White matter

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Basics

The white matter of the cerebral hemisphere is composed of axonal fibres which facilitate processing of information and tasks between cortical areas, and between cortex and subcortical areas (e.g., from basal ganglia downwards).

Fibre Categories

Association fibres

Short association fibres

  • Also known as U-fibres because of their shape
  • run between neurons of adjacent gyri within the same lobe
  • e.g., from the primary sensory (postcentral gyrus) to the adjacent sensory association area in the parietal lobe

Long association fibres

  • run between cortical areas of one lobe to another lobe within the same hemisphere
  • Specific, named bundles of long association fibres are:
    1. Superior longitudinal or superior occipito-frontal fasciculus: linking occipital to frontal lobe
    2. Inferior longitudinal fasciculus: linking occipital and temporal lobes
    3. Cingulum: linking the cingulate gyrus to parahippocampal gyrus
    4. Uncinate fasciculus: linking the paralimbic areas of the frontal and temporal poles
    5. Arcuate fasciculus: linking Wernicke's area to Broca's area

Commissural fibres

  • relay information between corresponding cortical areas of the left and right hemispheres
  • The Corpus callosum is the major bundle of commissural fibres
  • Other commissural fibres include the anterior commissure, posterior commissure, and the commissure between the fornices (axonal bundles of the limbic system)

Projection fibres

  • These fibres ascend and descend and synapse along the neuroaxis of the central nervous system
  • they connect the cortical areas with subcortical regions
  • Axonal fibres from different functional areas converge towards the region of the basal ganglia, forming the corona radiata
  • The corona radiata of the hemispheres form two compact bundles in the shape > <
  • Each bundle is named internal capsule and is situated between subcortical nuclei of the basal ganglia and diencephalon. The three anatomical regions of each internal capsule are: anterior limb, genu, and posterior limb. The internal capsules contain both ascending and descending fibres. The major descending tracts are the corticospinal and corticobulbar tracts, which can be mapped somatotopically to the genu and the posterior limb within the internal capsule (Fig. 3-12). 28 Cell bodies (upper motor neurons) of the corticospinal and corticobulbar tracts are located in the motor cortex (pre-central gyrus). Fibres from the upper motor neurons descend as part of the corona radiata and the internal capsule to reach the brain stem (midbrain, pons medulla) where fibres of the corticobulbar tract decussate (cross over) to the contralateral side and synapse with the lower motor neurons in the cranial nerve nuclei. Postsynaptic fibres then travel via cranial nerves to provide motor control of musculature mostly in the head and neck region. Fibres of the corticospinal tract travel through the brain stem, and descend to their corresponding spinal levels where they synapse with the alpha motor neurons (lower motor neurons) in the ventral horn of the contralateral side. Since the histological cell type of the upper motor neuron cell in the motor cortex is pyramidal, these tracts are also called pyramidal tract