The projection fibers consist of
efferent and
afferent fibers
Afferent nerve fibers are the axons (nerve fibers) carried by a sensory nerve that relay sensory information from sensory receptors to regions of the brain. Afferent projections ''arrive'' at a particular brain region. Efferent nerve fibers ar ...
uniting the
cortex with the lower parts of the brain and with the
spinal cord. 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 p ...
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 connec ...
.
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.
Efferent
The principal
efferent fibers
Efferent nerve fibers refer to axonal projections that ''exit'' a particular region; as opposed to afferent projections that ''arrive'' at the region. These terms have a slightly different meaning in the context of the peripheral nervous syst ...
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 gene ...
, 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
Decussation is used in biological contexts to describe a crossing (due to the shape of the Roman numeral for ten, an uppercase 'X' (), ). In Latin anatomical terms, the form is used, e.g. .
Similarly, the anatomical term chiasma is named aft ...
in the medulla, and end in the
motor nuclei
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 gene ...
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 f ...
of the opposite side; and
** (b) the
cerebrospinal fibers
The cerebrospinal fibers, derived from the cells of the motor area of the cerebral cortex, occupy the middle three-fifths of the base; they are continued partly to the nuclei of the motor cranial nerves, but mainly into the pyramids of the medu ...
, which are prolonged through the
medullary pyramids
In neuroanatomy, the medullary pyramids are paired white matter structures of the brainstem's medulla oblongata that contain motor fibers of the corticospinal and corticobulbar tracts – known together as the pyramidal tracts. The lower limi ...
into the
spinal cord:
* (2) the
corticopontine fibers
Corticopontine fibers are projections from the cerebral cortex to the pontine nuclei
Pontine may refer to:
* Having to do with the pons, a structure located in the brain stem (from ''pons'', "bridge")
* Pontine Marshes, a region of Italy ne ...
, ending in the
pontine nuclei
Pontine may refer to:
* Having to do with the pons, a structure located in the brain stem (from ''pons'', "bridge")
* Pontine Marshes, a region of Italy near Rome
* Pontine Islands
The Pontine Islands (, also ; it, Isole Ponziane ) are an ar ...
.
Afferent
The chief
afferent fibers
Afferent nerve fibers are the axons (nerve fibers) carried by a sensory nerve that relay sensory information from sensory receptors to regions of the brain. Afferent projections ''arrive'' at a particular brain region. Efferent nerve fibers ar ...
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
The red nucleus or nucleus ruber is a structure in the rostral midbrain involved in motor coordination. The red nucleus is pale pink, which is believed to be due to the presence of iron in at least two different forms: hemoglobin and ferritin. ...
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 occipital bone () is a cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone overlies the occipital lobes of the cere ...
, 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 pr ...
.
References
External links
Diagram at kennedykrieger.org
Cerebral white matter
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