Spinal Cord Anatomy

The spinal cord is the central nervous system’s only direct interface with the body below the head. It receives sensory input from skin, joints, muscles, and viscera; it sends motor output to skeletal muscle, smooth muscle, and glands; it executes the basic reflexes that keep us upright and protected; and it relays everything that the brain needs to know about the body, and everything the body needs to know from the brain. The cord is about 45 cm long in the adult and weighs about 30 grams — a slender cable of nervous tissue running from the foramen magnum to the conus medullaris at L1-L2, with thirty-one pairs of spinal nerves leaving along its length.

This page covers the gross anatomy of the spinal cord, its segmental organization, the relationship between cord segments and vertebral levels, the meninges and CSF compartment, and the vascular supply. Cord tracts and spinal nerves have their own dedicated pages.

Position and Extent

The spinal cord extends from the foramen magnum (continuous with the medulla) to its caudal end at the conus medullaris. In the adult, the conus is at the level of the L1-L2 vertebral bodies. In the infant, it extends lower, to L3; the difference reflects the differential growth of vertebral column versus cord during fetal and early postnatal development. The clinical implication: lumbar puncture in the adult is performed below L2 (typically L3-L4 or L4-L5), below the conus, to avoid injuring the cord.

Below the conus, the nerve roots that arose from the cord at higher levels but have not yet exited the vertebral canal continue caudally as a bundle of roots — the cauda equina (“horse’s tail”) — within the subarachnoid space, eventually exiting through their respective intervertebral foramina.

External Features

The cord has two enlargements where the nerves to the limbs originate:

• Cervical enlargement: from about C4 to T1 vertebral levels (cord segments C5-T1), giving rise to the brachial plexus.
• Lumbar enlargement: from about T10 to L1 vertebral levels (cord segments L1-S3), giving rise to the lumbosacral plexus.

External grooves run longitudinally along the cord:

• Anterior median fissure: a deep groove on the ventral surface.
• Posterior median sulcus: a shallow groove on the dorsal surface.
• Anterolateral and posterolateral sulci: marking the exits of ventral and entries of dorsal nerve rootlets.
• Posterior intermediate sulcus: in the cervical and upper thoracic cord, separating the gracile and cuneate columns.

Cord Segments and Vertebral Levels

The cord has 31 segments matching the 31 pairs of spinal nerves:

• 8 cervical (C1-C8)
• 12 thoracic (T1-T12)
• 5 lumbar (L1-L5)
• 5 sacral (S1-S5)
• 1 coccygeal

Note that there are 8 cervical cord segments but only 7 cervical vertebrae. The cervical nerve roots exit above their corresponding vertebra (C1 root exits above C1 vertebra, C7 root exits above C7 vertebra, C8 root exits between C7 and T1 vertebrae). From T1 downward, the roots exit below their corresponding vertebra.

Because the cord is shorter than the vertebral column, the cord segments do not correspond to the same numbered vertebrae:

This vertebra-segment offset is clinically important: a spinal cord lesion at the T10 vertebral level affects the L1 cord segment, with motor and sensory consequences in the L1 dermatome/myotome distribution. Always think in terms of cord segments for neurological localization, then translate to vertebral levels for surgical or imaging considerations.

Internal Architecture

The cord has a butterfly-shaped (or H-shaped) central core of gray matter surrounded by white matter — the opposite of the brain, where gray matter is mostly cortical (peripheral) and white matter mostly central.

Gray Matter

The gray matter is organized into:

• Dorsal (posterior) horn: receives sensory afferents from the dorsal roots. Major sensory relay nuclei here.
• Ventral (anterior) horn: contains the alpha motor neurons that project through the ventral roots to skeletal muscle. Topographically organized: medial neurons innervate axial muscles, lateral neurons innervate distal muscles.
• Lateral horn: present at T1-L2 (intermediolateral cell column, preganglionic sympathetic neurons) and at S2-S4 (sacral parasympathetic nucleus). The sympathetic outflow is therefore thoracolumbar; the parasympathetic outflow is craniosacral.

The gray matter is conventionally subdivided into ten Rexed laminae, numbered from dorsal to ventral. Laminae I-VI are dorsal horn; VII includes the intermediolateral and intermediomedial columns; VIII-IX are ventral horn (motor neurons); X surrounds the central canal.

White Matter

The white matter contains the long ascending and descending tracts, organized into three columns (funiculi):

• Dorsal (posterior) column: between the posterior horn and the dorsal median sulcus. Contains the dorsal column tracts (gracile fasciculus medially, cuneate fasciculus laterally above T6).
• Lateral column: between the dorsal and ventral horns. Contains the lateral corticospinal tract, the spinothalamic tract (lateral portion), dorsal and ventral spinocerebellar tracts, and others.
• Ventral (anterior) column: between the ventral horn and the ventral median fissure. Contains the anterior corticospinal tract, the anterior spinothalamic tract, and vestibulospinal tracts.

The white matter:gray matter ratio increases as you ascend the cord. At sacral levels, there is little white matter (because few fibers have ascended yet); at cervical levels, there is a lot (because all the tracts to and from the brain are present).

The Meninges and Compartments

The spinal cord is wrapped in the same three meningeal layers as the brain:

• Dura mater: a single layer surrounding the cord and roots, forming a tubular sheath. Unlike cranial dura, the spinal dura is not adherent to the bony canal — there is an epidural space between dura and vertebral bone, containing fat and the internal vertebral venous plexus. The epidural space is the target for epidural anesthesia and analgesia.
• Arachnoid mater: lying inside the dura. The subarachnoid space contains CSF and the spinal nerve roots.
• Pia mater: adherent to the cord surface. Extends as paired ligamentous slips (denticulate ligaments) that anchor the cord laterally to the dura.

The dural sac ends at about S2 vertebral level — below this, there is no subarachnoid space and no risk of dural puncture. The cord itself ends at L1-L2, so between L2 and S2 there is dural sac filled with CSF and cauda equina roots but no cord. This is the safe target for lumbar puncture.

Below S2, the spinal dura tapers to a fibrous strand — the filum terminale — that descends to attach to the coccyx, anchoring the cord caudally.

Vascular Supply

Arterial Supply

The cord is supplied by:

• Anterior spinal artery: a single vessel running along the anterior median fissure, formed by branches of the vertebral arteries at the cervical level. Supplies the anterior two-thirds of the cord (anterior horns, corticospinal tracts, spinothalamic tracts).
• Posterior spinal arteries: paired vessels running along the posterolateral surface, also from the vertebral arteries. Supply the dorsal columns and dorsal horns (posterior one-third of the cord).
• Radicular (segmental) arteries: branches from the aorta and other systemic arteries that enter the vertebral canal through the intervertebral foramina, sending feeders to the spinal arteries at various levels.

The longitudinal anterior and posterior spinal arteries are very narrow vessels that depend critically on the segmental feeders to maintain flow. The most clinically important segmental feeder is the artery of Adamkiewicz, usually arising from a left intercostal or lumbar artery between T9 and L2, and supplying a large fraction of the lower thoracic and lumbar cord. Injury to this artery (in aortic surgery, in aortic dissection, in interventional procedures) can produce devastating anterior cord syndrome with lower extremity paraplegia.

Watershed Zones

The cord has watershed regions where the arterial supply is more tenuous:

• Mid-thoracic (T4-T8 region): between the cervical anterior spinal artery territory and the artery of Adamkiewicz territory. Particularly vulnerable to global hypoperfusion.
• L1 region: between the artery of Adamkiewicz territory and the lower lumbosacral supply.

Venous Drainage

Venous drainage occurs through:

• Anterior and posterior spinal veins along the surface of the cord.
• Radicular veins exiting at each level.
• Connection to the internal vertebral venous plexus in the epidural space.
• Eventually to the segmental veins and from there to the systemic venous system.

The internal vertebral venous plexus has no valves and communicates extensively with pelvic and prostatic veins — providing a route for hematogenous spread of pelvic cancers to the vertebral column (the Batson plexus).

Spinal Cord Diseases by Anatomical Pattern

Complete Cord Transection

From trauma, severe inflammation, or massive vascular event. Loss of all sensation and motor function below the level, plus bladder, bowel, and (depending on level) autonomic function. Initial flaccid paralysis (spinal shock) lasts days to weeks; subsequently, the picture evolves to spasticity with hyperreflexia below the level.

Brown-Séquard Syndrome (Hemisection)

Classical pattern from cord hemisection:

• Ipsilateral motor weakness below the level (corticospinal tract).
• Ipsilateral loss of vibration and joint position sense (dorsal columns; still on same side until decussation in medulla).
• Contralateral loss of pain and temperature beginning a few segments below the level (spinothalamic tract; fibers crossed at the segmental level of entry).
• At the level of the lesion: ipsilateral lower motor neuron weakness in that segment’s myotome and ipsilateral loss of all modalities in that dermatome (from direct root involvement).

Central Cord Syndrome

Hyperextension injury, especially in older patients with cervical spondylosis. Damages the central cord disproportionately. Upper extremities are more affected than lower (corticospinal fibers to the arms run more medially), and pain/temperature loss may be “suspended” in a cape distribution. Bladder dysfunction common.

Anterior Cord Syndrome

Infarction of the anterior spinal artery territory. Loss of motor function and pain/temperature sensation below the level, with preservation of dorsal column function (vibration, joint position sense). Devastating but recognizable picture.

Posterior Cord Syndrome

Damage to the dorsal columns. Loss of vibration and joint position sense (sensory ataxia), with preservation of strength and pain/temperature. Causes include B12 deficiency (subacute combined degeneration), tabes dorsalis (now rare), demyelination.

Conus Medullaris Syndrome

Lesion of the conus (L1-L2 vertebral level affecting S2-Co1 cord segments). Mixed upper and lower motor neuron findings (some segments affected at cord level, some segmental motor neurons damaged). Early bladder and bowel dysfunction with saddle anesthesia. Symmetric, bilateral findings.

Cauda Equina Syndrome

Compression of the cauda equina roots below the conus. Asymmetric, lower motor neuron pattern (purely peripheral nerve roots). Severe radicular pain, saddle anesthesia, bladder and bowel dysfunction. Surgical emergency — decompression within 48 hours improves recovery of bowel and bladder function substantially.

Syringomyelia

A fluid-filled cavity (syrinx) in the central cord, expanding over time. Disrupts the crossing spinothalamic fibers in the anterior commissure, producing bilateral “suspended” loss of pain and temperature in a cape-like distribution over the shoulders and arms. As the syrinx expands, it can affect anterior horn cells (producing arm weakness with atrophy) and lateral corticospinal fibers (producing leg spasticity). Often associated with Chiari I malformation.

Subacute Combined Degeneration

B12 deficiency. Damage to the dorsal columns and lateral corticospinal tracts, with relative sparing of spinothalamic fibers and lower motor neurons. Combined picture: sensory ataxia (dorsal column), spastic paresis with upgoing toes (corticospinal), and mixed reflex pattern. Cognitive changes and macrocytic anemia may accompany but are not required.

Transverse Myelitis

An acute inflammatory lesion of the cord at a specific level, producing motor weakness, sensory level, and bladder dysfunction below the level. Causes: multiple sclerosis, NMOSD, post-infectious, lupus, sarcoid. Imaging shows T2 hyperintensity at the involved cord segments.

Spinal Cord Tumors

Subdivided by anatomical location:

• Extradural: most common, mostly metastatic (lung, breast, prostate, kidney).
• Intradural extramedullary: meningiomas, schwannomas, neurofibromas. Often present with radicular pain initially.
• Intramedullary: gliomas (astrocytomas, ependymomas). Often slowly progressive sensorimotor and bladder symptoms.

The Cord on Imaging

MRI is the imaging modality of choice for spinal cord disease. Key principles:

• The cord has uniform signal intensity on T1 and T2 in health.
• T2 hyperintensity within the cord suggests demyelination, inflammation, ischemia, or tumor.
• Contrast enhancement suggests active inflammation, infection, or tumor.
• Cord atrophy is best seen on sagittal images and may reflect chronic disease.
• Always image the relevant cord segment, not just the vertebral level corresponding to the clinical findings.

🔍 Did You Know?

The artery of Adamkiewicz is the largest segmental feeder to the anterior spinal artery in the lower thoracic and lumbar region. It typically arises from a single intercostal or lumbar artery, most often on the left side, between T9 and L2 vertebral levels. Because so much of the lower cord depends on this single vessel, the artery of Adamkiewicz has been a major source of clinical anxiety in vascular and spinal surgery. Inadvertent injury during thoracoabdominal aortic aneurysm repair, aortic dissection, or even high-risk interventional procedures can produce anterior cord syndrome with permanent lower extremity paraplegia. Various intraoperative monitoring techniques (somatosensory and motor evoked potentials) and surgical strategies (CSF drainage, distal aortic perfusion, sequential clamping) have been developed to reduce this risk. Preoperative angiographic identification of the artery has become routine in selected cases.

Pitfalls and Pearls

• The cord ends at L1-L2 in the adult. Lumbar puncture below L2 is safe.
• Cord segments and vertebral levels do not correspond. Always think in segments for neurological localization, then translate to vertebral level for surgical or imaging considerations.
• There are 8 cervical cord segments but 7 cervical vertebrae. Cervical roots exit above their corresponding vertebra; below T1, they exit below.
• Gray matter is central in the cord, peripheral in the brain. White matter is the opposite.
• The Brown-Séquard pattern is highly localizing: ipsilateral motor and dorsal column loss, contralateral pain/temperature loss starting a few segments below the lesion.
• Anterior cord syndrome spares dorsal column function while abolishing motor and pain/temperature. Classical picture of anterior spinal artery infarction.
• Central cord syndrome typically affects arms more than legs, often after hyperextension injury in older patients with spondylosis.
• Cauda equina syndrome with bowel/bladder dysfunction is a surgical emergency. Saddle anesthesia is the cardinal sign.
• Conus medullaris syndrome differs from cauda equina: symmetric, mixed upper and lower motor neuron findings, early bladder/bowel involvement.
• Subacute combined degeneration from B12 deficiency produces both dorsal column and corticospinal signs, with a characteristic mixed reflex pattern.
• The artery of Adamkiewicz is the vascular Achilles’ heel of the lower cord. Injury produces anterior cord syndrome.

References

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5. Fehlings MG, Tetreault LA, Wilson JR, et al. A clinical practice guideline for the management of acute spinal cord injury. Global Spine J. 2017;7(Suppl 3):203S-211S.
6. Tarulli AW, Raynor EM. Lumbosacral radiculopathy. Neurol Clin. 2007;25(2):387-405.