COMMISSURAL FIBERS & ASSOCIATION FIBERS
LEARNING OBJECTIVES
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At the end of the lecture the student should be able to:
Describe the commissures & their connections.
Define corpus callosum & its parts.
Explain the association fibers & their connections.
The nerve fibers which make up the white matter of the cerebral hemispheres are categorized
on the basis of their course and connections.
They are:
Association fibers
link different cortical areas in the same hemisphere
Commissural fibers
link corresponding cortical areas in the two hemispheres
Projection fibers
connect the cerebral cortex with the corpus striatum, diencephalon,
brain stem and the spinal cord.
COMMISSURAL FIBRES
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Commissural fibers cross the midline, may linking corresponding areas in the two cerebral
hemispheres.
the corpus callosum (largest commissure)
Others include the:
– Anterior commissure
– Posterior commissure
– Habenular commissure
CORPUS CALLOSUM
The largest fiber pathway of the brain.
It links the cerebral cortex of the two cerebral
hemispheres
It roofs much of the lateral ventricles.
It forms an arch approximately 10 cm long
– Anterior end approximately 4 cm from the
frontal poles
– Posterior end approximately 6 cm from the
occipital poles.
 CORPUS CALLOSUM
PARTS OF CORPUS CALLOSUM
• genu (anterior portion)
• Splenium (posterior portion)
• truncus, or "body“.
• isthmus (The part between the body and the
splenium)
• The rostrum(resemblance to a bird's beak) is the
part of the corpus callosum that projects
posteriorly and inferiorly from the anterior most
genu.
CORPUS CALLOSUM
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Thinner axons in the genu connect the prefrontal cortex between the two halves of the
brain.
Thicker axons in the midbody of the corpus callosum and in the splenium interconnect
areas of the premotor and supplementary motor regions and motor cortex.
The posterior body of the corpus communicates somato sensory information between the
two halves of the parietal lobe and visual center at the occipital lobe.
ANTERIOR COMMISSURE
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Also known as the precommissure
Oval in shape
Having a long vertical axis that measures about 5 mm.
Bundle of nerve fibers connecting the two cerebral hemispheres
across the midline
Placed in front of the columns of the fornix
CONNECTIONS
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The fibers of the anterior commissure can be traced laterally and posteriorly on either side
beneath the corpus striatum into the substance of the temporal lobe.
It serves in this way to connect the two temporal lobes
It also contains
decussating fibers
from the
olfactory tracts
and
neospinothalamic
tract for pain.
POSTERIOR COMMISSURE
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also known as the epithalamic commissure
a rounded band of white fibers crossing the middle line on
the dorsal aspect of the upper end of the cerebral
aqueduct.
It is important in the bilateral pupillary light reflex
Its fibers have their origin in a nucleus, the nucleus of the
posterior commissure (nucleus of Darkschewitsch),
CONNECTIONS
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nucleus lies in the central gray substance of the upper end of the cerebral aqueduct, in
front of the nucleus of the oculomotor nerve.
Some are probably derived from the thalamus and from the superior colliculus,
others are believed to be continued downward into the medial longitudinal fasciculus.
The posterior commissure interconnects the protectal nuclei, mediating the consensual
pupillary light reflex.
THE FORNIX
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Composed of myelinated nerve fibers
Constitutes the efferent system of hippocampus passes to the mammillary bodies.
PARTS OF THE FORNIX
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Alveus:
– Thin layer of white matter covering the ventricular surface of hippocampus then converge to
form the fimbria.
Posterior commissure of fornix:
– The fimbriae of two side increase in thickness, reach to the
posterior end of hippocampus, arches above the thalamus
below the corpus callosum
PARTS OF THE FORNIX
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Body of fornix:
– Formed by joining the two posterior columns in midline
Commissure of the fornix:
Consists of transverse fibers
Across the midline
From one column to another before formation of body
Function: to connect the hippocampal formation of two side.
CONNECTIONS
• Fibers pass posterior to the anterior commissure to enter the mammillary body, end in medial
nucleus
• Fibers pass posterior to the anterior commissure to end in the
anterior nuclei of the thalamus.
• Fibers pass posterior to the anterior commissure to enter the
tegmentum of midbrain.
• Fibers pass anterior to the anterior commissure to end in:
• Septal nuclei
• Lateral preoptic area
• Hypothalamus
• Fibers join the stria medullaris thalami to reach the habenular
nuclei
HABENULAR COMMISSURE
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A band of nerve fibers situated in front of the pineal gland.
Connects the habenular nuclei on both sides of the diencephalon.
Part of the trigonum habenulæ (a small depressed triangular area situated in front of the
superior colliculus and on the lateral aspect of the posterior part of the tæni thalami.
CONNECTIONS
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The trigonum habenulæ also contains groups of nerve cells termed the ganglion
habenulæ.
Fibers enter the trigonum habenulæ from the stalk of the pineal gland, and the habenular
commissure.
Most of the trigonum habenulæ's fibers are, directed downward and form a bundle, the
fasciculus retroflexus of Meynert, which passes medial to the red nucleus, and, after
decussating with the corresponding fasciculus of the opposite side, ends in the
interpeduncular ganglion.
THE ASSOCIATION FIBERS
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unite different parts of the same cerebral hemisphere
two kinds:
(1) those connecting adjacent gyri, short association fibers;
(2) those passing between more distant parts, long association fibers.
THE ASSOCIATION FIBERS
 Short association fibers
• also referred to as "U-fibers"
• lie immediately beneath the gray substance of the cortex of the hemispheres.
THE ASSOCIATION FIBERS
 Long association fibers include the following:
Name
From
To
uncinate fasciculus
frontal lobe
temporal lobe
cingulum
cingulate gyrus
entorhinal cortex
frontal lobe
occipital lobe
occipital lobe
temporal lobe
superior longitudinal
fasciculus
inferior longitudinal
fasciculus
perpendicular fasciculus inferior parietal lobule
fusiform gyrus
occipitofrontal fasciculus occipital lobe
frontal lobe
fornix
hippocampus
mammillary bodies
Arcuate fasciculus
frontal lobe
temporal lobe
-----------------------------THANK YOU--------------------------------