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Published February 11, 2015 | Published
Journal Article Open

Responses to Conflicting Stimuli in a Simple Stimulus–Response Pathway


The "local bend response" of the medicinal leech (Hirudo verbana) is a stimulus–response pathway that enables the animal to bend away from a pressure stimulus applied anywhere along its body. The neuronal circuitry that supports this behavior has been well described, and its responses to individual stimuli are understood in quantitative detail. We probed the local bend system with pairs of electrical stimuli to sensory neurons that could not logically be interpreted as a single touch to the body wall and used multiple suction electrodes to record simultaneously the responses in large numbers of motor neurons. In all cases, responses lasted much longer than the stimuli that triggered them, implying the presence of some form of positive feedback loop to sustain the response. When stimuli were delivered simultaneously, the resulting motor neuron output could be described as an evenly weighted linear combination of the responses to the constituent stimuli. However, when stimuli were delivered sequentially, the second stimulus had greater impact on the motor neuron output, implying that the positive feedback in the system is not strong enough to render it immune to further input.

Additional Information

© 2015 Baljon and Wagenaar. This article is freely available online through the J Neurosci Author Open Choice option. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution and reproduction in any medium provided that the original work is properly attributed. Received Sept. 12, 2014; revised Dec. 2, 2014; accepted Dec. 12, 2014. This work was supported by the Broad Foundations. D.A.W. is the recipient of a Career Award at the Scientific Interface from the Burroughs Wellcome Fund. We thank an anonymous reviewer for many valuable comments and in particular for the suggestion to consider bilateral pressure in the context of being pinched by a predator. The authors declare no competing financial interests. Author contributions: P.L.B. and D.A.W. designed research; P.L.B. performed research; P.L.B. and D.A.W. analyzed data; D.A.W. wrote the paper.

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