Neural tract

A tract (sometimes called a fascicle) or nerve tract, is a bundle of nerve fibers (axons) connecting nuclei of the central nervous system.

The nerve fibers in the central nervous system can be categorized into three groups on the basis of their course and connections.

The tracts that connect cortical areas within the same hemisphere are called association tracts. 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.

Commissural tracts connect corresponding cortical areas in the two hemispheres. They cross from one cerebral hemisphere to the other through bridges called commissures. The great majority of commissural tracts pass through the corpus callosum. A few tracts pass through the much smaller anterior and posterior commissures. Commissural tracts enable the left and right sides of the cerebrum to communicate with each other.

Projection tracts connect the cerebral cortex with the corpus striatum, diencephalon, brainstem and the spinal cord. The corticospinal tract for example, carry 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.

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