The corticospinal tracts decussate at the lower end of the medulla, although some of their fibers may cross above this level. The fibers destined for the upper limb neurons cross first (more rostrally). The proportion of crossed and uncrossed fibers varies to some extent from one person to another (Nyberg). About 75 to 80 percent of the fibers cross and the remaining fibers descend ipsilaterally, mostly in the uncrossed ventral corticospinal tract. In exceptional cases, these tracts cross completely; equally rarely, they remain uncrossed. These variations are probably of functional significance in determining the amount of neurologic deficit that results from a unilateral cerebral lesion such as capsular infarction. A few well-studied cases are found, such as the one described by Terakawa and colleagues, of acute stroke of the cerebral hemisphere causing hemiplegia on the same side. Also, Yakovlev found instances of completely uncrossed pyramids among 130 autopsies of developmentally delayed neonates but considering the maldevelopment of these brains, the finding may not be generalizable.
The corticospinal tract is phylogenetically relatively new, being found only in mammals, which probably accounts for its variability between individuals as compared to the older vestibulospinal, rubrospinal, and reticulospinalparapyramidal systems, which are invariant among persons. Uncrossed fibers in the corticospinal tract account for mirror movements that are seen during efforts at fine motor tasks, particularly in children, and also in some disorders of the nervous system such as the Klippel-Feil syndrome and the Kallmann syndrome. For a more complete discussion of the crossing of the various tracts of the nervous system, the reader is referred to the review by Vulliemoz et al.
Beyond their decussation, the corticospinal pathways descend as well-defined bundles in the anterior and posterolateral columns of white matter (funiculi) of the spinal cord (see Fig. 3-2). The course of the noncorticospinal motor pathways (vestibulospinal, reticulospinal, and descending propriospinal) has been traced in humans by Nathan and his colleagues. The lateral vestibulospinal tract lies at the periphery of the cord, where it occupies the most anterolateral portion of the anterior funiculus. The medial vestibulospinal fibers mingle with those of the medial longitudinal fasciculus. Reticulospinal fibers are less compact; they descend bilaterally, and most of them come to lie just anterior to the lateral corticospinal tract. The propriospinal pathway (also called spinal-spinal) consists of a series of short fibers (one or two segments long) lying next to the gray matter.
The somatotopic organization of the corticospinal system is of importance in clinical work, especially in relation to certain stroke syndromes. As the descending axons subserving limb and facial movements emerge from the cortical motor strip, they maintain the anatomic organization of the overlying cortex; therefore, a discrete cortical–subcortical lesion will result in a restricted weakness of the hand and arm or the foot and leg. More caudally, the descending motor fibers converge and are collected in the posterior limb of the internal capsule, so that even a small lesion there will cause a “pure motor hemiplegia,” in which the face, arm, hand, leg, and foot are affected to more or less the same degree (see Lacunar syndromes in Chap. 33). The axons subserving facial movement are situated anterior in the posterior limb of the capsule, those for hand and arm in the central portion and those for the foot and leg, posteriorly (as detailed by Brodal).