Projection fibers consist of efferent and afferent fibers uniting the cortex with the lower parts of the brain and with the spinal cord. In human neuroanatomy, bundles of axons (nerve fibers) called nerve tracts, within the brain, can be categorized by their function into association tracts, projection tracts, and commissural tracts.

In the neocortex, projection neurons are excitatory neurons that send axons to distant brain targets. Considering the six histologically distinct layers of the neocortex, associative projection neurons extend axons within one cortical hemisphere; commissural projection neurons extend axons across the midline to the contralateral hemisphere; and corticofugal projection neurons extend axons away from the cortex. That said, some neurons are multi-functional and can therefore be categorized into more than one such category.

The principal efferent fibers are:

The chief afferent fibers are:

This article incorporates text in the public domain from page 843 of the 20th edition of Gray's Anatomy (1918)