Neuromuscular patterns of stereotypic hindlimb behaviors in the first two postnatal months. I. Stepping in normal kittens.

Neuromuscular patterns associated with the development of hindlimb stepping behaviors were studied from birth to postnatal day 60 in normal kittens. Hindlimb muscles were chronically implanted with EMG electrodes at birth to characterize interlimb coordination and intralimb synergies during development of overground and treadmill stepping. Airstepping was also examined but seldom occurred after the second postnatal week. All kittens performed stepping under each condition, including weight-supported stepping, by postnatal day 3. The number of sequential steps on the treadmill and overground increased with age and cycle periods decreased. At onset, stepping behaviors were characterized by adult-like EMG patterns. Interlimb coordination was typified by alternating extensor bursts of similar duration. Extensors at the knee and ankle were coactive during the stance phase, and extensor burst durations were strongly correlated with the cycle periods over a wide range of stepping frequency. Ankle flexor and extensor muscles were reciprocally active during postural tremor, bouts of airstepping, and weight-supported steps on the treadmill and overground. The duration of the reciprocal flexor bust did not vary with cycle period or age. Observations of stepping behaviors and adult-like EMG patterns during initial postnatal development were contingent on optimal testing conditions. Taken together, the data suggest that pattern-generating circuits for regulating interlimb coordination and intralimb muscle synergies are potentially functional prior to the normal ontogenetic onset of locomotion. Perhaps the prolonged postnatal development of locomotion reflects the time required to establish adaptive mechanisms, such as postural control and agility, rather than spinal pattern-generating circuits for locomotion.

Neuromuscular patterns of stereotypic hindlimb behaviors in the first two postnatal months. II. Stepping in spinal kittens.

From birth to postnatal day 60, neuromuscular patterns for airstepping and treadmill stepping were assessed in kittens spinalized (T12) at birth (Day-1) or at the end of the second postnatal week (Day-14). Within 72 h after spinalization, all kittens displayed stepping motions, but exteroceptive facilitation (e.g. tail pinch) was required to initiate and sustain both behaviors. In Day-14 spinal kittens, the hindlimbs spontaneously and alternately airstepped, but in Day-1 spinal kittens exteroceptive stimulation was usually necessary to evoke airstepping, and the hindlimbs stepped synchronously. Kittens in both groups developed atypical neuromuscular patterns; flexor bursts were nearly twice as long in duration as extensor bursts. Development of bipedal treadmill stepping was similar for Day-1 and Day-14 spinal kittens, but differed from that for normal kittens. Tested at the same belt speeds, stepping was more easily elicited in spinal kittens, bouts of repetitive stepping were longer, and cycle periods were shorter than in normal kittens until postnatal week 6. Spinal kittens, however, seldom exhibited adequate weight support during hindlimbs stepping, and the neuromuscular patterns associated with bipedal stepping were atypical. For spinal kittens, the relationship between the extensor burst duration and the cycle period was reduced substantially, and flexor activity was initiated earlier in the step cycle and was longer in duration than that for normal kittens. These atypical intralimb synergies may have been the consequence of altered lumbosacral circuits produced by the spinal transection. It is also possible that these spinal circuits, lacking rostral input, were particularly susceptible to abnormal motion-dependent feedback resulting from reduced hindlimb weight support.

Neuromuscular patterns of stereotypic hindlimb behaviors in the first two postnatal months. III. Scratching and the paw-shake response in kittens.

Neuromuscular patterns of scratching and the paw-shake response were studied in normal kittens from birth to postnatal day 60. Onset of both behaviors coincided with the development of secure weight-bearing posture and occurred on postnatal day 21 for scratching and postnatal day 26 for paw shaking. At onset, cycle periods for scratching (5-6 Hz) and paw shaking (8-10 Hz) were similar to that for adult cats, and EMG patterns were adult-like. The scratch cycle consisted of reciprocal flexor and extensor bursts of equal duration, while the shake cycle consisted of coactive knee extensor and ankle flexor bursts alternately active with ankle extensor bursts. The lack of scratching and paw shaking during the first 3 postnatal weeks and the adult-like EMG patterns at onset are consistent with the hypothesis that pattern-generating circuits within lumbosacral segments are available early in development but inhibited by the rostral neuraxis until postural control is sufficient to accommodate the response. To eliminate rostral inputs, including descending input critical for postural control, kittens were spinalized at the T12 level, and onset of paw shaking was accelerated. In kittens spinalized at birth, paw-shake onset occurred on postnatal day 14, while in kittens spinalized on postnatal day 14, onset occurred 48 h after spinalization. In all spinal kittens, however, knee extensor activity was disrupted and not normal by postnatal day 60. Mature neuromuscular patterns for scratching and paw shaking are available at onset of the behavior during normal development. Spinalization hastens the onset of paw shaking but the normal neuromuscular synergy is disrupted as well as the temporal structure of the multi-cycle response. Disruptions following spinalization may be due to altered development of spinal pattern generators or aberrant feedback from atypical hindlimb motions due to a retardation of hindlimb growth and an alteration of muscle contractile properties in spinal kittens.

Early onset of hindlimb paw-shake responses in spinal kittens: new perspective in motor development.

Normal kittens and kittens spinalized (T12) at 14 days of age were tested for onset of paw-shake responses (PSR) in fore and hindlimbs. In normals, onset followed a cephalocaudal pattern that was coincident with the development of stable posture, 21-28 days of age. In spinals, onset of hindlimb PSRs preceded that of the forelimb and occurred soon after cordotomy, independent of posture development. These findings suggest that in the neonate, spinal networks responsible for coordinating PSRs are normally inhibited until the response can be supported by stable posture.

Reduction in buoyancy alters parameters of motility in E9 chick embryos.

Over the course of embryonic development, chick embryos express 3 different types of motility (I, II, III). Although neural pattern generators appear to control embryonic motility, the mechanisms responsible for the sequential emergence and/or transformations in these behaviors are not known. Given the early presence of functional sensory connections and substantial changes in movement dynamics associated with body growth in the fixed volume of an egg, it was hypothesized that changes in environmental constraints might contribute to shaping the transformations in motility. In this study, we tested the hypothesis that changes in buoyancy can alter parameters of motility in ovo at embryonic Day 9 (E9). Results demonstrate that attributes of Type I motility can be altered by a reduction in buoyancy. The possible contributions of the environment and experience to transformations in embryonic motor behavior are discussed.

Animal models offer the opportunity to acquire a new perspective on motor development.

During much of this century, our views of motor development were greatly influenced by speculations as to the role of reflexes in the control of movement. However, recent work casts a new view regarding the organization and control of movement. In this article, a review of recent studies of motor development in neonatal kittens and chick embryos is provided. The results of these studies suggest that very early in development there is considerable potential for coordinated movement. The potential for coordinated movement may also exist in early human development. Whether coordinated movement is observed appears to be dependent on a number of variables. For example, mechanical factors emerging during movement may impose demands that restrict the expression of coordinated movement. One possible implication for the practice of physical therapy is that researchers and clinicians will need to precisely characterize and quantify movement variables to advance our knowledge of the processes driving development and to establish effective means for addressing the movement problems of a pediatric population.

Correcting two-dimensional kinematic errors for chick embryonic movements in ovo.

In motor behavior studies of chick embryos in ovo, kinematic recording is limited to a single camera system and produces kinematic data that are distorted if out-of-plane movements are not considered. CONVERT is a rule based algorithm designed to calculate 3D limb movements given 2D kinematic data. CONVERT's calculations are based on a stationary reference point, limited translation of the chick embryo's trunk point, a multi-linked model of the body, and approximate limb segment lengths. Simulations indicate CONVERT calculates joint movements from 2D data with a maximum error of 6 degrees compared to a maximum error of 79 degrees if out-of-plane considerations are ignored. The approach used to correct two-dimensional kinematic measurement errors can be readily applied to other experimental conditions that restrict video recording to single camera systems.

Transformations in embryonic motility in chick: kinematic correlates of type I and II motility at E9 and E12.

Soon after hatching, chicks exhibit an array of adaptive, coordinated behaviors. Chick embryos also acquire nearly 18 days of movement experience, referred to as embryonic motility, before hatching. The chick expresses three forms of motility, types I, II, and III, and each emerges at a different stage of embryonic development. Although much is known about the mechanisms associated with motility at early embryonic stages and at the onset of hatching, the transformations in behavior and underlying mechanisms are not fully understood. Thus the purpose of this study was to determine how motility is modified during the first expected transformation, from type I to type II. It was hypothesized that kinematic features for motility at embryonic day 12 (E12) would differ significantly from features at E9 because type II motility emerges during E11. Embryos were video taped for extended intervals in ovo at E9 or E12 and entire sequences of motility were computer digitized for kinematic analyses. Results reported here indicate that several of the kinematic features characteristic of motility at E9 are also reliable features at E12. On the basis of these findings, a kinematic definition of type I motility is posed for use in subsequent behavioral studies. Several parameters distinguished motility at E12 from E9. The most notable difference between ages was the less regular timing of repetitive limb movements at E12, a finding consistent with recent reports suggesting early motility is an emergent product of a transient neural network rather than a specialized pattern generator. As predicted from established definitions for type II motility, startle-like movements were common at E12; however, they also were present in many kinematic plots at E9, suggesting the discreet age-dependent boundaries in the established definition for type II motility may require modification. Some age-related differences, such as increased intralimb coordination and excursion velocity, may be prerequisites for adaptive behavior after hatching.

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.

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.

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.

Development of coordinated movement in chicks: I. Temporal analysis of hindlimb muscle synergies at embryonic days 9 and 10.

In this study, we examined electromyographic activity for an ensemble of hindlimb muscles during spontaneous activity in chick embryos to advance understanding of early motor coordination and its relationship to later emerging behaviors. Four-channel recordings were obtained from 6 muscles in ovo at embryonic Days 9 and 10. Analyses indicated that when muscles are repetitively active, patterns during embryonic motility are distinct from those for other behaviors. For example, unlike the muscle patterns for locomotion, extensor muscles and flexor muscles are synchronously activated at 50% of the extensor cycle period. Furthermore, flexor and extensor bursts are similar in duration and show little correlation with extensor cycle period. Finally, our data suggest that the ensemble of muscles active can vary from cycle to cycle. This study provides the basis for future studies that will examine neural and biomechanical interactions underlying the development of coordinated movement.

Kinematic analysis of wing and leg movements for type I motility in E9 chick embryos.

Based on studies using direct observation methods, type I motility, the first motility pattern to emerge in chick embryos, is characterized as random, uncoordinated movement. Yet, electromyographic (EMG) studies indicate that leg muscles are recruited in orderly patterns of alternating flexor and extensor activity during type I motility. It has been suggested that this apparent paradox may be attributable to perturbations arising during movement in ovo under buoyant conditions. It is also possible that direct observation methods are insufficient to detect the extent of coordination between body parts during type I motility. To address the apparent discrepancy between random features reported in observational studies and reliable features reported in EMG studies, embryos were video recorded continuously for 60 min at embryonic day 9 and criteria were established to obtain homogeneous samples of motility for kinematic analysis of synchronous wing and leg movements. Limited to a single camera attached to a stereomicroscope, methods were developed to correct for out-of-plane movements of the ipsilateral wing and leg. Also, amniotic fluid was extracted from the egg in some recordings to test the possibility that movement under buoyant conditions may mask coordinated movement. Extended sequences of activity were digitized and analyzed. Results indicated that within a limb (wing or leg), direction and timing of excursions at adjacent joints co-varied and limb excursions were characterized by reliable patterns of alternating flexion and extension consistent with EMG studies.(ABSTRACT TRUNCATED AT 250 WORDS)

Absence of hind limb tactile placing in spinal cats and kittens.

Tactile placing and associated responses of the fore and hind paws to a light tactile stimulus were studied in normal young-adult cats and kittens and their spinal littermates. All spinal transections were performed at T12 on the 14th postnatal day. In the first study, responses of normal and spinal young-adult cats were compared at 9 to 10 months of age. Frequency of forelimb tactile placing (FL-TP) was similar for both groups, but hind limb tactile placing (HL-TP), seldom elicited in the normal cats, was never elicited in the spinal cats. Due to the absence of HL-TP in the spinal young-adult cats, a second study was conducted. Kittens, 1 to 62 days of age, were tested to determine if tactile placing was present prior to supraspinal maturation. The initial withdrawal response to the light tactile stimulus was equally developed in forelimbs and hind limbs at birth. In normal kittens, development of FL-TP preceded that of HL-TP by 2 weeks. During the first week after transection, forelimb responses of spinal kittens were more frequent than those of their normal littermates, suggesting an enhancement of motor responses proximal to the lesion, the difference decreasing thereafter. Frequency of hind limb withdrawal was not immediately altered after transection, and airstepping was easily triggered within 48 h after cordotomy. During tactile-triggered airstepping, the hind paw occasionally contacted the placing surface, but HL-TP was not observed in spinal kittens during the testing period. The absence of HL-TP in all spinal cats tested suggests that tactile placing is not a spinal reflex.