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Figure 2. Unilateral labyrinthectomy does not affect hindlimb intersegmental coordination during swimming.A. Both UL54 and UL66 juvenile frogs showed a strong caudo-rostral body twisting towards the lesioned side, which was measured as the angle δ (black arc) between the eye (pink line) and knee (orange line) axes. B. Colored markers placed on the three main joints of both hindlimbs and on the back along the axis (upper schematic) enabled bilateral angular variations of the hip, knee and ankle to be measured (lower schematic). Each angular variation was then rectified and plotted against time (traces at extreme right) to allow delay measurements. Color code: blue, left hindlimb, purple, right hindlimb. C: Detail of ankle, knee and hip angular variations on the left side over two consecutive cycles. The knee-ankle delay (d) and hip-ankle delay (dâ) were then calculated between maximal angular values (corresponding to the maximal joint aperture) for each cycle, and for both hindlimbs. o: open joint; c: closed joint. D. Knee and hip movement delays relative to maximal ankle excursion expressed as a percentage of cycle duration. No significant differences were found between control (unfilled) and UL54 (shaded) or UL66 (filled) juveniles. Error bars indicate SEM.
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Figure 3. Neither pre- nor post-metamorphic UL alters hindlimb motor burst coordination in vitro.Sample recordings from left (L) and right (R) hindlimb extensor (ext) and flexor (flex) motor nerves during fictive rectilinear swimming in isolated brainstem/spinal cord preparations from intact (A), UL54 (B) and UL66 (C) juveniles. Schematic at left indicates recording electrode positions. The circular plots below each raw recording panel illustrate the corresponding phase relationships of locomotor-related activity in bilateral flexor (LR flex), extensor (LR ext) and ipsilateral left and right flexor/extensor activity (L flex/ext and R flex/ext, respectively). Measurements were pooled for each group. Individual dots represent the mean activity phase value for one animal, while the direction and length of each vector indicate respectively that populationâs grand mean phase value and the concentration of individual phase values around that mean. An upward (vertical) projecting vector (0°) indicates burst synchrony, whereas a downward pointing vector (180°) indicates burst alternation. Note the preservation of a similar coordination in all three groups, consisting of bilateral synchrony between homologous extensor or flexor motor bursts, and ipsilateral alternation between antagonistic flexor/extensor bursts. Scale bars: 1s.
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Figure 4. 3D model of a UL juvenile with twisted body trunk.Geometrical model of a lesioned animalâs trunk using the 3D finite element (FE) method, based on anatomical characteristics and the assumption that the main body/water interactive forces occur at the level of the trunk (A). Once the initial FE model body was built, a 37° torsion () was applied in the antero-posterior axis (B right) in order to simulate the mean body twist towards the lesioned side observed in UL juveniles (B left). Red markers n (nose), r (right hip) and l (left hip) indicate model orientation and together with the two dashed lines (see C), illustrate the modelâs torsion. C: The two artificial front and rear rigid body components, respectively simulating the scapula and pelvis belts, were linked by an elastic portion to which the torsion was applied. The pink and orange dashed lines indicate the medial plans of the front and rear rigid body components (green plans), respectively, while the l and r red markers correspond to the linear left and right limits of the rear medial plan, and the n marker indicates the front of the anterior plan. Dorsalis muscles were simulated by two actuators (blue dashed lines) placed on both sides of the antero-posterior axis between the two rigid components. D: Lateral (right) view of the twisted FE model corresponding to UL-induced distortion in juvenile frogs. Arrowhead indicates that the left hip marker (l) is on the non-visible side of the model.
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Figure 5. Different patterns of actuator activation induce different body distortions and resultant changes in aquatic displacement.AâL: Frontal (A, D, G, J), lateral right (B, E, H, K) and exploded view (C, F, I, L) of the FE model at initial resting state (AâC) and at the end of either a left side (âcontralesionalâ; DâF), symmetrical (Bilateral; GâI) or right side (âipsilesionalâ; JâL) activation of the longitudinal actuators. The angle at the end of each simulation and the resulting angular displacement during propulsion (; obtained from the subsequent dynamic simulation) are indicated in panels D, G and J. Arrowhead in B indicates that the left hip marker (l) is on the non-visible side of the model. M: Theoretical explanation of lesioned animal movement according to simulation results. A high angular velocity associated with the strong body torsion of animals with symmetrically-activated postural back muscles (top) induces a complete disequilibrium of the body (red arrow; value taken from panel G) during each hindlimb extension, as observed experimentally in UL66 juvenile frogs. In contrast and in correspondence with UL54 juvenile behavior, a lower angular velocity associated with the much reduced body torsion of animals with an asymmetrical propulsion/posture coupling (bottom) causes only a slight disequilibrium (green arrow; taken from panel D) that is subsequently compensated for by passive water reaction forces (black arrows) during the remainder of the kick cycle. Color code: black, dorsal, white, ventral, and grey, lateral sides of the body.
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Figure 6. Solely a pre-metamorphic UL alters dorsal muscle/limb extensor muscle coordination in the post-metamorphic frog.Sample left (L) and right (R) electromyographic (EMG) recordings from dorsal back muscle dorsalis trunci (dt) and ankle extensor muscle plantaris longus (pl) in intact (A), UL54 (B) and UL66 (C) juveniles. The sites of the vestibular lesion (UL) and EMG electrode placements are shown at left. The corresponding circular plots (layout equivalent to Figure 3 except that each dot represents the mean for a single forward rectilinear swim episode) indicate the lack of bilateral dorsalis and right side (ipsilesional) dorsalis/plantaris coordination in the UL54 group only. Scale bars: 1s.
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Figure 7. Thoraco-lumbar coordination in vitro is also exclusively affected by a pre-metamorphic UL.Sample simultaneous recordings from left (L) and right (R) thoracic ventral roots (Th2) and lumbar nerve branches to the left and right ankle extensor muscles (ext) during fictive rectilinear swimming in isolated brainstem/spinal cord preparations from intact (A), UL54 (B) and UL66 (C) juveniles. Schematic at left indicates electrode placements for nerve recordings. The corresponding circular plots (same layout as Figure 3) show that the lumbo-thoracic coordination on the right (ipsilesional) side was altered solely in isolated preparations from UL54 animals. Scale bars: 1s.
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Figure 8. Summary of changes occurring in spinal locomotor-related networks during metamorphosis and after a right-side labyrinthectomy.A: Normal metamorphic modifications to spinal motor networks responsible for propulsion and dynamic postural adjustments during swimming (see Beyeler et al., 2008). Note the symmetrical left-right organization in the post-metamorphic juvenile frog. B: In already metamorphosed animals, UL causes asymmetry in the activity of descending brainstem commands to the spinal motor networks, producing an over-excitation on the lesioned side that leads to the expression of rolling behavior. This persistent descending imbalance during juvenile-to-adult maturation has no long-term influence on spinal network organization and animals never recover an effective locomotor capability. C: An acute UL in pre-metamorphic tadpoles also produces an asymmetric descending influence that now persists through metamorphosis (see Lambert et al. 2013). In such an unbalanced developmental environment, however, the adult spinal motor networks are built differently from normal through the establishment of a local asymmetry in propriospinal interactions that are somehow able to counterbalance the asymmetry in the descending commands to allow the restoration of swimming in the post-metamorphic frog. Red arrows: post-UL development; Black arrows: normal development; Double arrow: metamorphic development; Simple arrow: post-metamorphic maturation; Red cross: acute UL; Red dot: persistent UL. The widths of vertical arrows, arrowheads and circuit symbols are proportional to levels of activity.
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Figure 1. A unilateral labyrinthectomy at pre- or post-metamorphosis leads to distinct degrees of locomotor impairment in freely-behaving juvenile frogs.
A. Images of the hindlimb extension phase (near its termination) during three consecutive cycles of normal swimming (top) and rolling behavior (bottom) expressed by control and unilateral labyrinthectomy (UL; red dot)-lesioned stage 66 Xenopus. Each cycle consisted of alternate limb flexions (F) and extensions (E). UL-induced rolling behavior consisted of a semi-complete rotation of the animal around its longitudinal body axis during each hindlimb extension. B–D. Swimming behavior of intact control animals (n = 6) before and after metamorphosis (B), and acute and chronic effects of a right-side UL performed at stage 54 before (n = 9, UL54, C) or at stage 66 after (n = 12, UL66, D) metamorphosis. Histograms show the percentage of swim cycles in which normal rectilinear (unfilled), circling (light grey) or rolling (dark grey) trajectories were expressed in each animal group. Error bars indicate SEM.
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