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
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content when selected, and will be periodically checked to have remained so. Copyright © 2003-2018, Derek J. Smith.
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First published online 15:12 GMT 19th February 2003,
Copyright Derek J. Smith (Chartered Engineer). This version [2.0 - copyright] 09:00 6th BST July
2018.
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.
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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). If this diagram fails to load automatically,
it may be accessed separately at |
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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. |
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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. If this diagram fails to load automatically,
it may be accessed separately at |
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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. |
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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. If this diagram fails to load automatically,
it may be accessed separately at |
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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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