Somatotopy of the Corticospinal Tract at the Brainstem PONS The CST is located in the center of the pontine basis, which is surrounded by trasnspontine fibers. Several clini- co-radiological correlation studies have reported on the so- matotopic arrangement and location of the CST at the pons.8,17,19,21,35,41 In 1994, Schneidder and Gautier35 discov- ered that fibers for arm motor function were located an- tero-medially, while those for leg motor function were lo- cated more postero-laterally at the pons in 17 stroke patients with brainstem lesions. During the same year, Bassetti, et al.8 reported that 10 patients among 12 patients with ventro-medial pontine infarcts showed moderate to severe weakness of the distal hand; in contrast, nine pa- tients with ventrolateral pontine infarct showed no weak- ness or mild weakness. However, Tohgi, et al.21 reported different results, showing hand dominant weakness in pa- tients with infarct in the dorso-medial region of the pontine basis, and leg dominantweakness in patients with infarct in the ventro-medial region of the pontine basis in 36 patients with pontine basis infarct. In 2004, Schmahmann, et al.19 demonstrated that hand representation was located ventro- medially at the upper and mid-pons, and that leg represen- tation was located dorso-laterally at the lower pons in 25 patients with focal infarcts on the pontine basis. Soon after, using a three-dimensional brain mapping technique from brain MRI and motor-evoked potential, Marx, et al.17 re- vealed that lesion location was more dorsal in patients with hemiparesis affecting the proximal muscles and more ven- tral in patients with distal limb weakness, among 41 pa- tients with pontine infarct. However, they concluded that the arms and legs did not show significant somatotopical differences. Recently, using DTT, Hong, et al.11 investigat- ed the anatomical location of CST somatotopies for the hand and leg at the pons in 25 normal human brains. In the group analysis, they found that hand somatotopy descend- ed through the ventro-medial portion of the pontine basis and that leg somatotopy was located dorso-laterally to the hand somatotopy of the CST. Individual DTI data showed that the relative average location of the CST for the hand was 47.70% with the standard from the midline to the most lateral point of the upper pons, and 35.87% in the lower pons. For the leg, the average location of the CST was 56.82% in the upper pons and 40.63% in the lower pons. For the anteroposterior direction from the most ante- rior point of the pons to the most anterior point of the fourth ventricle, the location of the CST for the hand was 42.30% in the upper pons and 36.18% in the lower pons. For the leg, the location of the CST was 45.68% in the up- per pons and 39.01% in the lower pons. MEDULLA The CST descends through the medullary pyramid (MP) in the medulla. The MP is the narrowest area through which the CST descends in the human brain.1,22 The location of the whole CST in the MP can be easily identified on a con- ventional MRI or a color map of DTI. However, little is known about somatotopy in the MP. To the best of our knowledge, only one study has been reported on this topic. Using normalized DTT, Kwon, et al.16 investigated the so- matotopic arrangement of the CST at the MP in 30 normal human brains. They found that the hand somatotopy of the CST was located in the medial portion of the MP; in con- trast, the leg somatotopy occupied the lateral portion of the MP. CONCLUSIONS In the current review, I reviewed previous studies of somato- topy of the CST in the brainstem of the human brain. De- tailed knowledge of CST somatotopy is important in terms of rehabilitative management and invasive procedures for patients with brain injury; however, few DTI studies have been conducted on this topic. To the best of my knowledge, only nine studies on this topic have been reported: midbrain (2 studies), pons (6 studies), and medulla (1 study). There- fore, further DTI studies should be conducted in order to ex- pand the literature on this topic. In particular, research on midbrain and medulla should be encouraged, because only three studies have been reported. Recent developments in DTI allow depiction of various cranial nerves in the human brain; therefore, studies of the corticonuclear tract should also be encouraged.42,43 ACKNOWLEDGEMENTS This work was supported by a National Research Founda- tion of Korea Grant funded by the Korean Government (KRF-2008-314-E00173). Yonsei Med J http://www.eymj.org Volume 52 Number 4 July 2011 555