The association fibers in the brain link up the different cortical areas with one another. In human beings, they are very complex and work as the inner wiring, integrating every part of the cerebral cortex into one operating whole. Thus, the frontal lobe is connected to all the other lobes. These fibers get together to form fasciculi, which are thick bundles consisting of billions of myelinated neuron axons.
These association fibers in the brain vary considerably in length and thickness, making up the white matter of the cerebrum. Therefore, short and long association fibers can be distinguished in the cerebrum. The short association fibers provide connections within a cerebral lobe or from one gyrus, or convolution, to the next.
The shortest fibers connect adjacent cortical parts; they reenter the cortex after running just a short distance through the white matter; thus, the layer of arcuate fibers lies immediately beneath the cortex. On the other hand, the long association fibers connect different cerebral lobes and form compact bundles, each one called fasciculus, that can be recognized with the naked eye.
Long association fibers
The superior longitudinal fasciculus lying dorsolaterally to the putamen is a strong association bundle between frontal lobe and occipital lobe with fibers branching to the parietal lobe and temporal lobe. The inferior occipitofrontal fasciculus passes through the ventral part of the extreme capsule from the frontal lobe to the occipital lobe.
The inferior longitudinal fasciculus extends between occipital lobe and temporal lobe. The uncinate fasciculus connects the temporal cortex with the frontal cortex; its ventral part provides a connection between the entorhinal area and the orbital area of the frontal lobe.
The arcuate fasciculus, which is unique in humans, connects the two language areas, with the fibers projecting from the Wernicke's area on the back of the temporal lobe to the Brocca's area in the back of the 3rd frontal gyrus; this fiber bundle makes a big arch. Other fiber bundles are the vertical occipital fasciculus and the orbitofrontal fasciculus.
Below, schematic drawing exhibiting the association fibers that link up the different parts of the cerebral cortex. Number 5 and 4 represent the superior longitudinal fasciculus; # 6- the inferior occipitofrontal fasciculus; #7- inferior longitudianal fasciculus; # 8- uncinate fasciculus, which is made up of fronto-temporal myelinated fibers. # 1 and 2 are short association fibers, or intergyral fasciculi.
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