Anerve tract is a bundle of nerve fibers (axons) connectingnuclei of thecentral nervous system.[1][2][3] In theperipheral nervous system, this is known as anerve fascicle, and has associatedconnective tissue. The main nerve tracts in the central nervous system are of three types:association fibers,commissural fibers, andprojection fibers. A nerve tract may also be referred to as acommissure,decussation, orneural pathway.[4] A commissure connects the twocerebral hemispheres at the same levels, while a decussation connects at different levels (crosses obliquely).
The nerve fibers in the central nervous system can be categorized into three groups on the basis of their course and connections.[5] Different tracts may also be referred to asprojections orradiations such asthalamocortical radiations.
The tracts that connect cortical areas within the same hemisphere are calledassociation tracts.[5] Long association fibers connect different lobes of a hemisphere to each other whereas short association fibers connect different gyri within a single lobe. Among their roles, association tracts link perceptual and memory centers of the brain.[6]
Thecingulum is a major association tract. The cingulum forms the white matter core of thecingulate gyrus and links from this to theentorhinal cortex. Another major association tract is thesuperior longitudinal fasciculus (SLF) that has three parts.
Commissural tracts connect corresponding cortical areas in the two hemispheres.[5] They cross from one cerebral hemisphere to the other through bridges calledcommissures. The great majority of commissural tracts pass through the largest commissure thecorpus callosum. A few tracts pass through the much smalleranterior andposterior commissures. Commissural tracts enable the left and right sides of the cerebrum to communicate with each other. Other commissures are thehippocampal commissure, and thehabenular commissure.
Projection tracts connect the cerebral cortex with thecorpus striatum,diencephalon,brainstem and thespinal cord.[5] Thecorticospinal tract for example, carries motor signals from the cerebrum to the spinal cord. Other projection tracts carry signals upward to the cerebral cortex. Superior to the brainstem, such tracts form a broad, dense sheet called the internal capsule between the thalamus and basal nuclei, then radiate in a diverging, fanlike array to specific areas of the cortex.
A bundle of nerve fibers (axons) connecting neighboring or distant nuclei of the CNS is a tract.
The nerve fibres which make up the white matter of the cerebral hemispheres are categorized on the basis of their course and connections. They are association fibres, which link different cortical areas in the same hemisphere; commissural fibres, which link corresponding cortical areas in the two hemispheres; or projection fibres, which connect the cerebral cortex with the corpus striatum, diencephalon, brain stem and the spinal cord