Neuroscience: White Fibre Bundles

White Fibre Bundles

• Association Fibres
  • Link areas within the same cerebral hemisphere
  • Short Fibres: Connect adjacent gyri
  • Long Fibres: Connect different lobes
    • Uncinate Fasciculus: Connects Broca's area, orbital gyri and temporal lobe
    • Cingulum: Connects rostrum to temporal lobe
    • Superior Longitudinal Fasciculus: Connects frontal and occipital lobes
    • Inferior Longitudinal Fasciculus: Connects occipital and temporal lobes
    • Fronto-occipital Fasciculus: Connects frontal and occipital lobes, located lateral to the caudate nucleus and superior longitudinal fasciculus
• Commissural Fibres
  • Course across the midline
  • Link corresponding areas between cerebral hemispheres
  • Examples
    • Corpus Callosum: Links cerebral cortices between two hemispheres, forms the roof of the lateral ventricles
      • Rostrum: Anterior inferior part
      • Genu: Most anterior part
      • Body (Trunk): Arches posteriorly, convex superiorly
      • Splenium: Most posterior part, overhangs the pineal gland
      • Minor Forceps: Anterior extensions of the genu, connecting medial and lateral front lobe surfaces
      • Major Forceps: Posterior extensions of the splenium, courses between the occipital lobes
      • Tapetum: Lateral projections of the splenium and body
    • Anterior Commissure: Embedded within the lamina terminalis, anterior border of the third ventricle, connects amygdala and temporal lobes, important for the olfactory pathway
    • Posterior Commissure: Contains fibres from the thalamus and superior colliculus, important for language processing pathways
    • Habenular Commissure: Located anterior to the pineal gland, connects the habenular nuclei
    • Commissure of the Fornix: Connects the left and right hippocampus, fibres terminate in the thalamus and mammillary bodies
• Projection Fibres
  • Link cerebral cortex to lower levels of the brain and spinal cord
  • Internal Capsule:
    • Anterior Limb: Lateral to caudate nucleus
    • Genu: Between caudate nucleus and thalamus
    • Posterior Limb: Lateral to the thalamus
    • Lentiform Nucleus: Lateral to the internal capsule

Basal Nuclei (Basal Ganglia)

• Collection of gray matter structures responsible for posture and voluntary movements
• Nuclei connect to the thalamus, cortex, and neighbouring basal nuclei
• Nuclei in each Cerebral Hemisphere:
  • Putamen: Outermost part of the lentiform nucleus, separated from the globus pallidus via the lateral medullary lamina, located where the diencephalon and telencephalon fuse
  • Caudate Nucleus
  • Nucleus Accumbens
  • Globus Pallidus: Divided into medial (GPi) and lateral (GPe) parts, separated by the medial medullary lamina
• Other Sub-cortical Nuclei:
  • Subthalamic Nucleus: Located in the diencephalon
  • Substantia Nigra: Located in the rostral midbrain
• Lentiform Nucleus: Collective name for the globus pallidus and putamen, wedge-like shape (horizontal section)

White Matter Tracts

• Association fibres connect areas within the same cerebral hemisphere.

  • Short fibres connect adjacent gyri, possibly within the same lobe.
  • Long fibres connect different lobes.
    • Uncinate fasciculus connects Broca's area, orbital gyri, and the temporal lobe.
    • Cingulum connects the rostrum to the temporal lobe.
    • Superior longitudinal fasciculus connects frontal and occipital lobes.
    • Inferior longitudinal fasciculus connects occipital and temporal lobes.
    • Fronto-occipital fasciculus connects frontal and occipital lobes.
    • It lies lateral to the caudate nucleus and superior longitudinal fasciculus.

• Commissural fibres cross the midline, connecting the cerebral cortex of both hemispheres.

  • Corpus callosum links the cerebral cortices of both hemispheres.
    • It forms the roof of the lateral ventricles.
    • It has a rostrum, genu, body (trunk), and splenium.
    • The rostrum is anterioinferior, genu is the most anterior part, the body arcs posteriorly, and the splenium is the most posterior part.
    • The splenium overhangs the pineal gland.
    • The corpus callosum has minor forceps (anterior extensions of the genu connecting medial and lateral frontal lobe surfaces) and major forceps (posterior extensions of the splenium connecting occipital lobes).
    • The tapetum is lateral projections of the splenium and body.
  • Anterior and posterior commissure connect parts of the brain.
    • The anterior commissure connects the amygdala and temporal lobes, forming the anterior border of the third ventricle. It's important for the olfactory pathway.
    • The posterior commissure is important for language processing pathways.
  • Habenular commissure connects habenular nuclei, located anterior to the pineal gland.
  • Commissure of the fornix connects left and right hippocampi.
    • Fibres of the fornix terminate in the thalamus and mammillary bodies.

• Projection fibres link the cerebral cortex to lower levels of the brain and spinal cord.

  • They are located in the internal capsule.
  • Internal capsule is divided into:
    • Anterior limb lies lateral to the caudate nucleus.
    • Genu lies between the caudate nucleus and thalamus.
    • Posterior limb lies lateral to the thalamus.

Basal Ganglia

• Basal ganglia are collections of gray matter structures responsible for posture and voluntary movement.

  • They connect with the thalamus, cortex, and neighboring basal nuclei.
  • Each cerebral hemisphere has a putamen, caudate, nucleus accumbens, and globus pallidus.
  • Collectively, these structures are referred to as the corpus striatum.
  • The subthalamic nucleus lies in the diencephalon.
  • The substantia nigra lies in the rostral midbrain.

• Lentiform nucleus:

  • A collective term for the globus pallidus and putamen.
  • It has a wedge-like shape in a horizontal section.

• Putamen:

  • Outermost part of the lentiform nucleus.
  • Separated from the globus pallidus by the lateral medullary lamina.
  • It is located at the fusion of the diencephalon and telencephalon.

• Globus pallidus:

  • Divided into medial (GPi) and lateral (GPe) parts.
  • They are separated by the medial medullary lamina.