Activity of reticulospinal neurons during locomotion in the freely behaving Lamprey

The reticulospinal (RS) system is the main descending system transmitting c ommands from the brain to the spinal cord in the lamprey. It is responsible for initiation of locomotion, steering, and equilibrium control. In the pr esent study, we characterize the commands that are sent by the brain to the spinal cord in intact animals via the reticulospinal pathways during locom otion. We have developed a method for recording the activity of larger RS a xons in the spinal cord in freely behaving lampreys by means of chronically implanted macro-electrodes. In this paper, the mass activity in the right and left RS pathways is described and the correlations of this activity wit h different aspects of locomotion are discussed. In quiescent animals, the RS neurons had a low level of activity. A mild activation of RS neurons occ urred in response to different sensory stimuli. Unilateral eye illumination evoked activation of the ipsilateral RS neurons. Unilateral illumination o f the tail dermal photoreceptors evoked bilateral activation of RS neurons. Water vibration also evoked bilateral activation of RS neurons. Roll tilt evoked activation of the contralateral RS neurons. With longer or more inte nse sensory stimulation of any modality and laterality, a sharp, massive bi lateral activation of the RS system occurred, and the animal started to swi m. This high activity of RS neurons and swimming could last for many second s after termination of the stimulus. There was a positive correlation betwe en the level of activity of RS system and the intensity of locomotion. An a symmetry in the mass activity on the left and right sides occurred during l ateral turns with a 30% prevalence (on average) for the ipsilateral side.-R hythmic modulation of the activity in RS pathways, related to the locomotor cycle, often was observed, with its peak coinciding with the electromyogra phic (EMG) burst in the ipsilateral rostral myotomes. The pattern of vestib ular response of RS neurons observed in the quiescent state, that is, activ ation with contralateral roll tilt, was preserved during locomotion. In add ition; an inhibition of their activity with ipsilateral tilt was clearly se en. In the cases when the activity of individual neurons could be traced du ring swimming, it was found that rhythmic modulation of their firing rate w as superimposed on their tonic firing or on their vestibular responses. In conclusion, different aspects of locomotor activity-initiation and terminat ion, vigor of locomotion, steering and equilibrium control-are well reflect ed in the mass activity of the larger RS neurons.