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More than a century ago, the Spanish histologist Santiago Ramón y Cajal questioned the teleology of midline crossing. He provided the most comprehensive explanation to date on this topic in an article published in 1898.36 On the basis that the eye lens inverts images forming on the retina with respect to the outside world, Cajal suggested that crossing at the chiasm was necessary to restore image continuity in the brain (figure 1).36,37 Crossing is either complete or partial, depending on the existence of binocular vision, generating a representation of each visual hemiworld in the opposite side of the brain. The geometry of the visual tract being set by optical constraints, crossing of the tactile pathways is necessary to allow these two sensory inputs to gather in the brain, generating a global sensory representation contralateral to the stimulus. Motor decussation follows the crossed sensory representation to allow the correct limb to be activated upon sensory stimulation. Because the exquisite manual skills afforded by the CST are highly dependent on adequate visual and tactile inputs, the structure of this tract is particularly influenced by the anatomy of the visual and sensitive pathways, and extensive midline crossing occurs. The generalised crossing of output and input tracts has allowed the gathering of multiple sensory and motor modalities, culminating in the large associative brain areas that are the hallmark of the human cortex.
According to Cajal’s model, an inverse relation would exist throughout evolution between the proportion of fibres crossing at the chiasm and the proportion of decussating CST fibres. To our knowledge, this has never been assessed as such, but we believe another evolutionary argument may sustain the theory. In limbless primitive species, a threatening stimulus on the left side of the body perceived in the right hemisphere evokes a flight reaction through contraction of the ipsilateral (right) axial musculature (figure 2), which is mediated by the reticulospinal and vestibulospinal tracts, without the need for midline crossing. A limbed vertebrate, on the other hand, will attempt to escape a similar left-side threat by extending left limbs, pushing on the ground to turn to the right. In this case, the response occurs via the phylogenetically younger rubrospinal and corticospinal tracts, which cross the
midline (figure 2). Formulated more than 100 years ago, Cajal’s hypothesis remains seductive and self-standing, still waiting to be challenged. The use of modern molecular and imaging techniques could allow, as he stated, “more acute observers to dissipate the darkness”.
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