Multimodal sensory control of motor performance by glycinergic interneurons of the mouse spinal cord deep dorsal horn

Sensory feedback is integral for contextually appropriate motor output, yet the neural circuits responsible remain elusive. Here, we pinpoint the medial deep dorsal horn of the mouse spinal cord as a convergence point for proprioceptive and cutaneous input. Within this region, we identify a population of tonically active glycinergic inhibitory neurons expressing parvalbumin. Using anatomy and electrophysiology, we demonstrate that deep dorsal horn parvalbumin-expressing interneuron (dPV) activity is shaped by convergent proprioceptive, cutaneous, and descending input. Selectively targeting spinal dPVs, we reveal their widespread ipsilateral inhibition onto pre-motor and motor networks and demonstrate their role in gating sensory-evoked muscle activity using electromyography (EMG) recordings. dPV ablation altered limb kinematics and step-cycle timing during treadmill locomotion and reduced the transitions between sub-movements during spontaneous behavior. These findings reveal a circuit basis by which sensory convergence onto dorsal horn inhibitory neurons modulates motor output to facilitate smooth movement and context-appropriate transitions.

Recording Forelimb Muscle Activity in Head-Fixed Mice with Chronically Implanted EMG Electrodes

Powerful genetic and molecular tools available in mouse systems neuroscience research have enabled researchers to interrogate motor system function with unprecedented precision in head-fixed mice performing a variety of tasks. The small size of the mouse makes the measurement of motor output difficult, as the traditional method of electromyographic (EMG) recording of muscle activity was designed for larger animals like cats and primates. Pending commercially available EMG electrodes for mice, the current gold-standard method for recording muscle activity in mice is to make electrode sets in-house. This article describes a refinement of established procedures for hand fabrication of an electrode set, implantation of electrodes in the same surgery as headplate implantation, fixation of a connector on the headplate, and post-operative recovery care. Following recovery, millisecond-resolution EMG recordings can be obtained during head-fixed behavior for several weeks without noticeable changes in signal quality. These recordings enable precise measurement of forelimb muscle activity alongside in vivo neural recording and/or perturbation to probe mechanisms of motor control in mice.

Widespread innervation of motoneurons by spinal V3 neurons globally amplifies locomotor output in mice

While considerable progress has been made in understanding the neuronal circuits that underlie the patterning of locomotor behaviours such as walking, less is known about the circuits that amplify motoneuron output to enable adaptable increases in muscle force across different locomotor intensities. Here, we demonstrate that an excitatory propriospinal neuron population (V3 neurons, Sim1 + ) forms a large part of the total excitatory interneuron input to motoneurons (∼20%) across all hindlimb muscles. Additionally, V3 neurons make extensive connections among themselves and with other excitatory premotor neurons (such as V2a neurons). These circuits allow local activation of V3 neurons at just one segment (via optogenetics) to rapidly depolarize and amplify locomotor-related motoneuron output at all lumbar segments in both the in vitro spinal cord and the awake adult mouse. Interestingly, despite similar innervation from V3 neurons to flexor and extensor motoneuron pools, functionally, V3 neurons exhibit a pronounced bias towards activating extensor muscles. Furthermore, the V3 neurons appear essential to extensor activity during locomotion because genetically silencing them leads to slower and weaker mice with a poor ability to increase force with locomotor intensity, without much change in the timing of locomotion. Overall, V3 neurons increase the excitability of motoneurons and premotor neurons, thereby serving as global command neurons that amplify the locomotion intensity.

Distinct roles of spinal commissural interneurons in transmission of contralateral sensory information

Crossed reflexes are mediated by commissural pathways transmitting sensory information to the contralateral side of the body, but the underlying network is not fully understood. Commissural pathways coordinating the activities of spinal locomotor circuits during locomotion have been characterized in mice, but their relationship to crossed reflexes is unknown. We show the involvement of two genetically distinct groups of commissural interneurons (CINs) described in mice, V0 and V3 CINs, in the crossed reflex pathways. Our data suggest that the exclusively excitatory V3 CINs are directly involved in the excitatory crossed reflexes and show that they are essential for the inhibitory crossed reflexes. In contrast, the V0 CINs, a population that includes excitatory and inhibitory CINs, are not directly involved in excitatory or inhibitory crossed reflexes but downregulate the inhibitory crossed reflexes. Our data provide insights into the spinal circuitry underlying crossed reflexes in mice, describing the roles of V0 and V3 CINs in crossed reflexes.

Distinct roles of spinal commissural interneurons in transmission of contralateral sensory information

Crossed reflexes (CR) are mediated by commissural pathways transmitting sensory information to the contralateral side of the body, but the underlying network is not fully understood. Commissural pathways coordinating the activities of spinal locomotor circuits during locomotion have been characterized in mice, but their relationship to CR is unknown. We show the involvement of two genetically distinct groups of commissural interneurons (CINs) described in mice, V0 and V3 CINs, in the CR pathways. Our data suggest that the exclusively excitatory V3 CINs are directly involved in the excitatory CR, and show that they are essential for the inhibitory CR. In contrast, the V0 CINs, a population that includes excitatory and inhibitory CINs, are not directly involved in excitatory or inhibitory CRs but down-regulate the inhibitory CR. Our data provide insights into the spinal circuitry underlying CR in mice, describing the roles of V0 and V3 CINs in CR.

Muscle spindles and their role in maintaining robust locomotion

Muscle spindles, one of the two main classes of proprioceptors together with Golgi tendon organs, are sensory structures that keep the central nervous system updated about the position and movement of body parts. Although they were discovered more than 150 years ago, their function during movement is not yet fully understood. Here, we summarize the morphology and known functions of muscle spindles, with a particular focus on locomotion. Although certain properties such as the sensitivity to dynamic and static muscle stretch are long known, recent advances in molecular biology have allowed the characterization of the molecular mechanisms for signal transduction in muscle spindles. Building upon classic literature showing that a lack of sensory feedback is deleterious to locomotion, we bring to the discussion more recent findings that support a pivotal role of muscle spindles in maintaining murine and human locomotor robustness, defined as the ability to cope with perturbations. Yet, more research is needed to expand the existing mechanistic understanding of how muscle spindles contribute to the production of robust, functional locomotion in real world settings. Future investigations should focus on combining different animal models to identify, in health and disease, those peripheral, spinal and brain proprioceptive structures involved in the fine tuning of motor control when locomotion happens in challenging conditions.

ERR2 and ERR3 promote the development of gamma motor neuron functional properties required for proprioceptive movement control

The ability of terrestrial vertebrates to effectively move on land is integrally linked to the diversification of motor neurons into types that generate muscle force (alpha motor neurons) and types that modulate muscle proprioception, a task that in mammals is chiefly mediated by gamma motor neurons. The diversification of motor neurons into alpha and gamma types and their respective contributions to movement control have been firmly established in the past 7 decades, while recent studies identified gene expression signatures linked to both motor neuron types. However, the mechanisms that promote the specification of gamma motor neurons and/or their unique properties remained unaddressed. Here, we found that upon selective loss of the orphan nuclear receptors ERR2 and ERR3 (also known as ERRβ, ERRγ or NR3B2, NR3B3, respectively) in motor neurons in mice, morphologically distinguishable gamma motor neurons are generated but do not acquire characteristic functional properties necessary for regulating muscle proprioception, thus disrupting gait and precision movements. Complementary gain-of-function experiments in chick suggest that ERR2 and ERR3 could operate via transcriptional activation of neural activity modulators to promote a gamma motor neuron biophysical signature of low firing thresholds and high firing rates. Our work identifies a mechanism specifying gamma motor neuron functional properties essential for the regulation of proprioceptive movement control.

The brain integrates proprioceptive information to ensure robust locomotion

Robust locomotion relies on information from proprioceptors: sensory organs that communicate the position of body parts to the spinal cord and brain. Proprioceptive circuits in the spinal cord are known to coarsely regulate locomotion in the presence of perturbations. Yet, the regulatory importance of the brain in maintaining robust locomotion remains less clear. Here, through mouse genetic studies and in vivo electrophysiology, we examined the role of the brain in integrating proprioceptive information during perturbed locomotion. The systemic removal of proprioceptors left the mice in a constantly perturbed state, similar to that observed during mechanically perturbed locomotion in wild-type mice and characterised by longer and less accurate synergistic activation patterns. By contrast, after surgically interrupting the ascending proprioceptive projection to the brain through the dorsal column of the spinal cord, wild-type mice showed normal walking behaviour, yet lost the ability to respond to external perturbations. Our findings provide direct evidence of a pivotal role for ascending proprioceptive information in achieving robust, safe locomotion. KEY POINTS: Whether brain integration of proprioceptive feedback is crucial for coping with perturbed locomotion is not clear. We showed a crucial role of the brain for responding to external perturbations and ensure robust locomotion. We used mouse genetics to remove proprioceptors and a spinal lesion model to interrupt the flow of proprioceptive information to the brain through the dorsal column in wild-type animals. Using a custom-built treadmill, we administered sudden and random mechanical perturbations to mice during walking. External perturbations affected locomotion in wild-type mice similar to the absence of proprioceptors in genetically modified mice. Proprioceptive feedback from muscle spindles and Golgi tendon organs contributed to locomotor robustness. Wild-type mice lost the ability to respond to external perturbations after interruption of the ascending proprioceptive projection to the brainstem.

Crossed reflex responses to flexor nerve stimulation in mice

Motor responses in one leg to sensory stimulation of the contralateral leg have been named "crossed reflexes" and are extensively investigated in cats and humans. Despite this effort, a circuit-level understanding of the crossed reflexes has remained missing. In mice, advances in molecular genetics enabled insights into the "commissural spinal circuitry" that ensures coordinated leg movements during locomotion. Despite some common features between the commissural spinal circuitry and the circuit for the crossed reflexes, the degree to which they overlap has remained obscure. Here, we describe excitatory crossed reflex responses elicited by electrically stimulating the common peroneal nerve that mainly innervates ankle flexor muscles and the skin on anterolateral aspect of the hind leg. Stimulation of the peroneal nerve with low current intensity evoked low-amplitude motor responses in the contralateral flexor and extensor muscles. At higher current strengths, stimulation of the same nerve evoked stronger and more synchronous responses in the same contralateral muscles. In addition to the excitatory crossed reflex pathway indicated by muscle activation, we demonstrate the presence of an inhibitory crossed reflex pathway, which was modulated when the motor pools were active during walking. The results are compared with the crossed reflex responses initiated by stimulating proprioceptors from extensor muscles and cutaneous afferents from the posterior part of the leg. We anticipate that these findings will be essential for future research combining the in vivo experiments presented here with mouse genetics to understand crossed reflex pathways at the network level in vivo.NEW & NOTEWORTHY Insights into the mechanisms of crossed reflexes are essential for understanding coordinated leg movements that maintain stable locomotion. Advances in mouse genetics allow for the selective manipulation of spinal interneurons and provide opportunities to understand crossed reflexes. Crossed reflexes in mice, however, are poorly described. Here, we describe crossed reflex responses in mice initiated by stimulation of the common peroneal nerve, which serves as a starting point for investigating crossed reflexes at the cellular level.

Control of Mammalian Locomotion by Somatosensory Feedback

When animals walk overground, mechanical stimuli activate various receptors located in muscles, joints, and skin. Afferents from these mechanoreceptors project to neuronal networks controlling locomotion in the spinal cord and brain. The dynamic interactions between the control systems at different levels of the neuraxis ensure that locomotion adjusts to its environment and meets task demands. In this article, we describe and discuss the essential contribution of somatosensory feedback to locomotion. We start with a discussion of how biomechanical properties of the body affect somatosensory feedback. We follow with the different types of mechanoreceptors and somatosensory afferents and their activity during locomotion. We then describe central projections to locomotor networks and the modulation of somatosensory feedback during locomotion and its mechanisms. We then discuss experimental approaches and animal models used to investigate the control of locomotion by somatosensory feedback before providing an overview of the different functional roles of somatosensory feedback for locomotion. Lastly, we briefly describe the role of somatosensory feedback in the recovery of locomotion after neurological injury. We highlight the fact that somatosensory feedback is an essential component of a highly integrated system for locomotor control. © 2021 American Physiological Society. Compr Physiol 11:1-71, 2021.

Freely Behaving Mice Can Brake and Turn During Optogenetic Stimulation of the Mesencephalic Locomotor Region

A key function of the mesencephalic locomotor region (MLR) is to control the speed of forward symmetrical locomotor movements. However, the ability of freely moving mammals to integrate environmental cues to brake and turn during MLR stimulation is poorly documented. Here, we investigated whether freely behaving mice could brake or turn, based on environmental cues during MLR stimulation. We photostimulated the cuneiform nucleus (part of the MLR) in mice expressing channelrhodopsin in Vglut2-positive neurons in a Cre-dependent manner (Vglut2-ChR2-EYFP) using optogenetics. We detected locomotor movements using deep learning. We used patch-clamp recordings to validate the functional expression of channelrhodopsin and neuroanatomy to visualize the stimulation sites. In the linear corridor, gait diagram and limb kinematics were similar during spontaneous and optogenetic-evoked locomotion. In the open-field arena, optogenetic stimulation of the MLR evoked locomotion, and increasing laser power increased locomotor speed. Mice could brake and make sharp turns (~90°) when approaching a corner during MLR stimulation in the open-field arena. The speed during the turn was scaled with the speed before the turn, and with the turn angle. Patch-clamp recordings in Vglut2-ChR2-EYFP mice show that blue light evoked short-latency spiking in MLR neurons. Our results strengthen the idea that different brainstem neurons convey braking/turning and MLR speed commands in mammals. Our study also shows that Vglut2-positive neurons of the cuneiform nucleus are a relevant target to increase locomotor activity without impeding the ability to brake and turn when approaching obstacles, thus ensuring smooth and adaptable navigation. Our observations may have clinical relevance since cuneiform nucleus stimulation is increasingly considered to improve locomotion function in pathological states such as Parkinson's disease, spinal cord injury, or stroke.

Relative Contribution of Proprioceptive and Vestibular Sensory Systems to Locomotion: Opportunities for Discovery in the Age of Molecular Science

Locomotion is a fundamental animal behavior required for survival and has been the subject of neuroscience research for centuries. In terrestrial mammals, the rhythmic and coordinated leg movements during locomotion are controlled by a combination of interconnected neurons in the spinal cord, referred as to the central pattern generator, and sensory feedback from the segmental somatosensory system and supraspinal centers such as the vestibular system. How segmental somatosensory and the vestibular systems work in parallel to enable terrestrial mammals to locomote in a natural environment is still relatively obscure. In this review, we first briefly describe what is known about how the two sensory systems control locomotion and use this information to formulate a hypothesis that the weight of the role of segmental feedback is less important at slower speeds but increases at higher speeds, whereas the weight of the role of vestibular system has the opposite relation. The new avenues presented by the latest developments in molecular sciences using the mouse as the model system allow the direct testing of the hypothesis.

Fractal analysis of muscle activity patterns during locomotion: pitfalls and how to avoid them

Time-dependent physiological data sets are often difficult to interpret objectively. Biosignals such as electromyogram, electroencephalogram, or single-neuron recordings can be interpreted using various linear and nonlinear methods. Each analysis technique aims at the explanation of different data features that might be visible or not to the naked eye. Here, we used linear decomposition based on machine learning to extract motor primitives (the time-dependent coefficients of muscle synergies) from the hindlimb electromyographic activity of mice during normal and mechanically perturbed locomotion. We set out to investigate the effects of calculation parameters and data quality on two nonlinear metrics derived from fractal analysis: the Higuchi's fractal dimension (HFD) and the Hurst exponent (H). Both HFD and H proved to be exceptionally sensitive to changes in motor primitives induced by external perturbations to locomotion. We discuss the potential pitfalls that might arise from fractal analysis by using examples based on surrogate data. We conclude giving some simple, data-driven suggestions to reduce the chance of misinterpretations when metrics such as HFD and H are applied to any biological signal containing elements of periodicity.NEW & NOTEWORTHY Despite the lack of consensus on how to perform fractal analysis of physiological time series, many studies rely on this technique. Here, we shed light on the potential pitfalls of using the Higuchi's fractal dimension and the Hurst exponent. We expose and suggest how to solve the drawbacks of such methods when applied to data from normal and perturbed locomotion by combining in vivo recordings and computational approaches.

Sensory Feedback Control of Locomotor Pattern Generation in Cats and Mice

Traditionally, research aimed at the understanding of the sensory control of terrestrial mammalian locomotion has focused on cats as the animal model. But advances in molecular genetics and new methods to record movement in small animals have moved mice into the forefront of locomotor research. In this review article, I will first give an overview of what is known about sensory feedback control of locomotion, mainly emerged from experiments performed on cats. This overview will not be an exhaustive overview, but will rather aim to give a broad picture of what has been learned about the sensory control of locomotion using cats as the animal model. I will then give a brief summary of how the mouse is adding to these insights.

Balance Control Mediated by Vestibular Circuits Directing Limb Extension or Antagonist Muscle Co-activation

Maintaining balance after an external perturbation requires modification of ongoing motor plans and the selection of contextually appropriate muscle activation patterns that respect body and limb position. We have used the vestibular system to generate sensory-evoked transitions in motor programming. In the face of a rapid balance perturbation, the lateral vestibular nucleus (LVN) generates exclusive extensor muscle activation and selective early extension of the hindlimb, followed by the co-activation of extensor and flexor muscle groups. The temporal separation in EMG response to balance perturbation reflects two distinct cell types within the LVN that generate different phases of this motor program. Initially, an LVN_extensor population directs an extension movement that reflects connections with extensor, but not flexor, motor neurons. A distinct LVN_{co-activation} population initiates muscle co-activation via the pontine reticular nucleus. Thus, distinct circuits within the LVN generate different elements of a motor program involved in the maintenance of balance.

Excitatory and inhibitory crossed reflex pathways in mice

Sensory information from one leg has been known to elicit reflex responses in the contralateral leg, known as "crossed reflexes," and these have been investigated extensively in cats and humans. Furthermore, experiments with mice have shown commissural pathways in detail by using in vitro and in vivo physiological approaches combined with genetics. However, the relationship between these commissural pathways discovered in mice and crossed reflex pathways described in cats and humans is not known. In this study, we analyzed the crossed reflex in mice by using in vivo electromyographic recording techniques combined with peripheral nerve stimulation protocols to provide a detailed description of the crossed reflex pathways. We show that excitatory crossed reflexes are mediated by both proprioceptive and cutaneous afferent activation. In addition, we provide evidence for a short-latency inhibitory crossed reflex pathway likely mediated by cutaneous feedback. Furthermore, the short-latency crossed inhibition is downregulated in the knee extensor muscle and the ankle flexor muscle during locomotion. In conclusion, this article provides an analysis of excitatory and inhibitory crossed reflex pathways during resting and locomoting mice in vivo. The data presented in this article pave the way for future research aimed at understanding crossed reflexes using genetics in mice. NEW & NOTEWORTHY We describe for the first time excitatory and inhibitory crossed reflex pathways in mouse spinal cord in vivo and show that the inhibitory pathways are modulated during walking. This is a first step toward an understanding of crossed reflexes and their function during walking using in vivo recording techniques combined with mouse genetics.

Role of muscle spindle feedback in regulating muscle activity strength during walking at different speed in mice

Terrestrial animals increase their walking speed by increasing the activity of the extensor muscles. However, the mechanism underlying how this speed-dependent amplitude modulation is achieved remains obscure. Previous studies have shown that group Ib afferent feedback from Golgi tendon organs that signal force is one of the major regulators of the strength of muscle activity during walking in cats and humans. In contrast, the contribution of group Ia/II afferent feedback from muscle spindle stretch receptors that signal angular displacement of leg joints is unclear. Some studies indicate that group II afferent feedback may be important for amplitude regulation in humans, but the role of muscle spindle feedback in regulation of muscle activity strength in quadrupedal animals is very poorly understood. To examine the role of feedback from muscle spindles, we combined in vivo electrophysiology and motion analysis with mouse genetics and gene delivery with adeno-associated virus. We provide evidence that proprioceptive sensory feedback from muscle spindles is important for the regulation of the muscle activity strength and speed-dependent amplitude modulation. Furthermore, our data suggest that feedback from the muscle spindles of the ankle extensor muscles, the triceps surae, is the main source for this mechanism. In contrast, muscle spindle feedback from the knee extensor muscles, the quadriceps femoris, has no influence on speed-dependent amplitude modulation. We provide evidence that proprioceptive feedback from ankle extensor muscles is critical for regulating muscle activity strength as gait speed increases. NEW & NOTEWORTHY Animals upregulate the activity of extensor muscles to increase their walking speed, but the mechanism behind this is not known. We show that this speed-dependent amplitude modulation requires proprioceptive sensory feedback from muscle spindles of ankle extensor muscle. In the absence of muscle spindle feedback, animals cannot walk at higher speeds as they can when muscle spindle feedback is present.

Stumbling corrective reaction elicited by mechanical and electrical stimulation of the saphenous nerve in walking mice

The ability to walk around in a natural environment requires the capacity to cope with unexpected obstacles that may disrupt locomotion. One such mechanism is called the stumbling corrective reaction (SCR) that enables animals to step over obstacles that would otherwise disturb the progression of swing movement. Here we use in vivo motion analysis and physiological recording techniques to describe the SCR in mice. We show that SCR can be elicited consistently in mice during locomotion by inserting an obstacle along the path of leg movement during swing phase. Furthermore, we show that the same behavior can be elicited if the saphenous nerve, a cutaneous nerve that would detect contact of the leg with an object, is stimulated electrically. This suggests that cutaneous afferent feedback is sufficient to elicit SCR. We further show that the SCR is phase dependent, occurring only with stimulation during swing phase, but not during early stance. During SCR elicited by either method, the foot is lifted higher to clear the object by flexing the knee, via the semitendinosus muscle, and ankle joint, by tibialis anterior contraction. The tibialis anterior also exhibits a brief extension before flexion onset. Our data provide a detailed description of SCR in mice and will be crucial for future research that aims to identify the interneurons of the premotor network controlling SCR and its neuronal mechanisms by combining motion analysis, electrophysiology and mouse genetics.

Pioglitazone is superior to quetiapine, clozapine and tamoxifen at alleviating experimental autoimmune encephalomyelitis in mice

Recent evidence suggests that clozapine and quetiapine (atypical antipsychotics), tamoxifen (selective-estrogen receptor modulator) and pioglitazone (PPARγ agonist) may improve functional recovery in multiple sclerosis (MS). We have compared the effectiveness of oral administration of these drugs, beginning at peak disease, at reducing ascending paralysis, motor deficits and demyelination in mice subjected to experimental autoimmune encephalomyelitis (EAE). Mice were immunized with an immunogenic peptide corresponding to amino acids 35-55 of the myelin oligodendrocyte glycoprotein (MOG_35-55) in complete Freund's adjuvant and injected with pertussis toxin to induce EAE. Unlike clozapine, quetiapine and tamoxifen, administration of pioglitazone beginning at peak disease decreased both clinical scores and lumbar white matter loss in EAE mice. Using kinematic gait analysis, we found that pioglitazone also maintained normal movement of the hip, knee and ankle joints for at least 44 days after MOG_35-55 immunization. This long-lasting preservation of hindleg joint movements was accompanied by reduced white matter loss, microglial and macrophage activation and the expression of pro-inflammatory genes in the lumbar spinal cords of EAE mice. These results support clinical findings that suggest pioglitazone may reduce the progressive loss of motor function in MS by decreasing inflammation and myelin damage.

Are mice good models for human neuromuscular disease? Comparing muscle excursions in walking between mice and humans

Background

The mouse is one of the most widely used animal models to study neuromuscular diseases and test new therapeutic strategies. However, findings from successful pre-clinical studies using mouse models frequently fail to translate to humans due to various factors. Differences in muscle function between the two species could be crucial but often have been overlooked. The purpose of this study was to evaluate and compare muscle excursions in walking between mice and humans.

Methods

Recently published musculoskeletal models of the mouse hindlimb and human lower limb were used to simulate muscle-tendon dynamics during mouse and human walking, a key daily activity. Muscle fiber length changes (fiber excursions) of 25 muscle homologs in the two species were calculated from these simulations and then compared. To understand potential causes of differences in fiber excursions in walking, joint excursions and muscle moment arms were also compared across one gait cycle.

Results

Most muscles (19 out of 25 muscles) of the mouse hindlimb had much smaller fiber excursions as compared to human lower limb muscles during walking. For these muscles, fiber excursions in mice were only 48 ± 19% of those in humans. The differences in fiber excursion between the two species were primarily due to the reduced joint excursions and smaller muscle moment arms in mice as compared to humans.

Conclusions

Since progressive neuromuscular diseases, such as Duchenne muscular dystrophy, are known to be accelerated by damage accumulated from active muscle lengthening, these results suggest that biomechanical differences in muscle function during walking between mice and humans may impede the translations of knowledge gained from mouse models to humans. This knowledge would add a fresh perspective on how pre-clinical studies on mice might be better designed to improve translation to human clinical trials.

Electronic supplementary material

The online version of this article (10.1186/s13395-017-0143-9) contains supplementary material, which is available to authorized users.

Sagittal Plane Kinematic Gait Analysis in C57BL/6 Mice Subjected to MOG35-55 Induced Experimental Autoimmune Encephalomyelitis

Kinematic gait analysis in the sagittal plane has frequently been used to characterize motor deficits in multiple sclerosis (MS). We describe the application of these techniques to identify gait deficits in a mouse model of MS, known as experimental autoimmune encephalomyelitis (EAE). Paralysis and motor deficits in mice subjected to EAE are typically assessed using a clinical scoring scale. However, this scale yields only ordinal data that provides little information about the precise nature of the motor deficits. EAE disease severity has also been assessed by rotarod performance, which provides a measure of general motor coordination. By contrast, kinematic gait analysis of the hind limb in the sagittal plane generates highly precise information about how movement is impaired. To perform this procedure, reflective markers are placed on a hind limb to detect joint movement while a mouse is walking on a treadmill. Motion analysis software is used to measure movement of the markers during walking. Kinematic gait parameters are then derived from the resultant data. We show how these gait parameters can be used to quantify impaired movements of the hip, knee, and ankle joints in EAE. These techniques may be used to better understand disease mechanisms and identify potential treatments for MS and other neurodegenerative disorders that impair mobility.

Spinal microcircuits comprising dI3 interneurons are necessary for motor functional recovery following spinal cord transection

The spinal cord has the capacity to coordinate motor activities such as locomotion. Following spinal transection, functional activity can be regained, to a degree, following motor training. To identify microcircuits involved in this recovery, we studied a population of mouse spinal interneurons known to receive direct afferent inputs and project to intermediate and ventral regions of the spinal cord. We demonstrate that while dI3 interneurons are not necessary for normal locomotor activity, locomotor circuits rhythmically inhibit them and dI3 interneurons can activate these circuits. Removing dI3 interneurons from spinal microcircuits by eliminating their synaptic transmission left locomotion more or less unchanged, but abolished functional recovery, indicating that dI3 interneurons are a necessary cellular substrate for motor system plasticity following transection. We suggest that dI3 interneurons compare inputs from locomotor circuits with sensory afferent inputs to compute sensory prediction errors that then modify locomotor circuits to effect motor recovery.

Long-term measurement of muscle denervation and locomotor behavior in individual wild-type and ALS model mice

The increasing number of mouse models of human degenerative and injury-related diseases that affect motor behavior raises the importance of in vivo methodologies allowing measurement of physiological and behavioral changes over an extended period of time in individual animals. A method that provides long-term measurements of muscle denervation and its behavioral consequences in individual mice for several months is presented in this article. The method is applied to mSod1G93A mice, which model human amyotrophic lateral sclerosis (ALS). The denervation process of gastrocnemius and soleus muscles in mSod1G93A mice is demonstrated for up to 3 mo. The data suggest that as muscle denervation progresses, massive behavioral compensation occurs within the spinal cord that allows animals to walk almost normally until late ages. Only around the age of 84 days is the first sign of abnormal movement during walking behavior detected as an abnormal tibialis anterior activity profile that is manifested in subtle but abnormal swing movement during walking. Additionally, this method can be used with other mouse models of human diseases, such as spinal cord injury, intracerebral hemorrhage, Parkinson's diseases, and spinal muscular atrophy.

Requirement of enhanced Survival Motoneuron protein imposed during neuromuscular junction maturation

Spinal muscular atrophy is a common motor neuron disease caused by low survival motoneuron (SMN), a key protein in the proper splicing of genes. Restoring the protein is therefore a promising therapeutic strategy. Implementation of this strategy, however, depends on defining the temporal requirements for SMN. Here, we used controlled knockdown of SMN in transgenic mice to determine the precise postnatal stage requirements for this protein. Reducing SMN in neonatal mice resulted in a classic SMA-like phenotype. Unexpectedly, depletion of SMN in adults had relatively little effect. Insensitivity to low SMN emerged abruptly at postnatal day 17, which coincided with establishment of the fully mature neuromuscular junction (NMJ). Mature animals depleted of SMN eventually exhibited evidence of selective neuromuscular pathology that was made worse by traumatic injury. The ability to regenerate the mature NMJ in aged or injured SMN-depleted mice was grossly impaired, a likely consequence of the inability to meet the surge in demand for motoneuronal SMN that was seen in controls. Our results demonstrate that relative maturity of the NMJ determines the temporal requirement for the SMN protein. These observations suggest that the use of potent but potentially deleterious SMN-enhancing agents could be tapered in human patients once the neuromuscular system matures and reintroduced as needed to enhance SMN for remodeling aged or injured NMJs.

Circuits for grasping: spinal dI3 interneurons mediate cutaneous control of motor behavior

Accurate motor performance depends on the integration in spinal microcircuits of sensory feedback information. Hand grasp is a skilled motor behavior known to require cutaneous sensory feedback, but spinal microcircuits that process and relay this feedback to the motor system have not been defined. We sought to define classes of spinal interneurons involved in the cutaneous control of hand grasp in mice and to show that dI3 interneurons, a class of dorsal spinal interneurons marked by the expression of Isl1, convey input from low threshold cutaneous afferents to motoneurons. Mice in which the output of dI3 interneurons has been inactivated exhibit deficits in motor tasks that rely on cutaneous afferent input. Most strikingly, the ability to maintain grip strength in response to increasing load is lost following genetic silencing of dI3 interneuron output. Thus, spinal microcircuits that integrate cutaneous feedback crucial for paw grip rely on the intermediary role of dI3 interneurons.

Quantification of gait parameters in freely walking wild type and sensory deprived Drosophila melanogaster

Coordinated walking in vertebrates and multi-legged invertebrates [corrected] such as Drosophila melanogaster requires a complex neural network coupled to sensory feedback. An understanding of this network will benefit from systems such as Drosophila that have the ability to genetically manipulate neural activities. However, the fly's small size makes it challenging to analyze walking in this system. In order to overcome this limitation, we developed an optical method coupled with high-speed imaging that allows the tracking and quantification of gait parameters in freely walking flies with high temporal and spatial resolution. Using this method, we present a comprehensive description of many locomotion parameters, such as gait, tarsal positioning, and intersegmental and left-right coordination for wild type fruit flies. Surprisingly, we find that inactivation of sensory neurons in the fly's legs, to block proprioceptive feedback, led to deficient step precision, but interleg coordination and the ability to execute a tripod gait were unaffected.DOI:http://dx.doi.org/10.7554/eLife.00231.001.

Quantification of gait parameters in freely walking wild type and sensory deprived Drosophila melanogaster

Coordinated walking in vertebrates and multi-legged invertebrates such as Drosophila melanogaster requires a complex neural network coupled to sensory feedback. An understanding of this network will benefit from systems such as Drosophila that have the ability to genetically manipulate neural activities. However, the fly's small size makes it challenging to analyze walking in this system. In order to overcome this limitation, we developed an optical method coupled with high-speed imaging that allows the tracking and quantification of gait parameters in freely walking flies with high temporal and spatial resolution. Using this method, we present a comprehensive description of many locomotion parameters, such as gait, tarsal positioning, and intersegmental and left-right coordination for wild type fruit flies. Surprisingly, we find that inactivation of sensory neurons in the fly's legs, to block proprioceptive feedback, led to deficient step precision, but interleg coordination and the ability to execute a tripod gait were unaffected.

DOI:http://dx.doi.org/10.7554/eLife.00231.001

Most animals need to be able to move to survive. Animals without limbs, such as snakes, move by generating by wave-like contractions along their bodies, whereas limbed animals, such as vertebrates and arthropods, walk by coordinating the movements of multi-jointed arms and legs. Locomotion in limbed animals involves bending each joint within each arm or leg in a coordinated manner, while also ensuring that the movements of all the limbs are coordinated with each other. In bipeds such as humans, for example, it is critical that one leg is in the stance phase when the other leg is in the swing phase. The rules that govern the coordination of limbs also depend on the gait, so the rules for walking are not the same as the rules for running.

The nervous systems of bipeds and other animals that walk solve these problems by using complex neural circuits that coordinate the firing of the relevant motor neurons. Two general mechanisms are used to coordinate the firing of motor neurons. In one mechanism, local interneurons within the central nervous system coordinate motor neuron activities: in vertebrates these interneurons are found in the spinal cord. A second mechanism, termed proprioception, relies on sensory neurons that report the load and joint angles from the arms and legs back to the central nervous system, and thereby influence the firing of the motor neurons. Remarkably, both of these mechanisms, and also the types of neurons that comprise motor neuron circuits, are conserved from arthropods to vertebrates.

Mendes et al. describe a new approach that can be used to analyze how the fruit fly, D. melanogaster, walks on surfaces. They use a combination of an optical touch sensor and high-speed video imaging to follow the body of the fly as it walks, and also to record when and where it places each of its six feet on the surface as it moves. Then, using a software package called FlyWalker, they are able to extract a large of number of parameters that can be used to describe locomotion in adult fruit flies with high temporal and spatial resolution. Many of these parameters have never been measured or studied before.

Mendes et al. show that fruit flies do not display the abrupt transitions in gait that are typically observed in vertebrates. However, they do modify their neural circuits depending on their speed: indeed it appears that flies use subtly different neural circuitry for walking at slow, medium and fast speeds. Moreover, when genetic methods are used to block sensory feedback, the fly is still able to walk, albeit with reduced coordination and precision. Further, the data suggest that proprioception is less important when flies walk faster compared to when they walk more slowly. The next step in this research will be to combine this new method for analyzing locomotion in flies with the wide range of genetic tools that are available for the study of Drosophila: this will allow researchers to explore in greater detail the components of the motor neuron circuitry and their role in coordinated walking.

DOI:http://dx.doi.org/10.7554/eLife.00231.002

A cluster of cholinergic premotor interneurons modulates mouse locomotor activity

Mammalian motor programs are controlled by networks of spinal interneurons that set the rhythm and intensity of motor neuron firing. Motor neurons have long been known to receive prominent "C bouton" cholinergic inputs from spinal interneurons, but the source and function of these synaptic inputs have remained obscure. We show here that the transcription factor Pitx2 marks a small cluster of spinal cholinergic interneurons, V0(C) neurons, that represents the sole source of C bouton inputs to motor neurons. The activity of these cholinergic interneurons is tightly phase locked with motor neuron bursting during fictive locomotor activity, suggesting a role in the modulation of motor neuron firing frequency. Genetic inactivation of the output of these neurons impairs a locomotor task-dependent increase in motor neuron firing and muscle activation. Thus, V0(C) interneurons represent a defined class of spinal cholinergic interneurons with an intrinsic neuromodulatory role in the control of locomotor behavior.

Spinal 5-HT7 receptors are critical for alternating activity during locomotion: in vitro neonatal and in vivo adult studies using 5-HT7 receptor knockout mice

5-HT7 receptors have been implicated in the control of locomotion. Here we use 5-HT7 receptor knockout mice to rigorously test whether 5-HT acts at the 5-HT7 receptor to control locomotor-like activity in the neonatal mouse spinal cord in vitro and voluntary locomotion in adult mice. We found that 5-HT applied onto in vitro spinal cords of 5-HT7+/+ mice produced locomotor-like activity that was disrupted and subsequently blocked by the 5-HT7 receptor antagonist SB-269970. In spinal cords isolated from 5-HT7-/- mice, 5-HT produced either uncoordinated rhythmic activity or resulted in synchronous discharges of the ventral roots. SB-269970 had no effect on 5-HT-induced rhythmic activity in the 5-HT7-/- mice. In adult in vivo experiments, SB-269970 applied directly to the spinal cord consistently disrupted locomotion and produced prolonged-extension of the hindlimbs in 5-HT7+/+ but not 5-HT7-/- mice. Disrupted EMG activity produced by SB-269970 in vivo was similar to the uncoordinated rhythmic activity produced by the drug in vitro. Moreover, 5-HT7-/- mice displayed greater maximal extension at the hip and ankle joints than 5-HT7+/+ animals during voluntary locomotion. These results suggest that spinal 5-HT7 receptors are required for the production and coordination of 5-HT-induced locomotor-like activity in the neonatal mouse and are important for the coordination of voluntary locomotion in adult mice. We conclude that spinal 5-HT7 receptors are critical for alternating activity during locomotion.

V3 spinal neurons establish a robust and balanced locomotor rhythm during walking

A robust and well-organized rhythm is a key feature of many neuronal networks, including those that regulate essential behaviors such as circadian rhythmogenesis, breathing, and locomotion. Here we show that excitatory V3-derived neurons are necessary for a robust and organized locomotor rhythm during walking. When V3-mediated neurotransmission is selectively blocked by the expression of the tetanus toxin light chain subunit (TeNT), the regularity and robustness of the locomotor rhythm is severely perturbed. A similar degeneration in the locomotor rhythm occurs when the excitability of V3-derived neurons is reduced acutely by ligand-induced activation of the allatostatin receptor. The V3-derived neurons additionally function to balance the locomotor output between both halves of the spinal cord, thereby ensuring a symmetrical pattern of locomotor activity during walking. We propose that the V3 neurons establish a regular and balanced motor rhythm by distributing excitatory drive between both halves of the spinal cord.

Organizing network action for locomotion: insights from studying insect walking

The operational basis for the generation of a functional motor output during walking is formed by the interaction between central pattern generating networks, local feedback from sensory neurons about movements and forces generated in the locomotor organs and coordinating signals from neighboring segments or appendages. This review primarily addresses the current knowledge about network organization underlying the control of an insect walking leg and recent advances in understanding the ways by which modifications in the motor output for walking are generated. Here we focus especially on modifications of the walking motor pattern that are associated with changing walking speed and walking direction. We will place the current knowledge and new results into the broad context gained from other locomotor behaviors and organisms.

Segment specificity of load signal processing depends on walking direction in the stick insect leg muscle control system

In terrestrial locomotion, sensory feedback from load sensors is important for altering ongoing motor output on a step-by-step basis. We investigated the influence of load signals from the leg on motoneuron pools of the thorax-coxa (ThC) joint in the stick insect walking system. Load sensors were stimulated during rhythmic, alternating activity in protractor coxae (ProCx) and retractor coxae (RetCx) motoneuron pools. Alternating activity in the segment of interest was induced by mechanical stimulation of the animal or pharmacological activation of the isolated thoracic ganglia. Load signals from the legs altered the timing of ThC motoneuron activity by resetting and entraining the activity of the central rhythm generating network of the ThC joint. In the front and middle legs, load signals induced or promoted RetCx activity and decreased or terminated ProCx activity. In the hindleg, reverse transitions were elicited, with increasing load terminating RetCx and initiating ProCx activity. Studies in semi-intact walking animals showed that the effect of load on the ThC-joint motoneurons depended on walking direction, with increased load promoting the functional stance phase motoneuron pool (in forward walking, RetCx activity; in backward walking, ProCx activity). Thus, we show that modifications of sensory feedback in a locomotor system are related to walking direction. In a final set of ablation experiments, we show that the load influence is mediated by the three groups of trochanteral campaniform sensilla.

Load signals assist the generation of movement-dependent reflex reversal in the femur-tibia joint of stick insects

Reinforcement of movement is an important mechanism by which sensory feedback contributes to motor control for walking. We investigate how sensory signals from movement and load sensors interact in controlling the motor output of the stick insect femur-tibia (FT) joint. In stick insects, flexion signals from the femoral chordotonal organ (fCO) at the FT joint and load signals from the femoral campaniform sensilla (fCS) are known to individually reinforce stance-phase motor output of the FT joint by promoting flexor and inhibiting extensor motoneuron activity. We quantitatively compared the time course of inactivation in extensor tibiae motoneurons in response to selective stimulation of fCS and fCO. Stimulation of either sensor generates extensor activity in a qualitatively similar manner but with a significantly different time course and frequency of occurrence. Inactivation of extensor motoneurons arising from fCS stimulation was more reliable but more than threefold slower compared with the extensor inactivation in response to flexion signals from the fCO. In contrast, simultaneous stimulation of both sense organs produced inactivation in motoneurons with a time course typical for fCO stimulation alone, but with a frequency of occurrence characteristic for fCS stimulation. This increase in probability of occurrence was also accompanied by a delayed reactivation of the extensor motoneurons. Our results indicate for the first time that load signals from the leg affect the processing of movement-related feedback in controlling motor output.

Coordination of fore and hind leg stepping in cats on a transversely-split treadmill

To gain insight into the mechanism of coordination of stepping in the fore and hind legs of quadrupeds, we examined the kinematics of leg movements and the motor patterns in fore and hind leg flexor muscles in decerebrate walking cats when the two pairs of legs stepped on separate treadmills running at different speeds. When the front treadmill was slowed progressively from 0.6 to 0.3 m/s with the rear treadmill running at 0.6 m/s, the rate of stepping in both the fore and hind legs decreased and a 1:1 stepping ratio was maintained. The decrease in the rate of stepping in the hind legs was due primarily to an increase in the duration of the swing phase. Slowing the speed of the rear treadmill while keeping the front treadmill speed at 0.6 m/s decreased the rate of stepping of the hind legs, but had relatively little influence on the average rate of stepping in the forelegs. In this situation stepping in the fore and hind legs was uncoupled and the time of stepping in one hind leg relative to the ipsilateral foreleg progressively shifted during a walking sequence. Analysis of the timing of electromyographic (EMG) recordings from flexor muscles of the hip and elbow joints yielded insight into the neuronal mechanisms underlying the asymmetry in slowing either the front or rear treadmill. We propose that ipsilateral pattern generating networks are asymmetrically coupled via descending inhibitory pathways and an ascending excitatory pathway. We discuss how the characteristics of these linkages are functionally appropriate for establishing the normal timing of stepping in the hind and forelegs during slow walking.

Behavioral and electromyographic characterization of mice lacking EphA4 receptors

EphA4 receptors play an important role in axon guidance during development. Disrupting the expression of these receptors in mice has been shown to modify neuronal connections in the spinal cord and results in the production of a characteristic hopping gait. The EphA4-null mouse has been used in numerous investigations aimed at establishing mechanisms responsible for patterning motor activity during walking. However, there have been no detailed behavioral or electrophysiological studies on adult EphA4-null mice. We used high-speed video recordings to determine the coordination of leg movements during locomotion in adult EphA4-null mice. Our data show that the hopping movements of the hind legs are not always associated with synchronous movements of forelegs. The coupling between the forelegs is weak, resulting in changes in their phase relationship from step to step. The synchronous coordination of the hind legs can switch to an alternating pattern for a short period of time during recovery from isoflurane anesthesia. Comparison of the kinematics of hind leg movements in EphA4-null mice and wild-type animals shows that besides the synchronous coordination in EphA4-null mice, the swing durations and the swing amplitude are shorter. Electromyographic recordings from a knee extensor muscle show double bursting in the EphA4-null animals but single bursts in wild types. This double burst changes to single-burst activity during swimming and when hind legs are stepping in alternation. These observations suggest an influence of sensory feedback in shaping the pattern of muscle activity during locomotion in the mutant animals. Our data give the first detailed description of the locomotor behavior of an adult mouse with genetically manipulated spinal networks.

Our Experience in Cardiac Transplantation in Baskent University

Recently cardiac transplantation has an important place in treatment of end-stage cardiac failure. In Turkey between 2003 and 2005 at 10 centers 64 cardiac transplantations were performed including five at our facility. Herein we have presented our results. All patients were men of mean age 34.2 +/- 10.7 (17 to 44) years. Upon preoperative echocardiography their mean ejection fraction was 18% +/- 3.27% (17% to 23%). Pulmonary vascular resistance was 4.47 wood unit in one patient and in one case, there was Rh incompatibility between donor and recipient. We used HTK solution for protection of donor hearts. Mean ischemia time was 251.2 +/- 62.7 minutes (155 to 314). Mean aortic clamping time was 84 +/- 4.7 minutes (80 to 90). In all patients we performed a biatrial anastomosis technique. Hemofiltration was used to prevent hemodilution during operation. In the postoperative period four patients had acute renal dysfunction; one, a minor cerebrovascular accident; two, reoperated because of bleeding; one, cholestasis; one, temporary atrio-ventricular block; and one, mediastinitis. Mean follow-up time was 15.6 +/- 19.7 months (2 to 50). Neither early nor late mortality has occurred. All patients are in New York Heart Association class I. In all cases we used triple immunosuppressive therapy. In the follow-up period the mean number of cardiac biopsies per patient was 4.2 +/- 3.03 (2 to 8). Two cases had cardiac catheterization. As a complication of cardiac biopsy, pericardial tamponade developed in one patient; in another one we observed a right ventricular aneursym after cardiac biopsy. Cardiac transplantation was performed with low mortality and morbidity rates in end-stage cardiac failure patients with longer life expectancy and higher life quality. Unfortunately in our country, because of difficulties to find donor hearts, cardiac transplantations were small in number. For better results, we need a larger series.

Low-Dose Histidine-Tryptophan-Ketoglutarate Solution for Myocardial Protection

The effect of histidine-tryptophan-ketoglutarate (HTK) solution for myocardial protection has been shown in experimental and clinical studies using long ischemic times and high dosages. In our study we compared myocardial protection in isolated coronary bypass with a short period of ischemia using low dosage HTK and cold crystalloid cardioplegia. Each group contained 21 coronary artery disease patients. Cardioplegic solutions were administered antegrade in 10 to 15 mL/kg in one shot. This dosage of HTK was lower than that mentioned in the literature. We measured malondialdehyde, lactate, creatine kinase, creatine kinase-MB, and troponin-I levels. Aortic clamping time in the HTK group 33.9 +/- 8.2 minutes, versus 36.2 +/- 11.3 minutes in the crystalloid cardioplegia group (P > .05). Levels of creatine kinase and malondialdehyde were lower in HTK group at 24 hours and 2 minutes, respectively. Lactate levels were lower in the crystalloid cardioplegia group at 2 minutes in the coronary sinus serum sample, but there were no statistically differences among ischemic serum markers in both groups. Only intervals between aortic clamping and cardiac arrest were statistically meaningful (HTK 63.3 +/- 14.7 seconds versus crystalloid cardioplegia 53.6 +/- 15.6 seconds, P = .044). Our study shows that use of low-dose HTK for short clamping time operations is as successful for myocardial protection as crystalloid cardioplegia. Longer times for fibrillation can be explained with the low levels of potassium in HTK solution, but this length did not cause a biochemical or clinical difference.

Signals From Load Sensors Underlie Interjoint Coordination During Stepping Movements of the Stick Insect Leg

During stance and swing phase of a walking stick insect, the retractor coxae (RetCx) and protractor coxae (ProCx) motoneurons and muscles supplying the thorax-coxa (TC)-joint generate backward and forward movements of the leg. Their activity is tightly coupled to the movement of the more distal leg segments, i.e., femur, tibia, and tarsus. We used the single middle leg preparation to study how this coupling is generated. With only the distal leg segments of the middle leg being free to move, motoneuronal activity of the de-afferented and -efferented TC-joint is similarly coupled to leg stepping. RetCx motoneurons are active during stance and ProCx motoneurons during swing. We studied whether sensory signals are involved in this coordination of TC-joint motoneuronal activity. Ablation of the load measuring campaniform sensilla (CS) revealed that they substantially contribute to the coupling of TC-joint motoneuronal activity to leg stepping. Individually ablating trochanteral and femoral CS revealed the trochanteral CS to be necessary for establishing the coupling between leg stepping and coxal motoneuron activity. When the locomotor system was active and generated alternating bursts of activity in ProCx and RetCx motoneurons, stimulation of the CS by rearward bending of the femur in otherwise de-afferented mesothoracic ganglion terminated ongoing ProCx motoneuronal activity and initiated RetCx motoneuronal activity. We show that cuticular strain signals from the trochanteral CS play a major role in shaping TC-joint motoneuronal activity during walking and contribute to their coordination with the stepping pattern of the distal leg joints. We present a model for the sensory control of timing of motoneuronal activity in walking movements of the single middle leg.

Interjoint coordination in the stick insect leg-control system: the role of positional signaling

Interjoint coordination is essential for proper walking behavior in multi-jointed insect legs. We have shown previously that movement signals from the femur-tibia (FT) joint can shape motor activity of the adjacent coxa-trochanter (CT) joint in the stick insect, Carausius morosus. Here, we present data on the role of position signals from the FT-joint on activity generated in motoneurons (MNs) of the CT-joint. We show that the probability of occurrence of stance (with depression in the CT-joint) or swing movements (with levation in the CT-joint) at the start of walking sequences is influenced by the angle of the FT-joint in the resting animal. We tested the influence of FT-joint angle on pharmacologically induced rhythmic activity of CT-joint depressor (DprTr) and levator (LevTr) MNs. The burst duration, mean spike rate within bursts, and duty cycle for each MN pool were found to depend on FT position. For LevTr MNs, these parameters progressively increased as the FT-joint was moved from extension to flexion, and the opposite was true for DprTr MNs. The cycle period of CT-MN rhythmicity also depended on FT position. In addition, we sometimes observed that the motor output shifted completely to one MN pool at extreme positions, suggesting that the central rhythm-generating network for the CT-joint became locked in one phase. These results indicate that position signals from the FT-joint modulate rhythmic activity in CT-joint MNs partly by having access to central rhythm generating networks of the CT-joint.

The role of sensory signals from the insect coxa-trochanteral joint in controlling motor activity of the femur-tibia joint

Interjoint coordination in multi-jointed limbs is essential for the generation of functional locomotor patterns. Here we have focused on the role that sensory signals from the coxa-trochanteral (CT) joint play in patterning motoneuronal activity of the femur-tibia (FT) joint in the stick insect middle leg. This question is of interest because when the locomotor system is active, movement signals from the FT joint are known to contribute to patterning of activity of the central rhythm-generating networks governing the CT joint. We investigated the influence of femoral levation and depression on the activity of tibial motoneurons. When the locomotor system was active, levation of the femur often induced a decrease or inactivation of tibial extensor activity while flexor motoneurons were activated. Depression of the femur had no systematic influence on tibial motoneurons. Ablation experiments revealed that this interjoint influence was not mediated by signals from movement and/or position sensitive receptors at the CT joint, i.e., trochanteral hairplate, rhombal hairplate, or internal levator receptor organ. Instead the influence was initiated by sensory signals from a field of campaniform sensillae, situated on the proximal femur (fCS). Selective stimulation of these fCS produced barrages of inhibitory postsynaptic potentials (IPSPs) in tibial extensor motoneurons and activated tibial flexor motoneurons. During pharmacologically activated rhythmic activity of the otherwise isolated mesothoracic ganglion (pilocarpine, 5 x 10(-4) M), deafferented except for the CT joint, levation of the femur as well had an inhibitory influence on tibial extensor motoneurons. However, the influence of femoral levation on the rhythm generated was rather labile and only sometimes a reset of the rhythm was induced. In none of the preparations could entrainment of rhythmicity by femoral movement be achieved, suggesting that sensory signals from the CT joint only weakly affect central rhythm-generating networks of the FT joint. Finally, we analyzed the role of sensory signals from the fCS during walking by recording motoneuronal activity in the single middle leg preparation with fCS intact and after their removal. These experiments showed that fCS activity plays an important role in generating tibial motoneuron activity during the stance phase of walking.