Research

  • Research projects in the Motor Control Development Laboratory are addressing 3 critical questions in early sensorimotor development:

     

    • What are the behavioral and physiological attributes of prenatal movement experience?
    • Does the environment impact attributes of prenatal motor behavior?
    • Are there attributes of prenatal motor experience that shape the development of neonatal motor behaviors?
    Adapting and miniaturizing physiological and biomechanical methods for movement studies in humans, research projects in the Motor Control Development Laboratory are discovering how motor behavior is transformed over embryonic development. Concurrent with these transformations, the embryo's relationship to the environment changes dramatically. The embryo is initially buoyant and its movements are relatively unconstrained, but during the second half of development as body size increases, buoyancy diminishes, plus the rigid shell wall increasingly constrains movement, eventually forcing the embryo into an extremely flexed posture. The human fetal experiences similar changes relative to its environment during development. Our studies seek to determine whether these movement experiences play an instructive role as motor control is established.

    Our work has demonstrated that parameters of embryonic movement are altered by environmental perturbations, such as a reduction in buoyancy and fixation of a single limb joint. We have found two lines of evidence to suggest that alternations in motility patterns are attributable to more than transient mechanical phenomena. One, mechanical constraint of leg motions significantly alters the patterns of wing movement. Two, physical constraint can yield a net increase of activity. Recently, we have also found that the temporal pattern of exposure to light can alter the level of activity and accelerate development of breathing movements. These latter findings may indicate that environmental conditions can play an important role in preparing animals for hatching/birth. Recently we identified a select pattern of very rapid movements that emerge at a late embryonic age. Behavioral, physiological, and anatomical studies now in progress will test whether these unique movements under late-stage conditions are essential for successful hatching and postnatal mobility.

    For more information on the work performed in the Motor Control Development Laboratory click on one of the following:

    Posters

    Repetitive Limb Movements in the Late-Stage Chicken Embryo 
    The long-term objectives of our research are to determine if fetal movements and the environment in which they are generated contribute to postnatal motor behavior. In pursuit of these questions, we have asked if there is a developmental continuum in motor behavior during embryogenesis that links spontaneous motility early in embryonic development with the precocious walking behavior of chicks (gallus gallus) at hatching. We recently reported a motor behavior in chick embryos not previously studied, repetitive limb movement (RLM). RLM emerges in the finals days prior to hatching. During rhythmically stable sequences, leg muscles burst at frequencies (1-10 Hz) common to locomotion post hatching (Bradley et al, J Neurophysiol 2008). In this talk I will present recent evidence from our laboratory that intralimb muscle activity patterns for these movements share elements of both early motility and precocious walking. These patterns also exhibit burst deletions observed in adult fictive locomotor preparations. Findings from these studies appear to be of relevance to the notion of separate rhythm and pattern generation and that they may mature within different developmental time frames. In addition, data will be presented demonstrating that alternating interlimb coordination can also occur across the entire RLM frequency range, providing further evidence that RLM in the late-gestation embryo is a behavioral link between early motility and precocious locomotion in the chick. This work was supported by NIH NICHD grant R01HD053367.
     
    Evidence of a Fast Locomotor Burst Generator during Embryonic Motility in Chick 
    Chicks produce repetitive (0.2-2 Hz) leg movements by embryonic day 9 (E9). It is uncertain if a spinal pattern generator controls embryonic movements because transient immature network properties contribute to motility through at least E15. We recently described 2-10 Hz leg movements at E18 that had not been previously studied. In this study we examined muscle burst patterns and burst frequencies for these repetitive limb movements (RLM). The aim was to determine if burst frequencies for RLM are indicative of a rhythm burst generator and if maturing muscle afference can modulate the rhythm. Embryos were prepared for in ovo recording of spontaneous leg movements at E15, E18 or E20. Leg muscles were implanted with fine wire electrodes. Electromyograms and output of a force transducer tracking body movements were synchronized with video. The shell wall anterior to the leg was removed in some experiments to test if spatial constraints contributed to the faster frequencies. All procedures were approved by the university IACUC. Results summarize findings for 69 experiments producing 3232 RLM. Both leg flexor and extensor muscles were rhythmically active at E18 and E20. Rhythmic activity was present E15, but was less well formed. The ankle dorsi flexor was the most reliably rhythmic muscle at E18-E20; extensor muscles frequently dropped out. Thus sequences of stable dorsi flexor bursting (s.d. ≤ 1 Hz) were selected for analyses. Burst frequencies E18 largely fell between 2-6 Hz, with a mean frequency (± s.d.) of 4.0 ±1.0 Hz. Burst frequencies E20 were more broadly distributed between 2-11 Hz (5.5 ±1.9 Hz); the age-related difference was significant. Removing the shell wall anterior to the foot had little effect on leg posture or burst frequency at E18. At E20, embryos extended the foot outside the egg during 74% of RLM and burst frequencies shifted upward (6.5 ±1.7 Hz). Sequences of repetitive bursting were broadly distributed between 1-12 Hz, comparable to fast locomotor burst generation in lamprey. The slower frequencies are also observed during walking in hatchlings and the faster frequencies are observed during airstepping. Rhythmic flexor bursting persisted as extensor bursting dropped out, consistent with the modular burst generator proposed in fictive locomotion and scratching. Our data provide evidence that motility in chick may be controlled by a fast locomotor burst generator by E15 and modulation by proprioceptors may emerge between E18 and E20. This work was funded by NIH grant 1 R01 HD053367.

    Immutability and Flexibility of Muscle Activity Patterns during Rhythmic Leg Movements in Late Stage Chick Embryos 
    Previous studies in late stage chick embryos indicate that rhythmic leg movement (RLM) frequency ranges from slow to fast (1-12Hz). It is unknown how the central nervous system (CNS) exploits spinal motor networks to coordinate limb movements across the wide range of RLM frequencies in the late stage chick embryo. The purpose of the present study was to determine temporal features of muscle activity at embryonic day 20 during stable rhythmic RLM. To characterize muscle activity patterns electromyographic signals from sartorius (SA, hip flexor), femorotibialis (FT, knee extensor), and tibialis anterior (TA, ankle dorsiflexor) were assessed to measure burst duration, pause or cocontraction time, and relative phase of burst onsets between two muscles. TA was used to compute cycle frequency and as a reference to measure SA and FT burst patterns, i.e., TA vs. SA (TA-SA), and TA vs. FT (TA-FT). In TA-SA analyses, a cocontraction pattern was predominant (82.6%). A TA-SA pause (12.1%) and a SA double burst patterns (5.3%) were also observed. In the TA-SA cocontraction pattern, TA burst duration (63ms±39ms), SA burst duration (59ms±35ms), and cocontraction time (28ms±26ms) slightly decreased as cycle frequency increased. Relative phase (0.09±0.12) was preserved across all frequency ranges, suggesting that the temporal relationship between TA and SA in the cocontraction pattern was not affected by cycle frequency. In TA-FT analyses, a reciprocal pattern characterized by pause between TA and FT bursts was predominant (79.4%). TA-FT cocontraction (3.1%) and FT double burst patterns (17.5%) were also found. In the TA-FT reciprocal pattern, TA burst duration (63ms±42ms), FT burst duration (73ms±87ms), and pause time (92ms±200ms) slightly decreased with increasing cycle frequency. Relative phase (0.41±0.22) demonstrated that FT onset varied relative to TA onset at slow cycle frequencies, but merged more tightly around 0.4 at higher frequencies, suggesting that the TA-FT reciprocal pattern was flexible and frequency-dependent. The TA-SA pause and the TA-FT cocontraction patterns were observed only for 1-2 cycles within a burst sequence, suggesting that these were not preferred rhythmical patterns. The present study suggests that in late stage chick embryos, the spinal motor network can form different synergies and temporal combinations to produce various coordination patterns by exerting differential control of synergist and non-synergist muscles across a wide RLM frequency range. Also, the spinal network does not proportionally modulate burst duration with changes in cycle frequency. This study was funded by NIH grant 1 R01 HD053367.

    Impact of Postural Constraints on Muscle Activity during Rhythmic Leg Movements in Late Stage Chick Embryos 
    Late stage chick embryos produce spontaneous rhythmic leg movement (RLM) ranging 1-12 Hz during extremely flexed posture in ovo. It is not known how this flexed posture, common to many developing animals, impacts the neural control of RLM. The purpose of the present study was to determine if sensory modifications associated with changes in limb posture can alter RLM muscle characteristics at embryonic day 18 (E18) and 20 (E20). We compared RLM during natural posture (control) and ankle restraint (AR) in each embryo. AR was achieved by casting the ankle in an extended position (approximately 170º) so as to stretch muscle spindles in ankle dorsiflexors. Electromyographic signals from femorotibialis (FT, knee extensor), tibialis anterior (TA, ankle dorsiflexor), and lateral gastrocnemious (LG, ankle platarflexor) were assessed to measure three dependent variables, burst duration, cycle frequency, and mean burst amplitude. Preliminary data summarize findings for two E18 embryos and three E20 embryos, yielding 1557 bursts and 2337 bursts respectively. Cycle frequency of EMG bursting during RLM varied from slow to fast at E18 and E20. At E18, no differences between control and AR were found for any of the dependent variables across the 3 muscles, suggesting that restricting ankle motion in the extended posture did not alter the rhythmicity of muscle bursting or motor neuron excitability at this age. At E20, however, muscle-specific changes were observed between the two postural conditions. In TA, decreased mean cycle frequency was observed in two embryos, and increased burst duration and mean burst amplitude were observed across all embryos during AR compared to control. These results suggest that in more mature embryos, RLM rhythmicity and motor neuron excitability may be mutable in response to TA muscle spindle stretch. During AR, LG burst deletions were more common than in the control condition, and reduced LG mean burst amplitude was observed in one E18 embryo. These findings suggest that LG motor neuron excitation may be diminished by placing the muscle on slack at E20. There were no differences between control and AR across 3 dependent variables for FT. Taken together, preliminary findings suggest that by E20 sensory afferents are sufficiently mature to respond to postural constrains and to impact control of RLM, but may not be sufficiently mature to impact RLM at E18. This study was funded by NIH grant 1 R01 HD053367.

    Incubating Light Conditions Impact Precocious Locomotor Skill in Neonatal Chick 
    Chicken embryos typically hatch embryonic day 21 (E21), but they hatch a day earlier (E20) when exposed to continuous bright light during the incubation period, and a day later (E22) when incubated in complete darkness throughout incubation. Earlier studies have examined the effects of incubation lighting on general viability, such as hatchability and body weight, but have not considered its impact on motor control development. The objective of this study is to investigate if altered rates of embryonic development due to lighting conditions impact locomotor control within the first 24 hrs after hatching. Fertile chicken eggs obtained from a local hatchery were incubated at 36.9°C under 1 of 3 light conditions: bright fluorescent light 24 hrs daily (24L, 4000-7000 lux); moderate fluorescent light 12 hrs daily (12L, 650-3000 lux); darkness 24 hrs daily (24D). Upon hatching, chicks were trained to walk along a darkened, enclosed walkway (400cm X 90cm X 180 cm) with a Plexiglas floor. The plantar pads of both feet were marked with a white dot and walking performance was video recorded from below the walkway. Approximately 6-10 steps of walking were digitized (60 pictures/sec) within the middle portion of walkway. Foot coordinates were processed to obtain spatial (stride length, step length, step width) and temporal (swing, stance, double limb support duration) walking parameters. Chicks were tested approximately 2-4 hours post-hatching and again 4 hours later; 4 successful walk trials were recorded per session. All methods were approved by the university IACUC. Our pilot results summarize analyses for 3 hatchlings per group (N = 9). As predicted, 24L hatchlings hatched first (E20.5 ±0.3), followed by 12L (E21.8 ±0.5) and 24D animals (22.4 ±0.4). The last to hatch, 24D hatchlings exhibited the best walking performance at both test sessions. Swing duration, stride length and step length were greater for 24D hatchlings than 24L and 12L hatchlings. Double limb support duration was also least for 24D group. Thus, 24D hatchlings out-performed both 24L and 12L hatchlings in temporal and spatial parameters during the first postnatal day. These trends indicate that both moderate and intense light may negatively impact motor control development while continuous darkness may optimize development. Alternatively, the rate of maturation may be the critical light-related variable, for 24D hatchlings were the slowest to progress to hatching and this delay may benefit precocious locomotor skill. This work was supported by NIH grant 1 R01 HD053367.

    Video 
    Chicken Embryo Study Video  
    Web links