Role of spinal sensorimotor circuits in triphasic command: a simulation approach using Goal Exploration Process

During rapid voluntary limb movement about a single joint, a stereotyped triphasic pattern is typically observed in the electromyograms (EMGs) of antagonistic muscles acting at this joint. To explain the origin of such triphasic commands, two types of theories have been proposed. Peripheral theories consider that triphasic commands result from sensorimotor spinal networks, either through a combination of reflexes or through a spinal central pattern generator. Central theories consider that the triphasic command is elaborated in the brain. Although both theories were partially supported by physiological data, there is still no consensus about how exactly triphasic commands are elaborated. Moreover, capacities of simple spinal sensorimotor circuits to elaborate triphasic commands on their own have not been tested yet. In order to test this, we modelled arm musculoskeletal system, muscle activation dynamics, proprioceptive spindle and Golgi afferent activities and spinal sensorimotor circuits. Descending step commands were designed to modify the activity of spinal neurons and the strength of their synapses, either to prepare (SET) the network before movement onset, or to launch the movement (GO). Since these step commands do not contain any dynamics, changes in muscle activities responsible for arm movement rest entirely upon interactions between the spinal network and the musculo-skeletal system. Critically, we selected descending step commands using a Goal Exploration Process inspired from baby babbling during development. In this task, the Goal Exploration Process proved to be very efficient. It proficiently discovered step commands that enabled spinal circuits to handle a broad spectrum of functional behaviors. Notably, this accomplishment was predominantly realized while eliciting natural triphasic commands, thereby substantiating the inherent capacity of the spinal network. ### Competing Interest Statement The authors have declared no competing interest.