2.3 Spinal Cord and Spinal Nerves

Spinal Cord Gross Anatomy

The spinal cord is a cylindrical continuation of the medulla oblongata that begins at the foramen magnum and, in adults, terminates as the conus medullaris at the lower border of L1 or upper L2. This adult termination is shorter than the vertebral column because of differential growth — the cord "ascends" relative to the spine during development. Below the conus, the lumbar and sacral nerve roots stream downward within the lumbar cistern of the subarachnoid space, forming the cauda equina ("horse's tail"). The thin filum terminale anchors the conus to the coccyx.

The cord has two enlargements: the cervical enlargement (C4–T1) gives rise to the brachial plexus supplying the upper limb, and the lumbosacral enlargement (T11–S3) gives rise to the lumbar and sacral plexuses supplying the lower limb.

Meninges

Three membranes envelop the spinal cord:

1. Dura mater — the tough outermost layer; a tubular sheath extending from the foramen magnum to about S2, with lateral sleeves covering each spinal nerve root to the intervertebral foramen. The epidural space (between dura and vertebral canal) contains fat and the internal vertebral venous plexus; it is the target of epidural anesthesia.
2. Arachnoid mater — a delicate avascular layer against the inner dural surface. The subdural space (between dura and arachnoid) is a potential space.
3. Pia mater — a vascular membrane adherent to the cord surface. It forms the denticulate ligaments that anchor the cord laterally to the dura and the filum terminale inferiorly.

The subarachnoid space (between arachnoid and pia) contains cerebrospinal fluid (CSF). Because the cord ends at L1–L2 but the subarachnoid space continues to S2, the lumbar cistern (L2–S2) is the safe target for lumbar puncture — performed at the L3–L4 or L4–L5 interspinous space to avoid the cord.

Spinal Cord Segments and Roots

The cord is divided into 31 segments — 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal — each giving rise to a pair of spinal nerves. Each segment has:

• A dorsal (posterior) root — sensory; carries afferent fibers from the periphery; the dorsal root ganglion contains the cell bodies of these sensory neurons.
• A ventral (anterior) root — motor; carries efferent fibers from anterior horn cells to skeletal muscle.

The dorsal and ventral roots unite just distal to the dorsal root ganglion to form a spinal nerve, which immediately exits through the intervertebral foramen. Because there are 8 cervical nerves but only 7 cervical vertebrae, C1–C7 nerves exit above the corresponding vertebra, while C8 exits between C7 and T1; from T1 downward, each nerve exits below the corresponding vertebra.

Rami and Plexuses

Immediately outside the foramen, each spinal nerve divides into a dorsal ramus and a ventral ramus:

• Dorsal rami supply the intrinsic (deep) back muscles and the skin of the back in a segmental band near the midline.
• Ventral rami are larger. In thoracic regions they remain segmental as the intercostal nerves (T1–T11) and subcostal nerve (T12), supplying the intercostal muscles and overlying skin. Elsewhere, adjacent ventral rami unite to form plexuses:
  • Cervical plexus (C1–C4) — supplies the neck and diaphragm (via the phrenic nerve, C3–C5).
  • Brachial plexus (C5–T1) — supplies the upper limb.
  • Lumbar plexus (T12–L4) — supplies the anterior and medial thigh (e.g., femoral, obturator nerves).
  • Sacral plexus (L4–S4) — supplies the posterior thigh and most of the leg and foot (e.g., sciatic nerve).

Dermatomes

A dermatome is the strip of skin supplied by a single spinal nerve. Although adjacent dermatomes overlap, key landmarks are clinically useful and tested on the PA-CAT:

Loss of sensation in a dermatomal pattern localizes a lesion to a specific root or nerve.

Segmental Reflexes

A reflex arc requires a receptor, sensory neuron, synapse (usually in the spinal cord), motor neuron, and effector. Deep tendon reflexes are monosynaptic stretch reflexes whose segmental level helps localize cord and root lesions:

• Biceps reflex — C5–C6 (musculocutaneous nerve)
• Brachioradialis reflex — C5–C6 (radial nerve)
• Triceps reflex — C7–C8 (radial nerve)
• Patellar (knee-jerk) reflex — L2–L4 (femoral nerve)
• Achilles (ankle-jerk) reflex — S1–S2 (tibial nerve)

A depressed reflex suggests a lower motor neuron (LMN) lesion at that segment; a hyperactive reflex suggests an upper motor neuron (UMN) lesion above the segment.

Clinical Correlates

• Lumbar puncture — performed at L3–L4 or L4–L5 to sample CSF from the lumbar cistern without risking the cord (which ends at L1–L2).
• Spinal cord injury — a complete cord transection at a given level produces LMN signs at the level of the lesion (dorsal and ventral roots damaged) and UMN signs below (corticospinal tracts interrupted).
• Cauda equina syndrome — compression of the lumbosacral roots below the conus produces saddle anesthesia (S2–S4), bladder/bowel dysfunction, and asymmetric lower limb weakness; it is a surgical emergency.
• Shingles (herpes zoster) — reactivation of varicella-zoster in a dorsal root ganglion produces a painful vesicular rash in a single dermatome.

These clinical examples illustrate why the PA-CAT samples spinal cord anatomy through case vignettes rather than pure structure identification.