Adaptive plasticity of spino-extraocular motor coupling during locomotion in metamorphosing Xenopus laevis.

During swimming in the amphibian ITALIC! Xenopus laevis, efference copies of rhythmic locomotor commands produced by the spinal central pattern generator (CPG) can drive extraocular motor output appropriate for producing image-stabilizing eye movements to offset the disruptive effects of self-motion. During metamorphosis, ITALIC! X. laevisremodels its locomotor strategy from larval tail-based undulatory movements to bilaterally synchronous hindlimb kicking in the adult. This change in propulsive mode results in head/body motion with entirely different dynamics, necessitating a concomitant switch in compensatory ocular movements from conjugate left-right rotations to non-conjugate convergence during the linear forward acceleration produced during each kick cycle. Here, using semi-intact or isolated brainstem/spinal cord preparations at intermediate metamorphic stages, we monitored bilateral eye motion along with extraocular, spinal axial and limb motor nerve activity during episodes of spontaneous fictive swimming. Our results show a progressive transition in spinal efference copy control of extraocular motor output that remains adapted to offsetting visual disturbances during the combinatorial expression of bimodal propulsion when functional larval and adult locomotor systems co-exist within the same animal. In stages at metamorphic climax, spino-extraocular motor coupling, which previously derived from axial locomotor circuitry alone, can originate from both axial and ITALIC! de novohindlimb CPGs, although the latter's influence becomes progressively more dominant and eventually exclusive as metamorphosis terminates with tail resorption. Thus, adaptive interactions between locomotor and extraocular motor circuitry allows CPG-driven efference copy signaling to continuously match the changing spatio-temporal requirements for visual image stabilization throughout the transitional period when one propulsive mechanism emerges and replaces another.

Spinal efference copy signaling and gaze stabilization during locomotion in juvenile Xenopus frogs.

In swimming Xenopus laevis tadpoles, gaze stabilization is achieved by efference copies of spinal locomotory CPG output that produce rhythmic extraocular motor activity appropriate for minimizing motion-derived visual disturbances. During metamorphosis, Xenopus switches its locomotory mechanism from larval tail-based undulatory movements to bilaterally synchronous hindlimb kick propulsion in the adult. The change in locomotory mode leads to body motion dynamics that no longer require conjugate left-right eye rotations for effective retinal image stabilization. Using in vivo kinematic analyses, in vitro electrophysiological recordings and specific CNS lesions, we have investigated spino-extraocular motor coupling in the juvenile frog and the underlying neural pathways to understand how gaze control processes are altered in accordance with the animal's change in body plan and locomotor strategy. Recordings of extraocular and limb motor nerves during spontaneous "fictive" swimming in isolated CNS preparations revealed that there is indeed a corresponding change in spinal efference copy control of extraocular motor output. In contrast to fictive larval swimming where alternating bursts occur in bilateral antagonistic horizontal extraocular nerves, during adult fictive limb-kicking, these motor nerves are synchronously active in accordance with the production of convergent eye movements during the linear head accelerations resulting from forward propulsion. Correspondingly, the neural pathways mediating spino-extraocular coupling have switched from contralateral to strictly ipsilateral ascending influences that ensure a coactivation of bilateral extraocular motoneurons with synchronous left-right limb extensions. Thus, adaptive developmental plasticity during metamorphosis enables spinal CPG-driven extraocular motor activity to match the changing requirements for eye movement control during self-motion.

Premotor interneurons in the local control of stepping motor output for the stick insect single middle leg.

In insect walking systems, nonspiking interneurons (NSIs) play an important role in the control of posture and movement. As such NSIs are known to contribute to state-dependent modifications in processing of proprioceptive signals from the legs. For example, NSIs process a flexion of the femur-tibia (FTi) joint signaled by the femoral chordotonal organ (fCO) such that the stance phase motor output is reinforced in the active locomotor system. This phenomenon representing a reflex reversal is the first part of the "active reaction" (AR) and was hypothesized to functionally represent a major control feature by which sensory feedback supports stance generation. As NSIs are known to contribute to the AR, the question arises, whether they serve similar functions during stepping and whether the AR is generally part of the control system for walking. We studied these issues in vivo, in a single leg preparation of Carausius morosus with the leg kinematics being confined to changes in one plane, along the coxa-trochanteral and the FTi-joint. Following kinematic analysis, identified NSIs (E1-E8, I1, I2, and I4) were recorded intracellularly during single leg stepping at different velocities. We detected clear similarities between the activity pattern of NSIs during single leg stepping and their responses to fCO-stimulation during the generation of the AR. This strongly supports the notion that the motor output generated during the AR reflects part of the control regime for stepping. Furthermore, our experiments revealed that alterations in stepping velocity result from modifications in the activity of the premotor NSIs involved in stance phase generation.

Control of stepping velocity in the stick insect Carausius morosus.

We performed electrophysiological and behavioral experiments in single-leg preparations and intact animals of the stick insect Carausius morosus to understand mechanisms underlying the control of walking speed. At the level of the single leg, we found no significant correlation between stepping velocity and spike frequency of motor neurons (MNs) other than the previously shown modification in flexor (stance) MN activity. However, pauses between stance and swing motoneuron activity at the transition from stance to swing phase and stepping velocity are correlated. Pauses become shorter with increasing speed and completely disappear during fast stepping sequences. By means of extra- and intracellular recordings in single-leg stick insect preparations we found no systematic relationship between the velocity of a stepping front leg and the motoneuronal activity in the ipsi- or contralateral mesothoracic protractor and retractor, as well as flexor and extensor MNs. The observations on the lack of coordination of stepping velocity between legs in single-leg preparations were confirmed in behavioral experiments with intact stick insects tethered above a slippery surface, thereby effectively removing mechanical coupling through the ground. In this situation, there were again no systematic correlations between the stepping velocities of different legs, despite the finding that an increase in stepping velocity in a single front leg is correlated with a general increase in nerve activity in all connectives between the subesophageal and all thoracic ganglia. However, when the tethered animal increased walking speed due to a short tactile stimulus, provoking an escape-like response, stepping velocities of ipsilateral legs were found to be correlated for several steps. These results indicate that there is no permanent coordination of stepping velocities between legs, but that such coordination can be activated under certain circumstances.

Slow temporal filtering may largely explain the transformation of stick insect (Carausius morosus) extensor motor neuron activity into muscle movement.

Understanding how nervous systems generate behavior requires understanding how muscles transform neural input into movement. The stick insect extensor tibiae muscle is an excellent system in which to study this issue because extensor motor neuron activity is highly variable during single leg walking and extensor muscles driven with this activity produce highly variable movements. We showed earlier that spike number, not frequency, codes for extensor amplitude during contraction rises, which implies the muscle acts as a slow filter on the time scale of burst interspike intervals (5-10 ms). We examine here muscle response to spiking variation over entire bursts, a time scale of hundreds of milliseconds, and directly measure muscle time constants. Muscle time constants differ during contraction and relaxation, and contraction time constants, although variable, are always extremely slow (200-700 ms). Models using these data show that extremely slow temporal filtering alone can explain much of the observed transform properties. This work also revealed an unexpected (to us) ability of slow filtering to transform steadily declining inputs into constant amplitude outputs. Examination of the effects of time constant variability on model output showed that variation within an SD primarily altered output amplitude, but variation across the entire range also altered contraction shape. These substantial changes suggest that understanding the basis of this variation is central to predicting extensor activity and that the animal could theoretically vary muscle time constant to match extensor response to changing behavioral need.

Different motor neuron spike patterns produce contractions with very similar rises in graded slow muscles.

Graded muscles produce small twitches in response to individual motor neuron spikes. During the early part of their contractions, contraction amplitude in many such muscles depends primarily on the number of spikes the muscle has received, not the frequency or pattern with which they were delivered. Stick insect (Carausius morosus) extensor muscles are graded and thus would likely show spike-number dependency early in their contractions. Tonic stimulations of the extensor motor nerve showed that the response of the muscles differed from the simplest form of spike-number dependency. However, these differences actually increased the spike-number range over which spike-number dependency was present. When the motor nerve was stimulated with patterns mimicking the motor neuron activity present during walking, amplitude during contraction rises also depended much more on spike number than on spike frequency. A consequence of spike-number dependency is that brief changes in spike frequency do not alter contraction slope and we show here that extensor motor neuron bursts with different spike patterns give rise to contractions with very similar contraction rises. We also examined in detail the early portions of a large number of extensor motor neuron bursts recorded during single-leg walking and show that these portions of the bursts do not appear to have any common spike pattern. Although alternative explanations are possible, the simplest interpretation of these data is that extensor motor neuron firing during leg swing is not tightly controlled.

Natural neural output that produces highly variable locomotory movements.

We recorded fast extensor tibiae motor neuron activity during single-legged treadmill walking in the stick insect, Carausius morosus. We used this activity to stimulate the extensor muscle motor nerve, observed the resulting extensor muscle contractions under isotonic conditions, and quantified these contractions with a variety of measures. Extensor contractions induced in this manner were highly variable, with contraction measures having SDs of 12 to 51%, and ranges of 82 to 275%, when expressed as percentages of the means, an unexpectedly wide range for a locomotory pattern. Searches for correlations among the contraction measures showed that, in general, this high variability is not reduced by contraction measure covariation. Comparing responses (to identical input) across animals showed that extensor muscles from different animals generally significantly differed from one another. However, correlation analyses on these data suggested that these differences do not indicate that multiple extensor muscle subtypes exist. Extensor muscles instead appear to belong to a single class, albeit one with high animal to animal variability. These data thus provide another well-quantified example (along with Aplysia feeding) of a repetitive but highly variable motor pattern (in contrast to the high rhythmicity and stereotypy present in most other well-quantified repetitive motor patterns). We suggest this high variability could be an adaptive combination of locomotion, active sensing, and crypsis arising from the relatively low demand for locomotion in Carausius behavior, the highly fragmented environment the animal inhabits, and its need to avoid predatory attention.