The projection fibers consist of efferent and afferent fibers uniting the cortex with the lower parts of the brain and with the

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the spin ...

. In human neuroanatomy, bundles of

axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action ...

s (nerve fibers) called tracts, within the brain, can be categorized by their function into

association fibers Association fibers are axons that connect cortical areas within the same cerebral hemisphere. In human neuroanatomy, axons (nerve fibers) within the brain, can be categorized on the basis of their course and connections as association fibers, ...

, projection fibers, and

commissural fibers The commissural fibers or transverse fibers are axons that connect the two hemispheres of the brain. In contrast to commissural fibers, association fibers connect regions within the same hemisphere of the brain, and projection fibers connect ...

. In the

neocortex The neocortex, also called the neopallium, isocortex, or the six-layered cortex, is a set of layers of the mammalian cerebral cortex involved in higher-order brain functions such as sensory perception, cognition, generation of motor commands, ...

, 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.

Efferent

The principal efferent fibers are: * (1) the

motor tract An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy. Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power gen ...

, occupying the

genu Genu, a Latin word for "knee," may refer to: * Genu of internal capsule * Genu of the corpus callosum * Genu recurvatum * Genu valgum * Genu varum Genu varum (also called bow-leggedness, bandiness, bandy-leg, and tibia vara) is a varus deform ...

and anterior two-thirds of the occipital part of the internal capsule, and consisting of ** (a) the

geniculate fibers The geniculate fibers are the fibers in the region of the genu of the internal capsule; they originate in the motor part of the cerebral cortex, and, after passing downward through the base of the cerebral peduncle with the cerebrospinal fibers ...

, which decussate in the medulla, and end in the motor nuclei of the

cranial nerves Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and ...

of the opposite side; and ** (b) the cerebrospinal fibers, which are prolonged through the medullary pyramids into the

: * (2) the corticopontine fibers, ending in the pontine nuclei.

Afferent

The chief afferent fibers are: * (1) those of the lemniscus which are not interrupted in the

thalamus The thalamus (from Greek θάλαμος, "chamber") is a large mass of gray matter located in the dorsal part of the diencephalon (a division of the forebrain). Nerve fibers project out of the thalamus to the cerebral cortex in all direction ...

; * (2) those of the superior cerebellar peduncle which are not interrupted in the red nucleus and

; * (3) numerous fibers arising within the

, and passing through its stalks to the different parts of the cortex; * (4) optic and acoustic fibers, the former passing to the occipital, the latter to the

temporal lobe The temporal lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The temporal lobe is located beneath the lateral fissure on both cerebral hemispheres of the mammalian brain. The temporal lobe is involved in proc ...

References

External links

Diagram at kennedykrieger.org
Cerebral white matter {{Neuroanatomy-stub