A Neuromechanical Model of Spinal Control of Locomotion

We have developed a neuromechanical computational model of cat hindlimb locomotion controlled by spinal central pattern generators (CPGs, one per hindlimb) and motion-dependent afferent feedback. Each CPG represents an extension of previously developed two-level model (Rybak et al. J Physiol 577:617–639, 2006a, J Physiol 577:641–658, 2006b) and includes a half-center rhythm generator (RG), generating the locomotor rhythm, and a pattern formation (PF) network operating under control of RG and managing the synergetic activity of different hindlimb motoneuronal pools. The basic two-level CPG model was extended by incorporating additional neural circuits allowing the CPG to generate the complex activity patterns of motoneurons controlling proximal two-joint muscles (Shevtsova et al., Chap. 5, Neuromechanical modeling of posture and locomotion, Springer, New York, 2015). The spinal cord circuitry in the model includes reflex circuits mediating reciprocal inhibition between flexor and extensor motoneurons and disynaptic excitation of extensor motoneurons by load-sensitive afferents. The hindlimbs and trunk were modeled as a 2D system of rigid segments driven by Hill-type muscle actuators with force-length-velocity dependent properties. The musculoskeletal model has been tuned to reproduce the mechanics of locomotion; as a result, the computed motion-dependent activity of muscle group Ia, Ib, and II afferents and the paw-pad cutaneous afferents matched well the cat in vivo afferent recordings reported in the literature (Prilutsky et al., Chap. 10, Neuromechanical modeling of posture and locomotion, Springer, New York, 2015). In the neuromechanical model, the CPG operation is adjusted by afferent feedback from the moving hindlimbs. The 22 S. N. Markin et al. model demonstrates stable locomotion with realistic mechanical characteristics and exhibits realistic patterns of muscle activity. The model can be used as a testbed to study spinal control of locomotion in various normal and pathological conditions.