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Projection fiber
Details
Identifiers
Latin	fibrae projectionis
NeuroNames	1218
TA98	A14.1.00.018
TA2	5617
FMA	76745
Anatomical terms of neuroanatomy [edit on Wikidata]

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.^[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]

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.

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.
^ ^a ^b ^c 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

Nervous tissue

CNS

Tissue Types

Cell Types

Neuronal	Pyramidal Purkinje Granule Von Economo Medium spiny Interneuron
Glial	Astrocyte Ependymal cells Tanycyte Oligodendrocyte progenitor cell Oligodendrocyte Microglia

PNS

General	Dorsal Root Ganglion Ramus Ventral Root Ramus Ramus communicans Gray White Autonomic ganglion (Preganglionic nerve fibers Postganglionic nerve fibers) Nerve fascicle Funiculus
Connective tissues	Epineurium Perineurium Endoneurium
Neuroglia	Myelination: Schwann cell Neurilemma Myelin incisure Node of Ranvier Internodal segment Satellite glial cell

Neurons/
nerve fibers

Parts

Soma	Axon hillock
Axon	Telodendron Axon terminals Axoplasm Axolemma Neurofibril/neurofilament
Dendrite	Nissl body Dendritic spine Apical dendrite/Basal dendrite

Types

Afferent nerve fiber/
Sensory neuron

Efferent nerve fiber/
Motor neuron

Termination

Synapse	Electrical synapse/Gap junction Chemical synapse Synaptic vesicle Active zone Postsynaptic density Autapse Ribbon synapse Neuromuscular junction
Sensory receptors	Meissner's corpuscle Merkel nerve ending Pacinian corpuscle Ruffini ending Muscle spindle Free nerve ending Nociceptor Olfactory receptor neuron Photoreceptor cell Hair cell Taste receptor

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