ABSTRACT
The wiring of the nervous system arises from extensive directional migration of neuronal cell bodies and growth of processes that, somehow, end up forming functional circuits. Thus far, this feat of biological engineering appears to rely on sequences of pathfinding decisions upon local cues, each with little relationship to the anatomical and physiological outcome. Here, we uncover a straightforward cellular mechanism for circuit building whereby a neuronal type directs the development of its future partners. We show that visceral afferents of the head (that innervate taste buds) provide a scaffold for the establishment of visceral efferents (that innervate salivatory glands and blood vessels). In embryological terms, sensory neurons derived from an epibranchial placode--that we show to develop largely independently from the neural crest--guide the directional outgrowth of hindbrain visceral motoneurons and control the formation of neural crest-derived parasympathetic ganglia.
