Postural development in rats.

Mammals adopt a limited number of postures during their day-to-day activities. These stereotyped skeletal configurations are functionally adequate and limit the number of degrees of freedom to be controlled by the central nervous system. The temporal pattern of emergence of these configurations in altricial mammals is unknown. We therefore carried out an X-ray study in unrestrained rats from birth (P0) until postnatal day 23 (P23). The X-rays showed that many of the skeletal configurations described in adult rodents were already present at birth. By contrast, limb placement changed abruptly at around P10. These skeletal configurations, observed in anesthetized pups, required the maintenance of precise motor control. On the other hand, motor control continued to mature, as shown by progressive changes in resting posture and head movements from P0 to P23. We suggest that a few innate skeletal configurations provide the necessary frames of reference for the gradual construction of an adult motor repertoire in altricial mammals, such as the rat. The apparent absence of a requirement for external sensorial cues in the maturation of this repertoire may account for the maturation of postural and motor control in utero in precocial mammals (Muir et al., 2000 for a review on the locomotor behavior of altricial and precocial animals).

Mice deficient for the close homologue of the neural adhesion cell L1 (CHL1) display alterations in emotional reactivity and motor coordination.

Motor and cognitive phenotypes were assessed in mice deficient for the close homologue of the L1 adhesion molecule (CHL1). The CHL1-deficient mice displayed signs of decreased stress and a modification of exploratory behaviour. The mice also showed motor impairments on the Rotarod, but they were able to move as fast as controls in the alleys of a T-maze. The observed changes were assumed to be related to a deficit in attention. In addition, gender differences in CHL1 deficits were found and are discussed in view of a possible interaction with other cell adhesion molecules (CAMs) during development. The results are discussed in relation with motor and cognitive deficits in the human, caused by mutations of the distal part of the chromosome 3 which contains the CHL1 orthologue.

Air-stepping in neonatal rats: A comparison of L-dopa injection and olfactory stimulation.

The kinematic parameters of air-stepping induced by 2 methods known to elicit locomotion (olfactory stimulation vs. L-dopa injection) were compared in 3-day-old rats. In the 1st stage, suspended pups were induced to step with an olfactory stimulus of soiled shavings from the nest. In the 2nd stage, they received a subcutaneous injection of L-dopa. Their movements were faster, with a larger amplitude and a phase delay in ipsilateral coupling. Third, the olfactory stimulus was presented in conjunction with L-dopa. The characteristics of locomotion returned to the same level as with the olfactory stimulus alone. These results suggest that olfactory stimulation involves higher nerve centers able to modulate the dopaminergic pathways. They are discussed in relation to the neural structure involved in locomotion.

Behavioural profiling of a murine Charcot-Marie-Tooth disease type 1A model.

Different features of motor behaviour were studied on a transgenic mouse model of Charcot-Marie-Tooth's disease (CMT). Mutants with 4 or 7 copies of the human PMP22 gene leading to a phenotype significantly close to CMT's disease type 1A were compared with control animals. The aim of the study was to validate this transgenic model and to characterise the impairments occurring in the various lines. Three main types of analysis were performed in 2-month-old mice without any peculiar visible deficit: (i) a study of standardised clinical tests (SHIRPA protocol) demonstrated that only a few motor deficits were expressed; (ii) a measurement of general spontaneous activity by means of a commercial video-tracking system was performed and revealed that the main spontaneous activities were identical in the three lines with, however, some slight localised modifications; and, (iii) by contrast, the three lines respond very differently to the footprints, grip strength, splay test and rotarod test. Even in lines with a significantly limited copy number of the transgene, we observed and quantified impairments. In conclusion, mutants of CMT1A seem to be a very pertinent model of this human pathology and will certainly be useful for therapeutic procedures and for theoretical studies on this disease.

Kinematics of rotation in place during defense turning in the crayfish Procambarus clarkii.

The kinematic patterns of defense turning behavior in freely behaving specimens of the crayfish Procambarus clarkii were investigated with the aid of a video-analysis system. Movements of the body and all pereiopods, except the chelipeds, were analyzed. Because this behavior approximates to a rotation in place, this analysis extends previous studies on straight and curve walking in crustaceans. Specimens of P. clarkii responded to a tactile stimulus on a walking leg by turning accurately to face the source of the stimulation. Angular velocity profiles of the movement of the animal's carapace suggest that defense turn responses are executed in two phases: an initial stereotyped phase, in which the body twists on its legs and undergoes a rapid angular acceleration, followed by a more erratic phase of generally decreasing angular velocity that leads to the final orientation. Comparisons of contralateral members of each pair of legs reveal that defense turns are affected by changes in step geometry, rather than by changes in the timing parameters of leg motion, although inner legs 3 and 4 tend to take more steps than their outer counterparts during the course of a response. During the initial phase, outer legs 3 and 4 exhibit larger stance amplitudes than their inner partners, and all the outer legs produce larger stance amplitudes than their inner counterparts during the second stage of the response. Also, the net vectors of the initial stances, particularly, are angled with respect to the body, with the power strokes of the inner legs produced during promotion and those of the outer legs produced during remotion. Unlike straight and curve walking in the crayfish, there is no discernible pattern of contralateral leg coordination during defense turns. Similarities and differences between defense turns and curve walking are discussed. It is apparent that rotation in place, as in defense turns, is not a simple variation on straight or curve walking but a distinct locomotor pattern.

The relationship between leg stepping pattern and yaw torque oscillations in curve walking of two crayfish species

Curve walking in two species of crayfish, Procambarus clarkii and Astacus leptodactylus, was investigated to test whether the mechanism underlying curve walking is the synchronous action of a centrally pre-programmed leg tripod or whether it is the action of one principal leg that produces the main body yaw torque. Curve walking was induced by an optomotor visual stimulus, and the yaw torque produced by the tethered animals was measured in open-loop conditions. Our main results suggest that the yaw torque oscillations in both P. clarkii and A. leptodactylus are related to the movement of outer leg 4 (i.e. leg 4 on the outside of the turn). That is, the peaks in the yaw torque occur, on average, in synchrony with the power stroke of outer leg 4. When comparing the results of this open-loop experiment on P. clarkii with results previously obtained for curve walking in untethered individuals of the same species, we found a much higher variability in leg coordination in the open-loop situation. Similarly, here we did not find the same level of synchrony in the tripod (formed by outer leg 4 and inner legs 2 and 5) observed during untethered free walking. Therefore, we suggest that tethered conditions may diminish the need for stability and thus allow outer leg 4 to produce a body rotation regardless of the leg stepping configuration. The characteristics of leg 4 are in line with its major role in turning. According to previous studies, legs 4 provide the largest force and the largest step amplitude during walking, and their force includes both a pulling and a pushing component which can facilitate the control of turning. Although it is apparent that outer leg 4 is not the only leg that can produce an inward yaw torque, its major role in modulating the yaw torque suggests that there may be a specific, centrally generated control of outer leg 4 during curve walking in crayfish.

Early walking in the neonatal rat: a kinematic study.

The development of the early stage of locomotion (between Postnatal Days 3 and 10) was studied in newborn rats. At this age, rats are known to perform limited locomotor activities, consisting of an inefficient nonpostural gait termed crawling. By providing appropriate olfactory stimulation, it was possible to override the pups' reluctance to walk and to discover their actual locomotor abilities. The step period decreased from 1,200 ms to 900 ms from Postnatal Days 4 to 9, showing both a regular decrease in the swing and a discontinuous decrease in the stance phase. The fore- and hindlimb periods stabilized early on an alternate pattern of coupling. The ipsilateral coupling shifted progressively from 220 degrees to 260 degrees in relation with the change in the gait pattern. In parallel with the change in timing, the newborn rats showed gradual changes in the foot position and in the interlimb spatial coordination. These results show that quadruped locomotion develops before postural control is acquired, in a continuous process as the nervous system develops.

Role of gravity in the development of posture and locomotion in the neonatal rat.

This report describes the early motor behaviour in the neonatal rat in relation with the maturation of sensory and motor elements of the central nervous system (CNS). The role of vestibular information during the week before (E14-21) and the 2 weeks after (P0-15) birth will be considered. There is a rostro-caudal gradient in the maturation of posture and locomotion with a control of the head and forelimbs during the first postnatal week and then a sudden acceleration in the functional maturation of the hindlimb. At birth, the neonatal rat is blinded and deaf; despite the immaturity of the other sensory systems, the animal uses its olfactory system to find the mother nipple. Vestibular development takes place between E8 and P15. Most descending pathways from the brainstem start to reach the lumbar enlargement of the spinal cord a few days before birth (reticulo-, vestibulospinal pathways as well as the serotonergic and noradrenergic projections); their development is not completed until the end of the second postnatal week. At birth, in an in vitro preparation, a locomotor activity can be evoked by perfusing excitatory amino acids and serotonin over the lumbar region. The descending pathways which trigger the activity of the CPG are also partly functional. At the same age both air stepping and swimming can be induced. Complex locomotion such as walking, trotting and galloping start later because it requires the maturation of the vestibular system, descending pathways and postural reflex regulation. The period around birth is critical to properly define how the vestibular information is essential for the structuring of the motor behaviour. Different types of experiments (hypergravity, microgravity) are planned to test this hypothesis.

Early olfactory-induced rhythmic limb activity in the newborn rat.

Locomotor-like rhythmic movements without postural constraints were elicited in newborn rats aged from a few hours to five days, using an olfactory stimulus provided by bedding materials. The rats were held in a sling with the front and the hind legs hanging on each side. The step frequency increased between postnatal days 0 and 4 (P0-P4); the step period was around 1 s at P0 and decreased during the following days. This decrease was larger in the forelimbs (650 ms at P4) than in the hindlimbs (750 ms at P4) and was mainly due to a decrease in stance duration. Both ipsilateral and contralateral legs moved in an alternating pattern. Analysis of the regulation of this pattern when a 1:1 or a 1:2 inter-leg coordination (with double steps) occurred showed that both anterior and posterior locomotion pattern generators were coupled very early. Results are discussed in relation to the locomotor activities studied at this early stage of life in other behavioral situations (swimming and air stepping), and in relation to fictive locomotion induced in vitro.

Curve walking in freely moving crayfish (Procambarus clarkii)

The curve walking of freely moving crayfish trained to walk along a curved path during homing behaviour was investigated using a video-analysis system. The leg kinematics and leg phase relationships, as well as the relationship between stepping patterns and body axis rotation measured relative to external references, were studied.

The anterior and posterior extreme positions of the power stroke (AEP and PEP, respectively) and step amplitudes were analysed. As in a previous study on crayfish curve walking on a treadmill, PEPs were more posterior in outer legs (the legs on the outside of the turn) than in the inner legs. As a result, outer legs showed larger step amplitudes than inner legs. Leg kinematics varied within each walking sequence. AEP leg angles (the angles between the body and leg axes at the AEP) tended to decrease over time for inner legs and increase for outer legs. This leg angle drift was present mainly in the anterior legs and it suggests that these legs did not completely compensate for the body rotation after each step. In addition, leg angle asymmetries in a direction opposite to that of leg angle drift were observed at the start of each curve-walking sequence, suggesting that the extensive training (3 weeks) may have allowed crayfish to anticipate the leg angle drift.

The rotational component of curve walking showed a discontinuous pattern, with the animal's body axis turning towards the inside of the curve only periodically. Analysis of cross-correlation functions showed that the angular acceleration of the body axis in the direction of the turn occurred during the power strokes of inner legs 2 and 5 and outer leg 4. While the tripod formed by these three legs showed in-phase relationships, the legs of the corresponding contralateral tripod (outer legs 2 and 5 and inner leg 4) were not in phase. We hypothesize that inner legs 2 and 5 and outer leg 4 act synergically causing the inward body rotation observed in curve-walking crayfish and that some of the asymmetries found in step geometry may be a passive phenomenon due to the body rotation.

Variability of leg kinematics in free-walking crayfish, Procambarus clarkii, and related inter-joint coordination

The inter-joint coordination in leg 4 of the crayfish Procambarus clarkii was investigated while they travelled freely along straight paths. Video analysis of the kinematics of the leg's three-dimensional motion, combined with a statistical method based on conjugate cross-correlation functions, showed stable inter-joint coordination in the leg kinematics. In particular, the inter-joint coordination involved a strong movement in the distal mero-carpopodite (M-C) joint occurring between the movements of the proximal thoraco-coxopodite and coxo-basal joints; thus, the leg extended during the swing phase and flexed during the stance phase. This synchronisation was mainly independent of global changes in the locomotor pattern induced by variation in speed or contralateral inter-leg coupling which occurred during free walking. The main changes in inter-joint coordination were found to be related to the appearance of a biphasic flexion/extension movement during each stroke of the step cycle when the leg retracted far backwards. This movement was observed more frequently in large animals and was therefore possibly related to changes in postural control. The functional role of this distal M-C joint movement in the leg motion is discussed.

Locomotor patterns in freely moving crayfish (Procambarus clarkii)

Freely walking crayfish, Procambarus clarkii, were studied using a video analysis procedure adapted especially for use with crayfish. The animals were placed in a tank and their homing behaviour was filmed as they returned in a straight line to their shelter. Various sequences were studied at the two following levels. First, the trajectory of each pair of legs (from leg 2 to leg 5) during the step cycle (power stroke and return stroke) was studied to measure stride length and to analyse in detail changes in acceleration. Each leg was found to contribute in a specific manner to locomotion. Second, ipsi- and contralateral leg coordination was investigated. Ipsilateral coordination was found to involve a metachronal organization from front to back in all the walking sequences recorded, whereas contralateral coordination involved, in addition to the weak alternate coupling commonly observed in treadmill walking, another coordination pattern where the legs on each side (legs 3 and 4) are in phase. The results obtained in these free-walking sequences are discussed and compared with those obtained previously, in particular in treadmill situations.

Possible use of environmental gradients in orientation by homing wood mice, Apodemus sylvaticus.

51 wood mice, Apodemus sylvaticus, tagged with chemoluminescent bulbs, were visually tracked in a series of night homing experiments. The mice did not go straight towards home, but wandered around the release area and covered distances equal to twice the radial lengths recorded. In the course of their walk, they orientated preferentially towards landscapes which resembled their usual habitat, irrespective of where their actual homeward direction lay. In spite of the randomness of their paths, homing success was good, but detailed analysis of homing time showed that the homing performances of the mice depended on the direction in which they vanished in the course of the observation period. The good fit of observed orientations with those obtained by computer simulation of klinokinesis along an axial gradient, and field observation of a vegetational gradient in the study area, are both consistent with the hypothesis that homing may have resulted from wandering, locally biased by a klinokinetic effect.