Article Figures & Data
Figures
- Figure 1.
Kinematics of limb and eye movements during swimming activity of young postmetamorphic Xenopus frogs. A–C, Analysis of free swimming behavior. A1, Plots of displacement velocity versus time during three different swim episodes from a single animal. Episodes comprised 4–6 cycles and differed considerably in their peak velocities and durations. A2, Three episodes from different animals, again showing very different peak velocities and episode durations. B, Velocity versus time trajectory of a typical single swim cycle (indicated in A2) with corresponding pictograms showing body and limb positions at successive points during the initial propulsive (bilateral hindlimb extension), gliding (limbs held extended), and recovery (bilateral limb flexion) phases. C, Peak velocity versus cycle duration during swim episodes in one animal (blue data points) and pooled measurements from six different animals (black points). In both cases, the coefficients of linear regression were significantly negative (single animal, r2 = 0.62; group, r2 = 0.46). ***p < 0.001. D, Ocular movements associated with swimming-related hindlimb kicking. D1, Schematic of the right eye at resting position showing sequential positional changes during a single kick cycle analyzed from a high-speed video recording of a head-restrained froglet with freely moving hindlimbs. Eye motion during limb movement is indicated by changes in the x-y coordinates of two marker spots placed on either side of the eye that were tracked through individual frames of the video sequence and connected by lines for better visualization. The green line (1) indicates initial resting position; orange lines show positional changes of the eye related to hindlimb extension with the thick orange line (2) indicating the maximal inward rotation during limb extension; purple line (3) denotes fully retracted eye position at the end of limb extension; white lines show release from retraction during hindlimb flexion with the turquoise line (4) depicting the eye position at the end of the kick cycle. For the purpose of illustration, the two overlapping steps of ocular motion (rotation and retraction) during limb extension are depicted separately in the small pictograms at right. The maximal angle of medial (inward) rotation was 23.5° from resting position (0°; bottom pictograms). The numbered (and colored) eye positions in D1 correspond to the pictograms and trajectory colorations in B as well as to the numbering of the single video frames of eye positions in D2. D3, Wider angle images showing eye and associated right hindlimb positions during a leg extension (numbering corresponds to B,D1,D2). Positional changes of the right eye were mirrored by those of the left eye to produce coincident, convergent ocular movement in each cycle.
- Figure 2.
Spinal CPG and extraocular motor activity during spontaneous fictive swimming in vitro. A1, Photograph of the isolated brainstem/spinal cord preparation (ventral side up) showing recording sites from motor nerve branches innervating the left (le) and right (ri) hindlimb tibialis anterior (ankle flexor, Flex, c) and plantaris longus (ankle extensor, Ext, d) muscles, the bilateral ventral roots of the second thoracic (Th2) cord segment (b), and bilateral motor nerves to extraocular muscles (a). A2, During an episode of spontaneous fictive swimming, ipsilateral Ext and Flex limb nerves were active in alternation, whereas homologous bilateral nerves expressed in-phase bursting. Th2 ventral root activity was limb extensor burst-timed. B1, Simultaneous recordings of left and right medial rectus (MR) nerve branches (left) that arise from the cranial oculomotor nerves (Ocl) and both lateral rectus (LR) nerve branches (right) arising from the cranial abducens nerve (Abd) along with right limb extensor motor nerve during fictive swimming. In both cases, extraocular motor discharge occurred in bilateral coordination with rhythmic extensor nerve bursting. B2, Event correlation analysis of spino-extraocular motor coupling during fictive swimming in a different preparation. Left, Individual auto-correlograms for ri-MR, ri-LR, and ri-Ext nerve discharge. Right, Corresponding cross-correlograms showing a strong in-phase relationship between the Ext and MR nerve bursts (black plot; r2 = 0.56, p < 0.001, n = 16 cycles). In the same fictive swim episode, LR activity was not significantly coupled to Ext bursts (p > 0.05, n = 16), despite evidence of Ext-timed LR modulation in the corresponding cross-correlogram (gray plot). C, Bilaterally synchronous, extensor burst-timed activity recorded from the le-Ocl and ri-Abd nerves during fictive swimming. Spino-extraocular motor coupling occurred during swim episodes with strong extensor discharge and associated thoracic (Th2) ventral root bursting (left), but not during weaker episodes (right; compare instantaneous frequencies of Ext nerve firing; bottom) when Th2 activation also failed to occur.
- Figure 3.
A sagittal midline separation of the cervicothoracic segments does not affect lumbar CPG-extraocular motor coupling. A, Control. A1, Schematic of the intact isolated preparation indicating recording protocol (left) and motor activity recorded from the le-Ocl, bilateral Abd, and ri-Ext nerves during spontaneous fictive swimming (right). A2, Corresponding circular phase plots of burst onsets in left and right Ocl (top) and Abd (bottom) nerves relative to ri-Ext burst onsets showing strongly significant phase synchrony (i.e., near 0°) in all cases (ri-Ocl: mean vector direction μ = 355.12°, vector length r = 0.94, phase distribution Z = 15.85; le-Ocl: μ = 0.53°, r = 0.93, Z = 16.58; ri-Abd: μ = 0.73°, r = 0.88, Z = 14.59; le-Abd: μ = 357.41°, r = 0.92, Z = 15.92; p < 0.001, n = 19 cycles from 4 animals for all plots). B, Postlesion. B1, Schematic of the same preparation after a mid-cord (C1–Th3) separation (left) and samples of recordings (right) from the same motor nerves as in A. B2, Auto- (top) and cross-correlations (bottom) for le-Abd versus ri-Ext activity indicating rhythmicity and the maintenance of strong extensor-Abd burst coupling (r2 = 0.83, p < 0.001, n = 8 cycles). B3, Circular analysis (pooled data from 4 animals) showing no postlesion alterations in burst onset synchrony (compare with A2) (ri-Ocl: μ = 349.42°, r = 0.94, Z = 15.05; le-Ocl: μ = 352.07°, r = 0.81, Z = 10.45; ri-Abd: μ = 2.96°, r = 0.83, Z = 11.7; le-Abd: μ = 348.83°, r = 0.96, Z = 15.65; p < 0.001, n = 17 cycles for all plots).
- Figure 4.
Sagittal midline lesions to the brainstem do not alter lumbar CPG-extraocular motor coupling. A, Control. A1, Schematic of the isolated preparation indicating recording protocol (left) and motor activity recorded from the bilateral Ocl, ri-Abd, and ri-Ext nerves during spontaneous fictive swimming (right) before a restricted sagittal midline incision at rhombomere 5. A2, Corresponding circular phase plots (data from 4 animals) of burst onsets in left and right Ocl (top) and Abd (bottom) nerves relative to ri-Ext burst onsets showing strongly significant phase synchrony in all cases (ri-Ocl: μ = 355.71°, r = 0.93, Z = 10.28; le-Ocl: μ = 356.65°, r = 0.9, Z = 9.61; ri-Abd: μ = 354.99°, r = 0.96, Z = 8.24; le-Abd: μ = 345.88°, r = 0.91, Z = 9.88; p < 0.001, n = 12 cycles for all plots). B, Postlesion. B1, Schematic of the same preparation after the rhombomere 5 lesion (left) and sample recordings (right) from the same motor nerves as in A during fictive swimming. B2, Event correlation analysis confirming the expression of le-Ocl and ri-Ext rhythmic bursting that remained strongly coupled (r2 = 0.74, p < 0.001, n = 21 cycles). B3, Circular phase analysis (data from 4 animals) showing the persistence of burst onset synchrony (relative to ri-Ext) after the lesion (compare with A2) (ri-Ocl: μ = 346.24°, r = 0.94, Z = 16.92, n = 19; le-Ocl: μ = 349.3°, r = 0.93, Z = 15.51, n = 18; ri-Abd: μ = 340.85°, r = 0.96, Z = 12.92, n = 14; le-Abd: μ = 346.53°, r = 0.92, Z = 15.34, n = 18; p < 0.001 in each case). Coordinated rhythmic bursting in le-Ext, le-Abd, and bilateral Ocl nerves (C) during swimming activity remained unaffected by a complete sagittal separation of the midbrain (D1). D2, Postlesion phase analysis (data from 3 animals) of burst onsets relative to ri-Ext bursts showing maintained burst synchrony (ri-Ocl: μ = 348.04°, r = 0.86, Z = 4.46, p < 0.001, n = 6 cycles; le-Ocl: μ = 322.23°, r = 0.91, Z = 4.92, p < 0.001, n = 6; ri-Abd: μ = 33.94°, r = 0.79, Z = 3.08, p < 0.05, n = 5; le-Abd: μ = 343.11°, r = 0.91, Z = 5.85, p < 0.001, n = 7).
- Figure 5.
Effects of a rostral cord hemisection in combination with sagittal spinal and brainstem lesions on lumbar CPG-extraocular motor coupling. Swimming-related activity recorded from the indicated nerves of an isolated preparation swimming before (A) and after (B1) a hemisection at the right obex plus a sagittal separation of the cervicothoracic (C1–Th3) cord segments (see schematic). B2, Corresponding circular phase plots of postlesion burst onsets in bilateral Ocl (top) and Abd (bottom) relative to ri-Ext showing a persistence of synchronous bursting after the two lesions (ri-Ocl: μ = 345.25°, r = 0.67, Z = 30.71, n = 69; le-Ocl: μ = 342.62°, r = 0.76, Z = 41.8, n = 72; ri-Abd: μ = 349.62°, r = 0.78, Z = 44.58, n = 74; le-Abd: μ = 345.33°, r = 0.83, Z = 51.93, n = 76; p < 0.001 in each case). C, A further sagittal lesion at rhombomere 5 of the same preparation (see schematic) completely suppressed spinal-driven re-Abd activity, whereas the le-Abd nerve (on the intact cord side) continued to express ri-Ext timed bursts. D, A complete longitudinal separation of the midbrain now suppressed ri-Ext timed activity in ri-Ocl, but lumbar-driven bursting in le-Ocl persisted.
- Figure 6.
Effects of a longitudinal midline separation of the brainstem and spinal cord on spino-extraocular motor coupling. A, Control. A1, Schematic of recording protocol (left) and motor activity recorded from the le-Ocl, ri-Abd, and le-Ext nerves during spontaneous fictive swimming (right). A2, Corresponding circular plots of burst onsets in bilateral Ocl and Abd nerves, and Ocl and Abd onsets relative to ri-Ext bursts, showing phase synchrony in all cases (ri- vs le-Ocl: μ = 339.53°, r = 0.45, Z = 16.52, n = 82; ri- vs le-Abd: μ = 349.18°, r = 0.56, Z = 24.35, n = 79; le-Ocl vs ri-Ext: μ = 353.24°, r = 0.61, Z = 29.38, n = 80; ri-Abd vs ri-Ext: μ = 341.55°, r = 0.78, Z = 49.68, n = 82; p < 0.001 in all plots). B1, Sample recording of swimming activity in the same preparation after a midline separation from the midbrain until the last thoracic (Th3) segment (see schematic). B2, Postlesion phase analysis still showed a tendency for synchronous activity in ri- vs le-Ocl (μ = 358.44°, r = 0.48, Z = 7.91, p < 0.001, n = 34) and ri- vs le-Abd (μ = 54.92°, r = 0.23, Z = 2.44, p = 0.086, n = 32). Activity in le-Ocl (μ = 334.54°, r = 0.73, Z = 18.11, p < 0.001, n = 34) and ri-Abd (μ = 345.87°, r = 0.55, Z = 15.96, p < 0.001, n = 52) also remained in synchrony with ri-Ext bursts.
- Figure 7.
Schematic representation of an intrinsic, nonsensory mechanism for gaze stabilization during rhythmic swimming movements in young adult X. laevis. During linear forward acceleration in each cycle generated by bilaterally synchronous hindlimb extensions, a spinal efference copy from the lumbar CPG network is conveyed to the brainstem extraocular motor nuclei via homolaterally ascending pathways to drive phase-coupled contractions of the left and right horizontal extraocular muscles. This in turn produces overall convergent eye movements to offset the disruptive visual consequences of propulsive body/head motion.
