is rarely assessed clinically as an isolated structure. Its anatomical importance is greater as part of the erector spinae complex and as a landmark for understanding the layered organization of the paraspinal musculature.

Imaging anatomy

On CT and MRI, spinalis occupies the medial portion of the erector spinae mass near the spinous processes. Its distinction from longissimus is clearest where the fascial and muscular boundaries between the erector columns can be identified. Superiorly, separation from neighboring muscles becomes less distinct because of the variable development of spinalis cervicis and capitis.

Posterior spinal approaches

The erector spinae muscles form part of the muscular layers encountered during posterior approaches to the vertebral column. The medial position of spinalis places it close to the spinous processes and supraspinous ligaments, while the deeper transversospinalis muscles occupy the plane between the spinous and transverse processes.

Paraspinal muscle assessment

Changes in muscle bulk, symmetry, or signal within the paraspinal musculature may be visible on cross-sectional imaging. Because spinalis is relatively small and frequently blends with neighboring muscles, abnormalities are often interpreted in the context of the erector spinae or paraspinal muscle group rather than attributed specifically to spinalis.

The spinalis is therefore best understood as the small medial column of the erector spinae. Its close relationship with the spinous processes, predominantly longitudinal fibers, and variable superior portions distinguish it from the larger longissimus and iliocostalis columns.