Topographical Organization of the Pyramidal Fiber System - Diffusion Tensor MRI of the Human and Rhesus Monkey Brain

Sabine Hofer*, 1, 2, Jens Frahm1
1 Biomedizinische NMR Forschungs GmbH am Max-Planck-Institut für biophysikalische Chemie, 37070 Göttingen, Germany
2 Bernstein Center for Computational Neuroscience, 37073 Göttingen, Germany

© 2009 Hofer and Frahm;

open-access license: This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 International Public License (CC-BY 4.0), a copy of which is available at: This license permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

* Address correspondence to this author at the Biomedizinische NMR Forschungs GmbH, 37070 Göttingen, Germany; Tel: +49-551-201-1735; Fax: +49-551-201-1307; E-mail:


The fibers of the pyramidal tract (PT) that connect the precentral and postcentral gyrus with the spinal cord are organized in a topological manner. However, their exact arrangement and orientation in the internal capsule is still a matter of debate. Here, we applied magnetic resonance diffusion tensor imaging and tract tracing techniques to determine and compare the pyramidal fibers from the primary motor and somatosensory system in humans and rhesus monkeys in vivo. The results demonstrate that the pyramidal systems of the human and monkey brain differ in their orientation at the level of the internal capsule, whereas track orientations in the gyrus and brainstem appear similar. In the monkey internal capsule the somatotopic arrangement of PT fibers from the upper and lower extremities form an angle with the left-right axis of 146°± 23° (n=4). Thus, fibers from lateral areas of the gyrus are located more anterior-medial to fibers from medial cortical areas. In contrast, in humans the angle is only 50° ± 16° (n=9) yielding a more anterior-lateral orientation of PT fibers along the short axis of the internal capsule. These species dependencies may possibly be due to structural constraints of the smaller and differently shaped monkey brain.

Keywords: MRI, diffusion tensor imaging, pyramidal fiber, internal capsule, macaque, human.