Current Biology
Volume 25, Issue 15, 3 August 2015, Pages 2012-2017
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A Leg-Local Neural Mechanism Mediates the Decision to Search in Stick Insects

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Highlights

  • A non-spiking interneuron (I4) is necessary and sufficient for stick insect leg searching

  • Sensory input signaling ground contact blocks the effects of this command neuron

  • Each leg has its own I4

  • Command-based control of behavior can thus function on the level of single legs

Summary

In many animals, individual legs can either function independently, as in behaviors such as scratching or searching, or be used in coordinated patterns with other legs, as in walking or climbing. While the control of walking has been extensively investigated, the mechanisms mediating the behavioral choice to activate individual legs independently are poorly understood. We examined this issue in stick insects, in which each leg can independently produce a rhythmic searching motor pattern if it doesn’t find a foothold [1, 2, 3, 4]. We show here that one non-spiking interneuron, I4, controls searching behavior in individual legs. One I4 is present in each hemi-segment of the three thoracic ganglia [5, 6]. Search-inducing sensory input depolarizes I4. I4 activity was necessary and sufficient to initiate and maintain searching movements. When substrate contact was provided, I4 depolarization no longer induced searching. I4 therefore both integrates search-inducing sensory input and is gated out by other sensory input (substrate contact). Searching thus occurs only when it is behaviorally appropriate. I4 depolarization never elicited stepping. These data show that individual, locally activated neurons can mediate the behavioral choice to use individual legs independently. This mechanism may be particularly important in insects’ front legs, which can function independently like vertebrate arms and hands [7]. Similar local command mechanisms that selectively activate the pattern generators controlling repeated functional units such as legs or body segments may be present in other systems.

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Present address: Department of Neuroscience, Karolinska Institute, 17177 Stockholm, Sweden