|Anatomical terms of neuroanatomy|
The 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 tracts, within the brain, can be categorized by their function into association fibers, projection fibers, and commissural fibers.
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:
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.