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Dorsal ramus of spinal nerve

The dorsal ramus of the spinal nerve (also called the posterior primary ramus) is one of the two major branches of each spinal nerve, the other being the ventral ramus. After emerging from the spinal nerve at the intervertebral foramen, the dorsal ramus turns posteriorly to supply the intrinsic muscles of the back, the facet joints, ligaments of the vertebral column, and the overlying skin.

It is much smaller than the ventral ramus but plays an essential role in posture, spinal stability, and cutaneous sensation of the back.

Synonyms

  • Posterior ramus of spinal nerve

  • Posterior primary division of spinal nerve

  • Dorsal primary ramus

Origin, Course, and Branches

  • Origin:

    • Arises from the mixed spinal nerve just distal to the dorsal root ganglion and spinal nerve trunk

  • Course:

    • Passes posteriorly through the intertransverse muscles and fascia toward the back

    • Runs between adjacent transverse processes, often dividing into medial and lateral branches

  • Branches:

    • Medial branch: Supplies facet joints, periosteum, interspinous ligaments, and skin of paravertebral region (especially cervical and thoracic levels)

    • Lateral branch: Innervates deep back muscles and cutaneous regions of the posterior thorax and lumbar region

    • Intermediate branch: In some regions (notably lumbar), provides motor supply to erector spinae muscles

Relations

  • Anteriorly: Ventral ramus and sympathetic trunk

  • Posteriorly: Intrinsic back muscles (multifidus, erector spinae, semispinalis)

  • Laterally: Transverse processes of vertebrae

  • Medially: Spinous processes, interspinous ligaments, vertebral laminae

Function

  • Provides motor innervation to the intrinsic (deep) muscles of the back, including erector spinae, transversospinalis, and splenius groups

  • Provides sensory innervation to:

    • Zygapophyseal (facet) joints

    • Posterior vertebral ligaments and periosteum

    • Overlying skin of posterior neck, back, and gluteal region

  • Contributes to proprioception and spinal posture control

Clinical Significance

  • Facet joint pain: Medial branches of dorsal rami innervate zygapophyseal joints → frequent target in chronic back pain

  • Diagnostic/therapeutic procedures: Medial branch blocks and radiofrequency ablation for back pain syndromes

  • Trauma or surgery: Dorsal ramus injury can result in localized weakness of back extensors and numbness of back skin

  • Entrapment syndromes: May occur in paraspinal muscle fascia, producing focal back pain

MRI Appearance

T1-weighted images:

  • Dorsal ramus appears as a low-to-intermediate signal linear structure within bright fat of paraspinal spaces

T2-weighted images:

  • Appears as intermediate to mildly hyperintense compared to muscle

  • Pathology (neuritis, compression) shows higher hyperintensity

STIR (Short Tau Inversion Recovery):

  • Normal nerve: low signal

  • Pathology (edema, inflammation, trauma): bright hyperintensity

T1 Fat-Sat Post-Contrast:

  • Normal: minimal or no enhancement

  • Abnormal: focal or diffuse enhancement in neuritis, entrapment, or tumor infiltration

3D T2 SPACE / CISS:

  • Dorsal ramus appears as intermediate to mildly hyperintense compared to muscle

  • Surrounded by bright CSF or fat, aiding clear visualization of nerve roots and branches

  • Excellent for evaluating nerve root exits, foraminal entrapment, or compressive lesions

CT Appearance

Non-Contrast CT:

  • Dorsal ramus itself is not directly visible, inferred within fat and between paraspinal muscles

  • Bony landmarks (facet joints, transverse and spinous processes) define its course

  • Pathology (e.g., foraminal stenosis, bony encroachment) indirectly affects nerve appearance

Post-Contrast CT:

  • Normal nerve: not directly enhanced

  • Pathologic processes (tumor, inflammatory infiltration): may appear as enhancing soft tissue or thickening in paraspinal fat planes

  • Secondary signs include fat stranding, mass effect, or foraminal narrowing

MRI image