🎓 Prepared by students from Boğaziçi University

What are White Matter Tract Pathways?

White matter tracts are bundles of myelinated axons that connect brain regions, enabling communication across the CNS. Major tracts link cortical areas, thalamus, basal ganglia, cerebellum, and brainstem, forming the 'wiring' of cognition and motor control.

Short answer

White matter tracts are neural highways — grouped myelinated fibres carrying signals between brain regions. Key tracts include commissural (corpus callosum), association (within hemisphere), and projection (cortex ↔ subcortex) tracts.

Major White Matter Tracts and Their Functions

1
Commissural Tracts
Corpus callosum (CC) transfers information between hemispheres. Largest tract in the brain; split into genu, body, splenium. Also: anterior commissure, posterior commissure
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2
Association Tracts (Within Hemisphere)
Arcuate fasciculus (language, Broca ↔ Wernicke), superior longitudinal fasciculus (SLF), superior fronto-occipital fasciculus (SFOF), cingulum (emotion, memory)
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3
Projection Tracts (Cortex ↔ Subcortex)
Corona radiata → internal capsule (motor, sensory fibres between cortex and brainstem). Thalamocortical tracts (sensory relay). Corticospinal tract (motor to spinal cord)
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4
Brainstem Tracts
Medial lemniscus (fine touch, proprioception ascending). Spinothalamic tract (pain, temperature ascending). Corticospinal tract (motor descending). Pyramids contain ~90% corticospinal fibres

Step-by-step worked examples

A patient has a stroke in the genu of the internal capsule. What deficits occur?

Genu of internal capsule contains corticobulbar fibres (facial motor, speech):
- Facial weakness (especially lower face)
- Dysarthria (speech slurring)
- If large stroke, also arm/leg weakness (adjacent motor fibres in posterior limb)
- Contralateral deficits (fibre crossing in brainstem)

Why does damage to Broca's area sometimes spare speech production if the arcuate fasciculus is intact elsewhere?

Arcuate fasciculus connects Broca (motor speech) to Wernicke (speech comprehension):
- Damage to Broca area → expressive (non-fluent) aphasia
- If arcuate fasciculus intact below Broca → some compensation via alternate routes
- MRI diffusion tensor imaging (DTI) shows tract integrity

A patient has a lesion in the corpus callosum (anterior part). What symptom might occur?

Corpus callosum anterior genu transfers information between frontal lobes:
- Left hand cannot respond to commands given to right ear (only heard by right hemisphere)
- Interhemispheric disconnection syndrome
- Right hemisphere sees command but left hemisphere (speech) cannot access it
- Classic split-brain phenomenon

Flashcards

Quick quiz

Q1.What separates the corpus callosum into anterior and posterior regions?

Correct answer: D. The corpus callosum has four named regions: rostrum, genu, body, and splenium (posteriormost). The genu and body form the anterior part, splenium the posterior.

Q2.A patient has a left middle cerebral artery stroke affecting the superior longitudinal fasciculus. Which language deficit is likely?

Correct answer: C. The arcuate fasciculus (part of SLF) connects Broca and Wernicke. Damage → conduction aphasia: comprehension and production intact, but repetition impaired.

Q3.Which white matter tract carries motor signals from motor cortex to the spinal cord?

Correct answer: B. The corticospinal tract descends from motor cortex, through the internal capsule, pyramids of medulla, crosses at the pyramid base (~90% cross), and innervates spinal cord motor neurons.

Q4.A patient with a thalamic stroke loses sensation (pain, temperature) on the contralateral body. Which ascending tract is affected?

Correct answer: B. Pain and temperature ascend via the spinothalamic tract (decussates in spinal cord), reaching the thalamus (VPL nucleus). Thalamic lesion → contralateral pain/temperature loss.

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Common mistakes

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Confusing white matter tracts with the tracts themselves (e.g., 'corticospinal' as a single structure). — Correct: Tracts are bundles of parallel axons. The corticospinal tract is one such bundle, passing through multiple structures (cortex → internal capsule → pyramids → medulla → spinal cord).

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Assuming all motor fibres cross at the same level. — Correct: ~90% of corticospinal fibres cross at the medullary pyramids (decussatio pyramidum), but ~10% remain ipsilateral (ventral corticospinal tract).

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Thinking the corpus callosum only transfers language or motor information. — Correct: The corpus callosum transfers ALL types of information between hemispheres: sensory, motor, cognitive, and emotional. Different parts have functional specialization.

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Forgetting that white matter lesions can be 'silent' if they don't cross functional pathways. — Correct: Deep white matter lesions may not cause obvious deficits if they don't damage major tracts. DTI helps identify which tracts are affected.

FAQ

Why is white matter called 'white'?

Myelin (the insulation around axons) is composed of lipids, which appear white in fresh brain tissue. Grey matter (cell bodies, unmyelinated) appears grey or dark.

What is myelin and why does it matter?

Myelin is a fatty sheath wrapping around axons, produced by oligodendrocytes in the brain. It insulates fibres, speeding action potentials (saltatory conduction). Demyelination (MS, Guillain-Barré) slows conduction and causes deficits.

How are white matter tracts classified?

Three types: (1) Commissural — connect hemispheres (corpus callosum). (2) Association — connect cortical regions within one hemisphere (arcuate fasciculus). (3) Projection — connect cortex to subcortical structures (corticospinal, thalamocortical).

Why is diffusion tensor imaging (DTI) valuable in neurosurgery?

DTI visualizes white matter tract paths and integrity. Surgeons use it to avoid disrupting critical tracts (motor, language) during tumour removal or epilepsy surgery, reducing neurological deficits.