Course Handout - Cross-Sectional Anatomy of the Cerebrum

Copyright Notice: This material was written and published in Wales by Derek J. Smith (Chartered Engineer). It forms part of a multifile e-learning resource, and subject only to acknowledging Derek J. Smith's rights under international copyright law to be identified as author may be freely downloaded and printed off in single complete copies solely for the purposes of private study and/or review. Commercial exploitation rights are reserved. The remote hyperlinks have been selected for the academic appropriacy of their contents; they were free of offensive and litigious content when selected, and will be periodically checked to have remained so. Copyright 2010, High Tower Consultants Limited.

 

First published online 15:12 GMT 19th February 2003, Copyright Derek J. Smith (Chartered Engineer). This version [HT.1 - transfer of copyright] dated 18:00 14th January 2010

 

An earlier version of this material appeared in Smith (1997; Chapter 2). It is repeated here with minor amendments and supported with hyperlinks.

 

Cross-Sectional Anatomy of the Cerebrum

As we saw in the related microanatomy paper, the cerebral cortex contains the neural cell bodies, and the white matter beneath it contains the axons bringing information to and fro. This latter is what is exposed when the cerebrum is cut into to any depth, and is typically visibly fibrous. This is because axons typically travel as bundles of nerve fibres big enough to be visible to the naked eye. These are known as tracts or fasciculi (Latin fasciculus = "little bundle"). The usual classification of cerebral fibre tracts divides them into three main categories, as follows:

(a) Association Fibres: Firstly there are association fibre tracts. These are tracts linking one area of the cerebral cortex to another within the same hemisphere. Some are quite short, linking one gyrus to its immediate neighbours, but others are longer and more adventurous, linking regions to regions and lobes to lobes. The integrity of these pathways is vitally important to the healthy brain because they integrate its various sub-functions into a coherent whole. Figures 1 to 3 show the route taken by the main association fibre tracts. Note that the association tracts running to and from the prefrontal cortex are deliberately severed during a prefrontal lobotomy.

(b) Commissural Fibres: Secondly, there are commissural fibre tracts (or simply "commissures"). These are connections between (a) the two cerebral hemispheres, or (b) any pair of lateralised structures. The corpus callosum (or "great cerebral commissure") is the largest link between the cerebral hemispheres, but is assisted by the anterior commissure (a diencephalic commissural tract situated just anterior to the thalami, over the optic chiasm), and the posterior commissure (a midbrain commissure situated just anterior to the tectum). There are also commissures within the brainstem and spinal cord. As a rule, such tracts interconnect mirror-imaged locations (eg. Area 17-left with Area 17-right).

(c) Projection Fibres: And thirdly, there are projection fibre tracts. These are fibre tracts linking an area of cerebral cortex to a lower structure. For afferent fibres, the principal projections run upwards from the projection nuclei of the thalamus. The optic radiation, for example, runs from the lateral geniculate body to Area 17, and is a major feature in Figure 3. Efferent fibres, on the other hand, run downwards in one of two separate systems, the pyramidal tract and the extrapyramidal tract.

Figure 1 - Cross-Sectional Anatomy of the Cerebrum (Parasagittal Section): This diagram shows some of the main association and projection tracts. The superior longitudinal (or "arcuate") fasciculus runs between the frontal and parietotemporal regions, and is one of the most important language pathways [being a major part of Lichtheim's House, and just about every related model through to Crosson (1985)]. The inferior longitudinal fasciculus connects the temporal and occipital lobes, and the superior/inferior occipitofrontal fasciculi connect the frontal and occipital lobes by a high and a low route respectively. More medially, the uncinate fasciculus links the frontal lobe to the anterior portions of the temporal lobe (and is accordingly cramped quite tightly as it loops round the lateral fissure).

PICtracts-fig1.gif

Enhanced from a black and white original in Smith (1997; Figure 2.5). After Williams and Warwick (1975:971). This version Copyright 2003, Derek J. Smith.

 

Figure 2 - Cross-Sectional Anatomy of the Cerebrum (Coronal Section): Here are the main structures of the cerebral hemispheres in coronal section just behind the optic chiasm. Nuclei are shown in grey and CSF-filled cavities in black. The external capsule is a thin sheet of fibres passing between the putamen and the claustrum. Its partner, the internal capsule, is described in Figure 3. The corpus callosum connects "like with like" on a mirror-image basis. Thus point X connects with Y, P with Q, J with K, etc. The two points Z show where the internal capsule feeds down into the cerebral peduncles of the midbrain.

PICtracts-fig2.gif

Enhanced from a black and white original in Smith (1997; Figure 2.6). After Williams and Warwick (1975:967). This version Copyright 2003, Derek J. Smith.

 

 

Figure 3 - Cross-Sectional Anatomy of the Cerebrum (Horizontal Section): Here are the main structures of the cerebral hemispheres in horizontal section. The internal capsule is a very heavy and very widely fanned bundle of fibres connecting the midbrain to the cerebral cortex. It passes between the basal ganglia and the thalamus of each hemisphere, and is of varying origin and destination. Damage to it at about point E will interfere with the flow of information to and fro between the prefrontal region of the frontal lobe and the diencephalon, and is often deliberately inflicted by neurosurgeons during psychosurgery. The optic radiation (or geniculostriate or geniculocalcarine tract) contains the projection fibres of the optic system which run from the lateral geniculate body of the thalamus to the visual cortex.

PICtracts-fig3.gif

Enhanced from a black and white original in Smith (1997; Figure 2.7). After Williams and Warwick (1975:973). This version Copyright 2003, Derek J. Smith.

 

References

 Smith, D.J. (1997). Neuroanatomy for Students of Communication. Cardiff: UWIC. [ISBN: 190066609X - out of print]

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