Lateral turns in the lamprey. II. Activity of reticulospinal neurons during the generation of fictive turns

We studied the neural correlates of turning movements during fictive locomo tion in a lamprey in vitro brain-spinal cord preparation. Electrical stimul ation of the skin on one side of the head was used to evoke fictive turns. Intracellular recordings were performed from reticulospinal cells in the mi ddle (MRRN) and posterior (PRRN) rhombencephalic reticular nuclei, and from Mauthner cells, to characterize the pattern of activity in these cell grou ps, and their possible functional role for the generation of turns. All rec orded reticulospinal neurons modified their activity during turns. Many cel ls in both the rostral and the caudal MRRN, and Mauthner cells, were strong ly excited during turning. The level of activity of cells in rostral PRRN w as lower, while the lowest degree of activation was found in cells in cauda l PRRN, suggesting that MRRN may play a more important role for the generat ion of turning behavior. The sign of the response (i.e., excitation or inhi bition) to skin stimulation of a neuron during turns toward (ipsilateral), or away from (contralateral) the side of the cell body was always the same. The cells could thus be divided into four types: 1) cells that were excite d during ipsilateral turns and inhibited during contralateral turns; these cells provide an asymmetric excitatory bias to spinal networks and presumab ly play an important role for the generation of turns; these cells were com mon (n=35; 52%) in both MRRN and PRRN; 2) cells that were excited during tu rns in either direction; these cells were common (n=19; 28%), in particular in MRRN; they could be involved in a general activation of the locomotor s ystem after skin stimulation; some of the cells were also more activated du ring turns in one direction and could contribute to an asymmetric turn comm and; 3) one cell that was inhibited during ipsilateral turns and excited du ring contralateral turns; and 4) cells (n=12; 18%) that were inhibited duri ng turns in either direction. In summary, our results show that, in the lam prey, the large majority of reticulospinal cells have responses during late ral turns that are indicative of a causal role for these cells in turn gene ration. This also suggests a considerable overlap between the command syste m for lateral turns evoked by skin stimulation, which was studied here, and other reticulospinal command systems, e.g., for lateral turns evoked by ot her types of stimuli, initiation of locomotion, and turns in the vertical p lanes.