Fast Locomotor Burst Generation in Late Stage Embryonic Motility

Abstract

Bradley NS, Ryu YU, Lin J. Fast locomotor burst generation in late stage embryonic motility. J Neurophysiol 99: 1733-1742, 2008. First published February 13, 2008 doi:10.1152/jn.01393.2007. We examined muscle burst patterns and burst frequencies for a distinct form of repetitive leg movement recently identified in chick embryos at embryonic day (E)18 that had not been previously studied. The aim was to determine if burst frequencies during repetitive leg movements were indicative of a rhythm burst generator and if maturing muscle afferent mechanisms could modulate the rhythm. Electromyographic recordings synchronized with video were performed in ovo during spontaneous movement at E15, E18, and E20. Multiple leg muscles were rhythmically active during repetitive leg movements at E18 and E20. Rhythmic activity was present at E15 but less well formed. The ankle dorsi flexor, tibialis anterior, was the most reliably rhythmic muscle because extensor muscles frequently dropped out. Tibialis anterior burst frequencies ranged from 1 to 12 Hz, similar to frequencies during fast locomotor burst generation in lamprey. The distribution in burst frequencies at E18 was greatest at lower frequencies and similar to locomotor data in hatchlings. Relative distributions were more variable at E20 and shifted toward faster frequencies. The shell wall anterior to the leg was removed in some experiments to determine if environmental constraints associated with growth contributed to frequency distributions. Wall removal had minimal impact at E18. E20 embryos extended their foot outside the egg, during which faster frequencies were observed. Our findings provide evidence that embryonic motility in chick may be controlled by a fast locomotor burst generator by E15 and that modulation by proprioceptors may emerge between E18 and E20.

Spontaneous locomotor activity in late-stage chicken embryos is modified by stretch of leg muscles

“…1A) activates LG proprioceptors, or whether their central synapses are inhibited by other spinal and/or descending pathways. For example, as embryos outgrow egg volume, the shell wall potentially imposes passive mechanical constraints by limiting working ranges for posture and movement (Bradley et al, 2005;Sharp et al, 1999), but it is unclear whether extensive foot contact and limb loading occur during RLMs (Bradley et al, 2008). We have only observed sustained leg-shell contact, accompanied by tonic extensor EMG or extensor-flexor coactivity, during prehatching postural extensions associated with body rotation (N.S.B., unpublished).…”

“…Under control conditions, TA burst frequency can vary from 1 to 10 Hz without any apparent variation in postural context or modulation in TA burst duration (Bradley et al, 2008). While these features attest to the central generation of RLMs, they also suggest there is no sensorimotorrelated modulation, like that imposed by a changing belt speed during treadmill locomotion, to account for the parameter variations.…”

“…Limb movements were typically small in amplitude and not easily visualized in video recordings during ankle extension restraint. Therefore, all EMG sequences of four or more consecutive and similarly spaced bursts in at least one muscle were included in initial analyses, as these methods were previously demonstrated to reliably detect RLM sequences exhibiting locomotor-related EMG activity (Bradley et al, 2008). EMG was rectified to implement automated detection (Datapac 2K2, Run Technologies) of burst onsets and offsets based on predefined burst criteria (Fig.…”

“…These precocious locomotor abilities are evidence that proprioceptive afferents can accurately code the state of the leg for controlling the center of mass during stance and locomotion at hatching. During the final week in ovo prior to hatching, chicks begin producing repetitive limb movements (RLMs) having muscle patterns comparable to stepping during locomotor behaviors post-hatching (Bradley et al, 2008;Ryu and Bradley, 2009). Therefore, it is plausible that proprioceptive coding of limb posture and movement is established prior to hatching, and that proprioception may influence both embryonic behavior and preparations for precocious postnatal locomotion, a notion first proposed several decades ago (Narayanan and Malloy, 1974).…”

Age-related Changes in the Response of Embryonic Motility to Acute Hypoxia During the Third Quarter of Chick Embryogenesis

“…We studied the hypoxic effect on cyclic motility until D15 inclusive. Then the motility character was beginning to gradually change mainly due to the appearance of repetitive limb movements that have been described by Bradley et al [23], and the cyclic character of the motility became less pronounced. Other authors have also reported an increased variation of the parameters of cyclic motility (APh and IPh) with embryonic age [16,24,25].…”