Projection fiber

Projection fiber
Details
Identifiers
Latin	Fibrae projectionis
NeuroNames	1218
TA	A14.1.00.018
FMA	76745
	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.[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 cortex.[2] That said, some neurons are multi-functional and can therefore be categorized into more than one such category.[2]

Efferent

The principal efferent fibers are:

(1) the motor tract, occupying the genu and anterior two-thirds of the occipital part of the internal capsule, and consisting of
- (a) the geniculate fibers, which decussate in the medulla, and end in the motor nuclei of the cranial nerves of the opposite side; and
- (b) the cerebrospinal fibers, which are prolonged through the medullary pyramids into the spinal cord:
(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;
(2) those of the superior cerebellar peduncle which are not interrupted in the red nucleus and thalamus;
(3) numerous fibers arising within the thalamus, 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.

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References

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

Standring, Susan (2005). Gray's Anatomy: The Anatomical Basis of Clinical Practice (39th ed.). Churchill Livingstone. pp. 411. ISBN 9780443071683. 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.
Greig LC, Woodworth MB, Galazo MJ, Padmanabhan H, Macklis JD (November 2013). "Molecular logic of neocortical projection neuron specification, development and diversity". Nat Rev Neurosci. 14 (11): 755–69. doi:10.1038/nrn3586. PMC 3876965. PMID 24105342.

External links

Diagram at kennedykrieger.org

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[1] Standring, Susan (2005). Gray's Anatomy: The Anatomical Basis of Clinical Practice (39th ed.). Churchill Livingstone. pp. 411. ISBN 9780443071683. 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.

[Greig2013-2] Greig LC, Woodworth MB, Galazo MJ, Padmanabhan H, Macklis JD (November 2013). "Molecular logic of neocortical projection neuron specification, development and diversity". Nat Rev Neurosci. 14 (11): 755–69. doi:10.1038/nrn3586. PMC 3876965. PMID 24105342.