Back & spinal cord
First module — the body wallAlmost every gross-anatomy course opens on the back. It is the simplest region to dissect, it introduces the vocabulary the rest of the course is written in (dorsal rami, dermatomes, fascial layers), and it carries two of the highest-yield clinical procedures in all of medicine: the lumbar puncture and the epidural. Learn the vertebral levels cold — they are the coordinate system for everything that follows.
High-Yield Pearl
Every question about the back is secretly a question about a vertebral level. Learn the levels — cord ends L1–L2, dural sac S2, iliac crests L4, T4 nipple, T10 umbilicus — and the rest of the region organizes itself.
What the written exam asks
Relationships, supply, levels, and what happens when each fails.
- Vertebral column. 33 vertebrae: 7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral, 4 fused coccygeal. Primary curves (thoracic, sacral — kyphotic, present at birth); secondary curves (cervical, lumbar — lordotic, develop with head control and walking). Typical vertebra: body, pedicles, laminae, spinous and transverse processes, superior and inferior articular facets; the vertebral foramina stack into the vertebral canal.
- Atypical vertebrae. C1 (atlas) has no body and no spinous process; C2 (axis) carries the dens, around which the atlas rotates ("no" = atlantoaxial, "yes" = atlanto-occipital). C7 is the vertebra prominens. Cervical transverse processes carry the foramen transversarium for the vertebral artery (C6 and above).
- Intervertebral disc. Outer annulus fibrosus, inner nucleus pulposus (a notochord remnant). Herniation is posterolateral — the posterior longitudinal ligament is strong in the midline — and compresses the nerve root exiting ONE level below the disc in the lumbar spine (L4–L5 disc → L5 root; L5–S1 disc → S1 root).
- Ligaments. Anterior longitudinal (limits extension, on the vertebral bodies), posterior longitudinal (inside the canal, limits flexion), ligamentum flavum (elastic, yellow, between laminae — the "give" felt on a lumbar puncture), interspinous and supraspinous (continuing as the ligamentum nuchae in the neck).
- Spinal cord levels. The adult cord ends at L1–L2 as the conus medullaris; the nerve roots continue as the cauda equina; the dural sac ends at S2; the filum terminale anchors the cord to the coccyx. Because the cord is shorter than the canal, lumbar roots descend before exiting — which is why an L3–L4 needle is safe.
- Spinal nerves. 31 pairs: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal. C1–C7 exit ABOVE their vertebra, C8 exits below C7, and every nerve from T1 down exits below its vertebra. Dorsal root = sensory (with the dorsal root ganglion); ventral root = motor; they join to form the mixed spinal nerve, which immediately splits into a dorsal ramus (intrinsic back muscles + a strip of skin) and a much larger ventral ramus (everything else, including the plexuses).
- Back muscles by innervation. Extrinsic (move the limb; ventral rami or cranial nerves): trapezius — CN XI; latissimus dorsi — thoracodorsal nerve; rhomboids and levator scapulae — dorsal scapular nerve; serratus posterior — intercostal nerves. Intrinsic (move the column; dorsal rami): splenius, erector spinae (iliocostalis, longissimus, spinalis — lateral to medial), and the transversospinalis group (semispinalis, multifidus, rotatores). "Dorsal rami innervate the deep back" is the exam's favorite distinction.
- Blood supply and venous drainage. One anterior spinal artery (the anterior two-thirds of the cord) and two posterior spinal arteries, reinforced by segmental radicular arteries — the largest is the artery of Adamkiewicz (usually left, T9–T12), at risk in aortic surgery. The valveless internal vertebral venous plexus (Batson) connects pelvic veins to the vertebrae and skull — the route of prostate and breast metastases to the spine.
- Dermatome landmarks. C6 thumb, C7 middle finger, C8 little finger, T4 nipple, T10 umbilicus, L1 inguinal ligament, L4 medial malleolus, L5 dorsum of the foot and great toe, S1 lateral foot and little toe. Reflexes: biceps C5–C6, triceps C7, patellar L2–L4, Achilles S1.
What the practical tags · 33 structures
In the order a dissection meets them — each with the one relationship that identifies it. Your lab's study list is the authority; use this to check yours against.
- 1Spinous process — the midline ridge you palpate; bifid in most cervical vertebrae
- 2Transverse process — lateral; carries the foramen transversarium in the neck
- 3Lamina — joins spinous process to pedicle; removed in a laminectomy
- 4Pedicle — joins the body to the arch; its notches form the intervertebral foramen
- 5Vertebral foramen / canal — houses the cord and meninges
- 6Superior and inferior articular facets — the zygapophyseal (facet) joints
- 7Intervertebral foramen — where the spinal nerve exits, between adjacent pedicles
- 8Atlas (C1) — ring with no body; anterior and posterior arches
- 9Axis (C2) — the dens projecting upward into the atlas
- 10Sacrum and sacral hiatus — the caudal epidural entry point
- 11Trapezius — the superficial diamond; innervated by CN XI
- 12Latissimus dorsi — broad, inserts on the humerus; thoracodorsal nerve
- 13Rhomboid major and minor — deep to trapezius, medial scapular border
- 14Levator scapulae — superior angle of the scapula
- 15Serratus posterior superior and inferior — thin, easily destroyed; respiratory
- 16Thoracolumbar fascia — the glistening sheet over the lumbar erector spinae
- 17Erector spinae — iliocostalis (lateral), longissimus (middle), spinalis (medial)
- 18Splenius capitis and cervicis — the bandage-like muscle under trapezius
- 19Semispinalis capitis — the thick muscle of the posterior neck
- 20Multifidus — deep, filling the groove between spinous and transverse processes
- 21Suboccipital triangle — rectus capitis posterior major, obliquus capitis superior and inferior; the vertebral artery crosses its floor
- 22Greater occipital nerve — dorsal ramus of C2, piercing semispinalis
- 23Ligamentum nuchae and supraspinous ligament — the midline cord
- 24Ligamentum flavum — yellow, between laminae, seen after laminectomy
- 25Dura mater — the tough outer sac; the epidural space lies outside it
- 26Arachnoid mater — the film under the dura; CSF beneath it
- 27Denticulate ligaments — the pia's toothed lateral shelves anchoring the cord
- 28Spinal cord and conus medullaris — cord ends at L1–L2
- 29Cauda equina — the bundle of descending roots below the conus
- 30Filum terminale — the thread from the conus to the coccyx
- 31Dorsal root ganglion — the swelling on the dorsal root inside the foramen
- 32Dorsal root, ventral root, spinal nerve, dorsal and ventral rami — name all five on one level
- 33Sympathetic trunk and ganglia — the beaded chain lateral to the vertebral bodies
What everyone misses
The distinctions that lose points on the practical, year after year.
- Dorsal root ganglion vs sympathetic ganglion. The DRG sits ON the dorsal root inside the intervertebral foramen and contains sensory cell bodies; the sympathetic chain ganglion sits lateral to the vertebral body, connected to the spinal nerve by rami communicantes. Students tag one for the other constantly.
- Root vs ramus. Roots (dorsal, ventral) are inside the dura and join to make the nerve; rami (dorsal, ventral) are the nerve's branches after it leaves the foramen. Dorsal RAMUS goes to the intrinsic back; the dorsal ROOT is sensory.
- Denticulate ligaments. Lateral, toothed extensions of pia between the dorsal and ventral roots — they look like nothing until someone names them, then they are on every practical.
- Cauda equina vs filum terminale. The cauda is the bundle of nerve roots; the filum is the single glistening thread of pia in the middle of it, ending on the coccyx.
- C8. There are eight cervical nerves and seven cervical vertebrae. C8 exits between C7 and T1 — the favorite written-exam trap.
- Trapezius is not a back muscle by innervation. It covers the back but is supplied by the spinal accessory nerve (CN XI), not the dorsal rami. The same "extrinsic vs intrinsic" logic applies to latissimus, rhomboids, and levator scapulae.
- Three columns of erector spinae, medial to lateral. Spinalis, longissimus, iliocostalis — "I Love Spaghetti" lateral to medial. The practical tags a column, not the group.
Clinical correlations
Where this region shows up again — in clinical medicine, on rotations, and on the boards.
- Lumbar puncture. Between L3–L4 or L4–L5 (Tuffier's line — the iliac crests — crosses L4), below the conus. Needle passes skin → subcutaneous tissue → supraspinous → interspinous → ligamentum flavum (the give) → epidural space → dura → arachnoid → subarachnoid space (CSF). An epidural stops one layer earlier, outside the dura.
- Disc herniation and sciatica. Posterolateral herniation compresses the root exiting below the disc: L4–L5 → L5 (foot drop, weak dorsiflexion, numb dorsum of foot), L5–S1 → S1 (weak plantar flexion, lost Achilles reflex, numb lateral foot). A central herniation can take the whole cauda.
- Cauda equina syndrome. Saddle anesthesia, urinary retention/overflow, bilateral leg weakness, loss of anal tone — a surgical emergency. The roots, not the cord, are compressed, so signs are lower motor neuron.
- Compression fracture and spinal stenosis. Osteoporotic vertebral bodies wedge anteriorly (kyphosis, height loss); stenosis narrows the canal — neurogenic claudication relieved by flexion (leaning on the shopping cart).
- Vertebral metastasis. The valveless Batson plexus carries prostate (and breast, lung, renal, thyroid) cancer to the vertebral bodies — back pain that is worse at night and at rest, with cord compression as the emergency.
- Upper cervical fractures. Jefferson (burst) fracture of the atlas from axial load; hangman's fracture of the C2 pars from hyperextension; dens fractures in the elderly after a fall — the cord is at risk with any instability.
- Neural tube defects. Spina bifida occulta (tuft of hair, dimple) to meningocele and myelomeningocele — failure of the posterior neuropore to close; folate before conception prevents it.
Frequently asked
This guide is the map. Drop this region's lecture slides on the dashboard to get flashcards and questions from your lecture, group them into a module, and run a timed module exam the week before. The gross anatomy playbook has the weekly loop.
AI-authored study notes, not faculty-reviewed and not medical advice. Verify every structure against your atlas, your dissector, and your lab instructors; programs differ in what they tag.