Topics

Topic

design image
Spinal nerve Co1

The spinal nerve Co1, also known as the coccygeal nerve, is the terminal spinal nerve of the human body. It usually arises as a single pair from the coccygeal segment of the spinal cord. The anterior ramus of Co1 joins with the anterior rami of S4 and S5 to form the coccygeal plexus, which provides motor and sensory innervation to the coccygeal region, anococcygeal area, and part of the pelvic floor.

Although small, the Co1 nerve plays an important role in pelvic and perineal innervation, and is clinically relevant in conditions such as coccydynia, pelvic trauma, and in regional anesthesia.

Synonyms

  • Coccygeal nerve

  • Terminal spinal nerve

  • Co1 root

Origin, Course, and Branches

  • Origin:

    • Arises from the coccygeal segment of the spinal cord

    • Emerges from the vertebral canal via the sacral hiatus or coccygeal canal

  • Course:

    • Exits posterior to the sacrum and coccyx

    • Joins with anterior rami of S4 and S5 to form the coccygeal plexus

  • Branches:

    • Contributes to the anococcygeal nerve, which pierces the sacrotuberous ligament and supplies skin over the coccyx and anus

Relations

  • Anteriorly: Coccygeus and levator ani muscles

  • Posteriorly: Sacrum and coccyx

  • Laterally: Sacral nerves (S5, sacral plexus fibers)

  • Inferiorly: Anococcygeal body and skin of coccyx

Function

  • Provides sensory innervation to skin over the coccyx and anococcygeal region

  • Contributes to motor innervation of parts of the coccygeus and levator ani muscles via the coccygeal plexus

  • Helps stabilize and innervate the pelvic floor

Clinical Significance

  • Coccydynia: Pain syndromes involving the coccyx may involve Co1 nerve irritation

  • Trauma: Coccygeal fractures or dislocations may damage the nerve

  • Surgery: Important in sacral and coccygeal surgical approaches

  • Anesthesia: Coccygeal plexus block can provide analgesia for anorectal or perineal procedures

  • Oncology: Tumors of the sacrum/coccyx may infiltrate or compress Co1

MRI Appearance

T1-weighted images:

  • Co1 nerve appears as a thin, low-to-intermediate signal structure within bright epidural or perineural fat

T2-weighted images:

  • Nerve appears intermediate to mildly hyperintense compared to muscle

  • Edema, neuritis, or compression causes brighter signal intensity

STIR (Short Tau Inversion Recovery):

  • Normal nerve is low signal

  • Pathological nerve appears bright hyperintense

T1 Fat-Sat Post-Contrast:

  • Normal nerve: minimal or no enhancement

  • Pathology: focal or diffuse enhancement with neuritis, tumor infiltration, or inflammation

3D T2 SPACE / CISS:

  • Co1 nerve demonstrates intermediate to mildly hyperintense signal compared to muscle

  • Surrounded by bright CSF or fat, giving excellent contrast

  • Useful for tracing the small nerve root at sacral hiatus and coccygeal canal

CT Appearance

Non-Contrast CT:

  • Co1 nerve is not directly visualized due to small caliber

  • Inferred along coccygeal canal and surrounding fat planes

  • Bony landmarks (sacral hiatus, coccyx) assist in localization

Post-Contrast CT:

  • Nerve itself does not enhance significantly

  • Pathology (tumors, inflammation, infection) may appear as soft tissue thickening or enhancing lesion along coccyx

  • Adjacent fat stranding may indicate inflammatory involvement

MRI images

Spinal nerve Co1 mri axial anatomy image-img-00000-00000