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    The role of specific voltage-activated and calcium-activated potassium currents in shaping motor neuron firing output during rhythmic motor activity

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    Author
    McKiernan, Erin Christy
    Issue Date
    2010
    Advisor
    Levine, Richard
    Duch, Carsten
    
    Metadata
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    Publisher
    The University of Arizona.
    Rights
    Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
    Embargo
    Embargo: Release after 6/29/2011
    Abstract
    Rhythmic muscle contractions underlie a number of behaviors, such as respiration, ingestion, and locomotion, which are necessary for survival. Though in many cases motor neurons (MNs) themselves do not generate the basic motor rhythm, research suggests that the intrinsic properties of MNs may shape the timing of the final motor output. In particular, potassium (K⁺) currents can affect MN excitability and firing in a number of ways, including action potential repolarization, firing frequency, dendritic integration, repetitive firing, and burst termination. The roles played by K⁺ currents, therefore, make them ideal candidates to shape MN firing output during rhythmic activity. Just how crucial MN K⁺ currents are in shaping rhythmic motor output, which currents are crucial and what genes are responsible, and how these currents affect MN firing are areas of active investigation. Unfortunately, many previous studies attempting to answer such questions have relied on manipulations that affect large, and often unidentified, populations of neurons simultaneously. To effectively determine the role of MN currents in shaping rhythmic motor output we must target manipulations of specific ion channels to MNs. We used the Drosophila larval model system because the genetic tools available allow us to do just that. We targeted manipulations of specific calcium (Ca²⁺)-activated and voltageactivated K⁺ currents to identified populations of MNs. We hypothesized that MNs are not simply followers of network drive, but that MN firing output is further shaped during rhythmic motor activity by these K⁺ currents. Overall, our results show that though aspects of the motor output changed, such as burst duration, the system was remarkably robust in the face of K⁺ channel manipulations. Larvae expressing manipulations of the Ca²⁺-activated K⁺ channel, Slowpoke, and voltage-gated K⁺ channels Shal or eag/Shaker, continued to produce rhythmic motor output. Most manipulations had the strongest effect when expressed in cells other than just MNs, while restricting expression to smaller subsets of cells suggested that many of the effects were not intrinsic to the MNs. Even those manipulations which resulted in the strongest effects only changed burst or cycle durations by a few seconds, and did not affect the incidence or regularity of the rhythm. These results suggest that either these MN K⁺ currents minimally shape rhythmic motor output in this system, or compensation for the loss of K⁺ currents occurred. In sum, this work sheds light on the role of K⁺ currents in MNs during rhythmic activity, and points to a number of future directions examining the role of compensation in maintaining crucial rhythmic motor behaviors.
    Type
    text
    Electronic Dissertation
    Degree Name
    Ph.D.
    Degree Level
    doctoral
    Degree Program
    Graduate College
    Physiological Sciences
    Degree Grantor
    University of Arizona
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