Commissural circuits are brain and spinal cord connections which interconnect the two sides of the central nervous system (CNS). They play essential roles in brain and spinal cord processing, ensuring left-right coordination and synchronization of information and commands. During the formation of neuronal circuits, all commissural neurons of the central nervous system must accomplish a common task, which is to project their axon onto the other side of the nervous system, across the midline that delineates the two halves of the CNS. How this task is accomplished has been the topic of extensive studies over the last past 20 years and remains one of the best models to investigate axon guidance mechanisms. In the first part of this review, I will introduce the commissural circuits, their general role in the physiology of the nervous system, and their recognized or suspected pathogenic properties in human diseases. In the second part of the review, I will concentrate on two commissural circuits, the spinal commissures and the corpus callosum, to detail the cellular and molecular mechanisms governing their formation, mostly during their navigation at the midline. 1. Introduction 1.1. The Commissural Circuitry The commissural circuits form a complex network of projections interconnecting both sides of the CNS of bilaterals. Commissural neurons are present at all levels of the CNS and extend their axon across the midline to build circuits that are crucial for the integration of sensory modalities, motor commands, and processing of brain functions [1]. The telencephalon elaborates three major commissures: the anterior commissure, the hippocampal commissure, and the corpus callosum. The anterior commissure is composed of two branches interconnecting the temporal lobes and is implicated in emotional processing. The hippocampal commissure interconnects the left and right hippocampus, an enrolled gyrus flanking the lateral ventricle at temporal levels. The corpus callosum, which is in humans the largest tract of the brain, emerged in placental mammals [2]. It radiates dorsally from the whole cerebral cortex to interconnect left and right cortical areas (Figure 1(a)). Other brain structures elaborate additional commissures, such as the posterior commissure, which interconnects nuclei of the diencephalon. Finally, several axon tracts navigating long distances cross the midline at various places of the CNS. Organisms with bilateral vision have crossed and uncrossed visual pathways. The visual fibers arising from each eye cross the midline ventrally at the optic chiasm
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