Projection fibers consist of efferent and afferent fibers uniting the cortex with the lower parts of the brain and with the spinal cord.
[1] In the neocortex, projection neurons are excitatory neurons that send axons to distant brain targets.
[2] 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.
[2] That said, some neurons are multi-functional and can therefore be categorized into more than one such category.
[2] 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)