The spinal cord is the major neural pathway linking the brain to the body. It extends from the foramen magnum to the L1–L2 vertebral level in adults, and its internal organization reveals a remarkably precise arrangement of gray matter nuclei and white matter tracts.
Gross Anatomy
The adult spinal cord measures approximately 45 cm in length and weighs about 30 g. It is protected by the vertebral column, meninges (dura mater, arachnoid mater, pia mater), and cerebrospinal fluid within the subarachnoid space.
External Features
The spinal cord has two regions of enlargement corresponding to the limb plexuses:
- Cervical enlargement (C4–T1). Contains the neurons innervating the upper limbs.
- Lumbosacral enlargement (T9–T12). Contains the neurons innervating the lower limbs.
The spinal cord tapers to a conical point called the conus medullaris at approximately L1–L2. Below this, the cauda equina (horse’s tail) consists of the lumbar, sacral, and coccygeal nerve roots descending to their respective exit levels. The filum terminale, a pial extension, anchors the spinal cord to the coccyx.
Segmental Organization
The spinal 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. Because the vertebral column grows faster than the spinal cord, the lower spinal segments lie at higher vertebral levels than their corresponding vertebrae. In adults:
- Cervical segments align with cervical vertebrae (C1–C7)
- Thoracic segments are slightly above their corresponding vertebrae
- Lumbar segments lie at T9–T11
- Sacral segments lie at T12–L1
This means that a lumbar disc herniation at L4–L5 typically compresses the L5 nerve root, not the L4 root.
Internal Organization
In cross-section, the spinal cord has a central H-shaped or butterfly-shaped region of gray matter surrounded by white matter.
Gray Matter
The gray matter is divided into three regions on each side:
Dorsal (posterior) horn. Contains sensory processing neurons. Axons entering through the dorsal root synapse here or in deeper laminae. The dorsal horn is organized into six laminae (I–VI) according to Rexed’s cytoarchitectonic scheme:
- Lamina I (marginal zone). Receives input from pain and temperature fibers (Aδ and C fibers).
- Lamina II (substantia gelatinosa). Modulates pain input; contains many interneurons.
- Laminae III and IV (nucleus proprius). Process touch and pressure sensation.
- Lamina V. Receives input from visceral afferents and deep somatic structures.
- Lamina VI. Present only in the cervical and lumbar enlargements; involved in proprioception.
Lateral horn (intermediolateral cell column). Present only at thoracic (T1–L2) and sacral (S2–S4) levels. Contains the cell bodies of preganglionic autonomic neurons:
- T1–L2: Sympathetic preganglionic neurons.
- S2–S4: Parasympathetic preganglionic neurons.
Ventral (anterior) horn. Contains lower motor neurons (LMNs) that innervate skeletal muscle. These are organized somatotopically: medial nuclei innervate axial muscles, lateral nuclei innervate limb muscles, and the most lateral nuclei innervate the distal limbs. The ventral horn is largest at the cervical and lumbar enlargements, where the most motor neurons are needed.
Gray commissure. Connects the two halves of gray matter. The central canal (remnant of the neural tube) runs through the center of the gray commissure.
Rexed Laminae and Functional Organization
The gray matter is divided into ten laminae (I–X) based on cytoarchitectonic criteria. Laminae I–VI form the dorsal horn, lamina VII is the intermediate zone (including the lateral horn), laminae VIII–IX form the ventral horn, and lamina X surrounds the central canal.
White Matter
The white matter is organized into three columns (funiculi) on each side:
- Dorsal (posterior) funiculus. Between the dorsal median septum and the dorsal horn.
- Lateral funiculus. Between the dorsal and ventral horns.
- Ventral (anterior) funiculus. Between the ventral horn and the ventral median fissure.
Each funiculus contains ascending (sensory) and descending (motor) tracts. The tracts are named according to their origin and termination (e.g., spinothalamic = from spinal cord to thalamus; corticospinal = from cerebral cortex to spinal cord).
Ascending Tracts (Sensory Pathways)
Dorsal Column–Medial Lemniscal Pathway
This pathway carries conscious proprioception, vibration sense, and discriminative (fine) touch. It is the most precisely organized of the sensory pathways.
First-order neurons. Pseudounipolar neurons in the dorsal root ganglia. Their central processes enter the spinal cord through the dorsal root and ascend in the dorsal funiculus without synapsing. Fibers from the lower body (below T6) enter the fasciculus gracilis (medially); fibers from the upper body (above T6) enter the fasciculus cuneatus (laterally).
Second-order neurons. In the nucleus gracilis and nucleus cuneatus of the medulla. Their axons decussate (cross the midline) as the internal arcuate fibers and form the medial lemniscus, which ascends to the thalamus.
Third-order neurons. In the ventral posterolateral (VPL) nucleus of the thalamus. They project to the primary somatosensory cortex.
Clinical correlate. Dorsal column damage causes ipsilateral loss of vibration and proprioception below the lesion. Patients with dorsal column loss cannot feel the position of their limbs without looking at them (sensory ataxia) and have a positive Romberg sign.
Spinothalamic Tract (Anterolateral System)
This pathway carries pain, temperature, and crude touch sensation.
First-order neurons. In the dorsal root ganglia. Their central processes enter the spinal cord and synapse in the dorsal horn (laminae I, II, and V) within one or two segments of entry.
Second-order neurons. In the dorsal horn. Their axons decussate in the anterior white commissure (within one segment of entering) and ascend in the lateral spinothalamic tract (pain and temperature) or the anterior spinothalamic tract (crude touch) to the thalamus.
Third-order neurons. In the VPL nucleus of the thalamus. They project to the primary somatosensory cortex.
Clinical correlate. Spinothalamic damage causes contralateral loss of pain and temperature sensation beginning one to two segments below the lesion. This pattern is classic for spinal cord hemisection (see Brown-Séquard syndrome below).
Spinocerebellar Tracts
These tracts carry unconscious proprioceptive information to the cerebellum for coordination of movement:
- Dorsal (posterior) spinocerebellar tract. Arises from Clarke’s nucleus (C8–L2). Ascends ipsilaterally in the lateral funiculus. Carries information from muscle spindles and Golgi tendon organs in the lower body.
- Ventral (anterior) spinocerebellar tract. Arises from the ventral horn. Decussates and ascends, then decussates again in the cerebellum, effectively remaining ipsilateral.
Descending Tracts (Motor Pathways)
Corticospinal Tract (Pyramidal Tract)
This is the primary pathway for voluntary, skilled movement. It originates from the cerebral cortex:
Origin. Approximately one-third from the primary motor cortex (BA 4), one-third from the premotor and supplementary motor areas (BA 6), and one-third from the primary somatosensory cortex (BA 3, 1, 2).
Course. The fibers converge in the corona radiata, pass through the posterior limb of the internal capsule, the cerebral peduncle of the midbrain, the basis pontis of the pons, and form the pyramids of the medulla. At the caudal medulla, approximately 85–90% of fibers decussate at the pyramidal decussation and descend in the lateral funiculus as the lateral corticospinal tract. The uncrossed fibers descend in the ventral funiculus as the anterior corticospinal tract, which decussates at the spinal level of termination.
Termination. Lateral corticospinal tract fibers synapse on LMNs in the ventral horn, either directly (particularly for distal limb muscles) or via interneurons. Anterior corticospinal tract fibers innervate axial and proximal muscles.
Extrapyramidal Tracts
These tracts originate from brainstem nuclei and modulate movement, posture, and tone:
- Rubrospinal tract. Originates from the red nucleus (midbrain). Facilitates flexor tone. Small in humans.
- Reticulospinal tracts. Originate from the reticular formation. Pontine (medial) tract facilitates extensors; medullary (lateral) tract inhibits extensors.
- Vestibulospinal tracts. Originate from the vestibular nuclei. Lateral tract facilitates anti-gravity muscles (extensors); medial tract controls head position.
- Tectospinal tract. Originates from the superior colliculus. Coordinates head and eye movements toward visual stimuli.
Spinal Cord Syndromes
Understanding spinal cord anatomy makes it possible to localize lesions based on clinical findings:
Brown-Séquard syndrome (hemisection). Ipsilateral loss of motor function and proprioception/vibration (corticospinal and dorsal columns) with contralateral loss of pain and temperature (spinothalamic) one to two segments below the lesion.
Anterior cord syndrome. Loss of motor function and pain/temperature below the lesion, with preservation of proprioception and vibration (dorsal columns preserved). Typically from anterior spinal artery occlusion.
Central cord syndrome. Motor weakness greater in the upper limbs than lower limbs, with variable sensory loss. Classically from syringomyelia or hyperextension injury in cervical spondylosis. The spinothalamic fibers that cross in the anterior white commissure are affected earliest.
Posterior cord syndrome. Loss of proprioception and vibration below the lesion, with preserved motor function and pain/temperature sensation. Rare, from dorsal column damage (tabes dorsalis in tertiary syphilis).
Conus medullaris syndrome. Early bladder and bowel dysfunction, saddle anesthesia (S2–S4), and lower extremity weakness. From compression of the conus at L1–L2.
Cauda equina syndrome. Lower extremity weakness and sensory loss, radicular pain, and bladder/bowel dysfunction. Unlike conus syndrome, the onset is often unilateral and more gradual. This is a surgical emergency — decompression within 24–48 hours is critical.
Blood Supply of the Spinal Cord
The spinal cord receives blood from three longitudinal arteries:
Anterior spinal artery. Arises from the vertebral arteries at the foramen magnum. Supplies the anterior two-thirds of the spinal cord, including the ventral horns, spinothalamic tracts, and corticospinal tracts. Occlusion produces anterior cord syndrome.
Posterior spinal arteries (paired). Arise from the vertebral or posterior inferior cerebellar arteries. Supply the posterior one-third, including the dorsal columns and dorsal horns.
Segmental (radicular) arteries. Enter through the intervertebral foramina and reinforce the longitudinal arteries. The largest is the artery of Adamkiewicz (great radicular artery), which arises from a lower intercostal or lumbar artery (typically T9–L2 on the left side) and supplies the lower two-thirds of the spinal cord. Damage to this artery during aortic surgery can produce spinal cord infarction.
Summary
The spinal cord is a remarkably organized structure in which gray matter nuclei and white matter tracts are arranged in a precise, consistent pattern. Sensory information ascends through the dorsal columns (fine touch, vibration, proprioception) and spinothalamic tracts (pain, temperature). Motor commands descend through the corticospinal and extrapyramidal tracts. The segmental organization, with 31 pairs of spinal nerves, provides a detailed map for clinical localization. Understanding the internal anatomy allows clinicians to diagnose spinal cord lesions with remarkable precision based on the pattern of sensory, motor, and autonomic deficits.