Cerebral White Matter · 365 9
Cerebral White Matter
Each hemisphere contains a large amount of subcortical white matter, which is composed of myelinated nerve fibers of varying thickness and neuroglia (mainly oligodendrocytes, the cells that form myelin sheaths).
The subcortical white matter is bounded by the cerebral cortex, the lateral ventricles, and the striatum. Its nerve fibers are of three types:
1 Projection fibers
2 Association fibers
3 Commissural fibers
Projection Fibers
Projection fibers connect different parts of the central nervous system with each other over long distances.
Efferent fibers from the cerebral cortex traverse the subcortical white matter and then come together to form the internal capsule. As discussed in Chapter 3, these are the corticonuclear, corticospinal, and corticopontine fibers, as well as the fibers that link the cerebral cortex with the thalamus, striatum, reticular formation, substantia nigra, subthalamic nucleus, midbrain tectum, and red nucleus. The long efferent corticospinal fibers arise mainly in areas 4, 3, 1, and 2, and also in area 6, while fibers to other destinations, such as the corticopontine and corticothalamic fibers, arise from larger association areas of the cortex.
Afferent fibers travel from the thalamus to extensive areas of the cerebral cortex. These include fibers of all somatosensory modalities, which travel to areas 3, 1, 2, and 4, as well as other fibers carrying impulses from the cerebellum, globus pallidus, and mamillary body by way of the thalamus to the cerebral cortex. The thalamus is the last major relay station that sensory impulses must traverse before reaching their specific primary cortical areas and is therefore sometimes called the “gateway to consciousness.” Olfactory fibers are the only exception to this rule: they reach the cortex directly, without any thalamic relay.
Thalamocortical reciprocity. Most thalamocortical projections are reciprocal (i.e., there are fibers running in both directions). The cerebral cortex is thus presumed to modulate its own input by means of a feedback loop between the cortex and the thalamus. These massive thalamocortical and corticothalamic projections make up the large white matter tracts known as the anterior, su-
Baehr, Duus' Topical Diagnosis in Neurology © 2005 Thieme
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9366 · 9 Cerebrum
perior, posterior, and inferior thalamic peduncles, which are usually collectively termed the corona radiata. The topical organization of the thalamic projections is their most important feature.
Association Fibers
The association fibers (Figs. 9.15 and 9.16) make up most of the subcortical white matter. These fibers connect neighboring distant cortical areas of the same hemisphere with each other. The cerebral cortex is able to carry out its diverse associative and integrative functions only because all of its functionally important areas are tightly interconnected and neural impulses can travel easily from one cortical area to another. These extensive fiber connections between cortical areas may also be an important anatomical substrate for the partial recovery of function often seen in the aftermath of cortical injury (e. g., after trauma or stroke). Over time, as the individual practices the impaired activities, performance may improve because the corresponding neural impulses have been redirected along the remaining, intact pathways.
The superior longitudinal fasciculus runs dorsal to the insula and connects the frontal lobe with large parts of the parietal, occipital, and temporal lobes. An extension of it, the arcuate fasciculus, winds around the posterior end of the lateral sulcus (sylvian fissure) in the depths of the subcortical white matter. This fiber bundle connects the frontal and temporal language areas (of Broca and Wernicke, p. 387) with each other. Lesions of the arcuate fasciculus produce conduction aphasia (p. 389). The inferior longitudinal fasciculus connects the temporal lobe with the occipital lobe. The uncinate fasciculus travels around the anterior end of the lateral sulcus like a hook, connecting the orbital frontal gyri with the anterior portion of the temporal lobe.
Other important bundles of association fibers are the superior and inferior occipitofrontal fasciculi and the vertical occipital fasciculus. The cerebral arcuate fibers, also called U fibers, connect neighboring as well as distant gyri. Nerve fibers that travel exclusively within the cerebral cortex are called intracortical fibers, in contrast to the subcortical fibers that make up the cerebral white matter.
The cingulum is an association bundle of the limbic system. It runs from the subcallosal area to the parahippocampal gyrus (entorhinal area).
Commissural Fibers
Fibers linking cortical regions with their counterparts in the opposite cerebral hemisphere are called commissural fibers (Fig. 9.16c, d) and are found in the corpus callosum and the anterior commissure. The fibers of the corpus callo-
Baehr, Duus' Topical Diagnosis in Neurology © 2005 Thieme
All rights reserved. Usage subject to terms and conditions of license.
