Position-independent refinement of vagus motor neuron wiring

The vagus nerve (10 ^th cranial nerve) mediates brain-body communication by innervating and controlling various internal body parts, including the pharynx, larynx, and most visceral organs. Vagus sensory neurons send information about internal states to the brain, and vagus motor neurons return reflexive motor responses, such as gagging, swallowing, digestive enzyme secretion, gut peristalsis and heart rate adjustment. The diverse motor neurons underlying these bodily functions are topographically organized in the brainstem. However, the topographic map is continuous, with motor neurons innervating a common target, a “target group”, partly intermingled with neurons of other target groups, without clear boundaries. Particularly, motor neurons supplying different pharyngeal muscles are significantly overlapping in the topographic map throughout vertebrates. It remains unanswered how this intermingled diverse population of motor neurons can control different bodily functions. Through calcium imaging in larval zebrafish, we demonstrate that, when noxious stimulation is given focally to the larval pharynx, vagus motor neurons return stereotypic muscle activity patterns appropriate for the location of the stimulus. We show that vagus motor neurons that produce pharyngeal contraction following focal noxious stimulation are loosely distributed in the motor nucleus, consistent with the loose and overlapping distribution of anatomical target groups. This suggests that the connectivity of motor neurons is determined at the single-neuron level, not the neighboring population level. Remarkably, we show that pharynx-innervating motor neurons can maintain their appropriate response patterns to focal noxious stimulation even when their topographic organization is disrupted, and that mis-positioned motor neurons elaborate dendrites that extend towards the dendritic territory of their target group. We further identified that the motor activity patten is immature when motor neurons first initiate sensory-evoked responses and that maturation, dendrite extension and incorporation into the correct sensory motor circuit all depend on the efficiency of neurotransmission from the motor axons. Our data together suggest a position-independent wiring strategy that refines presynaptic connectivity of motor neurons through experience-dependent feedback regulation. We further demonstrate resilience to topographic manipulation in viscera-innervating vagus motor neurons, supporting that position-independent wiring is a general principle.