NervousSpinal structure

Spinal cord

Medulla spinalis

Also known as: cord, medulla spinalis

The caudal part of the CNS, a cylindrical cord ≈42–45 cm long running from the foramen magnum to about the L1–L2 vertebral level. It carries ascending sensory and descending motor tracts and contains the segmental circuits for spinal reflexes.

Show on body map Brain — Lateral View Brain — Midsagittal Section

Overview

Length (adult)≈42–45 cm

Mass≈30–35 g

Transverse diameter≈1 cm (≈1.3 cm at cervical enlargement)

Segments / spinal-nerve pairs31 (8 C, 12 T, 5 L, 5 S, 1 Co)

Termination (adult)L1–L2 disc (L3 at birth)

Dural sac endsS2 vertebra

The spinal cord continues the medulla oblongata below the foramen magnum and the pyramidal decussation. It occupies the upper two-thirds of the vertebral canal. In adults it ends as the conus medullaris at about the L1–L2 intervertebral disc (range T12–L3). It is slightly flattened anteroposteriorly, about 1 cm across, and swells at two enlargements where the limb plexuses arise: the cervical enlargement (C4–T1) for the brachial plexus and the lumbosacral enlargement (L2–S3) for the lumbar and sacral plexuses. Its surface is marked by the deep anterior median fissure, the shallow posterior median sulcus and the posterolateral sulci, where the dorsal rootlets enter. The cord is functionally divided into 31 segments, each giving a pair of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal.

In cross-section a butterfly-shaped core of grey matter (neuronal cell bodies, dendrites and synapses) surrounds the central canal. It is wrapped by white matter (myelinated tracts) organised into posterior, lateral and anterior funiculi. The proportion of white matter increases toward the head, because every ascending tract gains fibres and every descending tract has not yet lost any. Grey matter is most voluminous at the enlargements, where limb motor neurons are concentrated.

During development the vertebral column outgrows the cord (ascensus medullae). As a result, cord segments lie progressively higher than the vertebrae of the same number, and the lumbar and sacral roots must run steeply downward as the cauda equina to reach their foramina. This is why lumbar puncture below L2 is safe in adults. The cord is protected by the vertebral column, the three meninges, the CSF in the subarachnoid space, and the denticulate ligaments that suspend it laterally within the dural sac.

Location: Vertebral canal, from the foramen magnum (continuous with the medulla) to the L1–L2 vertebral level in adults (L3 at birth). The dural sac and subarachnoid space continue to S2.

Parts & subdivisions

Cervical segments (C1–C8)
Eight segments, but only seven cervical vertebrae: C1–C7 nerves exit above their vertebrae, and C8 exits between C7 and T1. They include the cervical enlargement (C4–T1) and the phrenic nucleus (C3–C5).
Thoracic segments (T1–T12)
The narrowest part of the cord, with relatively little grey matter. The lateral horn here (T1–L2) contains the sympathetic preganglionic neurons. Thoracic ventral rami form the intercostal nerves.
Lumbar segments (L1–L5)
They lie behind the T10–T12 vertebral bodies and form part of the lumbosacral enlargement, which supplies the lower limb.
Sacral and coccygeal segments (S1–S5, Co1)
Packed into the conus medullaris at about the L1 vertebra. S2–S4 contain the sacral parasympathetic nucleus and Onuf's nucleus (motor neurons for the external urethral and anal sphincters), which control bladder, bowel and sexual function.
Cervical and lumbosacral enlargements
Swellings at C4–T1 and L2–S3 caused by the extra anterior horn motor neurons and dorsal horn neurons serving the limbs. They are the origin of the brachial and lumbosacral plexuses.
Central canal
A narrow ependyma-lined channel that continues the fourth ventricle, often obliterated in adults. Pathological cavitation around it produces syringomyelia.
Denticulate ligaments
About 21 pairs of tooth-like lateral extensions of pia mater. They attach to the dura between the dorsal and ventral roots and anchor the cord in the middle of the subarachnoid space.

Anatomy

Arterial supply: One anterior spinal artery, formed from both vertebral arteries, runs in the anterior median fissure and supplies the anterior two-thirds of the cord (anterior and lateral horns, corticospinal and spinothalamic tracts). Two posterior spinal arteries, from the vertebral or posterior inferior cerebellar arteries, supply the posterior third, including the dorsal columns. These longitudinal channels are reinforced by segmental medullary arteries from the vertebral, deep cervical, posterior intercostal and lumbar arteries. The largest is the great anterior segmental medullary artery of Adamkiewicz. It usually arises on the left (≈75–90%) between T8 and L2, most often T9–T12, and supplies the lower two-thirds of the cord. The mid-thoracic cord is a watershed zone.
Venous drainage: Anterior and posterior spinal veins drain into the internal vertebral (epidural) venous plexus of Batson. This plexus is valveless and communicates with pelvic and thoracic veins, which is a route for metastatic spread from prostate and breast.
Relations: It is surrounded by pia mater, the subarachnoid space (CSF), arachnoid and dura mater. The epidural space, containing fat and the internal vertebral venous plexus, separates the dura from the vertebral canal. Anteriorly lie the vertebral bodies and intervertebral discs, covered by the posterior longitudinal ligament. Posteriorly lie the laminae and the ligamenta flava.
Histology: Grey matter holds neuronal cell bodies, dendrites, unmyelinated fibres, astrocytes and microglia, arranged into Rexed laminae I–X. White matter holds myelinated axons (oligodendrocyte myelin), fibrous astrocytes and blood vessels. The central canal is lined by ciliated ependymal cells.
Development: It forms from the caudal neural tube. The alar plates become the dorsal (sensory) horn and the basal plates become the ventral (motor) horn, separated by the sulcus limitans. Neural crest gives rise to the dorsal root ganglia. At 3 months of fetal life the cord fills the whole canal; it ends at about L3 at birth and reaches the adult L1–L2 level by about 2 years. Failure of posterior neuropore closure (day ≈26–28) causes spina bifida; maternal folate reduces the risk.

Function

Conducts ascending sensory information (touch, vibration, proprioception, pain, temperature) to the brain.
Conducts descending motor commands (corticospinal, reticulospinal, vestibulospinal, rubrospinal) to lower motor neurons.
Integrates spinal reflexes such as the stretch, withdrawal and crossed-extensor reflexes, independently of the brain.
Contains central pattern generators for rhythmic movements such as stepping.
Houses preganglionic autonomic neurons: sympathetic at T1–L2 and parasympathetic at S2–S4, controlling viscera, bladder, bowel and sexual function.

Clinical significance

Brown-Séquard syndrome (hemisection)

Below the lesion on the same side there is UMN weakness (spastic, hyperreflexic, Babinski +) and loss of vibration and proprioception, because the dorsal columns and lateral corticospinal tract are uncrossed in the cord. On the opposite side, pain and temperature are lost starting 1–2 segments below the lesion, because spinothalamic fibres cross near their entry level. At the level of the lesion there is ipsilateral LMN weakness and a band of total anaesthesia. It is usually caused by penetrating trauma, disc or tumour.

Anterior cord and central cord syndromes

Anterior spinal artery syndrome (aortic surgery, dissection, hypotension) causes bilateral paralysis and loss of pain and temperature below the lesion, with urinary retention. Vibration and proprioception are spared because the dorsal columns are supplied by the posterior spinal arteries. Central cord syndrome typically follows hyperextension in an elderly person with a spondylotic neck: the arms are weaker than the legs, with variable sensory loss. Syringomyelia damages the crossing spinothalamic fibres in the anterior white commissure, producing a bilateral "cape-like" loss of pain and temperature over the shoulders and arms. Touch is preserved (dissociated sensory loss), and anterior horn involvement adds hand wasting.

Complete transection and spinal shock

Immediately after acute transection there is flaccid paralysis, areflexia, loss of all sensation and autonomic failure below the level: this is spinal shock, lasting days to weeks. UMN signs (spasticity, hyperreflexia, Babinski) then emerge. Lesions at C3–C5 or above threaten the diaphragm ("C3, 4, 5 keeps the diaphragm alive"). Lesions above T6 risk autonomic dysreflexia: a noxious stimulus below the lesion, such as a full bladder, triggers a massive sympathetic discharge with severe hypertension and reflex bradycardia.

Test yourself

A man is stabbed in the back. Below the level of the wound he has right-sided spastic weakness and loss of vibration and joint-position sense on the right, with loss of pain and temperature sensation on the left. What is the lesion?

Anterior spinal artery occlusion
Central cord syndrome
Right hemisection of the spinal cord
Left dorsal column lesion

Answer: Right hemisection of the spinal cord

This is Brown-Séquard syndrome. The corticospinal tract and dorsal columns cross in the brainstem, so their loss is ipsilateral below the lesion. Spinothalamic fibres cross within 1–2 segments of entry, so pain and temperature are lost contralaterally.

Lumbar puncture in adults is performed at the L3/L4 or L4/L5 interspace because:

The dural sac ends at L2
Spinal nerves do not exist below L3
The subarachnoid space is widest at L5
The spinal cord usually ends at about L1–L2

Answer: The spinal cord usually ends at about L1–L2

The conus medullaris usually ends at L1–L2, but the dural sac and lumbar cistern continue to S2. Below L2 the needle enters a CSF space containing only the free-floating roots of the cauda equina. The line joining the iliac crests (Tuffier line) marks about L4.