White Matter Pathways and Tracts
White matter is the component of the central nervous system made up predominantly of myelinated axons and the supporting glia that wrap them, organised into discrete bundles called tracts or fasciculi that carry signals between distant regions of the brain and spinal cord. This area gives an orienting overview of how these pathways are classified, named, and studied, and links to the major functional groupings: descending motor tracts, ascending sensory and spinocerebellar pathways, commissural fibres joining the hemispheres, and the association fibres that interconnect cortical regions within a hemisphere.
Definition
White matter pathways are bundles of myelinated nerve fibres (tracts, fasciculi, lemnisci, peduncles, or commissures) within the central nervous system that convey nerve impulses between separated populations of neurons, conventionally classified as projection, commissural, or association fibres.
Scope
The area covers the general organisation of central nervous system white matter and the principal schemes used to classify its fibre systems, framed as reference anatomy and histology. It points to the four topic entries that treat the major tract families in detail. It does not cover peripheral nerves, grey-matter nuclei in their own right, or the clinical management of white-matter disease.
Sub-topics
Key concepts
- Projection, commissural, and association fibres
- Myelination and conduction velocity
- Tracts, fasciculi, lemnisci, and peduncles
- Decussation and crossing of pathways
- Somatotopic and topographic organisation
- Diffusion MRI tractography
Mechanisms
Central nervous system fibres are grouped by their connectional pattern: projection fibres run between the cortex and lower centres (for example the corticospinal tract and the ascending sensory lemnisci), commissural fibres cross the midline to join the two hemispheres (chiefly the corpus callosum and the anterior commissure), and association fibres link cortical areas within the same hemisphere (such as the superior longitudinal and uncinate fasciculi). Myelin produced by oligodendrocytes insulates the axons and supports rapid saltatory conduction, which gives white matter its pale appearance and its name. Many pathways decussate, so that one side of the brain relates to the opposite side of the body, and most carry an orderly somatotopic or topographic map of the territory they serve.
Clinical relevance
Knowing the course and grouping of white-matter pathways underlies the interpretation of lesion patterns and of diffusion MRI studies, because damage to a tract produces deficits referable to the regions it connects. This area describes anatomical and methodological organisation for reference and education and is not a basis for individual diagnosis or treatment.
Evidence & guidelines
Classical knowledge of tract anatomy derives from gross dissection, myelin-stained histology, and experimental tracer studies, codified in reference anatomy texts. In vivo, diffusion tensor imaging and tractography reconstruct fibre orientation from the directional diffusion of water and have produced standard atlases of human white matter; these methods are descriptive reconstructions whose limitations near crossing fibres are well recognised.
History
White matter was distinguished from grey matter by early anatomists, and nineteenth- and twentieth-century dissection and myelin-staining defined the major tracts. From the late 1990s, diffusion MRI tractography allowed non-invasive reconstruction of these pathways in living people, with Mori and colleagues demonstrating three-dimensional fibre tracking and Wakana, Catani, and others publishing tract atlases that mapped classical anatomy onto MRI.
Key figures
- Susumu Mori
- Marco Catani
- Denis Le Bihan
Related topics
Seminal works
- mori1999
- wakana2004
- catani2008
Frequently asked questions
- What is the difference between white matter and grey matter?
- Grey matter consists mainly of neuronal cell bodies and unmyelinated processes, whereas white matter consists mainly of myelinated axons in tracts; the myelin gives white matter its pale colour and its name.
- How are white-matter tracts classified?
- They are conventionally divided into projection fibres (between cortex and lower centres), commissural fibres (across the midline between hemispheres), and association fibres (between regions of the same hemisphere).