Sensory-motor transformation by individual command neurons.

Animals and humans maintain a definite body orientation in space during locomotion. Here we analyze the system for the control of body orientation in the lamprey (a lower vertebrate). In the swimming lamprey, commands for changing the body orientation are based on vestibular information; they are transmitted to the spinal cord by reticulospinal (RS) neurons. The aim of this study was to characterize the sensory-motor transformation performed by individual RS neurons. The brainstem-spinal cord preparation with vestibular organs was used. For each RS neuron, we recorded (1) its vestibular responses to turns in different planes and (2) responses in different motoneuron pools of the spinal cord to stimulation of the same RS neuron; the latter data allowed us to estimate the direction of torque (caused by the RS neuron) that will rotate the animal's body during swimming. For each of the three main planes (roll, pitch, and yaw), two groups of RS neurons were found; they were activated by rotation in opposite directions and caused the torques counteracting the rotation that activated the neuron. In each plane, the system will stabilize the orientation at which the two groups are equally active; any deviation from this orientation will evoke a corrective motor response. Thus, individual RS neurons transform sensory information about the body orientation into the motor commands that cause corrections of orientation. The closed-loop mechanisms formed by individual neurons of a group operate in parallel to generate the resulting motor responses.