title: Mechanics of mouse ocular motor plant quantified by optogenetic techniques.
abstract: Rigorous descriptions of ocular motor mechanics are often needed for models of ocular motor circuits. The mouse has become an important tool for ocular motor studies, yet most mechanical data come from larger species. Recordings of mouse abducens neurons indicate the mouse mechanics share basic viscoelastic properties with larger species but have considerably longer time constants. Time constants can also be extracted from the rate at which the eye re-centers when released from an eccentric position. The displacement can be accomplished by electrically stimulating ocular motor nuclei, but electrical stimulation may also activate nearby ocular motor circuitry. We achieved specific activation of abducens motoneurons through photostimulation in transgenic mice expressing channelrhodopsin in cholinergic neurons. Histology confirmed strong channelrhodopsin expression in the abducens nucleus with relatively little expression in nearby ocular motor structures. Stimulation was delivered as 20- to 1,000-ms pulses and 40-Hz trains. Relaxations were modeled best by a two-element viscoelastic system. Time constants were sensitive to stimulus duration. Analysis of isometric relaxation of isolated mouse extraocular muscles suggest the dependence is attributable to noninstantaneous decay of active forces in non-twitch fibers following stimulus offset. Time constants were several times longer than those obtained in primates, confirming that the mouse ocular motor mechanics are relatively sluggish. Finally, we explored the effects of 0.1- to 20-Hz sinusoidal photostimuli and demonstrated their potential usefulness in characterizing ocular motor mechanics, although this application will require further data on the temporal relationship between photostimulation and neuronal firing in extraocular motoneurons.
authors.affiliation: Neurology Division, Louis Stokes Cleveland Department of Veterans Affairs Medical Center, Cleveland, Ohio; Department of Neurology, Case Western Reserve University, Cleveland, Ohio; jss6@case.edu.
authors.forename: John S
authors.initials: JS
authors.lastname: Stahl
authors.affiliation: Neurology Division, Louis Stokes Cleveland Department of Veterans Affairs Medical Center, Cleveland, Ohio;
authors.forename: Zachary C
authors.initials: ZC
authors.lastname: Thumser
authors.affiliation: Department of Neurobiology and Anatomical Sciences, University of Mississippi, Jackson, Mississippi;
authors.forename: Paul J
authors.initials: PJ
authors.lastname: May
authors.affiliation: Department of Physiology, University of Kentucky, Lexington, Kentucky;
authors.forename: Francisco H
authors.initials: FH
authors.lastname: Andrade
authors.affiliation: Department of Automatic Control and Systems Engineering, University of Sheffield, Sheffield, United Kingdom; and.
authors.forename: Sean R
authors.initials: SR
authors.lastname: Anderson
authors.affiliation: Department of Psychology, University of Sheffield, Sheffield, United Kingdom.
authors.forename: Paul
authors.initials: P
authors.lastname: Dean
mesh_terms: D000010:Abducens Nerve / Q000502:physiology*; D000818:Animals; D059329:Cholinergic Neurons / Q000378:metabolism / Q000502:physiology; D005133:Eye Movements; D051379:Mice; D009046:Motor Neurons / Q000378:metabolism / Q000502:physiology*; D009119:Muscle Contraction; D062308:Optogenetics; D012243:Rhodopsin / Q000235:genetics* / Q000378:metabolism
chemical_list: D012243:Rhodopsin
keywords: extraocular muscle; mechanics; oculomotor; time constant; viscoelastic