Neural bottlenecks: axon count, distribution, and conduction in the Manduca sexta neck connective.

Large flying insects precisely control fast maneuvers, a demanding task made more difficult by the limitation that all information between the brain and body is transmitted through a single transmission line, the neck connective. Despite this neuroanatomical structure constraining both the amount and timing of all information between the brain and body, little is known about how severe these bottlenecks are. We sought to understand this structure in the hawkmoth Manduca sexta by directly measuring axon count and conduction velocities in their neck connective, using a nanometer-scale complete map of the neck connective in concert with microelectrode array recordings from hundreds of neurons. We hypothesized that Manduca opts for a large spatial bottleneck, with comparatively few neurons in their neck connective compared to their brain size, but latency constraints of agile flight necessitate adaptations for increased conduction velocity compared to small insects. Manduca had 8,874 total neck connective axons, a number similar to fruit flies despite Manduca’s order of magnitude greater body and brain size. Yet Manduca had far more giant axons, and the average conduction velocity of those axons exceeded 2 m/s, indicating a strong pressure on reducing neck connective latency. Both ascending and descending units were equally fast, and analyzing how velocity scales with diameter suggested adaptations beyond just axon size are increasing velocity. This data indicates Manduca’s neck connective faces similar requirements to other species in terms of number of neurons, but more acute pressures for higher conduction velocity and reduced latency in the neck connective.