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Medullary V2a RSNs are specifically recruited during MLR-induced forward swimming and encode the number of oscillations, TBF and amplitude. a, Schematic illustration of the experiment investigating medullary V2a RSN recruitment during MLR-induced forward swimming. b, Representation of behavioral periods during all experiments investigated; the color depicts the behavior type (blue: forward episode; pink: escape or struggle). c, Example bouts elicited during a 40-s MLR stimulation corresponding to either pure forward swimming (top trace), bout of mixed episodes (bottom left) or pure escape or struggle. d, Tail angle trace and calcium activity of the functional clusters of medullary V2a RSNs active during locomotion for an example fish (mean activity in the color trace, individual traces in black). e, Location of neurons in the medulla in dorsal view, color-coded according to the functional cluster. The empty circles represent neurons not active. f, Medullary V2a RSNs in the forward clusters were recruited more during forward swimming than during struggle behavior. Forward activity index calculated from calcium activity as (forward activity index = average (maximum ΔF/F during forward episodes) − average (maximum ΔF/F during escape or struggle) / average (maximum ΔF/F during forward episodes) + average (maximum ΔF/F during escape or struggle)). A positive index indicates an average maximum ΔF/F higher during forward than during escape or struggle behavior (5 fish; forward cluster: 1,438 cells (mean ± s.d.): 0.71 ± 0.37, one-sided t-test against μ = 0; t(1,437) = 72 *P < 0.001; rest of the clusters: 2,603 cells: −0.17 ± 0.72, one sample t-test against μ = 0, t(6,626) = −12.6, *P < 0.001). g1, Linear regression between the maximum ΔF/F of individual V2a RSNs from the forward cluster and the number of oscillations of the forward episodes (n = 132 neurons in n = 5 fish, n = 8 trials; the gray lines represent the regression for individual neurons, the blue line the regression for all neurons). g2, Location of neurons in the forward cluster; in blue are the ones whose response amplitude correlated the most with the number of oscillations (blue filled circles, n = 38 of 132 neurons in n = 5 fish, n = 6 of 8 trials, P < 0.12 and correlation coefficient ≥ 0.8; uncolored circles represents the other neurons). h, Motor regressors encoding distinct kinematic parameters: the iTBF, the iTBA and their binary positive derivative. Raw trace (black) and corresponding regressor (color) of each motor feature. Bottom trace: corresponding tail angle. i, Top: Traces from two example neurons that were recruited during the forward episodes and whose calcium activity differed during the episode. Middle: two motor regressors that best recapitulated the calcium activity of the two neurons above (matching color codes). Bottom: corresponding tail angle trace. j, Left: subsets of V2a medullary RSNs encode the iTBF (n = 5 fish, n = 6 experiments, n = 59 of 89 neurons). Right: distribution of V2a RSNs whose activity encoded TBF and either an increase in TBA (yellow outline) or number of oscillations (blue outline, from g2). k, Activity map comparison of medullary V2a RSNs obtained during the forward component of spontaneous locomotion (Fig. 7) and MLR-induced forward locomotion (Fig. 8g2). Note the similarity of the forward-component clusters located in the rostral and caudal medulla (dotted brackets). Source data
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