Simultaneous Intracellular Recording of a Lumbar Motoneuron and the Force Produced by its Motor Unit in the Adult Mouse <em>In vivo</em>

Abstract

The spinal motoneuron has long been a good model system for studying neural function because it is a neuron of the central nervous system with the unique properties of (1) having readily identifiable targets (the muscle fibers) and therefore having a very well-known function (to control muscle contraction); (2) being the convergent target of many spinal and descending networks, hence the name of "final common pathway"; and (3) having a large soma which makes it possible to penetrate them with sharp intracellular electrodes. Furthermore, when studied in vivo, it is possible to record simultaneously the electrical activity of the motoneurons and the force developed by their muscle targets. Performing intracellular recordings of motoneurons in vivo therefore put the experimentalist in the unique position of being able to study, at the same time, all the compartments of the "motor unit" (the name given to the motoneuron, its axon, and the muscle fibers it innervates 1 ): the inputs impinging on the motoneuron, the electrophysiological properties of the motoneuron, and the impact of these properties on the physiological function of the motoneurons, i.e. the force produced by its motor unit. However, this approach is very challenging because the preparation cannot be paralyzed and thus the mechanical stability for the intracellular recording is reduced. Thus, this kind of experiments has only been achieved in cats and in rats. However, the study of spinal motor systems could make a formidable leap if it was possible to perform similar experiments in normal and genetically modified mice.For technical reasons, the study of the spinal networks in mice has mostly been limited to neonatal in vitro preparations, where the motoneurons and the spinal networks are immature, the motoneurons are separated from their targets, and when studied in slices, the motoneurons are separated from most of their inputs. Until recently, only a few groups had managed to perform intracellular recordings of motoneurons in vivo [2][3][4] , including our team who published a new preparation which allowed us to obtain very stable recordings of motoneurons in vivo in adult mice 5,6 . However, these recordings were obtained in paralyzed animals, i.e. without the possibility to record the force output of these motoneurons. Here we present an extension of this original preparation in which we were able to obtain simultaneous recordings of the electrophysiological properties of the motoneurons and of the force developed by their motor unit. This is an important achievement, as it allows us to identify the different types of motoneurons based on their force profile, and thereby revealing their function. Coupled with genetic models disturbing spinal segmental circuitry [7][8][9] , or reproducting human disease 10,11 , we expect this technique to be an essential tool for the study of spinal motor system. Video LinkThe video component of this article can be found at http://www.jove.com/video/4312/ Protocol 1.Step One Pre-anesthetic medication: 1...

Escape from homeostasis: spinal microcircuits and progression of amyotrophic lateral sclerosis

“…A second type of diversification of MNs is that α-MNs themselves are not homogeneous. α-MN types can be defined by the contractile properties of the muscle fibers they innervate ( Bakels and Kernell 1993 ; Gardiner 1993 ; Manuel and Heckman 2012 ), with each MN forming synapses with muscle fibers of similar structural and functional properties ( Edström and Kugelberg 1968 ). Thus a single α-MN innervates multiple similar muscle fibers, which together constitute a motor unit, the functional unit of the motor system ( Buchthal and Schmalbruch 1980 ; Liddell and Sherrington 1925 ).…”

Muscle length-dependent contribution of motoneuron Cav1.3 channels to force production in model slow motor unit

“…viding systematic predictions that could be tested in animal experiments (51). First, the contribution of PIC activation to force production may be more effective for longer-than-optimal muscles as the motoneuron transitions from the quiescent to the firing state by a brief excitation either at the soma (Fig.…”

The Subprimary Range of Firing Is Present in Both Cat and Mouse Spinal Motoneurons and Its Relationship to Force Development Is Similar for the Two Species

“…Differences in the temperature of leg muscles during recording may therefore have contributed to the previously reported differences in muscle twitch dynamics between cats and mice because mice are extremely vulnerable to peripheral heat loss due to their high surface to weight ratio. Given that, in the previous SPR muscle force experiments described in mice Heckman, 2011, 2012), only core temperature was maintained at 38°C, this could help to explain their results of near maximal tetanic force during the SPR. In the current experiments, only by cooling the muscle to 25°C were we able replicate those findings (Manuel and Heckman, 2011), with stimulation frequencies corresponding to the mean end of the SPR producing 88% of maximal force.…”