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1Department of Physiology and 2Faculty of Rehabilitation Medicine, University of Alberta, Edmonton, Alberta, Canada
Submitted 16 February 2006; accepted in final form 29 March 2006
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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; Butt et al. 2005; Goldshmit et al. 2004; Kiehn and Butt 2003; Leblond et al. 2003; Whelan 2003). The increased use of mice to study walking has been fueled by three important factors: first, the possibility of eliciting sequences of fictive locomotion in reduced preparations of neonatal mice, allowing the recording of single cells within central pattern generator (CPG) networks (Bonnot et al. 1998; Nishimaru et al. 2000; Whelan et al. 2000); second, the possibility of genetically modifying neuronal networks in the spinal cord that pattern motor output (Dottori et al. 1998; Kullander et al. 2003; Lanuza et al. 2004); and third, the ability to identify groups of spinal neurons based on their patterns of gene expression during development (Edlund and Jessell 1999; Goulding et al. 2002; Jessell and Sanes 2000; Shirasaki and Pfaff 2002). Mutant mice with genetically modified locomotor networks where particular neurons are visible by fluorescent protein expression are now available (Kiehn and Butt 2003; Kiehn and Kullander 2004). Furthermore, quantitative analysis of the motor output of mutant mice in which specific interneuron types are absent (Dottori et al. 1998; Gosgnach et al. 2006; Goulding and Pfaff 2005; Leighton et al. 2001) or silenced (Gosgnach et al. 2006) has become a promising approach to understand the neuronal mechanism of locomotion.
Despite the increased use of mice, surprisingly little effort has been made to describe the walking behavior of adult mice. Only a few studies have described either leg kinematics (Fortier et al. 1987; Leblond et al. 2003) or interleg coordination during walking (Clarke and Still 1999; Kullander et al. 2001a; Starkey et al. 2005), and only one of these studies combined the analysis of kinematic and electromyographic (EMG) data (Leblond et al. 2003). To examine interleg coordination all but one investigation (Clarke and Still 1999) used techniques that provided only spatial parameters such as paw prints. However, understanding the neuronal mechanism underlying leg coordination additionally requires the examination of temporal patterns of movement and motor activity. Some information on motor patterns has been gained from recordings of EMG activity during walking (Leblond et al. 2003; Pearson et al. 2005; Scholle et al. 2005). If the potential of using molecular-genetic techniques in mice for gaining an understanding of the neurobiology of mammalian walking is to be fully realized, then it is essential that the behavior and motor patterns in genetically modified and normal animals must be characterized. In this study we have carried out such an analysis in EphA4-null mutant mice because neonatal EphA4-null mutants have been used extensively to gain information about the locomotor network (Butt et al. 2005; Kullander et al. 2003). In this mutant mouse the EphA4 receptors are not expressed (Dottori et al. 1998). This results in numerous abnormalities in axonal projections in the CNS, including a higher number of ipsilateral projections of corticospinal neurons and contralateral projections of putative local excitatory interneurons in the spinal cord (Dottori et al. 1998; Kullander et al. 2001b, 2003). The latter projections are considered to be primarily responsible for the characteristic hopping gait in this mutant that involves synchronous hind legs movements. It has been found that in in vitro preparations this synchronous pattern can be changed to an alternating pattern by blocking the reuptake of the transmitters glycine and 
-aminobutyric acid (GABA) (Kullander et al. 2003). This finding indicates that the inhibitory connections responsible for the normal left–right alternation still exist in EphA4-null mice and that the abnormal excitatory connections are dominating the coordination of activity in the hind legs.
In this investigation we compared leg movements of EphA4-null and wild-type mice during walking using high-speed video analysis, and recorded EMG activity from flexor and extensor muscles of the hind legs to assess possible abnormalities in the pattern-generating networks coordinating rhythmic movements of these legs. EMG recordings were also performed during swimming and scratching in mutant and wild-type animals to establish the extent to which coordination of hind leg movements is altered in behaviors other than walking. A preliminary report of our findings was previously published (Akay et al. 2005).
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONGRANTSACKNOWLEDGMENTSREFERENCES |
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] and six wild-type C57BL/6 (WT) mice (Charles River). The experiments were approved by the University of Alberta animal welfare committee and conducted in accordance with the guidelines set by the Canadian Council on Animal Care. Behavioral experiments
The procedure for high-speed videorecording during walking was previously described in Pearson et al. (2005). Except for two EphA4-null mice, animals were briefly anesthetized with Forane (Isoflurane, Baxter, Toronto, Ontario, Canada) and the custom-made three-dimensional reflective markers (2 mm diameters) were glued onto the shaved skin at the level of the iliac crest, hip, knee, ankle, paw, and tip of the fourth digit (toe) of the left hind leg, and one on the wrist of the left foreleg (Fig. 1A). Because of slippage of the skin above the knee joint during walking, the knee joint marker was placed only to estimate the knee position from the videorecordings. The actual knee position was calculated by triangulation from the position of hip and ankle joint markers, using the measured lengths of the femur and tibia. For the video recordings during free walking, the mice where placed into a custom-made Plexiglas walkway [90 x 5 x 13 cm (length x width x height)]. After recovery from anesthesia, the mice walked back and forth in the walkway and a section of 40 cm length in the center of the walkway was viewed with a high-speed camera (Photron Fastcam) set with the capture rate at 250 frames/s (Fig. 1B). Video data were stored directly to computer memory for later analysis. Coordination between the legs was determined from video images captured by a mirror placed underneath the walkway set at about 45° from vertical. These images allowed measurement of the time of swing onset of each leg, defined as time of the onset of forward movement of the paw.
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The fabrication and implantation of the EMG electrodes is described in Pearson et al. (2005). The animals were either anesthetized with intraperitoneal (ip) injection of a mixture of 0.2 ml Hypnorm (fentanyl citrate 0.315 mg/ml and fluanisone 10 mg/ml; Janssen-Cilag, Buckinghamshire, UK) and 0.2 ml Versed (Midazolam hydrochloride 5 mg/ml; SABEX 2002, Boucherville, Quebec, Canada) in 1 ml of sterile water, or with Forane. For the injectible anesthetic an initial dose of 0.2 ml was given ip and was supplemented as needed by subcutaneous injections.
After the mice were deeply anesthetized, the hind- and forelegs and the neck were shaved. Small incisions were made on the shaved areas and each bipolar electrode was led under the skin from the neck incision to the leg incisions. The needles on the distal end of each electrode were used to draw the pair of electrode wires through the muscle until the knot proximal to the bared regions was placed firmly against the surface of the muscle. The distal end of the pair of electrodes was loosely knotted and the knot moved to the muscle surface where it was tightened. The wires distal to the second knot were removed by cutting them close to the knot. The incisions on the legs were closed and the headpiece was stitched to the skin near the neck incision by 4–0 silk suture (Ethicon). After each surgery, 0.024 mg of Buprenex (buprenorphine hydrochloride; Reckitt Benckiser Healthcare, Hull, UK) was injected subcutaneously for analgesia. The mice were left at least 2 days in their cages to recover before any handling or experiments were performed.
In total, four wild-type mice and five EphA4-null mice were implanted with EMG electrodes. The remaining animals (two wild-type and three EphA4-null) were used only for videorecordings. In four wild-type mice and four EphA4-null mice, extensor muscles in all four legs (triceps: elbow extensor, Tr- in forelegs; and vastus lateralis: knee extensor, VL- in hind legs) were implanted. In one EphA4-null mouse, the ankle flexor muscles [tibialis anterior (TA)] of the left and right hind legs were recorded instead of foreleg extensor muscles. EMG activity was recorded during free walking, swimming, and scratching. The amplified EMG signals were stored on a magnetic tape (Vetter 4000A PCM recorder) for later analysis. Swimming behavior was elicited by placing the mice in a 33 cm-diameter plastic basin filled with lukewarm water. Scratching behavior occurred spontaneously in all mice.
Data analysis
All EMG recordings were digitized off-line using the Axotape (Axon Instruments) analog-to-digital conversion system (1 kHz). The digitized records were analyzed using custom-written software (Matlab) designed to analyze timing of various events in the EMG records. The video data were analyzed using Peak Motus 8.2 motion analysis software (ViconPeak, Denver, CO). Kinematic parameters of the stepping movements, such as swing and stance durations, were measured from data files created by the Peak Motus system. We defined the cycle period as the duration between one swing onset to the next swing onset. Swing amplitude was defined as the distance between the start of the swing onset (paw lift off) and the swing offset (after paw touch down).
Statistics
The differences in means were tested with the Student's t-test (for unpaired data). The significance level in the figures are shown as: *P < 0.05, **P < 0.01, or ***P < 0.001. In the text and figures N indicates the number of mice and n is the number of trials.
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RESULTS |
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The primary focus of this investigation was to describe the characteristics of hopping in EphA4-null mice and compare these with characteristics of walking in wild-type mice. We first investigated the coordination of all four legs during walking. In Fig. 2, each histogram shows the distribution of swing onsets in a chosen leg (as indicated by the arrow tip) relative to the step cycle of a reference leg (as indicated by the origin of the arrows). The histograms with black bars represent data measured from walking sequences obtained from wild-type mice and the histograms with the white bars represent data measured from EphA4-null mice. The two histograms at the bottom of Fig. 2 illustrate the most obvious differences between wild-type and EphA4-null: alternating and synchronous stepping of the hind legs, respectively.
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Our next objective was to examine the kinematics of movements in the hind legs of EphA4-null mice to establish whether there are significant differences in movements of individual legs compared with wild-type animals. In Fig. 4A, the durations of the swing (open circles) and stance (closed circle) phases were plotted versus the cycle periods of each step in the wild-type (left, N = 6 mice, n = 151 steps) and the EphA4-null (right, N = 6, n = 184) mice. These figures show that the EphA4-null mice generally tend to step with longer cycle periods indicated by the broader distribution of the data points toward the right side of the x-axis. Both graphs in Fig. 4A indicate a significant relationship between the stance durations and cycle period and no correlation between the swing durations and cycle period, whereas in the EphA4-null mice swing durations are shorter. The shorter swing durations in EphA4-null mice are also illustrated in Fig. 4B, where the mean and SDs of the swing durations from wild-type (N = 6, n = 151) and EphA4-null mice (N = 6, n = 184) are shown.
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Comparison of EMG activity during walking, swimming, and scratching in wild-type and EphA4-null mice
Given the differences in kinematics during walking/hopping we were interested to know whether these differences arose from major differences in the underlying motor pattern generated by spinal neuronal networks. We recorded the EMG activity of the VL muscle from the left and right hind legs because the VL muscle is easily accessible and is one of the main muscles extending the knee. Interestingly, we found that in EphA4-null mice the EMG showed two prominent bursts of activity with a low level of activity between (Fig. 8A, right). We refer to this pattern as double bursting. In contrast, the extensor phase consisted of only one fairly uniform burst in wild-type animals (Fig. 8A, left). The double burst in VL is also shown in Fig. 8B in recordings of another mouse, in which we also recorded EMG from a flexor muscle [Tibialis anterior (TA)] showing the flexor phases of the steps (gray area). The missing TA burst between the two VL bursts (arrows) shows that the double bursts are part of a single step cycle. In Fig. 8C, averaged EMG activity over the normalized cycle period from four mice is illustrated, showing that in all four animals there were two peaks of activity (double bursts) of VL activity during a step cycle. Finally, we never observed double bursting in the forelimb extensor muscles (Triceps) or in TA muscles in wild-type or in EphA4-null mice (not shown).
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Hind leg stepping in EphA4-null mice can alternate in certain circumstances
Synchrony in the hind legs in three behavioral situations in EphA4-null mice raises the issue of whether any circumstances exist where the hind legs step in alternation rather than in synchrony. If this can occur then it would indicate that the neuronal system responsible for the alternating left–right coordination still exists in adult EphA4-null mice, as has been demonstrated in neonates (Kullander et al. 2003). One situation in which the stepping in the hind legs of some adult EphA4-null mice alternated was for a short period of time (1 to 2 min) during recovery from isoflurane anesthesia. We observed this phenomenon in three of eight adult EphA4-null mice. This is illustrated in Fig. 10A, where the phase histograms of stepping in different pairs are presented for two EphA4-null mice when hind legs alternated during recovery from isoflurane anesthetic (gray bars) and in the same mice when the hind legs moved synchronously during walking. When stepping in the hind legs alternated, the forelegs also stepped in a coordinated, alternating manner (top gray histogram), and the coupling between ipsilateral fore- and hind legs was stronger (histograms on the left and right of Fig. 10A).
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DISCUSSION |
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Interleg coordination during rhythmic behaviors in EphA4-null mice
Until now the walking behavior of the adult EphA4-null mice has been described only anecdotally and documented only by means of footprint patterns (Dottori et al. 1998; Kullander et al. 2001b). All previous studies have noted the synchronous, hopping movements of the hind legs in these animals, but none has described movements of the forelegs. By using high-speed video recordings, we found that coupling between the forelegs was extremely variable, ranging from synchrony to alternation. There are a number of possible explanations for the loose coordination of the forelegs. First, the coupling of the central neuronal networks producing rhythmic foreleg movements might not be abnormal. In this case, these networks would tend to continue producing alternating stepping commands, although coupling might be modified by abnormal patterns of input from the hind leg pattern-generating networks and/or by abnormal sensory feedback arising from the modified movements of the body resulting from the hopping hind legs. Second, the coupling between pattern-generating networks controlling movement in the two forelegs may be similar to the hind legs; i.e., strongly excitatory and descending input may play a larger role in the control of the foreleg movements and thereby partly overcome the abnormal synchronous activity in the two legs. Finally, abnormal commissural excitation might be weaker in forelegs than in hind legs, thus allowing coupling between foreleg pattern-generating networks to depend on a variable mixture of activity in abnormal excitatory and normal inhibitory commissural connections. At this stage, we are unable to differentiate between these three explanations. However, our observation that movements of the forelegs are consistently synchronous during swimming in EphA4-null mice suggests that excitatory commissural connections do exist between the foreleg pattern-generating networks, thus arguing against the first possible explanation discussed above.
Another new finding of the present investigation is that alternating stepping movements of the hind legs can occur in adult EphA4-null mice as the animals are recovering from isoflurane anesthetic. This did not occur in all animals, but in those that it did, it was clear that the pattern of coordination of stepping in the legs was similar to that of wild-type animals (Fig. 10A), as was the motor pattern in the VL muscles (Fig. 10B). The fact that apparently normal alternating coupling of stepping in the hind legs can occur in adult EphaA4-null mice demonstrates that inhibitory commissural pathways between hind leg pattern-generating networks must exist in these animals. This conclusion parallels a similar conclusion from studies on neonatal EphA4-null mice (Kullander et al. 2003). Kullander et al. (2003) showed that EphA4 positive cells, which make ipsilateral projections only in wild-type animals, cross over the midline of the spinal cord in EphA4-null mice. The authors conclude that this abnormal commissural crossing is sufficient to explain the abnormal hopping gait of the EphA4-null mice and it provides an over-excitation between the two sides (Kullander et al. 2003). Moreover, they showed that superfusing the in vitro spinal cord from neonatal EphA4-null mice with glycine or GABA reuptake blockers can change the synchronous left–right pattern to alternation (Kullander et al. 2003). Because isoflurane is known to enhance GABAergic inhibition by increasing the potency of the GABAA receptors (Gyulai et al. 2001; Raines et al. 2003) it is likely that the alternating movements of the hind legs we observed during recovery from isoflurane anesthetic are the result of an enhancement of transmission in the normal inhibitory GABAergic commissural pathways and that this enhancement outweighs transmission in the abnormal excitatory commissural pathways.
We demonstrated that the synchronous hind leg stepping is also maintained in behaviors other than walking. This suggests that the abnormal commissural excitation in these mice is functional in different behaviors. A striking result is that the synchronous rhythmic movements of hind legs can also be observed in scratching behavior. Notice that during wild-type scratching the contralateral hind leg is not active at all. However, in EphA4-null mice the commissural innervation is apparently strong enough to initiate synchronous leg movement of the contralateral side.
Kinematics of hind leg movement in EphA4-null mice
An interesting issue is whether the individual pattern-generating networks controlling movements of each of the legs in EphA4-null mice develop abnormally, in addition to the obvious abnormal development of pathways coupling the two hind leg pattern-generating networks. Indications that EphA4 receptors also play a role in the development of the pattern-generating network controlling movements of one leg come from in vitro experiments, showing that abnormal EphA4 receptor function during development can lead to impaired flexor–extensor alternation (Egea et al. 2005). Therefore abnormal development and functioning of individual pattern-generating networks must be considered as a real possibility given the obvious differences in the kinematics of the hind leg movements during walking. Our observations have identified the following differences. First, the EphA4-null mice step with a shorter cycle period compared with wild-type mice at the same walking speed. The shorter cycle periods are produced by shortening the swing durations. Second, the swing amplitude is significantly smaller than normal in the EphA4-null mice, which explains the shorter cycle periods at a given walking speed. Third, the EphA4-null mice show larger ventral–dorsal undulation of the crest and much larger changes in forward velocity during a single step cycle resulting in the characteristic hopping movements of the hind quarters. Finally, movements at the hip, knee, and ankle joints occur at much more flexed positions in the EphA4-null mice, resulting in the hind legs being positioned more under the body compared with wild-type animals (Fig. 6).
Despite all these differences in the kinematics of movements in the hind legs, it is not necessarily the case that any of them reflect abnormal functioning of the pattern-generating network controlling a single leg. It remains possible that the changes in the kinematics of leg movement in EphA4-null mice are simply a secondary consequence of the abnormal intersegmental coupling between the two hind leg pattern-generating networks and perhaps between the two foreleg pattern-generating networks (see previous section). Indeed, we favor this possibility based on our findings from EMG recordings from hind leg muscles (see following text).
EMG pattern in extensor muscles during walking, swimming, and scratching
The EMG recordings from hind leg muscles revealed that the knee extensor muscle VL in EphA4-null generated two bursts of activity during the stance phase, compared with a single burst of activity in wild-type mice (Figs. 8 and 10B). One interpretation of this observation is that the individual pattern-generating networks do develop abnormally in the EphA4-null mice. An alternative interpretation is that the double bursting in the VL muscle results from a change in how activity in the VL motoneurons is regulated by sensory feedback signals. The dynamics of loading on the legs during walking and hopping would probably be different, and we know that load signals have a profound effect on patterning the locomotor output in different preparations, including vertebrates and invertebrates (reviewed in Duysens et al. 2000). The double bursts that occur in VL during one step cycle could be explained by differences in kinematics, such as the sudden load of the leg on touch down. This would generate larger than normal forces in leg extensor muscles, which would transiently inhibit the activity in the VL motoneurons, thus leading to the silencing of activity in VL early in the stance phase. The afferents responsible for the inhibition could arise from high-threshold force-sensitive free nerve endings in the VL muscle or other extensor muscles (Cleland and Rymer 1993), assuming these exist in mice as they do in cats. The second burst of activity in VL would emerge as this inhibitory signal wanes.
Evidence in support of the notion that changes in sensory feedback are primarily responsible for the abnormal pattern of activity in the VL muscles in EphA4-null mice comes from our observations on the pattern of activity in swimming mice and during recovery from isoflurane anesthetic. During swimming the VL muscles generated only a single burst per cycle that resembled the pattern during walking in wild-type animals (Fig. 9). Presumably during swimming the load variations during the extension phase of leg movement are much smaller than those during hopping. Similarly, when stepping of the hind legs of EphA4-null mice alternated during recovery from isoflurane, only single bursts of activity occurred in the VL muscles (Fig. 10B). This is strong evidence in support of the notion that the individual pattern-generating networks develop normally in EphA4-null mice. Interestingly, in the same animals exhibiting normal patterns of activity in VL during alternating stepping, the VL muscles began to generate double bursts per cycle as soon as the animals started hopping (Fig. 10B). Obviously, the observations we have made in this study cannot conclusively answer the question of whether individual pattern-generating networks develop abnormally in EphA4-null mice (this will require recording fictive motor patterns in adult animals). Nevertheless, our data indicate that major differences in the motor patterns in the mutant and normal animals could be a secondary consequence of the altered mechanics associated with the synchronous hopping behavior.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Address for reprint requests and other correspondence: T. Akay, Department of Physiology, University of Alberta, Edmonton, AB, Canada T6G 2H7 (E-mail: takay{at}ualberta.ca)
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REFERENCES |
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Bonnot A, Morin D, and Viala D. Organization of rhythmic motor patterns in the lumbosacral spinal cord of neonate mouse. Ann NY Acad Sci 860: 432–435, 1998.
Bonnot A, Whelan PJ, Mentis GZ, and O'Donovan MJ. Locomotor-like activity generated by neonatal mouse spinal cord. Brain Res Rev 40: 141–151, 2002.[CrossRef][Medline]
Butt SJ, Lundfald L, and Kiehn O. EphA4 defines a class of excitatory locomotor-related interneurons. Proc Natl Acad Sci USA 102: 14098–14103, 2005.
Clarke KA and Still J. Gait analysis in the mouse. Physiol Behav 66: 723–729, 1999.[CrossRef][Medline]
Cleland CL and Rymer WZ. Functional properties of spinal interneurons activated by muscular free nerve endings and their potential contributions to the clasp-knife reflex. J Neurophysiol 69: 1181–1191, 1993.
Dottori M, Hartley L, Galea M, Paxinos G, Polizzotto M, Kilpatrick T, Bartlett PF, Murphy M, Kontgen F, and Boyd AW. EphA4 (Sek1) receptor tyrosine kinase is required for the development of the corticospinal tract. Proc Natl Acad Sci USA 95: 13248–13253, 1998.
Duysens J, Clarac F, and Cruse H. Load-regulating mechanisms in gait and posture: comparative aspects. Physiol Rev 80: 83–133, 2000.
Edlund T and Jessell TM. Progression from extrinsic to intrinsic signaling in cell fate specification: a view from the nervous system. Cell 96: 211–224, 1999.[CrossRef][ISI][Medline]
Egea J, Nissen UV, Dufour A, Sahin M, Greeer P, Kullander K, Mrsic-Flogel TD, Greenberg ME, Kiehn O, Venderhaeghen P, and Klein R. Regulation of EphA4 kinase activity is required for a subset of axon guidance decisions suggesting a key role for receptor clustering in Eph function. Neuron 47: 515–528, 2005.[CrossRef][ISI][Medline]
Fortier PA, Smith AM, and Rossignol S. Locomotor deficits in the mutant mouse, Lurcher. Exp Brain Res 66: 271–286, 1987.[ISI][Medline]
Goldshmit Y, Galea MP, Wise G, Bartlett PF, and Turnley AM. Axonal regeneration and lack of asterocytic gliosis in EphA4-deficient mice. J Neurosci 24: 10064–10073, 2004.
Gosgnach S, Lanuza GM, Butt SJB, Saueressing H, Zhang Y, Velasquez T, Riethmacher D, Callaway EM, Kiehn O, and Goulding M. V1 spinal neurons regulate the speed of vertebrate locomotor outputs. Nature 440: 215–219, 2006.[CrossRef][Medline]
Goulding M, Lanuza G, Sapir T, and Narayan S. The formation of sensorimotor circuits. Curr Opin Nurobiol 12: 508–515, 2002.
Goulding M and Pfaff SJB. Development of circuits that generate simple rhythmic behaviors in vertebrates. Curr Opin Neurobiol 15: 14–20, 2005.[CrossRef][ISI][Medline]
Gyulai FE, Mintun MA, and Firestone LL. Dose-dependent enhancement of in vivo GABAA-benzodiazepine receptor binding by isoflurane. Anaesthesiology 95: 585–593, 2001.[ISI][Medline]
Jessell TM and Sanes JR. Development. The decade of the developing brain. Curr Opin Neurobiol 10: 599–611, 2000.[CrossRef][ISI][Medline]
Kiehn O and Butt SJ. Physiological, anatomical and genetic identification of CPG neurons in the developing mammalian spinal cord. Prog Neurobiol 70: 347–361, 2003.[CrossRef][ISI][Medline]
Kiehn O and Kullander K. Central pattern generators deciphered by molecular genetics. Neuron 41: 317–321, 2004.[CrossRef][ISI][Medline]
Kullander K, Butt SJB, Lebret JM, Lundfald L, Restrepo CE, Rydström A, Klein R, and Kiehn O. Role of EphA4 and EphrinB3 in local neuronal circuits that control walking. Science 299: 1889–1892, 2003.
Kullander K, Croll SD, Zimmer M, Pan L, McClain J, Hughes V, Zabski S, DeChiara TM, Klein R, Yancopoulos GD, and Gale NW. Ephrin-B3 is the midline barrier that prevents corticospinal tract axons from recrossing, allowing unilateral motor control. Genes Dev 15: 877–888, 2001a.
Kullander K, Mather NK, Diella F, Dottori M, Boyd AW, and Klein R. Kinase-dependent and kinase-independent functions of EphA4 receptors in major axon tract formation in vivo. Neuron 29: 73–84, 2001b.[CrossRef][ISI][Medline]
Lanuza GM, Gosgnach S, Pierani A, Jessell TM, and Goulding M. Genetic identification of spinal interneurons that coordinate left-right locomotor activity necessary for walking movements. Neuron 42: 375–386, 2004.[CrossRef][ISI][Medline]
Leblond H, L'Espérance M, Orsal D, and Rossignol S. Treadmill locomotion in the intact and spinal mouse. J Neurosci 23: 11411–11419, 2003.
Leighton PA, Mitchell KJ, Goodrich LV, Lu X, Pinson K, Scherz P, Skarnes WC, and Tessier-Lavigne M. Defining brain wiring patterns and mechanisms through gene trapping in mice. Nature 410: 174–179, 2001.[CrossRef][Medline]
Nishimaru H, Takizawa H, and Kudo N. 5-Hydroxytryptamine-induced locomotor rhythm in the neonatal mouse spinal cord in vitro. Neurosci Lett 280: 187–190, 2000.[CrossRef][ISI][Medline]
Pearson KG, Acharya H, and Fouad K. A new electrode configuration for recording electromyographic activity in behaving mice. J Neurosci Methods 148: 36–42, 2005.[CrossRef][ISI][Medline]
Raines DE, Claycomb RJ, and Forman SA. Modulation of GABAA receptor function by nonhalogenated alkane anesthetics: the effects on agonist enhancement, direct activation, inhibition. Anesth Analg 96: 112–118, 2003.
Scholle HC, Biedermann F, Arnold D, Jinnah HA, Grassme R, and Schumann NP. A surface EMG multi-electrode technique for characterizing muscle activation patterns in mice during treadmill locomotion. J Neurosci Methods 146: 174–182, 2005.[CrossRef][ISI][Medline]
Shirasaki R and Pfaff SL. Transcriptional codes and the control of neuronal identity. Ann Rev Neurosci 25: 251–281, 2002.[CrossRef][ISI][Medline]
Starkey ML, Barritt AW, Yip PK, Davies M, Hamers FPT, McMahon SB, and Bradbury EJ. Assessing behavioural function following a pyromidotomy lesion of the corticospinal tract in adult mice. Exp Neurol 195: 524–539, 2005.[CrossRef][ISI][Medline]
Whelan PJ. Developmental aspects of spinal locomotor function: insights from using the in vitro mouse spinal cord preparation. J Physiol 553: 695–706, 2003.
Whelan PJ, Bonnot A, and O'Donovan MJ. Properties of rhythmic activity generated by the isolated spinal cord of the neonatal mouse. J Neurophysiol 84: 2821–2833, 2000.
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