title: PICs in motoneurons do not scale with the size of the animal: a possible mechanism for faster speed of muscle contraction in smaller species.
abstract: The majority of studies on the electrical properties of neurons are carried out in rodents, and in particular in mice. However, the minute size of this animal compared with humans potentially limits the relevance of the resulting insights. To be able to extrapolate results obtained in a small animal such as a rodent, one needs to have proper knowledge of the rules governing how electrical properties of neurons scale with the size of the animal. Generally speaking, electrical resistances of neurons increase as cell size decreases, and thus maintenance of equal depolarization across cells of different sizes requires the underlying currents to decrease in proportion to the size decrease. Thus it would generally be expected that voltage-sensitive currents are smaller in smaller animals. In this study, we used in vivo preparations to record electrical properties of spinal motoneurons in deeply anesthetized adult mice and cats. We found that PICs do not scale with size, but instead are constant in their amplitudes across these species. This constancy, coupled with the threefold differences in electrical resistances, means that PICs contribute a threefold larger depolarization in the mouse than in the cat. As a consequence, motoneuronal firing rate sharply increases as animal size decreases. These differences in firing rates are likely essential in allowing different species to control muscles with widely different contraction speeds (smaller animals have faster muscle fibers). Thus from our results we have identified a possible new mechanism for how electrical properties are tuned to match mechanical properties within the motor output system.NEW & NOTEWORTHYThe small size of the mouse warrants concern over whether the properties of their neurons are a scaled version of those in larger animals or instead have unique features. Comparison of spinal motoneurons in mice to cats showed unique features. Firing rates in the mouse were much higher, in large part due to relatively larger persistent inward currents. These differences likely reflect adaptations for controlling much faster muscle fibers in mouse than cat.
authors.affiliation: Department of Physiology, Northwestern University, Feinberg School of Medicine, Chicago, Illinois.
authors.forename: Seoan
authors.initials: S
authors.lastname: Huh
authors.affiliation: Department of Biomedical Engineering, Georgia Institute of Technology, Atlanta, Georgia.
authors.forename: Ramamurthy
authors.initials: R
authors.lastname: Siripuram
authors.affiliation: Department of Biomedical Engineering, Georgia Institute of Technology, Atlanta, Georgia.
authors.forename: Robert H
authors.initials: RH
authors.lastname: Lee
authors.affiliation: Division of Neurobiology, Barrow Neurological Institute, St. Joseph's Hospital and Medical Center, Phoenix, Arizona; and.
authors.forename: Vladimir V
authors.initials: VV
authors.lastname: Turkin
authors.affiliation: Division of Neurobiology, Barrow Neurological Institute, St. Joseph's Hospital and Medical Center, Phoenix, Arizona; and.
authors.forename: Derek
authors.initials: D
authors.lastname: O'Neill
authors.affiliation: Division of Neurobiology, Barrow Neurological Institute, St. Joseph's Hospital and Medical Center, Phoenix, Arizona; and.
authors.forename: Thomas M
authors.initials: TM
authors.lastname: Hamm
authors.affiliation: Department of Physiology, Northwestern University, Feinberg School of Medicine, Chicago, Illinois.|Department of Physical Medicine and Rehabilitation, Northwestern University, Feinberg School of Medicine, Chicago, Illinois.|Department of Physical Therapy and Human Movement Science, Northwestern University, Feinberg School of Medicine, Chicago, Illinois.
authors.forename: Charles J
authors.initials: CJ
authors.lastname: Heckman
authors.affiliation: Department of Physiology, Northwestern University, Feinberg School of Medicine, Chicago, Illinois; marin.manuel@neurobio.org.|Centre de Neurophysique, Physiologie et Pathologie, UMR 8119, CNRS/Université Paris Descartes, Paris, France.
authors.forename: Marin
authors.identifier: 0000-0002-5344-3572
authors.initials: M
authors.lastname: Manuel
mesh_terms: D000200:Action Potentials; D000818:Animals; D049628:Body Size; D002415:Cats; D005260:Female; D008297:Male; D051379:Mice; D009046:Motor Neurons / Q000166:cytology / Q000502:physiology*; D009119:Muscle Contraction; D011930:Reaction Time; D013045:Species Specificity
keywords: adult spinal motoneurons; electrical properties; persistent inward currents; voltage clamp