Encoding and decoding of reticulospinal commands.

In the lamprey, the reticulospinal (RS) system is the main descending system transmitting commands to the spinal cord. We investigated these commands and their effect on the spinal mechanisms. The RS commands were studied by recording responses of RS neurons to sensory stimuli eliciting different motor behaviors. Initiation of locomotion was associated with symmetrical bilateral massive activation of RS neurons, whereas turns in different planes were associated with asymmetrical activation of corresponding neuronal groups. The sub-populations of RS neurons causing different motor behaviors partly overlap. We suggest that commands for initiation of locomotion and regulation of its vigour, encoded as the value of bilateral RS activity, are decoded in the spinal cord by integrating all RS signals arriving at the segmental locomotor networks. Commands for turns in different planes, encoded as an asymmetry in the activities of specific groups of RS neurons, are decoded by comparing the activities of those groups. This hypothesis was supported by the experiments on a neuro-mechanical model, where the difference between the activities in the left and right RS pathways was used to control a motor rotating the animal in the roll plane. Transformation of the descending commands into the motor responses was investigated by recording the effects of individual RS neurons on the motor output. Twenty patterns of influences have been found. This great diversity of the patterns allows the RS system to evoke body flexion in any plane. Since most neurons have asymmetrical projections we suggest that, for rectilinear swimming, RS neurons with opposite asymmetrical effects are co-activated.