Spinal cord

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Here is a comprehensive overview of the anatomy and structure of the spinal cord, drawn from multiple authoritative neuroscience and anatomy textbooks.

Spinal Cord - Anatomy & Structure

1. Overview and Position

The spinal cord is the cylindrical extension of the brain that runs within the vertebral canal. In adults, it is shorter than the vertebral column, terminating at the level of the L1-L2 vertebra as the conus medullaris (in neonates it extends to ~L3). Below the conus, a fibrous extension called the filum terminale tethers the cord to the first coccygeal segment.

The cord gives off 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.

Vertebral column regions - posterior view

The vertebral column: Occipital-C2, C3-C7, T1-T12, L1-L5, Sacrum, and Coccyx

2. Meningeal Coverings

The spinal cord is surrounded by three protective membranes (meninges):

Layer	Notes
Pia mater	Innermost; closely adherent to the cord surface
Arachnoid mater	Middle layer
Dura mater	Outermost; tough fibrous sheath

The space between the pia and arachnoid is the subarachnoid space, which contains cerebrospinal fluid (CSF). The epidural space (between dura and vertebral canal) contains a valveless venous plexus known as Batson's plexus, which communicates freely with abdominal and pelvic veins.

3. Spinal Cord Enlargements

The cord is not uniform in diameter. It has two enlargements corresponding to the nerve plexuses for the limbs:

Cervical enlargement (C5-T1): supplies the upper limbs via the brachial plexus
Lumbosacral enlargement (L1-S3): supplies the lower limbs via the lumbar and sacral plexuses

These enlargements contain more gray matter (especially in the ventral horns) than the thoracic cord.

4. Cross-Sectional Anatomy

The classic "butterfly" or H-shaped gray matter is surrounded by white matter:

Spinal cord cross section - gray and white matter organization

Gray matter horns, intermediate zone, and white matter columns (dorsal, lateral, ventral), with dorsal and ventral nerve roots

Gray Matter Horns

Horn	Function
Dorsal (posterior) horn	Sensory processing - receives afferent input from dorsal roots
Ventral (anterior) horn	Contains lower motor neurons (alpha and gamma) that send axons via ventral roots
Lateral horn (T1-L2 only)	Preganglionic sympathetic neurons (intermediolateral cell column)
Intermediate zone	Interneurons; Clarke's nucleus (T1-L2); sacral parasympathetics (S2-4)

White Matter Columns (Funiculi)

Three pairs of white matter columns carry the ascending (sensory) and descending (motor) tracts:

Column	Key Tracts
Dorsal (posterior)	Fasciculus gracilis + fasciculus cuneatus (fine touch, vibration, proprioception)
Lateral	Lateral corticospinal tract (voluntary motor); spinothalamic tract; spinocerebellar tracts
Ventral (anterior)	Ventral corticospinal tract; spinothalamic fibers

Cross-section showing major tracts with somatotopic lamination

Key tracts: Fasciculi gracilis/cuneatus (dorsal), lateral corticospinal, spinothalamic, spinocerebellar (lateral), and ventral corticospinal. Letters indicate somatotopic arrangement: C = cervical, Th = thoracic, L = lumbar, S = sacral. Note cervical fibers are most medial/central in descending tracts but most lateral in ascending dorsal columns.

5. Rexed's Laminae

The gray matter is subdivided into 10 laminae (named by Swedish anatomist Bror Rexed), each with distinct functions:

Lamina	Region	Key Nucleus/Function
I	Dorsal horn	Marginal zone (posterior marginal nucleus) - pain/temperature
II	Dorsal horn	Substantia gelatinosa of Rolando - pain modulation
III, IV	Dorsal horn	Nucleus proprius - sensory processing
V	Dorsal horn	Neck of dorsal horn (nucleus reticularis)
VI	Dorsal horn	Base of dorsal horn
VII	Intermediate zone	Clarke's nucleus, intermediolateral nucleus (sympathetics)
VIII	Ventral horn	Commissural nucleus
IX	Ventral horn	Alpha and gamma motor neurons (medial = axial; lateral = limb muscles)
X	Central gray	Surrounds the central canal (gray commissure)

The zone of Lissauer (posterolateral tract) separates the dorsal gray horn from the surface and carries pain/temperature fibers one or two levels before synapsing.

6. Regional Variation by Level

The cord looks different at each level - shown here in myelin-stained cross sections:

Myelin-stained sections at different spinal cord levels

From top: cervicomedullary junction, cervical (most white matter), thoracic (lateral horn visible), lumbar (large ventral horns), sacral (mostly gray matter)

Cervical cord: most white matter (all ascending fibers present + all descending yet to terminate); large gray matter at enlargement
Thoracic cord: smaller; lateral horn present; Nucleus dorsalis of Clarke present (T1-L2)
Lumbar cord: large ventral and dorsal horns; little white matter
Sacral cord: mostly gray matter; sacral parasympathetic nucleus

7. Spinal Nerve Formation

At each spinal level:

Dorsal (sensory) root: carries afferent fibers from peripheral sensory neurons whose cell bodies are in the dorsal root ganglion (DRG), located near the intervertebral foramen
Ventral (motor) root: carries efferent fibers (mainly motor) from ventral horn neurons
These unite to form the mixed spinal nerve

Exit level rules:

C1-C7: nerve exits above the correspondingly numbered vertebra
C8: exits between C7 and T1
T1 and below: all nerves exit below the correspondingly numbered vertebra

Because the cord is shorter than the column, lumbar and sacral nerve roots travel a long distance within the canal before exiting - this bundle of roots below the conus is the cauda equina ("horse's tail").

8. Vascular Supply

Artery	Territory
Anterior spinal artery (1 vessel, from vertebral arteries)	Anterior 2/3 of cord: anterior horns, lateral and anterior white matter columns
Posterior spinal arteries (2 vessels, from PICA or vertebral arteries)	Posterior 1/3: dorsal columns, part of dorsal horns
Artery of Adamkiewicz (great radicular artery, usually left T9-T12)	Major supply to lower thoracic and lumbar/sacral cord

The mid-thoracic region (T4-T8) is a watershed zone with relatively reduced perfusion, making it most vulnerable to infarction (e.g., during aortic surgery).

Venous drainage flows into the epidural venous plexus (Batson's plexus) - valveless, which allows retrograde spread of pelvic infections or metastases (e.g., prostate cancer) to the vertebral column.

Sources: Neuroanatomy through Clinical Cases, 3rd Ed. | Localization in Clinical Neurology, 8th Ed. | Campbell's Operative Orthopaedics, 15th Ed. | Neuroscience: Exploring the Brain, 5th Ed.

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