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.     

Central pattern generators and the control of rhythmic movements.

Central pattern generators are neuronal circuits that when activated can produce rhythmic motor patterns such as walking, breathing, flying, and swimming in the absence of sensory or descending inputs that carry specific timing information. General principles of the organization of these circuits and their control by higher brain centers have come from the study of smaller circuits found in invertebrates. Recent work on vertebrates highlights the importance of neuro-modulatory control pathways in enabling spinal cord and brain stem circuits to generate meaningful motor patterns. Because rhythmic motor patterns are easily quantified and studied, central pattern generators will provide important testing grounds for understanding the effects of numerous genetic mutations on behavior. Moreover, further understanding of the modulation of spinal cord circuitry used in rhythmic behaviors should facilitate the development of new treatments to enhance recovery after spinal cord damage.     

Development of coordinated movement in chicks: II. Temporal analysis of hindlimb muscle synergies at embryonic day 10 in embryos with spinal gap transections.

Spinal neural circuits can recruit muscles to produce organized patterns of activity early in embryonic development. In a previous study, using multichannel electromyographic (EMG) recordings, we characterized burst parameters for these patterns in the legs of chick embryos during spontaneous motility in ovo at embryonic days (E) 9 and E10 (Bradley and Bekoff, 1990). Results of the study suggested both neural and biomechanical factors play an important role in the development of coordinated limb movements. In this study, to explore the contribution of descending neural inputs to the control of leg movements during motility, we applied similar methods to characterize motor patterns produced by the spinal cord in the absence of descending inputs. Thoracic spinal gap transections were performed at E2 and EMG patterns were recorded at E10. Several EMG features for chronic spinal embryos were similar to those for normal embryos and demonstrate that lumbar spinal circuits can be correctly assembled to control limb movements in the absence of connectivity with more rostral neural structures during early differentiation processes. However, certain aspects of the EMG patterns in chronic spinal embryos were different from patterns in normal embryos and provide support for conclusions drawn earlier by Oppenheim (1975). Specifically, our data support the view that propriospinal and/or supraspinal inputs function to regulate the timing of cyclic limb movements controlled by spinal neural circuits. Finally, we consider the possible long-term effects of chronic spinal gap transections as compared to acute spinal transections on the development of motility.     

Development of coordinated movement in chicks: II. Temporal analysis of hindlimb muscle synergies at embryonic day 10 in embryos with spinal gap transections

Spinal neural circuits can recruit muscles to produce organized patterns of activity early in embryonic development. In a previous study, using multichannel electromyographic (EMG) recordings, we characterized burst parameters for these patterns in the legs of chick embryos during spontaneous motility in ovo at embryonic days (E) 9 and E10 (Bradley and Bekoff, 1990). Results of the study suggested both neural and biomechanical factors play an important role in the development of coordinated limb movements. In this study, to explore the contribution of descending neural inputs to the control of leg movements during motility, we applied similar methods to characterize motor patterns produced by the spinal cord in the absence of descending inputs. Thoracic spinal gap transections were performed at E2 and EMG patterns were recorded at E10. Several EMG features for chronic spinal embryos were similar to those for normal embryos and demonstrate that lumbar spinal circuits can be correctly assembled to control limb movements in the absence of connectivity with more rostral neural structures during early differentiation processes. However, certain aspects of the EMG patterns in chronic spinal embryos were different from patterns in normal embryos and provide support for conclusions drawn earlier by Oppenheim (1975). Specifically, our data support the view that propriospinal and/or supraspinal inputs function to regulate the timing of cyclic limb movements controlled by spinal neural circuits. Finally, we consider the possible long-term effects of chronic spinal gap transections as compared to acute spinal transections on the development of motility. © 1992 John Wiley & Sons, Inc.     

Hybrid isozymes of pyruvate kinase appear during avian cardiac development

Pyruvate kinase isozyme patterns in the ventricle of developing chicks shift gradually from one dominated by type K at ten days of embryonic development to the adult pattern, which is dominated by type M. Hybrid isozymes are apparent throughout development and are most prominent from two days before hatching until at least 14 days after hatching. These hybrid isozymes indicate simultaneous synthesis of the two subunit types in the same cells.The complex isozyme patterns of the chick heart probably limit the usefulness of simple kinetic analyses on tissue extracts for determing isozymic compositions during development.     

Segmentally distributed metamorphic changes in neural circuits controlling abdominal bending in the hawkmoth Manduca sexta

During the metamorphosis of Manduca sexta the larval nervous system is reorganized to allow the generation of behaviors that are specific to the pupal and adult stages. In some instances, metamorphic changes in neurons that persist from the larval stage are segment-specific and lead to expression of segment-specific behavior in later stages. At the larval-pupal transition, the larval abdominal bending behavior, which is distributed throughout the abdomen, changes to the pupal gin trap behavior which is restricted to three abdominal segments. This study suggests that the neural circuit that underlies larval bending undergoes segment specific modifications to produce the segmentally restricted gin trap behavior. We show, however, that non-gin trap segments go through a developmental change similar to that seen in gin trap segments. Pupal-specific motor patterns are produced by stimulation of sensory neurons in abdominal segments that do not have gin traps and cannot produce the gin trap behavior. In particular, sensory stimulation in non-gin trap pupal segments evokes a motor response that is faster than the larval response and that displays the triphasic contralateral-ipsilateral-contralateral activity pattern that is typical of the pupal gin trap behavior. Despite the alteration of reflex activity in all segments, developmental changes in sensory neuron morphology are restricted to those segments that form gin traps. In non-gin trap segments, persistent sensory neurons do not expand their terminal arbors, as do sensory neurons in gin trap segments, yet are capable of eliciting gin trap-like motor responses. Accepted: 10 January 1997     

Development of lipidic composition of neonatal chick liver and intestine microsomes: changes induced by cholesterol feeding

Changes in cholesterol and phospholipid content of chick liver and intestine microsomes were studied throughout the two first weeks of life. Differences observed throughout postnatal development were mainly due to the free cholesterol. Cholesterol feeding resulted in a clear increase of the amounts of both free and esterified cholesterol. Phospholipid content of chick liver and intestine microsomes did not change significantly after hatching. Phosphatidylcholine and phosphatidylethanolamine were found to be the major phospholipids. Although the amount of each phospholipid could be affected by cholesterol feeding, its relative percentage did not change by this treatment.     

Partly shared spinal cord networks for locomotion and scratching

Animals produce a variety of behaviors using a limited number of muscles and motor neurons. Rhythmic behaviors are often generated in basic form by networks of neurons within the central nervous system, or central pattern generators (CPGs). It is known from several invertebrates that different rhythmic behaviors involving the same muscles and motor neurons can be generated by a single CPG, multiple separate CPGs, or partly overlapping CPGs. Much less is known about how vertebrates generate multiple, rhythmic behaviors involving the same muscles. The spinal cord of limbed vertebrates contains CPGs for locomotion and multiple forms of scratching. We investigated the extent of sharing of CPGs for hind limb locomotion and for scratching. We used the spinal cord of adult red-eared turtles. Animals were immobilized to remove movement-related sensory feedback and were spinally transected to remove input from the brain. We took two approaches. First, we monitored individual spinal cord interneurons (i.e., neurons that are in between sensory neurons and motor neurons) during generation of each kind of rhythmic output of motor neurons (i.e., each motor pattern). Many spinal cord interneurons were rhythmically activated during the motor patterns for forward swimming and all three forms of scratching. Some of these scratch/swim interneurons had physiological and morphological properties consistent with their playing a role in the generation of motor patterns for all of these rhythmic behaviors. Other spinal cord interneurons, however, were rhythmically activated during scratching motor patterns but inhibited during swimming motor patterns. Thus, locomotion and scratching may be generated by partly shared spinal cord CPGs. Second, we delivered swim-evoking and scratch-evoking stimuli simultaneously and monitored the resulting motor patterns. Simultaneous stimulation could cause interactions of scratch inputs with subthreshold swim inputs to produce normal swimming, acceleration of the swimming rhythm, scratch-swim hybrid cycles, or complete cessation of the rhythm. The type of effect obtained depended on the level of swim-evoking stimulation. These effects suggest that swim-evoking and scratch-evoking inputs can interact strongly in the spinal cord to modify the rhythm and pattern of motor output. Collectively, the single-neuron recordings and the results of simultaneous stimulation suggest that important elements of the generation of rhythms and patterns are shared between locomotion and scratching in limbed vertebrates.     

Thoracic leg motoneurons in the isolated CNS of adult Manduca produce patterned activity in response to pilocarpine,which is distinct from that produced in larvae

In the hawkmoth, Manduca sexta, thoracic leg motoneurons survive the degeneration of the larval leg muscles to innervate new muscles of the adult legs. The same motoneurons, therefore, participate in the very different modes of terrestrial locomotion that are used by larvae (crawling) and adults (walking). Consequently, changes in locomotor behavior may reflect changes in both the CNS and periphery. The present study was undertaken to determine whether motor patterns produced by the isolated CNS of adult Manduca, in the absence of sensory feedback, would resemble adult specific patterns of coordination. Pilocarpine, which evokes a fictive crawling motor pattern from the isolated larval CNS, also evoked robust patterned activity from leg motoneurons in the isolated adult CNS. As in the larva, levator and depressor motoneurons innervating the same leg were active in antiphase. Unlike fictive crawling, however, bursts of activity in levator or depressor motoneurons of one leg alternated with bursts in the homologous motoneurons innervating the opposite leg of the same segment and the leg on the same side in the adjacent segment. The most common mode of intersegmental activity generated by the isolated adult CNS resembled an alternating tripod gait, which is displayed, albeit infrequently, during walking in intact adult Manduca. A detailed analysis revealed specific differences between the patterned motor activity that is evoked from the isolated adult CNS and activity patterns observed during walking in intact animals, perhaps indicating an important role for sensory feedback. Nevertheless, the basic similarity to adult walking and clear distinctions from the larval fictive crawling pattern suggest that changes within the CNS contribute to alterations in locomotor activity during metamorphosis. Electronic Publication     

Development of motor behaviour

The development of motor behaviour depends on the differentiation of underlying circuitry. Recent work with the zebrafish brings the simple swimming behaviour of lower vertebrates and their embryos into focus as a suitable model to study the development of motor circuitry and its genetic control. Changes in connectivity and excitability contribute to the development of swimming in this simple system. In the chick embryo, limb motor circuitry is spontaneously active before motor axons reach their muscle targets, and it has properties in common with the spontaneously active networks in the retina. The early rhythmic activity responsible for embryonic movement is probably a generalised property of developing spinal networks that precedes, and may be required for, the completion of functional locomotor circuitry.     

Motor pattern selection by combinatorial code of interneuronal pathways

We use a modeling approach to examine ideas derived from physiological network analyses, pertaining to the switch of a motor control network between two opposite control modes. We studied the femur–tibia joint control system of the insect leg, and its switch between resistance reflex in posture control and “active reaction” in walking, both elicited by the same sensory input. The femur–tibia network was modeled by fitting the responses of model neurons to those obtained in animals. The strengths of 16 interneuronal pathways that integrate sensory input were then assigned three different values and varied independently, generating a database of more than 43 million network variants. We demonstrate that the same neural network can produce the two different behaviors, depending on the combinatorial code of interneuronal pathways. That is, a switch between behaviors, such as standing to walking, can be brought about by altering the strengths of selected sensory integration pathways. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The synthesis of nerve growth factor (NGF) in developing skin is independent of innervation

The arrival of sensory fibers in developing mouse skin has been demonstrated to coincide precisely with the initiation of nerve growth factor (NGF) synthesis in the skin (Davies et al., 1987). This temporal correlation suggested that the arrival of sensory fibers might initiate NGF synthesis in their target tissues. Here we have eliminated the sensory and motor neurons projecting to the chick leg by the removal of the neural primordia in 3-day-old embryos. The levels of mRNA NGF of intact and denervated leg skin were identical, indicating that the developmental regulation of NGF synthesis in the skin of chick embryos is independent of its innervation.     

The neural basis of motivated behavior

A complete motivated or goal-oriented behavioral act can be viewed as consisting of initiation, procurement, and consummatory phases. In order to gain some insight into the organizing principles of neural circuitry that underlies the expression of motivated behavior, certain basic pathways thought to play an important role in two specific classes of such behavior, hypovolemic thirst and reproductive behavior, are reviewed. In both cases, humoral factors participate in the initiation phase, and their sites of action have been rather clearly defined. Circuitry underlying the procurement phase, which involves foraging behavior, is much more complex, but can be thought of as involving two fundamentally different systems, one concerned with the processing of specific sensory information and the production of refined motor responses, and the other concerned with modulating behavioral state. The former is associated primarily with the thalamocortical-lateral forebrain system whereas the latter is associated primarily with the medial forebrain system. Finally, evidence favoring the hypothesis that "biochemical switching" may take place in fixed neuroanatomical circuitry associated with ingestive and reproductive behaviors is reviewed.     

Retrofitting larval neuromuscular circuits in the metamorphosing frog

Maturation of vertebrate neuromuscular systems typically occurs in a continuous, orderly progression. After an initial period of developmental adjustment by means of cell death and axonal pruning, relatively stable relationships, with only subtle modifications, are maintained between motoneurons and their appropriate targets throughout life. However, among a restricted group of vertebrates (amphibians and especially the anuran amphibians) the sequential maturation of neuromuscular systems is altered by an abrupt reordering of the basic body plan that encompasses cellular changes in all tissues from skeleton to nervous system. Many anuran amphibians possess neuromuscular circuits that are remarkable by virtue of their complete reorganization during the brief span of metamorphosis. During this period motor systems initially designed for the behavioral patterns of aquatic tadpoles are adjusted to meet the drastically different motor activities of postmetamorphic terrestrial life. This adjustment involves the deletion of neural elements mediating larval specific activities, the accelerated maturation of neural circuits eliciting adult-specific activities and the retrofitting of larval neuromuscular components to serve postmetamorphic behaviors. This review focuses on the cellular events associated with the neuromuscular adaptation in the jaw complex during metamorphosis of the leopard frog, Rana pipiens. As part of the metamorphic reorganization of the jaw apparatus there is a complete turnover of the myofiber complement of the adductor mandibulae musculature. Trigeminal motoneurons initially deployed to the larval myofibers are redirected to new muscle fibers. Simultaneously the cellular geometry and synaptic input to these motoneurons is revamped. These changes suggest that trigeminal neuromuscular circuitry established during embryogenesis is updated during metamorphosis and reused to provide the basis for adult jaw motor activity that is far different than its larval counterpart.     

Skin sensory innervation patterns in embryonic chick hindlimb following dorsal root ganglion reversals

During embryonic development skin sensory neurons in lumbosacral dorsal root ganglia (DRGs) establish their dermatomes and axonal projections in a precise, orderly fashion in the chick. To investigate mechanisms responsible for this specific outgrowth, the rostrocaudal order of DRGs T7-LS3 was reversed by rotating the corresponding segments of neural crest, either alone or together with the underlying neural tube in St.15-16 embryos. The resulting skin sensory innervation patterns, mapped physiologically or anatomically at St.29-40, differed between the two experimental groups. Following neural tube rotations DRGs tended to establish innervation patterns that were consonant with their original position in the embryo. Axons from these rotated DRGs generally projected into the appropriate pathways and innervated the appropriate region of skin. Neural crest rotations left the ventral neural tube (including the motor neuron precursors) largely intact. In this case rotated DRGs tended to establish innervation patterns in accordance with their new position in the embryo, almost as if no rotation had been made. These results cannot be explained solely by the inherent specificity of sensory neurons. Instead, the results are largely consistent with the suggestion (Honig et al., 1986; Landmesser and Honig, 1986) that motor axons can direct the outgrowth of sensory axons and thereby influence the establishment of sensory innervation patterns. Other mechanisms that may also affect the development of sensory innervation patterns are discussed.     

Development of 1,25-dihydroxycholecalciferol receptor in the duodenal cytosol of chick embryo.

The development of 1,25-(OH)2D3 receptor in the duodenal cytosol of chick embryo was studied by the sucrose density gradient analysis. The binding profile for 1,25-(OH)2D3 in the cytosol of vitamin D-deficient chick duodenum on the sucrose density gradient revealed 3 binding components, and the sedimentation constant was estimated as 2.5, 3.5 and 5.5S respectively. The 3.5S binding component has high affinity and low capacity for 1,25-(OH)2D3 and is thought to be 1,25-(OH)2D3 receptor. During the development of chick embryo, the 3.5S binding component was not detected in 13-day embryonic duodenum, it appeared on 15th day of incubation and then gradually increased to the level of vitamin D-deficient chick on 19th day of incubation. The 5.5S binding component was specific for 25-OH-D3 and it was found even in 13-day embryo, but it did not show any significant change during development. On the other hand, the 2.5S component was not specific for either 1,25-(OH)2D3 or 25-OH-D3. However, it was main binding component in early stages of development and decreased during development. From these results, it is suggested that the receptor for 1,25-(OH)2D3 is available a few days before hatching and the inability to produce CaBP in the duodenum of chick embryo could not be ascribed to the absence of the receptor.     

Methods to Quantify Pharmacologically Induced Alterations in Motor Function in Human Incomplete SCI

Spinal cord injury (SCI) is a debilitating disorder, which produces profound deficits in volitional motor control. Following medical stabilization, recovery from SCI typically involves long term rehabilitation. While recovery of walking ability is a primary goal in many patients early after injury, those with a motor incomplete SCI, indicating partial preservation of volitional control, may have the sufficient residual descending pathways necessary to attain this goal. However, despite physical interventions, motor impairments including weakness, and the manifestation of abnormal involuntary reflex activity, called spasticity or spasms, are thought to contribute to reduced walking recovery. Doctrinaire thought suggests that remediation of this abnormal motor reflexes associated with SCI will produce functional benefits to the patient. For example, physicians and therapists will provide specific pharmacological or physical interventions directed towards reducing spasticity or spasms, although there continues to be little empirical data suggesting that these strategies improve walking ability.In the past few decades, accumulating data has suggested that specific neuromodulatory agents, including agents which mimic or facilitate the actions of the monoamines, including serotonin (5HT) and norepinephrine (NE), can initiate or augment walking behaviors in animal models of SCI. Interestingly, many of these agents, particularly 5HTergic agonists, can markedly increase spinal excitability, which in turn also increases reflex activity in these animals. Counterintuitive to traditional theories of recovery following human SCI, the empirical evidence from basic science experiments suggest that this reflex hyper excitability and generation of locomotor behaviors are driven in parallel by neuromodulatory inputs (5HT) and may be necessary for functional recovery following SCI. The application of this novel concept derived from basic scientific studies to promote recovery following human SCI would appear to be seamless, although the direct translation of the findings can be extremely challenging. Specifically, in the animal models, an implanted catheter facilitates delivery of very specific 5HT agonist compounds directly onto the spinal circuitry. The translation of this technique to humans is hindered by the lack of specific surgical techniques or available pharmacological agents directed towards 5HT receptor subtypes that are safe and effective for human clinical trials. However, oral administration of commonly available 5HTergic agents, such as selective serotonin reuptake inhibitors (SSRIs), may be a viable option to increase central 5HT concentrations in order to facilitate walking recovery in humans. Systematic quantification of how these SSRIs modulate human motor behaviors following SCI, with a specific focus on strength, reflexes, and the recovery of walking ability, are missing.This video demonstration is a progressive attempt to systematically and quantitatively assess the modulation of reflex activity, volitional strength and ambulation following the acute oral administration of an SSRI in human SCI. Agents are applied on single days to assess the immediate effects on motor function in this patient population, with long-term studies involving repeated drug administration combined with intensive physical interventions.     

Learning from sensory and reward prediction errors during motor adaptation

Voluntary motor commands produce two kinds of consequences. Initially, a sensory consequence is observed in terms of activity in our primary sensory organs (e.g., vision, proprioception). Subsequently, the brain evaluates the sensory feedback and produces a subjective measure of utility or usefulness of the motor commands (e.g., reward). As a result, comparisons between predicted and observed consequences of motor commands produce two forms of prediction error. How do these errors contribute to changes in motor commands? Here, we considered a reach adaptation protocol and found that when high quality sensory feedback was available, adaptation of motor commands was driven almost exclusively by sensory prediction errors. This form of learning had a distinct signature: as motor commands adapted, the subjects altered their predictions regarding sensory consequences of motor commands, and generalized this learning broadly to neighboring motor commands. In contrast, as the quality of the sensory feedback degraded, adaptation of motor commands became more dependent on reward prediction errors. Reward prediction errors produced comparable changes in the motor commands, but produced no change in the predicted sensory consequences of motor commands, and generalized only locally. Because we found that there was a within subject correlation between generalization patterns and sensory remapping, it is plausible that during adaptation an individual''s relative reliance on sensory vs. reward prediction errors could be inferred. We suggest that while motor commands change because of sensory and reward prediction errors, only sensory prediction errors produce a change in the neural system that predicts sensory consequences of motor commands.     

A pattern recognition technique to characterize the differential modulation of co-activating muscles at the performer/environment interface.

The purpose of this research was to develop and test an analytical tool that would recognize and classify the surface electromyographic (EMG) signal of co-activating muscles of the leg into pre-defined patterns of muscle activity: burst, tonic, and tonic-burst. Developed to study the task of landing from a jump in children, the pattern recognition technique (PRT) quantifies the full-wave rectified surface EMG signal over a short-duration sampling window by a single linear regression value. Shifting the sampling window across the data string ultimately defines the signal by a set of regression values that produce the recognizable burst, tonic and tonic-burst patterns on a least-squares surface plot. Statistical comparison of the PRT to the classical combination of threshold detection (+2 S.D. of mean baseline activity) and visual inspection proves the PRT to be more reliable on repeated measures for event detection and classification, with a Kappa statistic of 0.83 compared to 0.54 for threshold detection. Application of the PRT to motor control studies is presented for the regulation of the mechanical response of the leg during impact. Responsiveness of the PRT is tested, issues of accuracy and validity are addressed, and limitations in spatial-temporal resolution are identified.