Role of Synaptic and Non-Synaptic Mechanisms Underlying Motor Neuron Control

Persistent Link:
http://hdl.handle.net/10150/202994
Title:
Role of Synaptic and Non-Synaptic Mechanisms Underlying Motor Neuron Control
Author:
Revill, Ann
Issue Date:
2011
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.
Abstract:
While motor neuron activity has been studied for many decades, the relative contribution of synaptic and non-synaptic mechanisms underlying this activity during natural behaviors is not well understood. Thus, the goal of this dissertation was to further understand the role of non-synaptic properties of motor neurons during voluntary activity. In particular, I considered three non-synaptic properties: persistent inward currents (PICs) that boost synaptic inputs, spike-threshold accommodation that affects recruitment threshold as excitation rates of rise slow, and spike-frequency adaptation that leads to a decrease in firing rate despite constant excitation levels. Computer simulations were employed to understand the potential effect that these properties could have on firing rate behavior. In particular, the focus was on paired motor unit recordings where a lower threshold motor unit’s firing rate served as a proxy for synaptic drive, and differences in firing rate (ΔF) were compared at a higher threshold unit’s recruitment and derecruitment. While ΔF has been used by others to estimate PIC activation, the simulation results indicated that each of these non-synaptic mechanisms could lead to positive ΔF. Furthermore, by varying contraction speed and duration it seemed possible to determine which property contributes to ΔF in vivo. The results from human experiments indicated that adaptation is most likely the predominant contributor to ΔF during natural behaviors. Additionally, positive ΔF was even observed in the genioglossus muscle of the tongue, where the role of PICs has been debated. These results suggested that ΔF may not the best method to detect PICs during natural behaviors. As such, I also considered whether there might be another metric to infer PIC activation during natural behaviors. Motor unit firing rates tend to plateau, or saturate, despite continued force increase, and one hypothesis is that PICs contribute to this behavior. Indeed, motor unit firing rate saturation was diminished by the addition of inhibition, which should have limited PIC activation. Therefore, this final study provided possible evidence for PIC activation during natural behaviors. Overall, this dissertation highlights that non-synaptic properties of motor neurons are activated during natural behaviors and that they contribute significantly to firing rate output.
Type:
text; Electronic Dissertation
Keywords:
Motor unit; persistent inward current; spike-frequency adaptation; spike-threshold accommodation; Physiological Sciences; Motor control; Motor neuron
Degree Name:
Ph.D.
Degree Level:
doctoral
Degree Program:
Graduate College; Physiological Sciences
Degree Grantor:
University of Arizona
Advisor:
Fuglevand, Andrew J.

Full metadata record

DC FieldValue Language
dc.language.isoenen_US
dc.titleRole of Synaptic and Non-Synaptic Mechanisms Underlying Motor Neuron Controlen_US
dc.creatorRevill, Annen_US
dc.contributor.authorRevill, Annen_US
dc.date.issued2011-
dc.publisherThe University of Arizona.en_US
dc.rightsCopyright © 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.en_US
dc.description.abstractWhile motor neuron activity has been studied for many decades, the relative contribution of synaptic and non-synaptic mechanisms underlying this activity during natural behaviors is not well understood. Thus, the goal of this dissertation was to further understand the role of non-synaptic properties of motor neurons during voluntary activity. In particular, I considered three non-synaptic properties: persistent inward currents (PICs) that boost synaptic inputs, spike-threshold accommodation that affects recruitment threshold as excitation rates of rise slow, and spike-frequency adaptation that leads to a decrease in firing rate despite constant excitation levels. Computer simulations were employed to understand the potential effect that these properties could have on firing rate behavior. In particular, the focus was on paired motor unit recordings where a lower threshold motor unit’s firing rate served as a proxy for synaptic drive, and differences in firing rate (ΔF) were compared at a higher threshold unit’s recruitment and derecruitment. While ΔF has been used by others to estimate PIC activation, the simulation results indicated that each of these non-synaptic mechanisms could lead to positive ΔF. Furthermore, by varying contraction speed and duration it seemed possible to determine which property contributes to ΔF in vivo. The results from human experiments indicated that adaptation is most likely the predominant contributor to ΔF during natural behaviors. Additionally, positive ΔF was even observed in the genioglossus muscle of the tongue, where the role of PICs has been debated. These results suggested that ΔF may not the best method to detect PICs during natural behaviors. As such, I also considered whether there might be another metric to infer PIC activation during natural behaviors. Motor unit firing rates tend to plateau, or saturate, despite continued force increase, and one hypothesis is that PICs contribute to this behavior. Indeed, motor unit firing rate saturation was diminished by the addition of inhibition, which should have limited PIC activation. Therefore, this final study provided possible evidence for PIC activation during natural behaviors. Overall, this dissertation highlights that non-synaptic properties of motor neurons are activated during natural behaviors and that they contribute significantly to firing rate output.en_US
dc.typetexten_US
dc.typeElectronic Dissertationen_US
dc.subjectMotor uniten_US
dc.subjectpersistent inward currenten_US
dc.subjectspike-frequency adaptationen_US
dc.subjectspike-threshold accommodationen_US
dc.subjectPhysiological Sciencesen_US
dc.subjectMotor controlen_US
dc.subjectMotor neuronen_US
thesis.degree.namePh.D.en_US
thesis.degree.leveldoctoralen_US
thesis.degree.disciplineGraduate Collegeen_US
thesis.degree.disciplinePhysiological Sciencesen_US
thesis.degree.grantorUniversity of Arizonaen_US
dc.contributor.advisorFuglevand, Andrew J.en_US
dc.contributor.committeememberBailey, E. Fionaen_US
dc.contributor.committeememberFellous, Jean-Marcen_US
dc.contributor.committeememberFregosi, Ralphen_US
dc.contributor.committeememberLevine, Richarden_US
dc.contributor.committeememberFuglevand, Andrew J.en_US
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