Chapter

Connectional Neuroanatomy

Marco Catani and Michel Thiebaut de Schotten

in Atlas of Human Brain Connections

Published on behalf of Oxford University Press

Published in print March 2012 | ISBN: 9780199541164
Published online November 2012 | e-ISBN: 9780191753268 | DOI: http://dx.doi.org/10.1093/med/9780199541164.003.0041
Connectional Neuroanatomy

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Between the cortex, the lateral ventricles, and the deep nuclei lies a mass of fibres connecting distant regions of the brain. Most of the cerebral fibres have both their origin and termination within the cortex, either in the same hemisphere (association fibres) or opposite hemispheres (commissural fibres) (Figure 4.1). Other fibres connect the cortex to subcortical regions (projection fibres). There are some fibres that, although being located in the cerebral hemispheres do not connect the cortex, but only subcortical structures (e.g. fibres of the anterior commissure connecting the two amygdalae). Fibres group together to form bundles of different diameter and several bundles form larger pathways called fasciculi (or tracts). White matter fasciculi are classified into association, commissural, and projection tracts, although some tracts may contain more than one type of fibre (e.g. the uncinate fasciculus contains both association and projection fibres).

The association pathways connect cortical regions within the same hemisphere and have an anterior–posterior (or posterior–anterior) direction. The major association tracts are the arcuate fasciculus, the cingulum, the uncinate, the inferior longitudinal fasciculus, and the inferior fronto-occipital fasciculus (Figure 4.2). The terminology used to indicate these tracts refers to either their shape (e.g. the uncinate for ‘hook-like’, cingulum for ‘girdle’ or ‘belt’), their origin and termination (e.g. inferior fronto-occipital fasciculus), or their course and location (e.g. inferior longitudinal fasciculus). The long association tracts connect distant regions between lobes, while short U-shaped fibres connect neighbouring gyri within the same lobe (intralobar fibres) or different lobes (interlobar fibres). The association tracts are involved in higher cognitive functions, such as language, praxis, visuo-spatial processing, memory, and emotion (see also Chapters 7, 8, 11).

Commissural pathways are composed of fibres connecting the two halves of the brain. The major telencephalic commissures of the human brain include the corpus callosum, the anterior commissure, and the hippocampal commissure. The commissural pathways allow the transfer of inputs between the two halves of the brain and play a significant role in the functional integration of motor, perceptual, and cognitive functions (see also Chapter 9).

Projection pathways connect the cortex to subcortical structures, such as deep cerebral nuclei, brainstem nuclei, and spinal cord. Within the cerebral hemisphere the ascending projection fibres originate from subcortical nuclei (mainly thalamus) and terminate in the cortex, while descending projection fibres have the opposite orientation. Most of the projection fibres course through the corona radiata, internal capsule, cerebral peduncles, and brainstem. The fornix is also considered as a projection tract (see also Chapters 10 and 11).

Chapter.  8232 words.  Illustrated.

Subjects: Neuroscience

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