Age-related changes and condition-dependent modifications in distribution of limb movements during embryonic motility.

It has long been known that the chick initiates spontaneous motility early in embryogenesis, that the distribution of this activity is episodic, and that it varies both quantitatively and qualitatively with age. It is also well established that embryonic motility is controlled by spinal circuits and features of motility at early stages of development are likely the product of immature network properties. Over the course of embryonic development, however, the episodic distribution of motility becomes more variable. Because we are interested in determining whether movement experience in ovo is fundamental to the establishment of adaptive posthatching behaviors, this study examines the normal within-subject variability of episodic activity in embryos across ages under control and several experimental conditions. The distribution of activity, pause, and episode duration was obtained from video recordings of embryos prepared for electromyographic (EMG) and/or kinematic studies of motility in ovo at select ages (E9, E10, E12, E15, E18) under control conditions (control), acute reduction in buoyancy (ARB), ankle restraint (AR), thoracic spinal transection (spinal). Both control and ARB embryos exhibited significant age-related changes in the distribution of motility. Activity duration progressively increased with age and largely accounted for age-related increases in the variability of episodic behavior. Pause duration decreased markedly between E9 and E12 and did not appear to be a critical parameter in accounting for age-related changes in motility distribution. Activity duration was significantly lengthened in ARB embryos and decreased in spinal embryos. Pause duration was selectively lengthened in AR embryos. Collectively, age-related changes and selective effects of experimental preparations suggest that activity and pause duration are controlled by different mechanisms that operate independent of one another by E12. The results also suggest that the spinal network controlling motility becomes increasingly dependent on excitatory drive from supraspinal centers between E9 and E18. It is proposed that age-related increases in activity duration variability and condition-dependent effects on the distribution of activity are indicative of changing inputs weights for descending and sensory pathways and that they significantly impact spinal control of motility as the embryo's movement and posture are increasingly constrained by the fixed volume of the egg.