An experimental investigation of the possible role of tactile and proprioceptive stimulation in certain aspects of embryonic behavior in the chick

Sensory regulation of spontaneous limb movements in the midstage embryonic chick

It is becoming increasingly apparent that somatosensation plays an important role in regulating prenatal movement and developmental plasticity. Numerous studies performed on embryonic chicks and perinatal rats are beginning to implicate proprioception to be particularly important in modulating motility very soon after afferent connections are made in the spinal cord. In this report, we demonstrate new approaches in the chick embryo to explore the role of sensation in modulating embryonic movement. Force recordings from the legs of chick embryos on E9 and E11, during spontaneous motility, demonstrate changes in sensory regulation consistent with the concept that sensory regulation is functioning one day after sensory synapse formation and that the complexity of this regulation increases by E11. Additionally, we present new video data showing activation of embryonic motility on E5 and E9 in embryos expressing channelrhodopsin in the spinal cord as a novel way to approach the issues of sensorimotor development.

Spinal mediation of motor learning and memory in the rat fetus

Fetal rats can alter patterns of interlimb coordination after experience with a yoke that links two legs together. Yoke training results in a pronounced increase in conjugate limb movements (CLM). To determine whether yoke motor learning is mediated by spinal cord circuitry, fetal subjects at embryonic Day 20 (E20) received yoke training after mid-thoracic spinal cord transection or sham surgery. Both spinal and sham-treated fetuses exhibited an increase in CLM during training. In a second experiment, fetuses received initial yoke training, then were transected or sham treated before a 2nd training. Spinal and sham fetuses that were yoked during both training sessions exhibited a more rapid rise in CLM than those yoked only in the later session. These findings indicate that motor learning in fetal rats can be supported by spinal cord circuitry alone, and that savings implies a form of motor memory localized in the spinal cord.

Pyridoxine treatment alters embryonic motility in chicks: Implications for the role of proprioception

Somatosensory feedback is important for the modulation of normal locomotion in adult animals, but we do not have a good understanding of when somatosensory information is first used to modulate motility during embryogenesis or how somatosensation is first used to regulate motor output. We used pyridoxine administration (vitamin B6 ), which is known to mostly kill proprioceptive neurons in adult mammals and embryonic chicks, to explore the role of proprioceptive feedback during early embryonic motility in the chick. Injection of pyridoxine on embryonic day 7 (E7) and E8 reduced the amplitude of leg movements recorded on E9 and the number of large, healthy neurons in the ventral-lateral portion of the DRGs. We conclude that proprioception is initially used during embryogenesis to modulate the strength of motor output, but that it is not incorporated into other aspects of pattern generation until later in development as poly-synaptic pathways develop.

Teratogenic effects of pyridoxine on the spinal cord and dorsal root ganglia of embryonic chickens

Our understanding of the role of somatosensory feedback in regulating motility during chicken embryogenesis and fetal development in general has been hampered by the lack of an approach to selectively alter specific sensory modalities. In adult mammals, pyridoxine overdose has been shown to cause a peripheral sensory neuropathy characterized by a loss of both muscle and cutaneous afferents, but predominated by a loss of proprioception. We have begun to explore the sensitivity of the nervous system in chicken embryos to the application of pyridoxine on embryonic days 7 and 8, after sensory neurons in the lumbosacral region become post-mitotic. Upon examination of the spinal cord, dorsal root ganglion and peripheral nerves, we find that pyridoxine causes a loss of neurotrophic tyrosine kinase receptor type 3-positive neurons, a decrease in the diameter of the muscle innervating nerve tibialis, and a reduction in the number of large diameter axons in this nerve. However, we found no change in the number of Substance P or calcitonin gene-related peptide-positive neurons, the number of motor neurons or the diameter or axonal composition of the femoral cutaneous nerve. Therefore, pyridoxine causes a peripheral sensory neuropathy in embryonic chickens largely consistent with its effects in adult mammals. However, the lesion may be more restricted to proprioception in the chicken embryo. Therefore, pyridoxine lesion induced during embryogenesis in the chicken embryo can be used to assess how the loss of sensation, largely proprioception, alters spontaneous embryonic motility and subsequent motor development.

Prenatal Development of Interlimb Motor Learning in the Rat Fetus

The role of sensory feedback in the early ontogeny of motor coordination remains a topic of speculation and debate. On E20 of gestation (the 20th day after conception, 2 days before birth), rat fetuses can alter interlimb coordination after a period of training with an interlimb yoke, which constrains limb movement and promotes synchronized, conjugate movement of the yoked limbs. The aim of this study was to determine how the ability to express this form of motor learning may change during prenatal development. Fetal rats were prepared for in vivo study at 4 ages (E18-21) and tested in a 65-min training-and-testing session examining hind limb motor learning. A significant increase in conjugate hind limb activity was expressed by E19, but not E18 fetuses, with further increases in conjugate hind limb activity on E20 and E21. These findings suggest substantial development of the ability of fetal rats to modify patterns of interlimb coordination in response to kinesthetic feedback during motor training before birth.

Conjugate limb coordination after experience with an interlimb yoke: evidence for motor learning in the rat fetus

This study investigated the capacity of the E20 rat fetus to adaptively alter patterns of interlimb coordination in a prenatal model of motor learning. Fetal limb movement was manipulated with an interlimb yoke, consisting of a fine thread attached at the ankles, which created a physical linkage between two limbs. Exposure to the yoke resulted in a gradual increase in conjugate movements of the yoked limbs during a 30-min training period, which persisted after removal of the yoke. Training effects were evident when the yoke was applied to two hindlimbs, two forelimbs, or a homolateral forelimb-hindlimb pair. A savings in the rate of acquisition also was observed when fetuses experienced yoke training in a second session. These data argue that the rat fetus can respond to kinesthetic feedback resulting from variation in motor performance, which suggests that experience contributes to the development of coordinated motor behavior before birth.

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.

Cross-species investigations of prenatal experience, hatching behavior, and postnatal behavioral laterality

Turning biases have been reported in some mammalian species, but less is known about such biases in nonmammalians. This study investigated turning biases in domestic chicks, bobwhite and Japanese quail, leopard geckos, and snapping turtles. Domestic chicks (white leghorn and bantam) and bobwhite quail demonstrate strong group laterality. Japanese quail chicks, snapping turtles, and leopard geckos demonstrate no significant group bias. Results are discussed with regard to differences in embryonic experience, hatching behavior, and postnatal environment.

Asymmetrical hatching behaviors influence the development of postnatal laterality in domestic chicks (Gallus gallus)

Lateralized motor behaviors have been reported in some avian species. For instance, footedness has been reported in parrots and domestic chicks, and turning biases have been reported in such species as quail and domestic chicks. This study examined the effects of asymmetrical hatching behaviors on the development of lateralized turning bias and footedness in domestic chicks. Asymmetrical hatching behaviors are counter-clockwise full body turns that many precocial birds make to escape the egg. To study the role of such coordinated prenatal motor behaviors in the development of lateralization, hatching behaviors were systematically disrupted following pipping. Subjects were subsequently tested on two measures of laterality: footedness and turning bias. Results indicated a significant reduction in individual and group lateralization for both measures. These findings suggest that the hatching behaviors found in domestic chicks serve to induce the development of strong motor biases at both the individual and population level.

Ankle restraint modifies motility at E12 in chick embryos

The chick's relationship to its environment changes dramatically over 21 days of embryonic development. At early ages embryos are buoyant; their posture and movements are relatively unconstrained. As embryos grow and fluid level in ovo decreases, movements are increasingly constrained by gravitational forces and reactive forces due to body contact with the shell wall. The issue of how age-related changes in the constraints on movement in ovo may affect embryonic motility is addressed in this paper. Our long-term goal is to determine whether experience imposed by these conditions contributes to development of posthatching motor behaviors. Because previous work indicated that parameters of motility can be modified by a reduction in buoyancy at embryonic day (E) 9, we sought to determine whether a restraint localized to a single joint could also alter either the episodic distribution of activity or the spatiotemporal patterns of limb movement at either E9 or E12. Thus a restraint was applied to the right ankle of embryos prepared for kinematic recordings. Video and kinematic analyses indicated that the restraint had minimal effect at E9, but significantly modified several motility parameters in both the wing and leg at E12. Ankle restraint decreased episode duration. Restraint also decreased most joint excursion parameters, including excursion range, cycles per sequence, and excursion velocity. Restraint increased cycle period duration and signal frequency content under 1.0 Hz. Parameters of intralimb and interlimb coordination exhibited small mixed effects. Results provide support for the hypothesis that environmental conditions contribute to features of embryonic motility. Further, significant modifications of wing excursions in ankle restrained embryos suggest that sensory feedback arising from mechanical perturbations of leg movements may entrain rostral spinal circuits for preservation of interlimb coordination at E12. Potential mechanisms and implications are discussed.

Developmental changes in leg coordination of the chick at embryonic days 9, 11, and 13: uncoupling of ankle movements

To understand changes in motor behavior during development, kinematic measurements were made of the right leg during embryonic motility in chicks on embryonic (E) days 9, 11, and 13. This is an interesting developmental period during which the embryo first becomes large enough to be physically constrained by the shell. Additionally, sensory systems are incorporated at that time into the spinal motor circuitry. Previous electromyographic (EMG) recordings have shown that the basic pattern of muscle activity seen at E9, composed of half-center-type alternation of extensor and flexor activation, breaks down by E13. This breakdown in organization could be because of disruption of motor patterns by the immature sensory system and/or new spatial constraints on the embryo. The current article describes several changes in leg movement patterns during this period. Episodes of motility increase in duration and the intervals of time between episodes of motility decrease in length. The range of motion of the leg increases, but the overall posture of the leg becomes more flexed. It was found that in-phase coordination of movement among the hip, knee, and ankle decreased between E9 and E13 in agreement with the previous EMG recordings. However, it was also found that the decrease of in-phase coordination among the three joints was accompanied by an increase in the time any two joints were moving in the same manner. Furthermore, examination of in-phase coordination within pairs of joints showed that all three pairs were well coordinated at E9, but that at E13 the in-phase coordination of the ankle with the other two joints decreased, whereas the knee and hip coordination was maintained. This suggests that the hip-knee synergy was closely coupled and that coupling of the ankle with other joints was more labile. The authors conclude that embryos respond to the reduction of free space in the egg during this period not by decreasing the amplitude or coordination of leg movements in general, but instead by differentially controlling the movements of the ankle from those of the hip and knee. Additionally, the changes in movement patterns do not represent a decrease in organization, but rather an alteration of motor coordination possibly as the result of information from the newly acquired sensory systems. These data also support theories that limb central pattern generators (CPGs) are composed of unit CPGs for each joint that can be modulated individually and that this organization is already established early in embryogenesis.

Effects of designer drugs on the chicken embryo and 1-day-old chicken

The present study was conducted to examine the effects of d-amphetamine and the designer drugs 3,4-methylenedioxymethamphetamine (MDMA), N-methyl-3,4-methylenedioxyphenyl-3-butamine (HMDMA), 3,4-methylenedioxyphenyl-2-butanamine (BDB), 3,4-methylenedioxyphenyl-1-ethanamine (MDM1EA) in the chick embryo and the young chicken. HMDMA and MDM1EA had no effect on motility on day 14 of embryogenesis, while MDMA, BDB, and d-amphetamine decreased embryonic motility at one or more doses. On day 1 posthatch, chickens were challenged with cumulative injections of water or the same drug that they had received in ova. With the exception of MDM1EA, all of the drugs produced effects such as distress vocalization, wing extension, tremor, flat body posture, bursting forward movements, loss of righting reflex, and convulsant-like kicking. Pretreatment with drug in ova resulted in tolerance to certain drug effects and supersensitivity to other drug effects. Furthermore, BDB significantly decreased hatchability, MDM1EA decreased body weight, and HMDMA decreased liver weight. Further studies are needed to determine the mechanism(s) of toxicity in this species.

Neuroethological approaches to the study of motor development in chicks: achievements and challenges

Chicks and chick embryos provide a useful model system for the study of issues related to the development of motor behaviors. EMG and kinematic analyses of leg movements have been used to provide new data on the organization of embryonic motility. These data suggest that the circuitry needed to produce a basic, coordinated motor pattern is available early in development. This circuitry then appears to be retained throughout life. Evidence from analysis of EMG patterns and leg deafferentation studies suggest that the output of this basic circuit can be modulated by sensory input to produce the motor patterns of later behaviors, such as hatching and walking. If the same circuitry is present throughout life, then mechanisms for initiation and termination of particular behaviors must be available to ensure that specific behaviors are turned on and off at appropriate times. For example, hatching can be turned on by a specific sensory signal: proprioceptive signals from the bent neck. In addition to reviewing current research on the development of chick motor behaviors, methodological considerations and suggestions for future research are presented.

Development of central projections of lumbosacral sensory neurons in the chick

The development of central projections of sensory neurons in lumbosacral dorsal root ganglia (DRGs) was examined by using horseradish peroxidase labeling techniques in chick embryos from stage 23 (E4) to stage 39 (E13). Our results show that primary afferents reach the spinal cord by stage 23. Afferent axons extend in the primordium of the dorsal funiculus for several segments rostral and caudal to their segment of entry for over 24 hours before invading the gray matter at stage 28 (E6). Sensory fibers grow into the vicinity of motoneuron dendrites by stage 32 (E7.5), about the time that reflexes and apparent monosynaptic EPSPs can first be elicited. Dense projections into the dorsal laminae of the spinal cord, presumably representing cutaneous afferents, appear somewhat later, at about stage 39 (E13), when the segmental projection pattern begins to resemble the mature pattern.

Sensory control of the initiation of hatching in chicks: effects of a local anesthetic injected into the neck

Previous work shows that folding a posthatching chick into the hatching position results in the re-initiation of hatching. Furthermore, bending the neck to the right or left serves as a selective signal for turning on hatching behavior. The present study addresses the issue of whether sensory receptors located in the neck provide this signal. Three groups of chicks were folded into the hatching position and placed in glass eggs. In the experimental group, sensory input from the neck was eliminated with a local anesthetic, lidocaine. In these chicks, hatching was initiated only after a long latency, correlated with the time at which the anesthetic wore off. In the two control groups, in which saline was injected into the neck or lidocaine was injected into the thigh, the latency was much shorter. Therefore sensory receptors located in the neck appear to provide input that serves as a selective signal for initiating hatching.

Distribution and development of proenkephalin-like immunoreactivity in the lumbar spinal cord of the chicken

Met5-enkephalin- (Met-ENK), Leu5-enkephalin-, Met5-enkephalin-Arg6-Phe7-, metorphamide- and BAM 22P-like peptides could be detected in the lumbar spinal cord of the chicken by immunocytochemistry and/or high performance liquid chromatography. However, a peptide identical to Met5-enkephalin-Arg6-Gly7-Leu8 could not be detected in the lumbar spinal cord of the chicken using an antiserum that was capable of detecting the octapeptide in mammalian tissues. Nerve fiber- and terminal-like processes containing proenkephalin-derived peptides were concentrated in the superficial laminae of the dorsal horn and along the midline rostral to the central canal. A lesser concentration of processes containing proenkephalin-derived peptides occurred in the medial and lateral motor columns of the ventral horn. The level of total radioimmunoassayable Met-ENK in the lumbar spinal cord of the chicken embryo increased more than 1000-fold between day 4.5 and day 18. A schedule of curare administration that had previously been shown to prevent naturally occurring somatic motoneuron death in the chicken lumbar spinal cord resulted in a two-fold increase in total radioimmunoassayable Met-ENK in the lumbar spinal cord.

Behavioral analysis of opiate-mediated inhibition in the early chick embryo

Morphine (opiate agonist) produced a dose-dependent decrease in the spontaneous motility of 5- and 9-day chick embryos. Naloxone (opiate antagonist) appeared to reverse competitively the inhibition of motility caused by morphine. The effects of morphine on spontaneous motility in 5-day embryos were also reversed stereospecifically by the opiate antagonist pairs WIN 44441-3/WIN 44441-2 and levallorphan/dextrallorphan. Levorphanol (opiate agonist) also produced a dose-dependent decrease in the motility of 5-day embryos while its inactive (+)-isomer, dextrophan, was not effective. Etorphine (opiate agonist) was more than 1000-fold more effective than morphine in inhibiting the motility of 5-day embryos. The effectiveness of several opiate agonists and antagonists on the spontaneous motility of 5-day embryos was similar to their effectiveness in radioligand-binding studies on isolated membrane receptors from either adult mammalian brain or ileum. Levorphanol was more effective than dextrophan and etorphine was substantially more effective than morphine in decreasing the spontaneous motility of 4-day embryos. WIN 44441-3 was more effective than WIN 44441-2 in reversing the inhibition of motility in 4-day embryos caused by morphine. Morphine inhibited spontaneous hind-limb motility in both thoracic spinal and sham-operated 7-day embryos; the inhibition of motility caused by morphine was reversed by WIN 44441-3 in both thoracic spinal and sham-operated 7-day embryos. [Leu5]enkephalin-like immunoreactivity in the lumbar spinal cord was concentrated in the superficial laminae of the dorsal horn and along the midline rostral to the central canal. A lesser concentration of immunoreactive processes occurred in the medial and lateral motor columns where labelled varicosities appeared to contact motoneurons. Opiate receptors appear to be present at least as early as day 5 (and perhaps as early as day 4) in the chick embryo. Opiate receptors appear to be present in the lumbar spinal cord of the chick embryo at least as early as day 7. The structural requirements for ligand binding to opiate receptors in the 5-day chick embryo are similar to the requirements for ligand binding to opiate receptors in the adult.

Behavioral analysis of benzodiazepine-mediated inhibition in the early chick embryo

Diazepam, a depressant benzodiazepine, produced a dose-dependent decrease in the spontaneous motility of 5-day embryos while Ro 5-3663, a convulsant benzodiazepine, had no apparent effect. Diazepam and 4 other benzodiazepines inhibited motility in 5-day embryos with a potency that paralleled their effectiveness in ligand-binding studies. Ro 11-5073/Ro 11-5231, depressant benzodiazepine enantiomers, stereospecifically inhibited motility in 4- and 5-day embryos. Ro 15-1788, a benzodiazepine antagonist, reversed the decrease in the motility of 4- and 5-day embryos caused by a depressant benzodiazepine, clonazepam. Ro 5-6945, a potent agonist for the non-neuronal (but not the neuronal) benzodiazepine receptor, had no apparent effect on motility in 5-day embryos. Benzodiazepine receptors appear at least as early as day 5 (and perhaps as early as day 4) in the chick embryo.

Uptake, intra-axonal transport and fate of horseradish peroxidase in embryonic spinal neurons of the chick

Horseradish peroxidase (HRP) was injected in ovo into the ventral muscle mass of the hind limb of 5- to 7-day-old chick embryos or into the gastrocnemius muscle of 8- to 18-day embryos and localized histochemically. HRP is extensively incorporated via endocytosis into axonal growth cones or presynaptic terminals in the proximity of the injection site. Much of the tracer is taken up in vesicles and small vacuoles. Most of these are smooth-surfaced and only a few are bristle-coated. A small amount of the tracer is also incorporated into the axon terminal through the openings between the axolemma and an intricate membrane channel. The majority of the tracer-laden vesicles and vacuoles rapidly fuse with one another to become large vacuoles, some of which are transformed into multivesicular bodies (MVBs). In axon shafts, many labeled vacuoles and MVBs are transferred to tubule-like organelles, which appear to be the primary carrier for transporting the tracer back to the cell bodies in the lumbar spinal cord. HRP arrives in the sensory ganglia about 0.5-1 hour earlier than in the motoneurons of the lateral motor column. The maximal rate of the retrograde axoplasmic transport is about 3.5 mm/hour. After arriving in the cell bodies, HRP is transferred from tubule-like organelles to discrete vacuoles of various sizes and appearance. Lysosomal dense bodies and HRP-labeled vacuoles can be distinguished ultrastructurally. A fusion of HRP-labeled vacuoles with lysosomal dense bodies or Golgi vesicles was occasionally observed and the density of HRP-labeled vacuoles diminished after 2 to 3 days. Most of the HRP-labeled organelles were found to contain acid phosphatase activity. Therefore, the complete disappearance of HRP by 4 days postinjection is most likely related to lysosomal degradation. Neuronal cell bodies diffusely labeled with HRP were only observed prior to day 6. After day 6, despite various attempts to injure the peripheral axons, only granularly labeled cell bodies were found. This difference may imply that "mature" neurons have a more efficient mechanism for the sequestration of "free" HRP in the cytoplasmic matrix into membrane-bounded organelles. A mature-like retrograde transport mechanism appears to exist at the earliest stages of axonal growth in vivo.

Behavioral and biochemical analysis of GABA-mediated inhibition in the early chick embryo

Exogenous gamma-aminobutyric acid (GABA) decreased spontaneous motility in 4-, 6-, 7-, 9-, and 13-day chick embryos; the younger embryos were more sensitive. Neither the positional isomers of GABA, alpha-aminobutyric acid (AABA) and beta-aminobutyric acid, nor the principle GABA catabolite, succinic acid, decreased motility in 4-day embryos. Several semi-rigid GABA analogues decreased motility in 4-day embryos with a potency that paralleled their effectiveness in displacing [3H]GABA in ligand-binding studies. The effects of AABA and GABA on hind-limb motility were quantitatively similar in thoracic spinal and sham-operated 7-day embryos. Bicuculline and picrotoxin elicited absolute motility increases at 6, 7 and 9 days of incubation. Picrotoxin and two bicyclophosphate GABA antagnoists elicited relative motility increases while bicuculline elicited an absolute motility increase at 4 days. The two bicyclophosphates increased motility with a potency that paralleled their electrophysiological effectiveness. L-Glutamic acid decarboxylase (GAD) activity was detected in the embryonic lumbar spinal cord at all ages examined (3--7 days) using a new radiometric cation-exchange method. Gamma-Aminobutyric acid transaminase (GABA-T) activity was detected in the lumbar spinal cord at the earliest age examined (day 5). Both GAD and GABA-T activity were detected at earlier ages than previously reported. GABA receptors, and the enzymes necessary for the synthesis and degradation of GABA, all appear to be present at (or before) the onset of spontaneous motility. GABA-mediated transmission appears to be present at 6 days and perhaps as early as 4 days.

Ontogeny of leg motor output in the chick embryo: a neural analysis

The motor output of the leg of the chick embryo during hatching (20 days of incubation) was characterized using electromyographic (EMG) recordings from identified knee and ankle muscles. A highly coordinated pattern of motor output was found. It was therefore used as a standard against which to compare the motor output from younger embryos (7, 9, 13, 17 and 19 days of incubation). Despite large differences in some aspects of the EMG records from embryos of different ages, consistent patterns of muscle activation were observed. Quantitative analysis of phase and latency relationships between pairs of muscles indicated that at least some elements of the neural circuitry involved in generating the hatching motor output may be laid down very early in development. Duration vs. latency plots revealed that there is a gradual refinement in the temporal pattern of alternation between antagonist muscles during development.

Coordinated motor output in the hindlimb of the 7-day chick embryo

Electromyographic recordings from individual identified ankle muscles of the 7-day chick embryo (stage 31) were used to determine the organization of motor output at a developmental stage shortly after the onset of spontaneous motility in the leg. During spontaneous motility of the embryo, the electromyographic recordings from the gastrocnemius, peroneus, and tibialis muscles displayed bursts of motor unit activity which alternated with periods of little or no activity. Since the control of skeletal muscle in the chick embryo is neurogenic rather than myogenic, these findings imply that the motoneurons to a given muscle are driven by a common source. Since flexor and extensor muscles are attivated at different times, different central connections to flexor and extensor motoneurons must be present in the central nerbous system of the 7-day embryo. Moreover, since inhibition is known to play an important role in the selective activation of agonist and antagonist muscles, the present results suggest that functional inhibitory synapses may be present in the lumbosacral central nervous system at this stage of development. The basic pattern of muscle activation observed in the 7-day embryo is similar to that seen in older embryos. Since these patterns appear prior to the time at which motor responses to sensory stimulation of the leg can be demonstrated, it is likely that the neural pattern generating circuits for selective activation of muscles are established in the central nervous system without reliance on functional reflexes.