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Semispinalis muscle

The semispinalis muscle is part of the transversospinalis group of the deep back muscles, located in the epaxial compartment of the vertebral column. It consists of three parts:

  • Semispinalis capitis – arises from transverse processes of C4–T6 and inserts between the superior and inferior nuchal lines of the occipital bone.

  • Semispinalis cervicis – arises from transverse processes of T1–T6 and inserts into spinous processes of C2–C5.

  • Semispinalis thoracis – arises from transverse processes of T6–T10 and inserts into spinous processes of C6–T4.

The fibers run superomedially across 5–6 vertebrae before inserting, forming a key part of the transversospinalis system along with multifidus and rotatores.

Functionally, the semispinalis muscles are responsible for extending the head, neck, and thoracic spine, and for contralateral rotation of these regions. They provide postural stability to the vertebral column.

Synonyms

  • Musculus semispinalis

  • Deep paraspinal extensor

  • Transversospinalis muscle group component

Function

  • Bilateral contraction: extension of head, neck, and thoracic spine

  • Unilateral contraction: contralateral rotation of vertebral column and head

  • Provides dynamic stabilization of cervical and thoracic spine during movement

Nerve Supply

  • Dorsal rami of spinal nerves (segmental innervation)

Arterial Supply

  • Deep cervical artery (branch of costocervical trunk)

  • Occipital artery (for capitis portion)

  • Posterior intercostal arteries (for thoracic portion)

  • Lumbar arteries (lower fibers)

Venous Drainage

  • Mirrors arterial supply:

    • Deep cervical vein → brachiocephalic vein

    • Occipital vein → internal jugular vein

    • Posterior intercostal veins → azygos/hemiazygos system

    • Lumbar veins → ascending lumbar vein → IVC

MRI Appearance

T1-weighted images:

  • Normal muscle appears as intermediate signal intensity

  • Fat infiltration (chronic atrophy) appears hyperintense

T2-weighted images:

  • Normal muscle: intermediate-to-low signal

  • Acute strain, edema, or myositis: bright hyperintensity

STIR:

  • Fat suppression makes edema, inflammation, or trauma appear hyperintense

  • Excellent for detecting acute injury and denervation changes

T1 Post-Gadolinium (Gd-enhanced MRI):

  • Mild, homogeneous enhancement of normal muscle

  • Pathological states (tumor, myositis, abscess) show intense or heterogeneous enhancement

MRI Non-Contrast 3D Imaging:

  • Useful for reconstructing muscle bulk and orientation relative to vertebrae

  • Helpful in spine surgery planning and atrophy assessment

CT Appearance

Non-contrast CT:

  • Appears as a paraspinal soft-tissue density

  • Detects muscle atrophy, calcification, or fatty replacement

CT Post-Contrast:

  • Enhances muscle vascularity

  • Useful in identifying abscesses, neoplastic infiltration, or inflammatory changes

MRI image

Semispinalis muscle anatomy MRI axial image -img-00000-00000

CT image

Semispinalis muscle anatomy CT axial image -img-00000-00000