WHITE MATTER LECTURE PDF

White Matter Tract Anatomy

Overview

• Presented by: Damjan Veljanoski, Developmental Neurosciences, Great Ormond Street, Institute of Child Health.

• Outline:

  • Definitions

  • DTI (Diffusion Tensor Imaging)

  • Types of fibers: Projection, Association, Commissural

Definitions

1. Projection Fibers:

   • Connect cortical areas with:

     • Deep grey nuclei

     • Brain stem

     • Cerebellum

     • Spinal cord

2. Association Fibers:

   • Connect different cortical areas within the same hemisphere.

3. Commissural Fibers:

   • Connect similar cortical areas across the two hemispheres.

4. Fasciculus:

   • A bundle of myelinated nerve fibers (axons).

Types of White Matter Tracts

1. Projection Fibers:

• Major tracts include:

  • Corticospinal

  • Corticobulbar

  • Corticopontine

  • Thalamic radiations

  • Optic radiation (geniculocalcarine fibers)

2. Association Fibers:

• Major tracts include:

  • Short-range subcortical ‘U-fibres’

  • Long-range fibers:

  • Cingulum

  • Superior and Inferior Occipitofrontal Fasciculus

  • Superior Longitudinal Fasciculus

  • Arcuate Fasciculus

  • Inferior Longitudinal Fasciculus

3. Commissural Fibers:

• Major tracts include:

  • Corpus Callosum

  • Anterior Commissure

Diffusion Tensor Imaging (DTI)

• Color Coding of Fibers:

  • Green: anteroposteriorly oriented fibers

  • Red: right-left oriented fibers

  • Blue: superior-inferior oriented fibers

• Important Note:

  • Corticospinal, corticobulbar, and corticopontine tracts run together, making individual identification challenging on DTI.

Specific White Matter Tracts

Corticospinal Tract:

• Function: Motor area to spinal cord.

• Origin: In 71% of children, fibers originate from both precentral and postcentral gyrus.

• Color Coding for DTI Visualization:

  • Green: anteroposteriorly oriented fibers

  • Red: right-left oriented fibers

  • Blue: superior-inferior oriented fibers.

• Reference: Wycoco V, Shroff M, Sudhakar S, Lee W. Neuroimaging Clin N Am. 2013.

Internal Capsule:

• Description: Main conduit for projection fibers.

• Sections: Divided into three distinct sections.

Case Examples:

• Acute Weakness in Young Adult: Right-sided weakness highlighted.

• Corticobulbar Tract:

  • Connects cortical areas to cranial nerve nuclei in the brainstem.

• Symptoms in Case:

  • 36 y/o female with SLE presenting bulbar symptoms including:

  • Impaired memory

  • Slurred speech

  • Swallowing difficulties.

  • Reference: Kim HK, Han M, Lee HJ. Iran J Radiol. 2016.

Corticopontine Tract:

• Pathway: From cortex to pons and then to cerebellum.

Geniculocalcarine Tract (Optic Radiation):

• Connection: Links the lateral geniculate body (LGB) to the primary visual cortex.

• Bundles: Composed of three bundles:

  • Anterior

  • Meyer’s loop

  • Central

  • Posterior

• Surgical Consideration: Damage can lead to homonymous upper quadrantanopia.

Thalamic Radiations:

• Reference: Details not elaborated but mentioned in Wycoco V et al. 2013.

Association Fibers Overview

• Short-Range U Fibers and Long Fibers:

  • Components:

  • Superior longitudinal fasciculus (SLF)

  • Inferior longitudinal fasciculus (ILF)

  • Middle longitudinal fasciculus (MLF)

  • Uncinate fasciculus

  • Superior fronto-occipital fasciculus (SFOF)

  • Inferior fronto-occipital fasciculus (IFOF)

Arcuate Fasciculus:

• Connections: Connects frontal lobe, supramarginal gyrus, posterior part of the superior temporal gyrus, and temporo-occipital region.

• Impact of Damage: Decreases the ability to repeat spoken language; may cause unilateral neglect.

Subcomponents of Long Association Fibers:

• SLF:

  • Characteristics: Largest, bidirectional, consists of 4 components including the arcuate fasciculus.

  • Connections: Connects frontal lobe with parietal, temporal, and occipital lobes.

• ILF:

  • Function: Links the cortices to anterior temporal lobe and posterior occipital lobe. joins the inferior aspect of the SLF,

    optic radiations, and inferior

    longitudinal fasciculus to form the

    sagittal stratum traversing the occipital lobe.

  • Consequences of Disruption: May lead to unilateral visual neglect, visual amnesia, hallucinations, and visual hypoemotionality.

• MLF:

  • Pathway: From the inferior parietal lobule (angular gyrus) to the white matter of the superior temporal gyrus.

  • Role: Central in language processing (dominant hemisphere) and attention (non-dominant hemisphere).

Cognitive and Linguistic Processes: (Hickok and Poeppel)

• Dorsal Stream:

  • Function: Repetition of speech, maps acoustic speech to articulatory representations.

  • Involves the arcuate fasciculus and superior longitudinal fasciculus.

• Ventral Stream:

  • Function: Listening to meaningful speech, maps sound-based representation of speech to conceptual representations.

  • Involves EmC, MLF, and ILF.

SFOF and IFOF:

• SFOF Misnomer:

  • Connects the frontal and parietal lobes, aids spatial awareness and symmetric processing.

• IFOF:

  • Involvement in disease spread between frontal and temporal lobes, potentially triggering temporal lobe syndromes (e.g., amnesia, schizophrenia, Alzheimer’s).

  • Reference: Yeh, F. C., et al. NeuroImage, 2018.

Uncinate Fasciculus:

• Description: Hook-shaped fiber connecting the orbital and inferior frontal gyri to the anterior temporal lobe.

• Development: Longest period of development, only white matter tract to develop beyond age 30.

• Function: Possibly linked to auditory-verbal memory;

  • Clinical Relevance: Associated with posttraumatic retrograde amnesia.

Cingulum:

• Function: One of the earliest identified brain structures, facilitates communication within the limbic system.

• Pathway: Follows cingulate gyrus from the subcallosal gyrus to the parahippocampal gyrus.

• Clinical Application: Cingulotomy, a surgical procedure to sever the anterior cingulum for the treatment of depression and OCD.

Corpus Callosum:

• Development: Appears at 15 weeks gestation, extends anteriorly and posteriorly, with more anterior development by 19 weeks.

• Significance: Largest white matter tract in the brain, containing approximately 300 million axons.

• Clinical Considerations: Agenesis of the corpus callosum can lead to language delays and challenges integrating visual and tactile stimuli. Pathologies include MS, lymphoma, and butterfly GBM.

Case Study: Split Brain Phenomenon:

• Illustration: The split brain, exploring the two halves of brain function.

• Reference: Gazzaniga, M. Nature. 2012.

Anterior Commissure:

• Connections Involved: Links various brain regions associated with memory, emotion, speech, hearing, and olfaction (including amygdala, olfactory system, orbital frontal cortex).

• Clinical Relevance: Potential implications in conditions affecting these functions (e.g., mood disorders).

Conclusion

• Inquiries and Feedback: Contact Damjan Veljanoski at d.veljanoski@ucl.ac.uk for any questions. Feedback on Neuroanatomy Sessions is encouraged.

• Interactive Engagement: Access feedback through menti.com using code 3711 5577 or QR code provided.

Overview and Fundamentals

• Presented by: Damjan Veljanoski, Developmental Neurosciences, Great Ormond Street, Institute of Child Health.

• Objective: To provide a comprehensive understanding of the structural connectivity of the human brain through Diffusion Tensor Imaging ($DTI$) and anatomical dissection.

• Tractography Principles: White matter tracts are bundles of myelinated axons. $DTI$ measures the anisotropic diffusion of water molecules along these axons, allowing for the reconstruction of fiber pathways in vivo.

Classification of Nerve Fibers

1. Projection Fibers:

   • These fibers serve as the primary communication link between the cerebral cortex and lower centers, including:

     • Deep Grey Nuclei: Such as the thalamus and basal ganglia.

     • Brain Stem: Relaying motor and sensory signals.

     • Cerebellum: Facilitating motor coordination.

     • Spinal Cord: Executing voluntary movements via the corticospinal tract.

2. Association Fibers:

   • Facilitate communication within a single hemisphere, linking different cortical regions to permit high-level cognitive integration.

3. Commissural Fibers:

   • These cross the midline to connect homologous areas of the two hemispheres, ensuring bilateral coordination and information transfer.

4. Fasciculus Definitions:

   • A Fasciculus refers specifically to a distinct bundle of myelinated nerve fibers ($axons$) that travel together to a common destination.

1. Detailed Projection Fibers

• Major Components:

  • Corticospinal Tract ($CST$): Originates primarily from the primary motor cortex ($Brodmann area 4$). Recent studies indicate that in $71\%$ of pediatric cases, fibers also originate from the postcentral gyrus. It descends through the posterior limb of the internal capsule.

  • Corticobulbar Tract: Connects the motor cortex to the cranial nerve nuclei in the brainstem, controlling the muscles of the face and neck.

  • Internal Capsule: A massive white matter structure containing nearly all projection fibers. It is anatomically divided into:

    • Anterior Limb: Between the head of the caudate and the lentiform nucleus.

    • Genu: The 'bend' containing corticobulbar fibers.

    • Posterior Limb: Between the thalamus and lentiform nucleus, containing the $CST$ and sensory radiations.

  • Optic Radiation ($Geniculocalcarine Tract$): Routes visual information from the Lateral Geniculate Body ($LGB$) to the primary visual cortex ($V1$).

    • Meyer’s Loop: The anterior part of the radiation that loops into the temporal lobe; damage leads to 'pie in the sky' or superior homonymous quadrantanopia.

    • Central/Posterior Bundles: Targeted toward the superior and inferior banks of the calcarine fissure.

2. Detailed Association Fibers

• Short-Range Fibers:

  • Known as U-fibers, these connect adjacent gyri by dipping into the white matter and curving back up to the cortex.

• Long-Range tracts:

  • Superior Longitudinal Fasciculus ($SLF$): The largest association tract. It is bidirectional and consists of $4$ components. It links the frontal, parietal, temporal, and occipital lobes.

  • Arcuate Fasciculus: A subcomponent of the $SLF$ connecting Wernicke’s area (temporal) to Broca’s area (frontal). Damage causes Conduction Aphasia, where speech is fluent but repetition is severely impaired.

  • Inferior Longitudinal Fasciculus ($ILF$): Connects the occipital lobe to the anterior temporal lobe. Disruptions are linked to visual amnesia and prosopagnosia (face blindness).

  • Uncinate Fasciculus: A hook-shaped tract linking the orbitofrontal cortex to the anterior temporal pole. It is unique because it continues to develop and myelinate beyond the age of $30$. It is heavily involved in social behavior and emotional processing.

  • Cingulum: A curved bundle within the cingulate gyrus, acting as a core component of the Limic System (Papez Circuit). It connects the subcallosal gyrus to the parahippocampal gyrus.

3. Detailed Commissural Fibers

• Corpus Callosum:

  • The largest white matter structure with approximately $300$ million axons.

  • Development: Begins at $15$ weeks gestation, reaching a more mature configuration by $19$ weeks.

  • Subdivisions: From anterior to posterior: Rostrum, Genu, Body, and Splenium.

  • Clinical Correlate: Agenesis of the corpus callosum results in 'disconnection syndromes' where the two hemispheres cannot integrate information (e.g., tactile information from the left hand cannot be named by the left-hemisphere language centers).

• Anterior Commissure:

  • Crosses the midline in the lamina terminalis. It predominantly connects the temporal lobes and is involved in olfaction, emotion, and memory.

Diffusion Tensor Imaging ($DTI$) Methodology

• Fractional Anisotropy ($FA$): A scalar value between $0$ and $1$ that describes the degree of anisotropy. High $FA$ indicates healthy, organized white matter tracts.

• Visual Coding:

  • Green: Anteroposterior fibers (e.g., $SLF$, $IFOF$).

  • Red: Left-Right/Commissural fibers (e.g., Corpus Callosum).

  • Blue: Superior-Inferior/Projection fibers (e.g., $CST$).

• Clinical Mapping: Because tracts like the corticospinal and corticopontine run in close proximity, they often appear as a single blue bundle on standard $DTI$ maps, requiring advanced tractography for separation.

Cognitive Models: Hickok and Poeppel

• Dorsal Stream:

  • Anatomy: $SLF$ and Arcuate Fasciculus.

  • Function: Sensory-motor mapping of speech; involves 'how' we say things and repetition.

• Ventral Stream:

  • Anatomy: $ILF$, $MLF$, and Extreme Capsule ($EmC$).

  • Function: Semantic processing; mapping sound to meaning ('what' is being said).

Surgical and Pathological Considerations

• Cingulotomy: A neurosurgical procedure where the anterior cingulum is severed; used as a last resort for treatment-resistant Depression and Obsessive-Compulsive Disorder ($OCD$).

• Tumor Spread: The Inferior Fronto-Occipital Fasciculus ($IFOF$) is a known pathway for the infiltration of gliomas between the frontal and temporal lobes, potentially causing rapid cognitive decline or early amnesia.