A kinematic and kinetic analysis of locomotion during voluntary gait modification in the cat

Neuromechanical Strategies for Obstacle Negotiation during Overground Locomotion following Incomplete Spinal Cord Injury in Adult Cats

Following incomplete spinal cord injury in animals, including humans, substantial locomotor recovery can occur. However, functional aspects of locomotion, such as negotiating obstacles, remains challenging. We collected kinematic and electromyography data in 10 adult cats (5 males, 5 females) before and at weeks 1-2 and 7-8 after a lateral mid-thoracic hemisection on the right side of the cord while they negotiated obstacles of three different heights. Intact cats always cleared obstacles without contact. At weeks 1-2 after hemisection, the ipsilesional right hindlimb contacted obstacles in ∼50% of trials, triggering a stumbling corrective reaction or absent responses, which we termed Other. When complete clearance occurred, we observed exaggerated ipsilesional hindlimb flexion when crossing the obstacle with contralesional Left limbs leading. At weeks 7-8 after hemisection, the proportion of complete clearance increased, Other responses decreased, and stumbling corrective reactions remained relatively unchanged. We found redistribution of weight support after hemisection, with reduced diagonal supports and increased homolateral supports, particularly on the left contralesional side. The main neural strategy for complete clearance in intact cats consisted of increased knee flexor activation. After hemisection, ipsilesional knee flexor activation remained, but it was insufficient or more variable as the limb approached the obstacle. Intact cats also increased their speed when stepping over an obstacle, an increase that disappeared after hemisection. The increase in complete clearance over time after hemisection paralleled the recovery of muscle activation patterns or new strategies. Our results suggest partial recovery of anticipatory control through neuroplastic changes in the locomotor control system.SIGNIFICANCE STATEMENT Most spinal cord injuries (SCIs) are incomplete and people can recover some walking functions. However, the main challenge for people with SCIs that do recover a high level of function is to produce a gait that can adjust to everyday occurrences, such as turning, stepping over an obstacle, etc. Here, we use the cat model to answer two basic questions: How does an animal negotiate an obstacle after an incomplete SCI and why does it fail to safely clear it? We show that the inability to clear an obstacle is because of improper activation of muscles that flex the knee. Animals recover a certain amount of function thanks to new strategies and changes within the nervous system.

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.

A Validation of Supervised Deep Learning for Gait Analysis in the Cat

Gait analysis in cats and other animals is generally performed with custom-made or commercially developed software to track reflective markers placed on bony landmarks. This often involves costly motion tracking systems. However, deep learning, and in particular DeepLabCut^TM (DLC), allows motion tracking without requiring placing reflective markers or an expensive system. The purpose of this study was to validate the accuracy of DLC for gait analysis in the adult cat by comparing results obtained with DLC and a custom-made software (Expresso) that has been used in several cat studies. Four intact adult cats performed tied-belt (both belts at same speed) and split-belt (belts operating at different speeds) locomotion at different speeds and left-right speed differences on a split-belt treadmill. We calculated several kinematic variables, such as step/stride lengths and joint angles from the estimates made by the two software and assessed the agreement between the two measurements using intraclass correlation coefficient or Lin's concordance correlation coefficient as well as Pearson's correlation coefficients. The results showed that DLC is at least as precise as Expresso with good to excellent agreement for all variables. Indeed, all 12 variables showed an agreement above 0.75, considered good, while nine showed an agreement above 0.9, considered excellent. Therefore, deep learning, specifically DLC, is valid for measuring kinematic variables during locomotion in cats, without requiring reflective markers and using a relatively low-cost system.

Contribution of the Entopeduncular Nucleus and the Globus Pallidus to the Control of Locomotion and Visually Guided Gait Modifications in the Cat

We tested the hypothesis that the entopeduncular (EP) nucleus (feline equivalent of the primate GPi) and the globus pallidus (GPe) contribute to both the planning and execution of locomotion and voluntary gait modifications in the cat. We recorded from 414 cells distributed throughout these two nuclei (referred to together as the pallidum) while cats walked on a treadmill and stepped over an obstacle that advanced towards them. Neuronal activity in many cells in both structures was modulated on a step-by-step basis during unobstructed locomotion and was modified in the step over the obstacle. On a population basis, the most frequently observed change, in both the EP and the GPe, was an increase in activity prior to and/or during the swing phase of the step over the obstacle by the contralateral forelimb, when it was the first limb to pass over the obstacle. Our results support a contribution of the pallidum, in concert with cortical structures, to the control of both the planning and the execution of the gait modifications. We discuss the results in the context of current models of pallidal action on thalamic activity, including the possibility that cells in the EP with increased activity may sculpt thalamo-cortical activity.

Strategies for obstacle avoidance during walking in the cat

Avoiding obstacles is essential for successful navigation through complex environments. This study aimed to clarify what strategies are used by a typical quadruped, the cat, to avoid obstacles during walking. Four cats walked along a corridor 2.5 m long and 25 or 15 cm wide. Obstacles, small round objects 2.5 cm in diameter and 1 cm in height, were placed on the floor in various locations. Movements of the paw were recorded with a motion capture and analysis system (Visualeyez, PTI). During walking in the wide corridor, cats' preferred strategy for avoiding a single obstacle was circumvention, during which the stride direction changed while stride duration and swing-to-stride duration ratio were preserved. Another strategy, stepping over the obstacle, was used during walking in the narrow corridor, when lateral deviations of walking trajectory were restricted. Stepping over the obstacle involved changes in two consecutive strides. The stride preceding the obstacle was shortened, and swing-to-stride ratio was reduced. The obstacle was negotiated in the next stride of increased height and normal duration and swing-to-stride ratio. During walking on a surface with multiple obstacles, both strategies were used. To avoid contact with the obstacle, cats placed the paw away from the object at a distance roughly equal to the diameter of the paw. During obstacle avoidance cats prefer to alter muscle activities without altering the locomotor rhythm. We hypothesize that a choice of the strategy for obstacle avoidance is determined by minimizing the complexity of neuro-motor processes required to achieve the behavioral goal.NEW & NOTEWORTHY In a study of feline locomotor behavior we found that the preferred strategy to avoid a small obstacle is circumvention. During circumvention, stride direction changes but length and temporal structure are preserved. Another strategy, stepping over the obstacle, is used in narrow walkways. During overstepping, two strides adjust. A stride preceding the obstacle decreases in length and duration. The following stride negotiating the obstacle increases in height while retaining normal temporal structure and nearly normal length.

Ladder Treadmill: A Method to Assess Locomotion in Cats with an Intact or Lesioned Spinal Cord

After lesions of the CNS, locomotor abilities of animals (mainly cats) are often assessed on a simple flat treadmill (FTM), which imposes little demands on supraspinal structures as is the case when walking on targets. Therefore, the aims of the present work were as follows: (1) to develop a treadmill allowing the assessment of locomotion of intact cats required to place the paws on the rungs of a moving ladder treadmill (LTM); (2) to assess the capability of cats after a unilateral spinal hemisection at T10 to cope with such a demanding locomotor task; and (3) to regularly train cats for 6 weeks on the LTM to determine whether such regular training improves locomotor recovery on the FTM. A significant improvement would indicate that LTM training maximizes the contribution of spinal locomotor circuits as well as remnant supraspinal inputs. Together, we used 9 cats (7 females, 2 males). Six were used to compare the EMG and kinematic locomotor characteristics during walking on the FTM and LTM. We found that the swing phase during LTM walking was slightly enhanced as well as some specific activity of knee flexor muscles. Fore-hindlimb coupling favored a more stable diagonal coupling. These 6 cats were then hemispinalized and trained for 6 weeks on the LTM, whereas the 3 other cats were hemispinalized and trained solely on the FTM to compare the two training regimens. Intensive LTM training after hemisection was found to change features of locomotion, such as the foot trajectory as well as diminished paw drag often observed after hemisection.SIGNIFICANCE STATEMENT This paper introduces a method (ladder treadmill [LTM]) to study the locomotor ability of cats with an intact spinal cord or after a unilateral hemisection to walk with a precise foot placement on the rungs fixed to an ordinary flat treadmill (FTM). Because cats are compared in various conditions (intact or hemisected at different time points) in the same enclosure on the FTM and the LTM, the changes in averaged locomotor characteristics must reflect the specificity of the task and the neurological states. Furthermore, the ladder treadmill permits to train cats repetitively for weeks and observe whether training regimens (FTM or LTM) can induce durable changes in the parameters of locomotion.

Intraspinal microstimulation produces over-ground walking in anesthetized cats

OBJECTIVE

Spinal cord injury causes a drastic loss of motor, sensory and autonomic function. The goal of this project was to investigate the use of intraspinal microstimulation (ISMS) for producing long distances of walking over ground. ISMS is an electrical stimulation method developed for restoring motor function by activating spinal networks below the level of an injury. It produces movements of the legs by stimulating the ventral horn of the lumbar enlargement using fine penetrating electrodes (≤50 μm diameter).

APPROACH

In each of five adult cats (4.2-5.5 kg), ISMS was applied through 16 electrodes implanted with tips targeting lamina IX in the ventral horn bilaterally. A desktop system implemented a physiologically-based control strategy that delivered different stimulation patterns through groups of electrodes to evoke walking movements with appropriate limb kinematics and forces corresponding to swing and stance. Each cat walked over an instrumented 2.9 m walkway and limb kinematics and forces were recorded.

MAIN RESULTS

Both propulsive and supportive forces were required for over-ground walking. Cumulative walking distances ranging from 609 to 835 m (longest tested) were achieved in three animals. In these three cats, the mean peak supportive force was 3.5 ± 0.6 N corresponding to full-weight-support of the hind legs, while the angular range of the hip, knee, and ankle joints were 23.1 ± 2.0°, 29.1 ± 0.2°, and 60.3 ± 5.2°, respectively. To further demonstrate the viability of ISMS for future clinical use, a prototype implantable module was successfully implemented in a subset of trials and produced comparable walking performance.

SIGNIFICANCE

By activating inherent locomotor networks within the lumbosacral spinal cord, ISMS was capable of producing bilaterally coordinated and functional over-ground walking with current amplitudes <100 μA. These exciting results suggest that ISMS may be an effective intervention for restoring functional walking after spinal cord injury.

Accurate stepping on a narrow path: mechanics, EMG, and motor cortex activity in the cat

How do cats manage to walk so graciously on top of narrow fences or windowsills high above the ground while apparently exerting little effort? In this study we investigated cat full-body mechanics and the activity of limb muscles and motor cortex during walking along a narrow 5-cm path on the ground. We tested the hypotheses that during narrow walking 1) lateral stability would be lower because of the decreased base-of-support area and 2) the motor cortex activity would increase stride-related modulation because of imposed demands on lateral stability and paw placement accuracy. We measured medio-lateral and rostro-caudal dynamic stability derived from the extrapolated center of mass position with respect to the boundaries of the support area. We found that cats were statically stable in the frontal plane during both unconstrained and narrow-path walking. During narrow-path walking, cats walked slightly slower with more adducted limbs, produced smaller lateral forces by hindlimbs, and had elevated muscle activities. Of 174 neurons recorded in cortical layer V, 87% of forelimb-related neurons (from 114) and 90% of hindlimb-related neurons (from 60) had activities during narrow-path walking distinct from unconstrained walking: more often they had a higher mean discharge rate, lower depth of stride-related modulation, and/or longer period of activation during the stride. These activity changes appeared to contribute to control of accurate paw placement in the medio-lateral direction, the width of the stride, rather than to lateral stability control, as the stability demands on narrow-path and unconstrained walking were similar.

Deficits in memory-guided limb movements impair obstacle avoidance locomotion in Alzheimer's disease mouse model

Memory function deficits induced by Alzheimer's disease (AD) are believed to be one of the causes of an increased risk of tripping in patients. Working memory contributes to accurate stepping over obstacles during locomotion, and AD-induced deficits of this memory function may lead to an increased risk of contact with obstacles. We used the triple transgenic (3xTg) mice to examine the effects of memory deficits in terms of tripping and contact with obstacles. We found that the frequency of contact of the hindlimbs during an obstacle avoidance task increased significantly in 10–13 month-old 3xTg (Old-3xTg) mice compared with control mice. However, no changes in limb kinematics during unobstructed locomotion or successful obstacle avoidance locomotion were observed in the Old-3xTg mice. Furthermore, we found that memory-based movements in stepping over an obstacle were impaired in these mice. Our findings suggest that working memory deficits as a result of AD are associated with an increased risk of tripping during locomotion.

Gait and jump analysis in healthy cats using a pressure mat system

Physical orthopaedic examination in cats does not always reveal signs of lameness and no objective gait analysis method has yet been standardised for use in cats. The aims of the present study were to define appropriate parameters for pressure mat analyses during walk and jump, and to define reference values for gait parameters of healthy cats. Further, the distribution of the vertical force within the paws and the influence of a non-centred head position were investigated. The hypothesis was that cats have a symmetrical gait, a front/hindlimb asymmetry similar to dogs, and that peak vertical force (PVF) and vertical impulse (VI) have high intraclass correlation coefficients, confirming the reliability of these parameters. Data for walking (n = 46) showed gait symmetry indices of close to 1.0, besides PVF front/hind (1.3 ± 0.2). The PVF front/hind for jumping cats (n = 16) was 1.7 ± 0.6. Results from the distribution of the vertical force within the paw (n = 39) showed that the main weight during a strike is transferred from the caudal towards the craniomedial part of the paw. The findings support the hypothesis that healthy cats have similar gait symmetry to healthy dogs and that PVF and VI are reliable gait parameters. In conclusion, the present study provides a reference interval for healthy cats. Further studies are needed to investigate gait parameters in cats with orthopaedic disease.

EphA4-mediated ipsilateral corticospinal tract misprojections are necessary for bilateral voluntary movements but not bilateral stereotypic locomotion

In this study, we took advantage of the reported role of EphA4 in determining the contralateral spinal projection of the corticospinal tract (CST) to investigate the effects of ipsilateral misprojections on voluntary movements and stereotypic locomotion. Null EphA4 mutations produce robust ipsilateral CST misprojections, resulting in bilateral corticospinal tracts. We hypothesize that a unilateral voluntary limb movement, not a stereotypic locomotor movement, will become a bilateral movement in EphA4 knock-out mice with a bilateral CST. However, in EphA4 full knock-outs, spinal interneurons also develop bilateral misprojections. Aberrant bilateral spinal circuits could thus transform unilateral corticospinal control signals into bilateral movements. We therefore studied mice with conditional forebrain deletion of the EphA4 gene under control by Emx1, a gene expressed in the forebrain that affects the developing CST but spares brainstem motor pathways and spinal motor circuits. We examined two conditional knock-outs targeting forebrain EphA4 during performance of stereotypic locomotion and voluntary movement: adaptive locomotion over obstacles and exploratory reaching. We found that the conditional knock-outs used alternate stepping, not hopping, during overground locomotion, suggesting normal central pattern generator function and supporting our hypothesis of minimal CST involvement in the moment-to-moment control of stereotypic locomotion. In contrast, the conditional knock-outs showed bilateral voluntary movements under conditions when single limb movements are normally produced and, as a basis for this aberrant control, developed a bilateral motor map in motor cortex that is driven by the aberrant ipsilateral CST misprojections. Therefore, a specific change in CST connectivity is associated with and explains a change in voluntary movement.

Stabilization of cat paw trajectory during locomotion

We investigated which of cat limb kinematic variables during swing of regular walking and accurate stepping along a horizontal ladder are stabilized by coordinated changes of limb segment angles. Three hypotheses were tested: 1) animals stabilize the entire swing trajectory of specific kinematic variables (performance variables); and 2) the level of trajectory stabilization is similar between regular and ladder walking and 3) is higher for forelimbs compared with hindlimbs. We used the framework of the uncontrolled manifold (UCM) hypothesis to quantify the structure of variance of limb kinematics in the limb segment orientation space across steps. Two components of variance were quantified for each potential performance variable, one of which affected it ("bad variance," variance orthogonal to the UCM, VORT) while the other one did not ("good variance," variance within the UCM, VUCM). The analysis of five candidate performance variables revealed that cats during both locomotor behaviors stabilize 1) paw vertical position during the entire swing (VUCM > VORT, except in mid-hindpaw swing of ladder walking) and 2) horizontal paw position in initial and terminal swing (except for the entire forepaw swing of regular walking). We also found that the limb length was typically stabilized in midswing, whereas limb orientation was not (VUCM ≤ VORT) for both limbs and behaviors during entire swing. We conclude that stabilization of paw position in early and terminal swing enables accurate and stable locomotion, while stabilization of vertical paw position in midswing helps paw clearance. This study is the first to demonstrate the applicability of the UCM-based analysis to nonhuman movement.

Motor cortical regulation of sparse synergies provides a framework for the flexible control of precision walking

We have previously described a modular organization of the locomotor step cycle in the cat in which a number of sparse synergies are activated sequentially during the swing phase of the step cycle (Krouchev et al., 2006). Here, we address how these synergies are modified during voluntary gait modifications. Data were analysed from 27 bursts of muscle activity (recorded from 18 muscles) recorded in the forelimb of the cat during locomotion. These were grouped into 10 clusters, or synergies, during unobstructed locomotion. Each synergy was comprised of only a small number of muscles bursts (sparse synergies), some of which included both proximal and distal muscles. Eight (8/10) of these synergies were active during the swing phase of locomotion. Synergies observed during the gait modifications were very similar to those observed during unobstructed locomotion. Constraining these synergies to be identical in both the lead (first forelimb to pass over the obstacle) and the trail (second limb) conditions allowed us to compare the changes in phase and magnitude of the synergies required to modify gait. In the lead condition, changes were observed particularly in those synergies responsible for transport of the limb and preparation for landing. During the trail condition, changes were particularly evident in those synergies responsible for lifting the limb from the ground at the onset of the swing phase. These changes in phase and magnitude were adapted to the size and shape of the obstacle over which the cat stepped. These results demonstrate that by modifying the phase and magnitude of a finite number of muscle synergies, each comprised of a small number of simultaneously active muscles, descending control signals could produce very specific modifications in limb trajectory during locomotion. We discuss the possibility that these changes in phase and magnitude could be produced by changes in the activity of neurones in the motor cortex.

Features of hand-foot crawling behavior in human adults

Interlimb coordination of crawling kinematics in humans shares features with other primates and nonprimate quadrupeds, and it has been suggested that this is due to a similar organization of the locomotor pattern generators (CPGs). To extend the previous findings and to further explore the neural control of bipedal vs. quadrupedal locomotion, we used a crawling paradigm in which healthy adults crawled on their hands and feet at different speeds and at different surface inclinations (13°, 27°, and 35°). Ground reaction forces, limb kinematics, and electromyographic (EMG) activity from 26 upper and lower limb muscles on the right side of the body were collected. The EMG activity was mapped onto the spinal cord in approximate rostrocaudal locations of the motoneuron pools to characterize the general features of cervical and lumbosacral spinal cord activation. The spatiotemporal pattern of spinal cord activity significantly differed between quadrupedal and bipedal gaits. In addition, participants exhibited a large range of kinematic coordination styles (diagonal vs. lateral patterns), which is in contrast to the stereotypical kinematics of upright bipedal walking, suggesting flexible coupling of cervical and lumbosacral pattern generators. Results showed strikingly dissimilar directional horizontal forces for the arms and legs, considerably retracted average leg orientation, and substantially smaller sacral vs. lumbar motoneuron activity compared with quadrupedal gait in animals. A gradual transition to a more vertical body orientation (increasing the inclination of the treadmill) led to the appearance of more prominent sacral activity (related to activation of ankle plantar flexors), typical of bipedal walking. The findings highlight the reorganization and adaptation of CPG networks involved in the control of quadrupedal human locomotion and a high specialization of the musculoskeletal apparatus to specific gaits.

Altered obstacle negotiation after low thoracic hemisection in the cat

Following a lateralized spinal cord injury (SCI) in humans, substantial walking recovery occurs; however, deficits persist in adaptive features of locomotion critical for community ambulation, including obstacle negotiation. Normal obstacle negotiation is accomplished by an increase in flexion during swing. If an object is unanticipated or supraspinal input is absent, obstacle negotiation may involve the spinally organized stumbling corrective response. How these voluntary and reflex components are affected following partial SCI is not well studied. This study is the first to characterize recovery of obstacle negotiation following low-thoracic spinal hemisection in the cat. Cats were trained pre- and post-injury to cross a runway with an obstacle. Assessments focused on the hindlimb ipsilateral to the lesion. Pre-injury, cats efficiently cleared an obstacle by increasing knee flexion during swing. Post-injury, obstacle clearance permanently changed. At 2 weeks, when basic overground walking ability been recovered, the hindlimb was dragged over the obstacle (∼90%). Surprisingly, the stumbling corrective response was not elicited until after 2 weeks. Despite a notable increase, between 4 and 8 weeks, in the ability to modify limb trajectory when approaching an obstacle, limb lift during obstacle approach was insufficient during ∼50% of encounters and continued to evoke the stumbling corrective response even at 16 weeks. A post-injury lead limb bias identified during negotiations with complete clearance, suggests a potential training strategy to increase the number of successful clearances. Therefore, following complete severing of half of the spinal cord, the ability to modify ipsilateral hindlimb trajectory shows significant recovery and by 16 weeks permits effective clearing of an obstacle, without contact, ∼50% of the time. Although this suggests plasticity of supporting circuitry, it is insufficient to support consistent clearance. This inconsistency, even at the most chronic time point assessed (16 weeks), is probably a contributing factor to falls reported for people with SCI.

Spinal-like regulator facilitates control of a two-degree-of-freedom wrist

The performance of motor tasks requires the coordinated control and continuous adjustment of myriad individual muscles. The basic commands for the successful performance of a sensorimotor task originate in "higher" centers such as the motor cortex, but the actual muscle activation and resulting torques and motion are considerably shaped by the integrative function of the spinal interneurons. The relative contributions of brain and spinal cord are less clear for reaching movements than for automatic tasks such as locomotion. We have modeled a two-axis, four-muscle wrist joint with realistic musculoskeletal mechanics and proprioceptors and a network of regulatory circuitry based on the classical types of spinal interneurons (propriospinal, monosynaptic Ia-excitatory, reciprocal Ia-inhibitory, Renshaw inhibitory, and Ib-inhibitory pathways) and their supraspinal control (via biasing activity, presynaptic inhibition, and fusimotor gain). The modeled system has a very large number of control inputs, not unlike the real spinal cord that the brain must learn to control to produce desired behaviors. It was surprisingly easy to program this model to emulate actual performance in four very different but well described behaviors: (1) stabilizing responses to force perturbations; (2) rapid movement to position target; (3) isometric force to a target level; and (4) adaptation to viscous curl force fields. Our general hypothesis is that, despite its complexity, such regulatory circuitry substantially simplifies the tasks of learning and producing complex movements.

How spinalized rats can walk: biomechanics, cortex, and hindlimb muscle scaling--implications for rehabilitation

Neonatal spinalized (NST) rats can achieve autonomous weight-supported locomotion never seen after adult injury. Mechanisms that support function in NST rats include increased importance of cortical trunk control and altered biomechanical control strategies for stance and locomotion. Hindlimbs are isolated from perturbations in quiet stance and act in opposition to forelimbs in locomotion in NST rats. Control of roll and yaw of the hindlimbs is crucial in their locomotion. The biomechanics of the hind limbs of NST rats are also likely crucial. We present new data showing the whole leg musculature scales proportional to normal rat musculature in NST rats, regardless of function. This scaling is a prerequisite for the NST rats to most effectively use pattern generation mechanisms and motor patterns that are similar to those present in intact rats. Pattern generation may be built into the lumbar spinal cord by evolution and matched to the limb biomechanics, so preserved muscle scaling may be essential to the NST function observed.

Keeping it together: mechanisms of intersegmental coordination for a flexible locomotor behavior

The coordination of multiple neural oscillators is key for the generation of productive locomotor movements. In the medicinal leech, we determined that activation and coordination of the segmental crawl oscillators, or unit burst generators, are dependent on signals descending from the cephalic ganglion. In nearly intact animals, removing descending input (reversibly with a sucrose block) prevented overt crawling, but not swimming. Cephalic depolarization was sufficient for coordination. To determine whether descending signals were necessary for the generation and maintenance of posterior-directed intersegmental phase delays, we induced fictive crawling in isolated whole nerve cords using dopamine (DA) and blocked descending inputs. After blockade, we observed a significant loss of intersegmental coordination. Appropriate phase delays were also absent in DA-treated chains of ganglia. In chains, when one ganglion was removed from its neighbors, crawling in that ganglion emerged robust and stable, underscoring that these oscillators operate best with either all or none of their intersegmental inputs. To study local oscillator coupling, we induced fictive crawling (with DA) in a single oscillator within a chain. Although appropriate intersegmental phase delays were always absent, when one ganglion was treated with DA, neighboring ganglia began to show crawl-like bursting, with motoneuron spikes/burst greatest in untreated posterior ganglia. We further determined that this local excitatory drive excluded the swim-gating cell, 204. In conclusion, both long-distance descending and local interoscillator coupling contribute to crawling. This dual contribution helps to explain the inherent flexibility of crawling, and provides a foundation for understanding other dynamic locomotor behaviors across animal groups.

Object avoidance during locomotion

Many animals rely on vision for navigating through complex environments and for avoiding specific obstacles during locomotion. Navigation and obstacle avoidance are tasks that depend on gathering information about the environment by vision and using this information at later times to guide limb and body movements. Here we review studies demonstrating the use of short-term visual memory during walking in humans and cats. Our own investigations have demonstrated that cats have the ability to retain a memory of an obstacle they have stepped over with the forelegs for many minutes and to use this memory to guide stepping of the hindlegs to avoid the remembered obstacle. A brain region that may be critically involved in the retention of memories of the location of obstacles is the posterior parietal cortex. Recordings from neurons in area 5 in the posterior parietal cortex in freely walking cats have revealed the existence of neurons whose activity is strongly correlated with the location of an obstacle relative to the body. How these neurons might be used to regulate motor commands remains to be established. We believe that studies on obstacle avoidance in walking cats have the potential to significantly advance our understanding of visuo-motor transformations. Current knowledge about the brain regions and pathways underlying visuo-motor transformations during walking are reviewed.

Coordination strategies for limb forces during weight-bearing locomotion in normal rats, and in rats spinalized as neonates

Some rats spinally transected as neonates (ST rats) achieve weight-supporting independent locomotion. The mechanisms of coordinated hind-limb weight support in such rats are not well understood. To examine these we compared ST rats (with better than 60% of weight supported steps) and normal rats that were trained to cross an instrumented runway. Ground reaction forces, coordination of hind-limb and forelimb forces and the motions of the center of pressure (CoP) were assessed. Normal rats crossed the runway with a diagonal trot. On average hind-limbs bore about 80% of the vertical load carried by forelimbs (45% body weight on hind-limbs 55% on forelimbs), although this varied. Forelimbs and hind-limbs acted synergistically to generate decelerative and propulsive rostrocaudal forces, which averaged 15% of body weight with maximums of 50%. Lateral forces were very small (<8% of body weight). Center of pressure progressed in jumps along a straight line with mean lateral deviations <1 cm. ST rats hind-limbs bore about 60% of the vertical load of forelimbs (37% body weight on hind-limbs, 63% on forelimbs), significantly less compared to intact rats (P < 0.05). ST rats showed similar mean rostrocaudal forces, but with significantly larger maximum fluctuations of up to 80% of body weight (P < 0.05). Joint force-plate recordings showed forelimbs and hind-limb rostrocaudal forces in ST rats were opposing and significantly different from intact rats (P < 0.05). Lateral forces were approximately 20% of body weight and significantly larger than in normal rats (P < 0.05). Center of pressure zig-zagged, with mean lateral deviations of approximately 2 cm and a significantly larger range (P < 0.05). The haunches were also observed to roll more than normal rats. The locomotor strategy of injured rats using limbs in opposition was presumably less efficient but their complex gait was statically stable. Because forelimbs and hind-limbs acted in opposition, the trunk was held compressed. Force coordination was likely managed largely by the voluntary control in forelimbs and trunk.

Dynamic sensorimotor interactions in locomotion

Locomotion results from intricate dynamic interactions between a central program and feedback mechanisms. The central program relies fundamentally on a genetically determined spinal circuitry (central pattern generator) capable of generating the basic locomotor pattern and on various descending pathways that can trigger, stop, and steer locomotion. The feedback originates from muscles and skin afferents as well as from special senses (vision, audition, vestibular) and dynamically adapts the locomotor pattern to the requirements of the environment. The dynamic interactions are ensured by modulating transmission in locomotor pathways in a state- and phase-dependent manner. For instance, proprioceptive inputs from extensors can, during stance, adjust the timing and amplitude of muscle activities of the limbs to the speed of locomotion but be silenced during the opposite phase of the cycle. Similarly, skin afferents participate predominantly in the correction of limb and foot placement during stance on uneven terrain, but skin stimuli can evoke different types of responses depending on when they occur within the step cycle. Similarly, stimulation of descending pathways may affect the locomotor pattern in only certain phases of the step cycle. Section ii reviews dynamic sensorimotor interactions mainly through spinal pathways. Section iii describes how similar sensory inputs from the spinal or supraspinal levels can modify locomotion through descending pathways. The sensorimotor interactions occur obviously at several levels of the nervous system. Section iv summarizes presynaptic, interneuronal, and motoneuronal mechanisms that are common at these various levels. Together these mechanisms contribute to the continuous dynamic adjustment of sensorimotor interactions, ensuring that the central program and feedback mechanisms are congruous during locomotion.

Quantification of motor cortex activity and full-body biomechanics during unconstrained locomotion

Recent progress in the understanding of motor cortex function has been achieved primarily by simultaneously recording motor cortex neuron activity and the movement kinematics of the corresponding limb. We have expanded this approach by combining high-quality cortical single-unit activity recordings with synchronized recordings of full-body kinematics and kinetics in the freely behaving cat. The method is illustrated by selected results obtained from two cats tested while walking on a flat surface. Using this method, the activity of 43 pyramidal tract neurons (PTNs) was recorded, averaged over 10 bins of a locomotion cycle, and compared with full-body mechanics by means of principal component and multivariate linear regression analyses. Patterns of 24 PTNs (56%) and 219 biomechanical variables (73%) were classified into just four groups of inter-correlated variables that accounted for 91% of the total variance, indicating that many of the recorded variables had similar patterns. The ensemble activity of different groups of two to eight PTNs accurately predicted the 10-bin patterns of all biomechanical variables (neural decoding) and vice versa; different small groups of mechanical variables accurately predicted the 10-bin pattern of each PTN (neural encoding). We conclude that comparison of motor cortex activity with full-body biomechanics may be a useful tool in further elucidating the function of the motor cortex.

Kinematic and EMG determinants in quadrupedal locomotion of a non-human primate (Rhesus)

We hypothesized that the activation patterns of flexor and extensor muscles and the resulting kinematics of the forelimbs and hindlimbs during locomotion in the Rhesus would have unique characteristics relative to other quadrupedal mammals. Adaptations of limb movements and in motor pool recruitment patterns in accommodating a range of treadmill speeds similar to other terrestrial animals in both the hindlimb and forelimb were observed. Flexor and extensor motor neurons from motor pools in the lumbar segments, however, were more highly coordinated than in the cervical segments. Unlike the lateral sequence characterizing subprimate quadrupedal locomotion, non-human primates use diagonal coordination between the hindlimbs and forelimbs, similar to that observed in humans between the legs and arms. Although there was a high level of coordination between hind- and forelimb locomotion kinematics, limb-specific neural control strategies were evident in the intersegmental coordination patterns and limb endpoint trajectories. Based on limb kinematics and muscle recruitment patterns, it appears that the hindlimbs, and notably the distal extremities, contribute more to body propulsion than the forelimbs. Furthermore, we found adaptive changes in the recruitment patterns of distal muscles in the hind- and forelimb with increased treadmill speed that likely correlate with the anatomical and functional evolution of hand and foot digits in monkeys. Changes in the properties of both the spinal and supraspinal circuitry related to stepping, probably account for the peculiarities in the kinematic and EMG properties during non-human primate locomotion. We suggest that such adaptive changes may have facilitated evolution toward bipedal locomotion.

Cortical and brainstem control of locomotion

While a basic locomotor rhythm is centrally generated by spinal circuits, descending pathways are critical for ensuring appropriate anticipatory modifications of gait to accommodate uneven terrain. Neurons in the motor cortex command the changes in muscle activity required to modify limb trajectory when stepping over obstacles. Simultaneously, neurons in the brainstem reticular formation ensure that these modifications are superimposed on an appropriate base of postural support. Recent experiments suggest that the same neurons in the same structures also provide similar information during reaching movements. It is suggested that, during both locomotion and reaching movements, the final expression of descending signals is influenced by the state and excitability of the spinal circuits upon which they impinge.

Contribution of cutaneous inputs from the hindpaw to the control of locomotion. I. Intact cats

The goal of this study was to evaluate the role of hindpaw cutaneous feedback in the control of locomotion, by cutting some (in one cat) or all (in 2 cats) cutaneous nerves bilaterally at ankle level. Kinematic and electromyographic (EMG) recordings were obtained before and for several weeks after denervation during level and incline (15 degrees up and down) treadmill walking. Ladder walking and ground reaction forces were also documented sporadically. Early after the denervation (1-3 days), cats could not walk across a ladder, although deficits were small during level treadmill walking. Increased knee flexion velocity caused a 14% reduction in swing phase duration. EMG activity was consistently increased in knee, ankle, and toe flexors, and in at least one knee or ankle extensor. The adaptive changes during walking on the incline were much reduced after denervation. Ladder walking gradually recovered within 3-7 wk. By this time, level treadmill walking kinematics had completely returned to normal, but EMG activity in flexors remained above control. Incline walking improved but did not return to normal. Mediolateral ground reaction forces during overground walking were increased by 200%. It is concluded that in intact cats, cutaneous inputs contribute more to demanding situations such as walking on a ladder or on inclines than to level walking. Active adaptive mechanisms are likely involved given that the EMG locomotor pattern never returned to control level. The companion paper shows on the other hand that when the same cats are spinalized, these cutaneous inputs become critical for foot placement during locomotion.

Strategies for the integration of posture and movement during reaching in the cat

We have examined the relationship between the movement and the anticipatory postural adjustments (APAs) that precede that movement during a reaching task in the cat. We recorded ground reaction forces in all 3 planes from all 4 limbs as well as electromyographic (EMG) activity from limb and axial muscles. The reaching movement was always preceded by an APA that was characterized by a loading of the reaching forelimb and an unloading of the support forelimb. This loading of the reaching forelimb was preceded, and accompanied, by increased activity in shoulder and limb extensor muscles of the reaching limb; extensor muscle activity in the supporting limb was simultaneously decreased. An important finding from this study was that the onset of the APA and of the movement was temporally decoupled. Analyses of the onset of EMG activity showed that most of the muscles that we recorded could be classified as either related to the APA or related to the movement. These results support the idea of distributed, and perhaps independent, systems for the execution of the APA and of the prime movement. There was also postural activity in the supporting limb during the movement. Analysis of this activity, which is also anticipatory in nature, suggests that it was tightly linked to the movement. We suggest that this postural response is signaled as part of the command for movement. Some muscles, particularly the extensors of the reaching limb, received convergent input from the command signals for the APA and for the movement.

Discharge characteristics of neurons in the red nucleus during voluntary gait modifications: a comparison with the motor cortex

We have examined the contribution of the red nucleus to the control of locomotion in the cat. Neuronal activity was recorded from 157 rubral neurons, including identified rubrospinal neurons, in three cats trained to walk on a treadmill and to step over obstacles attached to the moving belt. Of 72 neurons with a receptive field confined to the contralateral forelimb, 66 were phasically active during unobstructed locomotion. The maximal activity of the majority of neurons (59/66) was centered around the swing phase of locomotion. Slightly more than half of the neurons (36/66) were phasically activity during both swing and stance. In addition, some rubral neurons (14/66) showed multiple periods of phasic activity within the swing phase of the locomotor cycle. Periods of phasic discharge temporally coincident with the swing phase of the ipsilateral limb were observed in 7/66 neurons. During voluntary gait modifications, most forelimb-related neurons (70/72) showed a significant increase in their discharge activity when the contralateral limb was the first to step over the obstacle (lead condition). Maximal activity in nearly all cells (63/70) was observed during the swing phase, and 23/63 rubral neurons exhibited multiple increases of activity during the modified swing phase. A number of cells (18/70) showed multiple periods of increased activity during swing and stance. Many of the neurons (35/63, 56%) showed an increase in activity at the end of the swing phase; this period of activity was temporally coincident with the period of activity in wrist dorsiflexors, such as the extensor digitorum communis. A smaller proportion of neurons with receptive fields restricted to the hindlimbs showed similar characteristics to those observed in the population of forelimb-related neurons. The overall characteristics of these rubral neurons are similar to those that we obtained previously from pyramidal tract neurons recorded from the motor cortex during an identical task. However, in contrast to the results obtained in the rubral neurons, most motor cortical neurons showed only one period of increased activity during the step cycle. We suggest that both structures contribute to the modifications of the pattern of EMG activity that are required to produce the change in limb trajectory needed to step over an obstacle. However, the results suggest an additional role for the red nucleus in regulating intra- and interlimb coordination.

Contributions of the reticulospinal system to the postural adjustments occurring during voluntary gait modifications

To test the hypothesis that reticulospinal neurons (RSNs) are involved in the formation of the dynamic postural adjustments that accompany visually triggered, voluntary modifications of limb trajectory during locomotion, we recorded the activity of 400 cells (183 RSNs; 217 unidentified reticular cells) in the pontomedullary reticular formation (PMRF) during a locomotor task in which intact cats were required to step over an obstacle attached to a moving treadmill belt. Approximately one half of the RSNs (97/183, 53%) showed significant changes in cell activity as the cat stepped over the obstacle; most of these cells exhibited either single (26/97, 26.8%) or multiple (63/97, 65.0%) increases of activity. There was a range of discharge patterns that varied in the number, timing, and sequencing of the bursts of modified activity, although individual bursts in different cells tended to occur at similar phases of the gait cycle. Most modified cells, regardless of the number of bursts of increased discharge, or of the discharge activity of the cell during unobstructed, control, locomotion, discharged during the passage of the lead forelimb over the obstacle. Thus, 86.9% of the modified cells increased their discharge when the forelimb ipsilateral to the recording site was the first to pass over the obstacle, and 72.2% when the contralateral limb was the first. Approximately one quarter of the RSNs increased their discharge during the passage of each of the four limbs over the obstacle in both the lead (27.1%) and trail (27.9%) conditions. In general, in any one cell, the number and relative sequencing of the subsequent bursts (with respect to the lead forelimb) was maintained during both lead and trail conditions. Patterns of activity observed in unidentified cells were very similar to the RSN activity despite the diverse population of cells this unidentified group may represent. We suggest that the increased discharge that we observed in these reticular neurons reflects the integration of afferent activity from several sources, including the motor cortex, and that this increased discharge signals the timing and the relative magnitude of the postural patterns that accompany the voluntary gait modification. However, based on the characteristics of the patterns of neuronal activity in these cells, we further suggest that while individual RSNs probably contribute to the selection of different patterns of postural activity, the ultimate expression of the postural response may be determined by the excitability of the locomotor circuits within the spinal cord.

Kinematics and modeling of leech crawling: evidence for an oscillatory behavior produced by propagating waves of excitation

Many well characterized central pattern generators (CPGs) underlie behaviors (e.g., swimming, flight, heartbeat) that require regular rhythmicity and strict phase relationships. Here, we examine the organization of a CPG for leech crawling, a behavior whose success depends more on its flexibility than on its precise coordination. We examined the organization of this CPG by first characterizing the kinematics of crawling steps in normal and surgically manipulated animals, then by exploring its features in a simple neuronal model. The behavioral observations revealed the following. (1) Intersegmental coordination varied considerably with step duration, whereas the rates of elongation and contraction within individual segments were relatively constant. (2) Steps were generated in the absence of both head and tail brains, implying that midbody ganglia contain a CPG for step production. (3) Removal of sensory feedback did not affect step coordination or timing. (4) Imposed stretch greatly lengthened transitions between elongation and contraction, indicating that sensory pathways feed back onto the CPG. A simple model reproduced essential features of the observed kinematics. This model consisted of an oscillator that initiates propagating segmental waves of activity in excitatory neuronal chains, along with a parallel descending projection; together, these pathways could produce the observed intersegmental lags, coordination between phases, and step duration. We suggest that the proposed model is well suited to be modified on a step-by-step basis and that crawling may differ substantially from other described CPGs, such as that for swimming in segmented animals, where individual segments produce oscillations that are strongly phase-locked to one another.

Corticoreticular pathways in the cat. II. Discharge activity of neurons in area 4 during voluntary gait modifications

We propose that the descending command from area 4 that is responsible, in part, for the change in limb trajectory required to step over an obstacle in one's path also plays a role in triggering the anticipatory postural modifications that accompany this movement. To test this hypothesis, we recorded the discharge characteristics of identified classes of corticofugal neurons in area 4 of the cat. Neurons were identified either as: pryamidal tract neurons (PTNs) if their axon projected to the caudal pyramidal tract (PT) but not to the pontomedullary reticular formation (PMRF); as corticoreticular neurons (CRNs) if their axon projected to the PMRF but not to the PT; and as PTN/CRNs if their axon projected to both structures. Altogether, the discharge properties of 212 corticofugal neurons (109 PTNs, 66 PTN/CRNs, and 37 CRNs) within area 4 were recorded during voluntary gait modifications. Neurons in all three classes showed increases in their discharge frequency during locomotion and included groups that increased their discharge either during the swing phase of the modified step, during the subsequent stance phase, or in the stance phase of the cycle preceding the step over the obstacle. A slightly higher percentage of CRNs (39%) discharged in the stance phase prior to the gait modification than did the PTNs or PTN/CRNs (20% and 17% respectively). In 37 electrode penetrations, we were able to record clusters of 3 or more neurons within 500 micro(m) of each other. In most cases, PTN/CRNs recorded in close proximity to PTNs had similar receptive fields and discharged in a similar, but not identical, manner during the gait modifications. Compared with adjacent PTNs, CRNs normally showed a more variable pattern of activity and frequently discharged earlier in the step cycle than did the PTNs or PTN/CRNs. We interpret the results as providing support for the original hypothesis. We suggest that the collateral branches to the PMRF from corticofugal neurons with axons that continue at least as far as the caudal PT provide a signal that could be used to trigger dynamic postural responses that are appropriately organized and scaled for the movements that are being undertaken. We suggest that the more variable and earlier discharge activity observed in CRNs might be used to modify the postural support on which the movements and the dynamic postural adjustments are superimposed.

Corticoreticular pathways in the cat. I. Projection patterns and collaterization

This paper summarizes and compares the projection patterns and the receptive fields of cortical neurons in areas 4 and 6 that project to the pontomedullary reticular formation (PMRF). A total of 326 neurons were recorded in area 4 and 129 in area 6 in four awake, unrestrained cats that were chronically implanted with arrays of electrodes in the PMRF and the pyramidal tract (PT). In area 4, 47% of the neurons projected to the caudal PT but not to the PMRF (PTNs); 19% were activated only from the PMRF [corticoreticular neurons (CRNs)], whereas 27% were activated from both the PT and the PMRF (PTN/CRNs). More PTN/CRNs conducted at velocities >20 m/s (82%) than did CRNs (23%). In area 6, only 19% of the neurons were identified as PTNs, 12% were PTN/CRNs and 31% were CRNs; a further 38% could not be activated from either structure. Collateral branches within the PMRF conducted at maximum velocities of 20 m/s (average = 6.5 m/s). No significant differences in the conduction velocities of the collateral branches were found either between fast and slow PTNs or between area 4 and area 6 neurons. A large proportion of neurons in area 4 (85/173, 49%) were activated by passive manipulation of the more distal, contralateral forelimb, with approximately equal numbers being classed as PTNs, PTN/CRNs and CRNs. Most neurons in area 6 for which a receptive field could be found were excited by lightly touching or tapping the face and neck; a receptive field could not be determined for 39% of the area 6 neurons compared with only 5% of those in area 4. Finally, there was evidence that neurons in quite widespread areas of the pericruciate cortex, including both areas 4 and 6 projected onto similar, restricted regions of the PMRF. The fact that the cortical projection from area 4 to the PMRF includes a high percentage of fast PTNs with a receptive field on the distal forelimb is consistent with the view that this projection may serve to integrate movement and the dynamic postural adjustments that accompany them. The fact that the cortical projection from area 6 to the PMRF is primarily from slow PTNs with receptive fields on the face, neck and back is consistent with a role for this cortical area in adjusting the general posture of the animal on which movements are superimposed.

Organization of the projections from the pericruciate cortex to the pontomedullary reticular formation of the cat: a quantitative retrograde tracing study

Dextran-amines were used as retrograde tracers to investigate the organization of cortical projections to different cytoarchitectonic regions of the pontomedullary reticular formation of the cat. Injections into the nucleus reticularis pontis oralis resulted in labelling of neurones in the proreus cortex and area 6a beta of the premotor cortex, with little labelling in the motor cortex (area 4). This labelling was predominantly ipsilateral to the injection site. In contrast, injections into the nucleus reticularis pontis caudalis (NRPc), nucleus reticularis gigantocellularis (NRGc), and nucleus reticularis magnocellularis (NRMc) resulted in bilateral labelling--primarily in areas 6a beta, 6a gamma, and in the rostromedial region of area 4--with little labelling in the proreus cortex. In general, the cortical projections to the caudal NRGc and the NRMc were larger than those to the NRPc. More than 25% of the total projections to each of the latter three reticular regions arose from the medial part of area 4. Labelling in the hindlimb regions of area 4 was largest following the NRMc injections and smallest after injections in the NRPc. The projections to the NRPc originated from more medial parts of areas 4 and 6 than did the projections to the caudal region of the NRGc. These results suggest that areas 4 and 6 may be able to differentially activate different regions of the pontomedullary reticular formation depending on the movement that is made and perhaps also on the context of that movement.