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Abstract

For the great majority of animals—certainly those of neurobiological interest—the body is bilateral, comprising a left side and a right side, which are near-mirror images across the plane of the midline. Since the nervous system is accordingly also bilateral, information must be transmitted from one side to the other in order for the organism to integrate sensory input and coordinate its movement. This requires some neurons to project to and then across the midline. Multiple decussation (projection across the midline) is very rare: axons usually cross the midline either one or zero times. In the vertebrate spinal cord and invertebrate ventral nerve cord, at least some of the molecules that are responsible for attracting growing axons to the midline are now known. Diffusible molecules from the UNC-6/netrin family form a gradient emanating from the midline, which is then detected by receptors from the UNC-40/DCC/frazzled family (for review, seeTessier-Lavigne and Goodman 1996).