Abstract
In this study, intraspinal axonal trajectories of corticospinal tract (CST) neurons in the digit area of the monkey motor cortex were quantitatively analyzed. In two adult rhesus monkeys, biotinylated dextran amine was injected into the digit area of the motor cortex, which was identified by intracortical microstimulation technique. Among the stained CST axons, those in the lateral funiculus contralateral to injection occupied 93 ± 6% (statistics in 54 slices from the 2 monkeys) in the C2-Th2 segments, and those on the ipsilateral side comprised 4.2 ± 4.0%. The CST axons descending in the ipsilateral ventral funiculus comprised 2.8 ± 2.6%, whereas no CST axons were found in the ventral funiculus on the contralateral side. No CST axon was found in the dorsal column on either side. The CST axons on the contralateral side to injection were mainly terminated in Rexed lamia VII of all the segments, and lamina IX in cervical enlargement (C6-Th1). On the ipsilateral side to injection, CST axons were mainly terminated in lamia VII of the C1-C4 segments and in lamina VIII of the C6-C8 segments, respectively. CST axons crossing the midline, were found mainly in the cervical enlargement. Furthermore, the crossed axons originating from bilateral CST axons terminated in laminae VII-IX after crossing the midline. These results clarified a novel aspect of ipsilateral corticospinal control of spinal circuits. [J Physiol Sci. 2007;57 Suppl:S159]
