Neuroanatomy
Spinal Cord
Anatomy and tracts — central highway carrying ascending sensation and descending motor commands between brain and body
The spinal cord runs from the medulla to about L1-L2 in adults (~45 cm), housed in the vertebral canal and ending as the conus medullaris with cauda equina nerve roots below. It has 31 segments (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal) each giving paired spinal nerves. Cross-section: central H-shaped gray matter (cell bodies) surrounded by white matter (myelinated tracts). Ascending tracts: dorsal columns (fine touch, vibration, proprioception), spinothalamic (pain, temperature, crude touch), spinocerebellar (proprioception to cerebellum). Descending tracts: corticospinal (voluntary motor), corticobulbar, rubrospinal, reticulospinal, vestibulospinal. Injury patterns (complete, hemisection/Brown-Séquard, central cord, anterior cord) localize from selective deficits.
- Length and termination~45 cm; ends at L1-L2 in adults
- Segments31 (C8, T12, L5, S5, Co1)
- Gray matterCentral, H-shaped, contains neuron cell bodies
- White matterPeripheral, contains myelinated ascending/descending tracts
- Major motor tractLateral corticospinal (decussates at medulla)
- Pain tractSpinothalamic (decussates within 1-2 segments)
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Why the spinal cord matters
- Trauma triage. Cervical injury threatens diaphragm function; immobilization is critical until cleared.
- Localization. Sensory level, motor pattern, and reflex changes pinpoint the cord level for imaging.
- Multiple sclerosis. Demyelinating plaques in cord cause sensory levels, Lhermitte sign, neurogenic bladder.
- Disc disease. Lumbar herniations cause radiculopathy (sciatica); cervical can cause myelopathy if cord is compressed.
- Cauda equina syndrome. Saddle anesthesia + urinary retention + bilateral weakness = surgical emergency.
- Anesthesia. Spinal blocks anesthetize below level; epidural delivers labor analgesia at L3-L4.
- Stroke and vascular. Anterior spinal artery infarcts spare dorsal columns but lose motor and pain/temp.
Common misconceptions
- Cord runs the full vertebral column. It ends at L1-L2 in adults; below is cauda equina.
- Lumbar puncture risks spinal cord injury. Performed below L2 to avoid the cord — only mobile nerve roots are present.
- All paralysis is the same. Upper motor neuron (spasticity) and lower motor neuron (flaccidity) patterns differ.
- Pain and touch share the same tract. Pain/temperature use spinothalamic; fine touch uses dorsal columns.
- Spinal injuries always present with paralysis from the start. Spinal shock initially produces flaccid areflexia even with upper motor neuron injury.
- Cord injury is irreversible. Incomplete injuries can recover substantially; central cord syndrome especially has good upper-extremity prognosis.
Frequently asked questions
What is the difference between dorsal columns and spinothalamic tracts?
Dorsal columns (gracilis from lower body, cuneatus from upper) carry fine touch, vibration, and proprioception. They ascend ipsilaterally, decussate in the medulla (medial lemniscus), and reach VPL of thalamus. Spinothalamic carries pain, temperature, crude touch — second-order neurons cross within 1-2 segments and ascend contralaterally. Hemisection (Brown-Séquard) loses ipsilateral fine touch and contralateral pain/temperature below the lesion.
How does the corticospinal tract work?
Upper motor neurons originate in primary motor cortex (Betz cells in M1), descend through internal capsule and brainstem, and ~85% decussate at the medullary pyramids to form the lateral corticospinal tract. They synapse on lower motor neurons in anterior horn at every spinal level. Lower motor neurons project via ventral roots to skeletal muscle. Upper motor neuron lesions cause spasticity, hyperreflexia, Babinski sign; lower motor neuron lesions cause flaccidity, atrophy, fasciculations.
Why does the cord end at L1-L2?
The vertebral column grows faster than the cord during development, so the cord lengthens proportionally less. Lumbar and sacral roots elongate within the canal as the cauda equina (horse's tail) below the conus medullaris. Lumbar puncture is performed at L3-L4 or L4-L5 to sample CSF without spearing the cord — only mobile nerve roots reside there. Cauda equina syndrome (massive disc herniation, tumor) presents with saddle anesthesia, urinary retention, bilateral leg weakness — surgical emergency.
What's a dermatome and why does it matter?
A dermatome is the strip of skin innervated by a single dorsal root. Key landmarks: C4 shoulder, C6 thumb, C7 middle finger, T4 nipples, T10 umbilicus, L1 inguinal, L4 medial calf, L5 dorsum of foot/great toe, S1 lateral foot, S2-S5 perineum/genitals. Localizes radiculopathy (nerve root compression by herniated disc) and shingles (varicella-zoster reactivates in dorsal root ganglion → unilateral dermatomal rash). Spinal cord level sensory loss differs from peripheral nerve loss.
What happens in spinal cord injury?
Acute injury causes spinal shock — flaccid paralysis, areflexia, autonomic dysfunction lasting days to weeks. Then upper motor neuron features emerge below the lesion. Complete injury: no motor or sensory function below level. Cervical injury (C5 most common) causes quadriplegia; high cervical (C3-C5) impairs phrenic nerve and diaphragm — ventilator dependence. Methylprednisolone (NASCIS protocol) is largely abandoned. Surgical decompression within 24 hours improves outcomes. Function is graded by ASIA scale.
What is the autonomic nervous system contribution from cord?
Sympathetic preganglionics arise from T1-L2 lateral horn (intermediolateral cell column) and project to paravertebral and prevertebral ganglia. Parasympathetic preganglionics arise from cranial nuclei (CN III, VII, IX, X) and S2-S4. Cord injury above T6 risks autonomic dysreflexia — noxious stimuli (bladder distention) trigger massive sympathetic discharge causing hypertension, bradycardia, headache, sweating above the lesion. Treatment: sit patient up, identify and remove the trigger.
How does the spinal cord generate reflexes?
Monosynaptic stretch reflex: muscle spindle Ia afferent → alpha motor neuron in same segment → muscle contraction (knee jerk via L3-L4). Polysynaptic withdrawal: nociceptive afferent → interneurons → flexor activation plus crossed extension. Reflexes test specific cord levels: biceps C5-C6, brachioradialis C6, triceps C7-C8, patellar L3-L4, Achilles S1-S2. Hyperreflexia indicates upper motor neuron lesion above the reflex arc; areflexia indicates lower motor neuron or sensory lesion at it.