There is a direct relationship between the velocity of electrical conduction and the diameter of nerve fibers, such that thick fibers conduct their impulses faster than thin ones. Another factor that increases conduction velocity is myelin wrapping. Myelinated fibers are much faster conductors than unmyelinated ones. As seen in the light microscope, human callosal fiber diameters range from 0.4 to 15 mm in diameter, but the most commonly observed fiber diameters are between 0.6 and 1 mm. Unmyelinated fibers seem to be scarce (about 5% of the total, except in the genu, where they may make up about 16%; see Aboitiz et al., 1992a), although, in general, postmortem human tissue is of suboptimal quality for determining the numbers of very thin unmyelinated fibers. The fiber composition of the corpus callosum shows regional differences that match the topographic arrangement of cortical areas (Aboitiz et al., 1992a) (see Figure 2.3). Fast-conducting, large-diameter fibers (larger than 3 mm in diameter) are especially abundant in the posterior body and the posterior splenium, which correspond in rostrocaudal sequence to the representation of motor, somatosensory, and auditory areas (posterior body) and visual cortex (posterior splenium). On the other hand, callosal regions representing higherorder areas are mainly composed of small-diameter, slower-conducting fibers. The callosal region with a highest density of thin, lightly myelinated and non-myelinated fibers is the genu, which, as has been mentioned, connects frontal areas. This is of interest because, as we said above, callosal cells connecting primary and secondary sensorimotor areas are relatively few, restricted to a narrow strip in the border of the area that represents the sensory or motor midline; in higher-order areas, callosal cells can be found all along the respective regions (see Innocenti, 1986). In primary and secondary visual and somatosensory cortices, fast-conducting, gigantic callosal fibers may be involved in fusing the two
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