Cerebral tracts are association, commissural, or projection in nature. In the brain, commissures run from one part to the corresponding part on the opposite side of the brain. Projection fibers carry information to and from the cortex. Association fibers connect different cortical areas.
The corpus callosum is the major commissure of the cerebrum. It is a massive band of myelinated nerve fibers and most of them interconnect symmetrical regions of the cerebral cortex. The different regions of the corpus callosum are termed the splenium, at the posterior end, the body, which is the main part, and the genu, which is the Latin word meaning ‘knee’ and is the bend at the anterior part of the corpus callosum. From the corpus callosum, fibers radiate out to the cerebral cortex. The corpus callosum forms part of the roof of the lateral ventricle and also the floor of the longitudinal fissure. The corpus callosum carries the interhemispheric transfer of memory, sensory experience, and learned discrimination. Damage to the corpus callosum does not appear to affect performance, except that destruction of the splenium causes alexia, or the inability to understand written words. This may be due to disconnection of the verbal processing in the left hemisphere from visual processing in the right hemisphere.
The anterior commissure is a compact fiber bundle that crosses the midline in front of the columns of the fornix and connects the olfactory bulbs and regions of the temporal gyri. The hippocampal commissure is a transverse commissure linking the posterior columns of the fornix.
Projection fibers are afferents carrying information to the cerebral cortex, and efferents carrying information away from it. The most prominent are the corona radiata, which radiate out from the cortex and then come together in the brain stem. These fibers become highly condensed in the internal capsule, which runs medially between the caudate nucleus and the thalamus and laterally between the thalamus and the lentiform nucleus. The anterior limb of the internal capsule carries connections between the frontal lobe and the basal part of the pons and between the prefrontal cortex and the mediodorsal nucleus of the thalamus. The posterior limb of the internal capsule carries fibers between the ventral posterior nucleus of the thalamus and the primary somatosensory cortex and also carries corticospinal and corticobulbar fibers.
The association fibers connect different areas of the cerebral cortex. Some are relatively large, such as the superior longitudinal fasciculus, which connects the occipital and frontal lobes. Part of the fasciculus, the arcuate fasciculus connects temporal and frontal lobes, and is important for language. The inferior longitudinal fasciculus connects the temporal and occipital lobes, and is involved in visual recognition function.