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Behavioral Health & Nervous System
Major White Matter Tracts: Corticospinal, Dorsal Column, Spinothalamic
Core Principle of Major White Matter Tracts
White matter tracts are bundles of myelinated axons that connect different regions of the CNS, organized into ascending (sensory) and descending (motor) pathways.
The three highest-yield tracts for boards are the corticospinal tract (voluntary motor), dorsal column-medial lemniscal pathway (fine touch/vibration/proprioception), and spinothalamic tract (pain/temperature/crude touch).
Each tract has a characteristic decussation point, creating the fundamental principle: lesions above the decussation produce contralateral deficits, while lesions below produce ipsilateral deficits.
Understanding tract anatomy allows precise localization of CNS lesions based on the pattern of sensory and motor deficits.
Corticospinal Tract: Voluntary Motor Control
The corticospinal tract originates from the primary motor cortex (precentral gyrus), with contributions from premotor and supplementary motor areas.
Upper motor neurons descend through the posterior limb of the internal capsule → cerebral peduncle → basis pontis → medullary pyramid.
At the pyramidal decussation (cervicomedullary junction), 85-90% of fibers cross to form the lateral corticospinal tract; 10-15% remain ipsilateral as the anterior corticospinal tract.
Lateral corticospinal tract synapses on lower motor neurons in the ventral horn → exit via ventral roots to innervate skeletal muscle.
Board pearl: Corticospinal tract lesions produce upper motor neuron signs: hyperreflexia, spasticity, Babinski sign, but no fasciculations or atrophy.
Dorsal Column-Medial Lemniscal Pathway: Fine Touch and Proprioception
First-order neurons have cell bodies in dorsal root ganglia and ascend ipsilaterally in the dorsal columns: fasciculus gracilis (lower extremity, medial) and fasciculus cuneatus (upper extremity, lateral).
These synapse in the medulla at nucleus gracilis and nucleus cuneatus → second-order neurons decussate as internal arcuate fibers to form the medial lemniscus.
The medial lemniscus ascends through the brainstem to synapse in the ventral posterior lateral (VPL) nucleus of the thalamus.
Third-order neurons project from VPL to the primary somatosensory cortex (postcentral gyrus).
Board pearl: Dorsal column lesions produce ipsilateral loss of vibration, proprioception, and two-point discrimination below the lesion.
Spinothalamic Tract: Pain and Temperature
First-order neurons (cell bodies in dorsal root ganglia) synapse immediately upon entering the spinal cord in the substantia gelatinosa (Rexed laminae I-II).
Second-order neurons decussate in the anterior white commissure, ascending 1-2 segments before crossing.
The lateral spinothalamic tract (pain/temperature) and anterior spinothalamic tract (crude touch) ascend through the lateral brainstem.
These synapse in the VPL nucleus of the thalamus → third-order neurons project to somatosensory cortex.
Board pearl: Spinothalamic lesions produce contralateral loss of pain and temperature beginning 1-2 segments below the lesion due to the ascending decussation pattern.
Somatotopic Organization and the Sensory Homunculus
All three tracts maintain precise somatotopic organization throughout their course — neighboring body regions are represented in neighboring tract regions.
In the spinal cord, sacral fibers are most lateral in both spinothalamic and corticospinal tracts, while cervical fibers are most medial — creating the "onion skin" pattern.
This explains why central cord lesions affect upper extremities more than lower extremities (cervical fibers are central).
At the cortical level, the sensory and motor homunculi show disproportionate representation: face, hands, and feet have large cortical areas relative to their body surface area.
Board clue: A lesion affecting only the legs suggests parasagittal cortical involvement (leg area draped over the interhemispheric fissure).
Syringomyelia and Tract-Specific Deficits
Syringomyelia is a fluid-filled cavity (syrinx) within the spinal cord, classically associated with Chiari malformations.
The expanding syrinx first damages the anterior white commissure, interrupting decussating spinothalamic fibers → bilateral loss of pain and temperature in a cape-like distribution.
Dorsal columns and corticospinal tracts are initially spared, preserving vibration/proprioception and motor function — this dissociated sensory loss is pathognomonic.
As the syrinx expands, it may involve anterior horn cells (LMN signs in hands) and lateral corticospinal tracts (UMN signs in legs).
Board pearl: Bilateral loss of pain/temperature with preserved touch/vibration in the upper extremities = syringomyelia until proven otherwise.
Brown-Séquard Syndrome: Hemisection Pattern
Hemisection of the spinal cord produces a characteristic triad of deficits that perfectly demonstrates tract anatomy.
Ipsilateral findings below the lesion: corticospinal tract damage → weakness/UMN signs; dorsal column damage → loss of vibration/proprioception.
Contralateral findings below the lesion: spinothalamic tract damage → loss of pain/temperature beginning 1-2 segments below.
At the level of the lesion: ipsilateral LMN signs (damaged anterior horn) and bilateral pain/temperature loss (damaged crossing fibers).
Board pearl: The combination of ipsilateral motor/vibration loss with contralateral pain/temperature loss = Brown-Séquard syndrome.
Anterior Cord Syndrome and Vascular Anatomy
The anterior spinal artery supplies the anterior two-thirds of the spinal cord, including both corticospinal and spinothalamic tracts.
Anterior spinal artery occlusion → bilateral loss of motor function (corticospinal) and pain/temperature (spinothalamic) below the lesion.
The dorsal columns are supplied by posterior spinal arteries and remain intact → preserved vibration and proprioception.
Common causes include aortic surgery, hypotension, vasculitis, and sickle cell disease.
Board distinction: Anterior cord syndrome preserves dorsal column function; central cord syndrome preferentially affects upper extremities; Brown-Séquard shows ipsilateral/contralateral dissociation.
Central Cord Syndrome: Understanding Somatotopy
Central cord lesions (trauma, syrinx, tumor) damage the central gray matter and innermost white matter tracts.
Due to somatotopic organization (cervical fibers central, sacral fibers peripheral), upper extremity tracts are preferentially damaged.
Classic presentation: weakness and sensory loss worse in arms than legs, with bladder dysfunction (S2-S4 fibers are lateral and relatively spared).
Most commonly seen in elderly patients with cervical spondylosis who suffer hyperextension injuries.
Board pearl: Greater motor impairment in upper > lower extremities after neck trauma in an elderly patient = central cord syndrome.
Tabes Dorsalis and Selective Dorsal Column Damage
Tabes dorsalis (tertiary syphilis) selectively damages dorsal columns and dorsal roots → loss of proprioception and vibration.
Loss of proprioception produces sensory ataxia: wide-based gait, positive Romberg sign, and steppage gait (high-stepping to compensate for position sense loss).
Dorsal root involvement → absent deep tendon reflexes (afferent arc disrupted) and lightning pains (sharp, shooting pains in legs).
Argyll Robertson pupils (accommodate but don't react to light) are a classic associated finding.
Board pearl: Absent reflexes + positive Romberg + lightning pains in a patient with history of untreated syphilis = tabes dorsalis.
Vitamin B12 Deficiency and Subacute Combined Degeneration
B12 deficiency causes demyelination of both dorsal columns and lateral corticospinal tracts → "combined" degeneration.
Dorsal column involvement: loss of vibration/proprioception, sensory ataxia, positive Romberg.
Lateral corticospinal tract involvement: weakness, hyperreflexia, Babinski signs (UMN pattern).
The combination of absent ankle reflexes (peripheral neuropathy) with extensor plantar responses (UMN signs) is pathognomonic.
Board distinction: B12 deficiency affects both sensory and motor tracts; tabes dorsalis affects only dorsal columns; copper deficiency can mimic B12 but is much rarer.
Internal Capsule Anatomy and Lacunar Strokes
The posterior limb of the internal capsule contains corticospinal fibers arranged somatotopically: face (anterior), arm (middle), leg (posterior).
Small vessel disease → lacunar infarcts in the internal capsule → pure motor stroke affecting face, arm, and leg equally on the contralateral side.
No sensory deficits occur because thalamocortical sensory fibers run separately in the ventral posterior internal capsule.
Other lacunar syndromes: pure sensory (VPL thalamus), ataxic hemiparesis (basis pontis), dysarthria-clumsy hand (basis pontis).
Board pearl: Sudden onset of face/arm/leg weakness without sensory loss or cortical signs = lacunar stroke in posterior limb of internal capsule.
Medial Medullary Syndrome and Tract Convergence
Occlusion of the anterior spinal artery or paramedian branches → infarction of medial medulla.
Structures affected: pyramid (contralateral weakness), medial lemniscus (contralateral vibration/proprioception loss), hypoglossal nucleus/nerve (ipsilateral tongue deviation).
The combination of contralateral body weakness/sensory loss with ipsilateral tongue weakness localizes precisely to the medial medulla.
Lateral medullary syndrome (Wallenberg) spares these tracts but affects spinothalamic → dissociated sensory loss.
Board pearl: Contralateral hemiplegia + contralateral vibration loss + ipsilateral tongue deviation = medial medullary syndrome.
Lateral Medullary Syndrome (Wallenberg)
PICA occlusion → lateral medullary infarction affecting spinothalamic tract but sparing corticospinal and dorsal column pathways.
Ipsilateral findings: facial pain/temperature loss (spinal trigeminal tract), Horner syndrome, ataxia (inferior cerebellar peduncle), dysphagia/hoarseness (nucleus ambiguus).
Contralateral findings: body pain/temperature loss (lateral spinothalamic tract).
No weakness occurs because the corticospinal tract runs medially in the pyramid.
Board pearl: Loss of pain/temperature on ipsilateral face and contralateral body = lateral medullary syndrome.
Spinal Cord Compression and Tract Vulnerability
External compression (tumor, abscess, hematoma) affects different tracts in a predictable sequence based on their anatomical position.
Corticospinal tracts (lateral) are affected first → weakness and hyperreflexia below the lesion.
Spinothalamic tracts are affected next → establishment of a sensory level for pain/temperature.
Dorsal columns (posterior) are affected last → late loss of proprioception/vibration.
Autonomic fibers (lateral) → early bladder dysfunction is a red flag for cord compression.
Board pearl: New back pain + bilateral leg weakness + urinary retention + sensory level = spinal cord compression requiring emergent MRI.
Multiple Sclerosis and Tract-Specific Plaques
MS plaques have predilection sites that commonly affect white matter tracts: periventricular white matter, corpus callosum, brainstem, and spinal cord.
Dorsal column involvement → sensory ataxia, Lhermitte sign (electric shock down spine with neck flexion).
Corticospinal tract involvement → weakness, spasticity, hyperreflexia, Babinski signs.
Incomplete transverse myelitis may affect tracts asymmetrically → partial Brown-Séquard patterns.
Board pearl: Young woman with Lhermitte sign and internuclear ophthalmoplegia = MS affecting dorsal columns and MLF.
Friedreich Ataxia and Combined Tract Degeneration
Autosomal recessive trinucleotide repeat disorder causing degeneration of multiple tracts: dorsal columns, spinocerebellar tracts, and corticospinal tracts.
Dorsal column loss → sensory ataxia, absent vibration/position sense, positive Romberg.
Spinocerebellar tract loss → cerebellar ataxia, dysmetria, intention tremor.
Corticospinal involvement is mild initially but progresses → eventual wheelchair dependence.
Associated findings: hypertrophic cardiomyopathy, diabetes, pes cavus, hammer toes.
Board pearl: Teenage onset of ataxia + absent reflexes + Babinski signs + cardiomyopathy = Friedreich ataxia.
Amyotrophic Lateral Sclerosis (ALS) and Motor Tracts
ALS affects both upper motor neurons (corticospinal tract) and lower motor neurons (anterior horn cells) but spares sensory tracts entirely.
UMN signs: hyperreflexia, spasticity, Babinski signs, pseudobulbar affect.
LMN signs: muscle atrophy, fasciculations, weakness, hyporeflexia.
The combination of UMN and LMN signs in the same region is pathognomonic for ALS.
Cognition, sensation, and eye movements remain intact (oculomotor nuclei are spared).
Board pearl: Weakness with both fasciculations and hyperreflexia in the same limb = ALS; sensory involvement excludes the diagnosis.
Board Question Stem Patterns
Elderly patient with neck trauma and arm > leg weakness → central cord syndrome.
Ipsilateral weakness/vibration loss with contralateral pain/temperature loss → Brown-Séquard syndrome.
Bilateral loss of pain/temperature with preserved vibration in cape distribution → syringomyelia.
Pure motor stroke affecting face/arm/leg equally → lacunar infarct in internal capsule.
Lightning pains + absent reflexes + positive Romberg → tabes dorsalis.
Absent ankle reflexes with extensor plantar response → B12 deficiency (subacute combined degeneration).
Young woman with Lhermitte sign and optic neuritis → MS affecting dorsal columns.
One-Line Recap
The corticospinal (voluntary motor), dorsal column (vibration/proprioception), and spinothalamic (pain/temperature) tracts each have characteristic decussation points that create predictable patterns of ipsilateral versus contralateral deficits, enabling precise neuroanatomical localization through classic syndromes like Brown-Séquard (hemisection), syringomyelia (central cavity), and anterior cord syndrome (vascular compromise).
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