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The Spinal Cord: Complete Guide to Structure, Tracts, and Clinical Correlations

Sources: Gray's Anatomy for Students; Neuroanatomy through Clinical Cases 3rd Ed; Medical Physiology (Boron & Boulpaep); Adams & Victor's Principles of Neurology 12th Ed

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PART 1: GROSS ANATOMY AND EXTERNAL FEATURES

• Cervical enlargement (C5-T1): Accommodates the brachial plexus innervating the upper limb
• Lumbosacral enlargement (L1-S3): Accommodates the lumbosacral plexus innervating the lower limb

• Anterior median fissure: Deep midline groove anteriorly, contains the anterior spinal artery
• Posterior median sulcus: Shallow midline groove posteriorly
• Posterolateral sulcus (bilateral): Where dorsal roots enter; separates posterior and lateral funiculi
• Anterior lateral sulcus: Where ventral roots exit
• Posterior intermediate sulcus (cervical only): Divides the posterior funiculus into fasciculus gracilis (medial) and fasciculus cuneatus (lateral)
• Lissauer's tract (posterolateral tract): Small, visible zone at the dorsal root entry zone

• Dura mater (tough outer layer)
• Arachnoid mater
• Subarachnoid space (contains CSF - clinically important for lumbar puncture)
• Pia mater (closely applied to cord surface)

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PART 2: INTERNAL STRUCTURE

2A. Gray Matter (Inner H-shape or "Butterfly")

• Medial motor nuclei: innervate axial and girdle muscles
• Lateral motor nuclei: present in cervical and lumbosacral enlargements; innervate distal limb muscles

• The T1-L2 lateral horn gives rise to preganglionic sympathetic fibers
• S2-S4 gives rise to preganglionic parasympathetic fibers (pelvic splanchnic nerves)

2B. Rexed's Laminae (10 zones of cytoarchitecture)

2C. White Matter

• Anterior (ventral) funiculus: Between the anterior median fissure and anterior lateral sulcus. Contains mainly motor (descending) axons.
• Posterior (dorsal) funiculus: Between the posterior median sulcus and posterior lateral sulcus. Contains sensory (ascending) axons.
• Lateral funiculus: Between anterior and posterior lateral sulci. Contains both ascending sensory and descending motor tracts.

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PART 3: ASCENDING TRACTS (Sensory Pathways)

"Two somatosensory pathways ascend within the spinal cord to reach the cortex: the anterolateral pathways, which convey sensations of pain, temperature, and crude touch; and the posterior column-medial lemniscal pathway, which conveys sensations of discriminative or fine touch, vibration, and conscious proprioception." - Gray's Anatomy for Students

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3A. ANTEROLATERAL PATHWAYS (Spinothalamic System)

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3A.1 Lateral Spinothalamic Tract (Neospinothalamic)

• Cell body in the dorsal root ganglion (DRG)
• Peripheral process: receives input from free nerve endings (pain, temperature)
  • A-delta (thinly myelinated): sharp, fast pain and cold temperature
  • C fibers (unmyelinated): dull, burning pain and warm temperature
• Central process enters spinal cord via the posterolateral sulcus (dorsal root entry zone)
• Travels 1-2 segments up or down in Lissauer's tract (posterolateral tract of Lissauer) before synapsing
• Synapses on 2nd-order neurons in dorsal horn laminae I and V (and some in laminae IV and VII)

• Cell body in dorsal horn (laminae I and V)
• Axon crosses obliquely through the anterior (ventral) white commissure to the contralateral side
• Critically: this crossing takes 2-3 spinal cord segments, so the spinothalamic tract represents contralateral pain/temperature beginning 2-3 segments below the level of entry
• After crossing, axons ascend in the anterolateral white matter as the spinothalamic tract (lateral portion)
• Somatotopic arrangement: sacral fibers are MOST lateral, cervical fibers are most medial (fibers from the anterior commissure add on medially as they ascend)
• Continues through medulla (lateral position, between olive and inferior cerebellar peduncle), pons (just lateral to medial lemniscus), midbrain tegmentum
• Synapses in the ventral posterior lateral (VPL) nucleus of the thalamus

• Cell body in VPL thalamus
• Projects via posterior limb of the internal capsule and corona radiata
• Terminates in primary somatosensory cortex (postcentral gyrus, Brodmann areas 3, 1, 2)
• Also sends collaterals to secondary somatosensory cortex in the parietal operculum

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3A.2 Spinoreticular Tract (Paleospinothalamic)

• 1st-order neuron: same as spinothalamic
• 2nd-order neuron: Cell body in dorsal horn laminae I, V, VII, VIII
• Axons cross (mostly) to the contralateral side and ascend in the anterolateral system
• Terminate in the medullary and pontine reticular formation
• From there, neurons project to intralaminar thalamic nuclei (central lateral nucleus) and medial thalamic nuclei (mediodorsal nuclei)
• Final projections reach widespread cortical areas and the limbic system (accounting for the emotional component of pain)

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3A.3 Spinomesencephalic Tract (Spinotectal)

• 2nd-order neurons in dorsal horn laminae I, IV, V
• Cross and ascend with the spinothalamic tract
• Terminate in the periaqueductal gray matter (PAG) and the superior colliculus of the midbrain
• The PAG is critical for descending pain modulation (stimulation produces analgesia via serotonergic and noradrenergic descending pathways)
• The superior colliculus component helps mediate visual-nociceptive reflexes

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3B. POSTERIOR COLUMN - MEDIAL LEMNISCAL PATHWAY (PCML)

• Cell body in the DRG
• Peripheral process from specialized mechanoreceptors (Meissner's corpuscles, Pacinian corpuscles, Merkel's discs, muscle spindles, Golgi tendon organs)
• Central process enters the spinal cord and immediately ascends ipsilaterally in the posterior funiculus (no synapse in the cord itself)
• Fasciculus gracilis (medial): Carries information from the lower limb and lower trunk (T7 and below). Present throughout the cord.
• Fasciculus cuneatus (lateral): Carries information from the upper limb, upper trunk, and neck (T6 and above). Only present above T6.

• Fasciculus gracilis terminates in nucleus gracilis in the caudal medulla
• Fasciculus cuneatus terminates in nucleus cuneatus in the caudal medulla
• 2nd-order axons sweep anteromedially as internal arcuate fibers and decussate in the caudal medulla
• After decussation, these fibers form the medial lemniscus on the contralateral side
• The medial lemniscus ascends through the brainstem (lies medially in the medulla, rotates laterally as it ascends through pons and midbrain)
• Terminates in the VPL nucleus of the thalamus

• VPL projects via posterior limb of internal capsule to primary somatosensory cortex (postcentral gyrus)
• Somatotopic map: foot and leg most medial (near interhemispheric fissure), face most lateral

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3C. SPINOCEREBELLAR TRACTS (Subconscious Proprioception)

3C.1 Dorsal (Posterior) Spinocerebellar Tract

• 1st-order neuron: DRG
• 2nd-order neuron: Clarke's nucleus (nucleus thoracicus dorsal) - located in lamina VII at C8-L3
• Axons ascend ipsilaterally (uncrossed) in the lateral funiculus as the dorsal spinocerebellar tract
• Enters the cerebellum via the inferior cerebellar peduncle
• Terminates in the ipsilateral cerebellar cortex (vermis)

3C.2 Ventral (Anterior) Spinocerebellar Tract

• 2nd-order neurons in laminae V-VII (spinal border cells)
• Most axons cross in the anterior commissure and ascend contralaterally in the lateral funiculus
• Ascend all the way to the pons and midbrain
• Enter the cerebellum via the superior cerebellar peduncle, then re-cross within the cerebellum
• Net result: the information reaches the ipsilateral cerebellum (double crossover)

3C.3 Cuneocerebellar Tract (Rostral Spinocerebellar Tract)

• 2nd-order neurons in the accessory (external) cuneate nucleus in the medulla
• Ascends ipsilaterally via the inferior cerebellar peduncle to the cerebellar cortex

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3D. SPINOCERVICAL TRACT

• Axons ascend ipsilaterally in the dorsolateral funiculus to the lateral cervical nucleus (at C1-C2)
• Then cross and join the medial lemniscus to reach the thalamus
• Function: fine touch and proprioception (a phylogenetically older alternative to the PCML)

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PART 4: DESCENDING TRACTS (Motor Pathways)

"Descending tracts through the spinal cord are involved in voluntary movements; postural movements; and coordination of head, neck, and eye movements. These pathways originate from the cerebral cortex and brainstem and are influenced by sensory input and feedback from the cerebellum and basal ganglia." - Gray's Anatomy for Students

1. Lateral motor system - controls distal limb muscles (skilled voluntary movement)
2. Medial motor system - controls axial and proximal muscles (posture, balance, gait)

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LATERAL MOTOR SYSTEM

4A. Lateral Corticospinal Tract (LCST) - THE PYRAMIDAL TRACT

1. Axons descend through the corona radiata → posterior limb of the internal capsule (somatotopically: face anterior, arm middle, leg posterior) → crus cerebri (middle three-fifths) of the midbrain
2. As the tract passes through the pons, it breaks up into small fascicles to accommodate transverse pontocerebellar fibers
3. Re-forms into the pyramids at the ventral medulla (visible as longitudinal ridges on the anterior surface of the medulla)
4. At the cervicomedullary junction, approximately 85-90% of fibers decussate as the pyramidal (motor) decussation → become the lateral corticospinal tract in the contralateral lateral funiculus
5. Remaining 10-15% do NOT decussate and instead descend as the anterior corticospinal tract (see medial system)
6. LCST descends the entire length of the cord in the lateral funiculus
7. Synapses:
   • Directly on alpha motor neurons (lamina IX) - especially for fine distal movements
   • On interneurons (lamina VII, VIII) - for most proximal movements

• Cell bodies in lamina IX (anterior horn)
• Exit via ventral roots → spinal nerves → peripheral nerves → neuromuscular junction

• Cervical fibers (arm): medial
• Lumbar fibers (leg): lateral

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4B. Rubrospinal Tract

• Origin: Magnocellular division of the red nucleus in the midbrain tegmentum
• Axons immediately decussate at the level of origin as the ventral tegmental decussation (in the midbrain)
• Descend in the lateral funiculus
• Terminate only on cervical cord interneurons (anterior horn, lamina V-VII)
• Synapse on interneurons facilitating flexor muscles and inhibiting extensor muscles of the upper limb

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MEDIAL MOTOR SYSTEM

4C. Anterior (Ventral) Corticospinal Tract

• Origin: Primary motor cortex and SMA
• The 10-15% of corticospinal fibers that do NOT decussate at the pyramidal decussation
• Descend ipsilaterally in the anterior funiculus
• Terminate only in cervical and upper thoracic cord
• Most fibers eventually cross in the anterior white commissure at the segmental level before synapsing on interneurons in lamina VIII (and some lamina IX)
• Projects bilaterally to medial anterior horn motor neurons controlling axial muscles

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4D. Vestibulospinal Tracts

• Origin: Lateral vestibular nucleus (Deiters' nucleus) in the pons/medulla
• Input to lateral vestibular nucleus: ipsilateral cerebellum and vestibular labyrinth
• Course: Descends ipsilaterally (uncrossed) through the entire length of the cord
• Terminates on interneurons in laminae VII-VIII (facilitates extensor alpha and gamma motor neurons)
• When you stumble, the LVST activates extensors to prevent falling

• Origin: Medial and inferior vestibular nuclei
• Travels within the medial longitudinal fasciculus (MLF)
• Projects bilaterally to cervical and upper thoracic cord only
• Function: Reflexive control of head and neck position

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4E. Reticulospinal Tracts

• Origin: Nucleus reticularis pontis caudalis and oralis (pontine reticular formation)
• Descends ipsilaterally (largely) in the anterior funiculus
• Facilitates extensor tone (reinforces vestibulospinal facilitation of extensors)
• Terminates on laminae VII-VIII interneurons throughout the cord

• Origin: Nucleus reticularis gigantocellularis (medullary reticular formation)
• Descends bilaterally in the lateral funiculus
• Inhibits extensor tone (opposes the pontine reticulospinal)
• Also important for transmission of descending pain modulation signals

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4F. Tectospinal Tract

• Origin: Superior colliculus of the midbrain (visual reflex center)
• Axons decussate immediately as the dorsal tegmental decussation in the midbrain
• Descend only to cervical cord (C4 and above)
• Terminate on interneurons in laminae VI-VIII
• When a sudden visual stimulus appears to your side, the tectospinal tract reflexively turns your head toward it

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4G. Medial Longitudinal Fasciculus (MLF) in the Cord

• Descending vestibulospinal fibers (medial vestibulospinal tract)
• Connections from cranial nerve nuclei to cervical cord
• Function: Integrates eye, head, and neck movements for coordinated gaze

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Motor Tract Summary Tables

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PART 5: INTERSEGMENTAL TRACTS (Propriospinal Fibers)

What Are They?

Location

• Anterior fasciculus proprius: around the anterior horn
• Lateral fasciculus proprius: around the lateral horn
• Posterior fasciculus proprius: between the posterior horn and the posterior funiculus

Types Based on Length

1. Short intersegmental fibers: Connect adjacent 1-2 segments. Coordinate movements of closely related spinal cord segments (e.g., shoulder and elbow muscles during reach-and-grasp)
2. Long intersegmental fibers: Connect many segments, including connections between cervical and lumbosacral cord. Coordinate upper and lower limb movements (e.g., arm-leg coupling during walking/swimming)

Functions

1. Locomotor pattern generation: Propriospinal neurons are a key component of central pattern generators (CPGs) - neural circuits in the spinal cord that generate rhythmic, alternating limb movements for walking. The CPG can produce coordinated stepping without input from the brain (as demonstrated in spinalized animal preparations).
2. Interlimb coordination: Long propriospinal neurons in the cervical cord project to the lumbosacral cord to coordinate upper and lower limb movements during gait (e.g., the natural arm-swing that coordinates with leg movement during walking).
3. Reflex integration: Intersegmental reflexes (e.g., crossed extension reflex, long-spinal reflexes) are mediated through these fibers.
4. Relay for descending commands: Some descending motor commands (e.g., from the reticulospinal tract) reach spinal motor neurons not directly but via propriospinal relay neurons. This is particularly important for proximal limb movements.
5. Relay for ascending sensory: Some sensory information ascends via propriospinal chains rather than the dedicated sensory tracts.

The C3-C4 Propriospinal System (Clinically Important)

• Cell bodies in the C3-C4 segments
• Receive direct input from the corticospinal tract as well as reticulospinal inputs
• Project to motor neurons in the C6-Th1 segments controlling arm muscles
• Proposed to mediate a "propriospinal relay" in humans for reaching movements
• After corticospinal tract injury (e.g., stroke, spinal cord injury), these neurons can potentially mediate motor recovery by acting as an alternate relay

Lissauer's Tract (Posterolateral Tract)

• Located at the tip of the posterior horn (dorsal root entry zone)
• Contains:
  • Collateral branches of entering primary sensory axons (A-delta and C fibers from DRG) that ascend or descend 1-2 segments before entering the dorsal horn
  • Propriospinal fibers from dorsal horn neurons
• Function: Allows one nociceptive stimulus to activate a broader zone of dorsal horn cells ("spreading" of pain signals). This is the basis for referred pain and pain radiation patterns.

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PART 6: VASCULAR SUPPLY TO THE SPINAL CORD (Clinical Relevance)

• Anterior spinal artery (ASA): Single midline vessel; formed by union of two branches from the vertebral arteries; runs in the anterior median fissure; supplies the anterior 2/3 of the cord, including the anterior horns, spinothalamic tracts, and corticospinal tracts (but NOT the posterior columns)
• Posterior spinal arteries (PSA): Two vessels (bilateral) running along the posterolateral sulci; branches of PICA or vertebral artery; supply the posterior 1/3 of the cord, including the posterior columns
• Artery of Adamkiewicz (arteria radicularis magna): The largest segmental medullary artery, usually at T9-L2 (predominantly left side); provides the dominant supply to the lower thoracic and lumbar cord

• Anterior and posterior spinal veins drain into a venous plexus (Batson's plexus), which connects to the azygous system

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PART 7: CLINICAL CORRELATIONS

7A. Spinal Cord Injury (SCI) - Level and Completeness

• Loss of all sensory modalities (both posterior column and spinothalamic)
• Upper motor neuron signs (spasticity, hyperreflexia, extensor plantar response/Babinski sign) - develop after initial spinal shock
• Loss of bladder, bowel, and sexual function
• Autonomic dysfunction

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7B. Brown-Sequard Syndrome (Hemisection of Spinal Cord)

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7C. Anterior Cord Syndrome (Anterior Spinal Artery Syndrome)

• Bilateral loss of pain and temperature below the lesion (both spinothalamic tracts)
• Bilateral motor weakness (UMN below; LMN at level)
• PRESERVED: Fine touch, vibration, and proprioception (posterior columns supplied by posterior spinal arteries are SPARED)

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7D. Posterior Cord Syndrome

• Loss of ipsilateral proprioception, vibration, fine touch
• Preserved: Pain, temperature, and motor function
• Results in sensory ataxia - wide-based gait, positive Romberg sign (falls when eyes closed)

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7E. Central Cord Syndrome

• Bilateral loss of pain and temperature at the level of the lesion (crossing spinothalamic fibers in anterior commissure are damaged) - "cape-like" distribution
• Bilateral weakness: arms > legs (because corticospinal tract is somatotopically organized: arm fibers are more central/medial, leg fibers are more peripheral/lateral)
• Bladder dysfunction (urinary retention)
• Partial preservation of sacral sensation (sacral LCST fibers are peripheral, spared)

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7F. Syringomyelia

1. Early: Suspended (cape-like) bilateral loss of pain and temperature at the level of the syrinx (crossing spinothalamic fibers in anterior commissure destroyed) - with PRESERVED touch (posterior columns intact). This selective loss of pain/temperature with preserved touch is called dissociated sensory loss.
2. As syrinx expands:
   • LMN signs at level of lesion (wasting, fasciculations, weakness) due to anterior horn involvement
   • UMN signs below due to lateral corticospinal involvement
   • Autonomic features: Horner's syndrome (if cervical: ptosis, miosis, anhidrosis) due to damage to the intermediolateral column/descending sympathetic fibers
3. Late: Posterior column involvement - proprioceptive loss

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7G. Subacute Combined Degeneration of the Spinal Cord (SCD)

• Posterior column: Proprioceptive loss, vibration loss, sensory ataxia, Romberg positive
• Lateral corticospinal: UMN weakness, hyperreflexia, Babinski sign
• Initially: Absent knee jerks may occur due to associated peripheral neuropathy (LMN), giving a mixed UMN/LMN picture
• Peripheral neuropathy (stocking-glove sensory loss)

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7H. Tabes Dorsalis

• Severe proprioceptive loss → tabetic gait (high-stepping, stamping gait)
• Positive Romberg sign
• Loss of vibration and joint position sense
• Lancinating (lightning) pains (sudden, severe, brief pains in limbs/trunk)
• Charcot's joints (neuropathic arthropathy due to proprioception loss)
• Loss of deep tendon reflexes (dorsal root involvement)
• Argyll Robertson pupils (miosis, irregular, no reaction to light but react to accommodation)

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7I. Amyotrophic Lateral Sclerosis (ALS)

• UMN signs: Spasticity, hyperreflexia, Babinski, pseudobulbar palsy
• LMN signs: Fasciculations, muscle wasting, weakness
• Bulbar signs: Dysarthria, dysphagia (corticobulbar involvement)
• Sensory spared (pure motor system disease)
• The coexistence of UMN and LMN signs in the same muscle group is the diagnostic hallmark

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7J. Conus Medullaris Syndrome vs. Cauda Equina Syndrome

• Mixed UMN + LMN bladder (hyperreflexic initially, then flaccid)
• Saddle anesthesia (S2-S5 dermatomes)
• Erectile dysfunction
• Absent bulbocavernosus reflex
• Relatively preserved lower limb strength

• Pure LMN: Flaccid paralysis, areflexia
• Severe saddle anesthesia
• Urinary retention (overflow incontinence)
• Bilateral sciatica
• Less symmetric pattern than conus

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7K. Spinal Cord Compression (Epidural)

1. Pain (local and radicular)
2. Motor weakness (UMN below, LMN at level)
3. Sensory loss (posterior column and spinothalamic)

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7L. Horner's Syndrome from Spinal Cord Lesion

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7M. Upper vs. Lower Motor Neuron Lesion - Key Clinical Differences

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7N. Spinothalamic Tract - Clinical Testing

• Pinprick and temperature testing are used clinically to test the spinothalamic/anterolateral system
• Loss of pain/temperature with preserved touch = dissociated sensory loss → suggests central cord pathology (syrinx) or Brown-Sequard
• The 2-3 segment lag: After a spinal cord lesion, the spinothalamic sensory level is typically 2-3 segments BELOW the actual lesion level (because fibers cross obliquely over 2-3 segments before ascending)

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7O. Descending Pain Modulation - Gate Control Theory

• Large myelinated fibers (A-beta: carrying touch) activate inhibitory interneurons in lamina II that "close the gate" to pain signals from C fibers
• This is why rubbing a bruised area reduces pain
• Descending pathways from the PAG (via spinomesencephalic tract afferents) and the raphe nuclei release serotonin and enkephalins to suppress pain at lamina I, II, V
• The locus coeruleus descending noradrenergic fibers similarly suppress pain
• This system is the basis for opioid analgesia (mu receptors abundant in laminae I, II, and PAG)

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Summary: Tract Quick Reference

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