A model of the feline medial gastrocnemius motoneuron-muscle system subjected to recurrent inhibition

Abstract

Recurrent inhibition in the mammalian spinal cord is complex, and its functions are not yet well understood. Skeletomotoneurons (alpha-MNs) excite, via recurrent axon collaterals, inhibitory Renshaw cells (RCs), which in turn inhibit alpha-MNs and other neurons. The anatomical and functional structure of the recurrent inhibitory network is nonhomogeneous, and the gain and filtering characteristics of RCs are modulated by inputs circumventing alpha-MNs. This complex organization is likely to play important roles for the discharge and recruitment properties of alpha-MNs. Modeling this system is a way of investigating hypothesized roles for normal functioning including muscle fatigue and different forms of physiological pathological tremor. In this paper, a detailed model including alpha-MNs, RCs, and the muscle fibers innervated by the alpha-MNs is presented. Outlines of the experimental data underlying the model and the modeling philosophy and procedure are presented. Then the behavior of a RC model is compared with experimental data reported in the literature. Model and experimental data agree well for burst responses elicited by synchronous single-pulse activation of different numbers of motor axons. In addition, the static relation between motor-axon activation rate and RC firing rate agree fairly well in model and experiment, and the same applies to the dynamic responses to step changes in motor-axon rate. The ultimate objective is to use this model in probing the role of recurrent inhibition in the control and stability of (isometric) muscular force under normal and altered conditions occurring during fatigue and muscle pain.

Simulation system of spinal cord motor nuclei and associated nerves and muscles, in a Web-based architecture

“…For Renshaw cells, these values were adapted so that the membrane time constant could make the cell fire a burst of action potentials in response to a supra-threshold input (Hultborn and Pierrot-Deseilligny 1979). The values of parameters such as rate constants and peak conductances (for default values, see V-d conductances panel in the Configuration module of the simulator) were determined so that the AHP, the f×I relation and the generation of action potential bursts followed data from the literature (Hultborn and Pierrot-Deseilligny 1979;Cleveland et al 1981;Walmsley and Tracey 1981;Windhorst 1990;Windhorst 1996;Uchiyama et al 2003). Parameters related to the IaIn and IbIn interneuron conductances were chosen so that these interneurons would fire isolated action potentials in response to a volley coming from the sensory afferents (Jankowska 1992).…”

Differential Postnatal Maturation of GABAA, Glycine Receptor, and Mixed Synaptic Currents in Renshaw Cells and Ventral Spinal Interneurons

“…In the adult spinal cord, RC-mediated recurrent inhibition modulates the dynamic behavior of motoneuron firing by acting as a gain regulator of their discharge and recruitment (Windhorst, 1996;Uchiyama et al, 2003;Hultborn et al, 2004). The fine organization of recurrent inhibition on different motoneuron types and pools allows RCs to influence motoneuron firing variability, force gain, and motor discharge synchronization in homonymous and synergistic motoneuron pools and muscles.…”

Effects of spinal recurrent inhibition on motoneuron short-term synchronization

“…An extensive description of the present model, including its justification by experimental data, was published earlier (Uchiyama et al 2003). The pertinent principles are summarized as follows.…”

“…In particular, the RC firing pattern depends on synaptic activation level and "after-effects" of previous activations (Windhorst 1990). Necessarily, our model of RC discharge in response to α-MN activation is complex as well and is not repeated here in mathematical detail for lack of space (full description in Uchiyama et al 2003). Rather, the basic ideas are qualitatively summarized as follows.…”