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AN42.1-3 | Back Region — Self-Directed Learning

Contents of the Vertebral Canal (AN42.1)

The vertebral (spinal) canal is the cylindrical tunnel formed by the successive vertebral foramina. From the foramen magnum (C1) to the sacral hiatus.

Contents:
1. Spinal cord — from foramen magnum to L1–L2 (conus medullaris)
2. Cauda equina — L2 to the sacral hiatus; nerve roots of L2–S5 and the filum terminale
3. Meninges (3 layers):
- Dura mater (outermost, thickest): forms the dural sac (extends to S2)
- Arachnoid mater (middle): closely applied to dura; avascular
- Pia mater (innermost): closely adherent to the spinal cord; continuous with the filum terminale
4. CSF — in the subarachnoid space (between arachnoid and pia); the lumbar cistern (L2–S2) is the site of lumbar puncture
5. Epidural (extradural) space — between the dura and the periosteum of the vertebral canal; contains: epidural fat, internal vertebral venous plexus (Batson's plexus), spinal arteries and veins
6. Filum terminale — a strand of pia extending from the conus medullaris to the coccyx (anchors the cord)

Spinal nerves:
- 31 pairs: C1–C8, T1–T12, L1–L5, S1–S5, Co1
- Each spinal nerve has a dorsal (sensory, via dorsal root ganglion) and ventral (motor) root
- They exit through the intervertebral foramina (between adjacent pedicles)

Arterial supply of the spinal cord:
- Anterior spinal artery (single): supplies the anterior 2/3 of cord (motor pathways)
- Posterior spinal arteries (paired): posterior 1/3 (sensory pathways)
- Reinforced by radicular arteries — the most important is the artery of Adamkiewicz (radicularis magna, arises from T8–L2, usually left-sided) — supplies the thoracolumbar cord
- Occlusion of the artery of Adamkiewicz (e.g., aortic surgery, aortic dissection) → anterior spinal artery syndrome: paraplegia + loss of pain/temperature (preserved vibration/proprioception — posterior column spared)

Venous drainage:
- Internal vertebral venous plexus (Batson's plexus) in the epidural space — valveless, communicates with pelvic, thoracic, and cranial veins — route for metastatic spread to vertebrae from prostate, breast, lung cancers

The Suboccipital Triangle (AN42.2)

Location: Deep to the posterior neck musculature, between C1 (atlas) and C2 (axis), beneath the semispinalis capitis.

Boundaries — 3 muscles form the triangle:

Side	Muscle
Superomedial	Rectus capitis posterior major (C2 spinous process → occipital bone)
Superolateral	Obliquus capitis superior (transverse process of atlas → occipital bone)
Inferior	Obliquus capitis inferior (C2 spinous process → transverse process of atlas)

Contents:
1. Vertebral artery (V3 segment) — emerges from the foramen transversarium of C1, winds around the posterior arch of the atlas, and pierces the posterior atlantooccipital membrane to enter the skull
2. Suboccipital nerve (dorsal ramus of C1) — purely motor; supplies the muscles forming the triangle + rectus capitis posterior minor
3. Greater occipital nerve (dorsal ramus of C2) — sensory; emerges below the inferior oblique, pierces semispinalis capitis and trapezius → supplies the posterior scalp to the vertex
4. Dense suboccipital venous plexus

Actions of suboccipital muscles:
- Extension, rotation (ipsilateral), and lateral flexion of the head — fine postural adjustment at the atlanto-axial and atlanto-occipital joints

Clinical relevance:
- Greater occipital nerve entrapment (occipital neuralgia): Compression or irritation of the greater occipital nerve (C2) as it pierces the semispinalis capitis + trapezius → pain radiating from the occiput over the vertex to the forehead; common in desk workers, post-whiplash. Treated with nerve block (injection of local anaesthetic + corticosteroid at the superior nuchal line) or physiotherapy
- Vertebral artery injury during posterior cervical surgery: V3 segment of the vertebral artery is exposed in the suboccipital triangle; at risk during dissection of the posterior atlantoaxial/atlantooccipital region
- C1–C2 instability (rheumatoid arthritis, Down syndrome): atlantoaxial subluxation can compress the spinal cord — evaluate with flexion/extension neck X-rays

Back Muscles — Semispinalis Capitis and Splenius Capitis (AN42.3)

Splenius capitis ("bandage muscle"):
- Origin: Ligamentum nuchae + spinous processes of C7–T4
- Insertion: Mastoid process and lateral superior nuchal line of occipital bone
- Nerve supply: Lateral branches of dorsal rami of middle cervical nerves (C3–C5)
- Action:
- Bilaterally: extension of the head and neck
- Unilaterally: lateral flexion and rotation of the head ipsilaterally
- Fibre direction: Spiral — passes upward and laterally (like a bandage)

Semispinalis capitis:
- Origin: Transverse processes of C4–T7 (and articular processes of C4–C6)
- Insertion: Occipital bone (between superior and inferior nuchal lines)
- Nerve supply: Medial branches of dorsal rami of cervical nerves
- Action:
- Bilaterally: extension of the head
- Unilaterally: lateral flexion; contralateral rotation (the semispinalis group rotates the spine to the OPPOSITE side — unlike splenius which rotates IPSILATERALLY)
- Layer: Deep to splenius capitis; superficial to suboccipital triangle and greater occipital nerve
- Key anatomical point: The greater occipital nerve (C2 dorsal ramus) pierces through the semispinalis capitis to reach the scalp — entrapment here causes occipital neuralgia

Clinical pearl — back muscle layering from superficial to deep:
1. Trapezius + latissimus dorsi (superficial extrinsic — move upper limb)
2. Serratus posterior (intermediate extrinsic — move ribs)
3. Splenius capitis/cervicis (deep extrinsic)
4. Erector spinae group: iliocostalis + longissimus + spinalis (deep intrinsic — extend spine)
5. Transversospinales: semispinalis + multifidus + rotatores (deepest intrinsic)
6. Suboccipital muscles (deepest — fine head position)