title: Characterization of motor units in behaving adult mice shows a wide primary range.
abstract: The mouse is essential for genetic studies of motor function in both normal and pathological states. Thus it is important to consider whether the structure of motor output from the mouse is in fact analogous to that recorded in other animals. There is a striking difference in the basic electrical properties of mouse motoneurons compared with those in rats, cats, and humans. The firing evoked by injected currents produces a unique frequency-current (F-I) function that emphasizes recruitment of motor units at their maximum force. These F-I functions, however, were measured in anesthetized preparations that lacked two key components of normal synaptic input: high levels of synaptic noise and neuromodulatory inputs. Recent studies suggest that the alterations in the F-I function due to these two components are essential for recreating firing behavior of motor units in human subjects. In this study we provide the first data on firing patterns of motor units in the awake mouse, focusing on steady output in quiet stance. The resulting firing patterns did not match the predictions from the mouse F-I behaviors but instead revealed rate modulation across a remarkably wide range (10-60 Hz). The low end of the firing range may be due to changes in the F-I relation induced by synaptic noise and neuromodulatory inputs. The high end of the range may indicate that, unlike other species, quiet standing in the mouse involves recruitment of relatively fast-twitch motor units.
authors.affiliation: McCormick Biomedical Engineering Department, Northwestern University, Evanston, Illinois;
authors.forename: Laura K
authors.initials: LK
authors.lastname: Ritter
authors.affiliation: McCormick Biomedical Engineering Department, Northwestern University, Evanston, Illinois; Department of Physiology, Feinberg School of Medicine, Northwestern University, Chicago, Illinois; Department of Physical Medicine and Rehabilitation, Feinberg School of Medicine, Northwestern University, Chicago, Illinois;
authors.forename: Matthew C
authors.initials: MC
authors.lastname: Tresch
authors.affiliation: Department of Physiology, Feinberg School of Medicine, Northwestern University, Chicago, Illinois; Department of Physical Medicine and Rehabilitation, Feinberg School of Medicine, Northwestern University, Chicago, Illinois; Department of Physical Therapy and Human Movement Sciences, Feinberg School of Medicine, Northwestern University, Chicago, Illinois; and.
authors.forename: C J
authors.initials: CJ
authors.lastname: Heckman
authors.affiliation: Laboratoire de Neurophysique et Physiologie, Université Paris Descartes, Institut des Neurosciences et de la Cognition, Centre National de la Recherche Scientifique UMR 8119, Paris, France.
authors.forename: Marin
authors.initials: M
authors.lastname: Manuel
authors.affiliation: Department of Physiology, Feinberg School of Medicine, Northwestern University, Chicago, Illinois; Department of Physical Therapy and Human Movement Sciences, Feinberg School of Medicine, Northwestern University, Chicago, Illinois; and v-tysseling@northwestern.edu.
authors.forename: Vicki M
authors.initials: VM
authors.lastname: Tysseling
mesh_terms: D000200:Action Potentials; D000818:Animals; D004567:Electrodes, Implanted; D004576:Electromyography; D005260:Female; D051379:Mice; D009046:Motor Neurons / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology; D011187:Posture / Q000502:physiology*; D012146:Rest / Q000502:physiology
keywords: electromyographic recordings; motoneuron; motor unit recordings; mouse