Activity of individual reticulospinal neurons during different forms of locomotion in the lamprey

Lamprey (a lower vertebrate) can employ different modes of locomotion, i.e. swimming in open water and crawling in tight places. Swimming is due to the periodic waves of lateral undulations with reciprocal activity of right and left muscles. In contrast, crawling (forward and backward) is based on single waves with coactivation of muscles on two sides. Basic mechanisms of swimming and, most likely, crawling reside in the spinal cord, and are activated by supraspinal commands. The main source of these commands is the reticulospinal (RS) system. The goal of the present experiments was to characterize the activity of individual RS neurons during swimming and during crawling in a U-shaped tunnel. The activity was recorded by means of chronically implanted electrodes in freely behaving animals. All recorded RS neurons were active during swimming but silent in quiescent animals. Many of them (61%) showed phasic modulation of their firing rate approximately in phase with the activity of ipsilateral rostral muscles. The majority of the neurons (80%) were also active during crawling. Many of them either increased or decreased their activity during crawling as compared to the background activity. These changes were better correlated with the direction of progression (forward or backward) than with the direction of turning in the tunnel (right or left). No correlation of the activity of RS neurons during locomotion and their sensory inputs was found. The results of this study suggest that different modes of locomotion in lampreys can be caused by considerably overlapping groups of RS neurons.