Contributions of Parietal Cortex to the Working Memory of an Obstacle Acquired Visually or Tactilely in the Locomoting Cat.

A working memory of obstacles is essential for navigating complex, cluttered terrain. In quadrupeds, it has been proposed that parietal cortical areas related to movement planning and working memory may be important for guiding the hindlegs over an obstacle previously cleared by the forelegs. To test this hypothesis, parietal areas 5 and 7 were reversibly deactivated in walking cats. The working memory of an obstacle was assessed in both a visually dependent and tactilely dependent paradigm. Reversible bilateral deactivation of area 5, but not area 7, altered hindleg stepping in a manner indicating that the animals did not recall the obstacle over which their forelegs had stepped. Similar deficits were observed when area 5 deactivation was restricted to the delay during which obstacle memory must be maintained. Furthermore, partial memory recovery observed when area 5 function was deactivated and restored within this maintenance period suggests that the deactivation may suppress, but not eliminate, the working memory of an obstacle. As area 5 deactivations incurred similar memory deficits in both visual and tactile obstacle working memory paradigms, parietal area 5 is critical for maintaining the working memory of an obstacle acquired via vision or touch that is used to modify stepping for avoidance.

Memory-Guided Stumbling Correction in the Hindlimb of Quadrupeds Relies on Parietal Area 5.

In complex environments, tripping over an unexpected obstacle evokes the stumbling corrective reaction, eliciting rapid limb hyperflexion to lift the leg over the obstruction. While stumbling correction has been characterized within a single limb in the cat, this response must extend to both forelegs and hindlegs for successful avoidance in naturalistic settings. Furthermore, the ability to remember an obstacle over which the forelegs have tripped is necessary for hindleg clearance if locomotion is delayed. Therefore, memory-guided stumbling correction was studied in walking cats after the forelegs tripped over an unexpected obstacle. Tactile input to only one foreleg was often sufficient in modulating stepping of all four legs when locomotion was continuous, or when hindleg clearance was delayed. When obstacle height was varied, animals appropriately scaled step height to obstacle height. As tactile input without foreleg clearance was insufficient in reliably modulating stepping, efference, or proprioceptive information about modulated foreleg stepping may be important for producing a robust, long-lasting memory. Finally, cooling-induced deactivation of parietal area 5 altered hindleg stepping in a manner indicating that animals no longer recalled the obstacle over which they had tripped. Altogether, these results demonstrate the integral role area 5 plays in memory-guided stumbling correction.

Leg mechanics contribute to establishing swing phase trajectories during memory-guided stepping movements in walking cats: a computational analysis.

When quadrupeds stop walking after stepping over a barrier with their forelegs, the memory of barrier height and location is retained for many minutes. This memory is subsequently used to guide hind leg movements over the barrier when walking is resumed. The upslope of the initial trajectory of hind leg paw movements is strongly dependent on the initial location of the paw relative to the barrier. In this study, we have attempted to determine whether mechanical factors contribute significantly in establishing the slope of the paw trajectories by creating a four-link biomechanical model of a cat hind leg and driving this model with a variety of joint-torque profiles, including average torques for a range of initial paw positions relative to the barrier. Torque profiles for individual steps were determined by an inverse dynamic analysis of leg movements in three normal cats. Our study demonstrates that limb mechanics can contribute to establishing the dependency of trajectory slope on the initial position of the paw relative to the barrier. However, an additional contribution of neuronal motor commands was indicated by the fact that the simulated slopes of paw trajectories were significantly less than the observed slopes. A neuronal contribution to the modification of paw trajectories was also revealed by our observations that both the magnitudes of knee flexor muscle EMG bursts and the initial knee flexion torques depended on initial paw position. Previous studies have shown that a shift in paw position prior to stepping over a barrier changes the paw trajectory to be appropriate for the new paw position. Our data indicate that both mechanical and neuronal factors contribute to this updating process, and that any shift in leg position during the delay period modifies the working memory of barrier location.

Long-lasting working memories of obstacles established by foreleg stepping in walking cats require area 5 of the posterior parietal cortex.

Walking animals rely on working memory to avoid obstacles. One example is the stepping of the hindlegs of quadrupeds over an obstacle. In this case, the obstacle is not visible at the time of hindleg stepping, because of its position between the fore and hindlegs, and working memory must be used to avoid it. We have previously shown that this memory is very precise and surprisingly long-lasting and that it depends on the stepping of the forelegs over the obstacle for its initiation. In this study, we test the hypothesis that area 5 in the posterior parietal cortex of cats is necessary for the maintenance of this long-lasting working memory. We report that small bilateral lesions to area 5 do not affect the amplitude of normal stepping of the hindlegs over obstacles, but they profoundly reduce the long-lasting working memory of obstacles. We propose that inputs to area 5 associated with foreleg stepping initiate long-lasting activity that maintains the memory of obstacle height in another brain region to guide the hindlegs over obstacles.

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.

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.

New technique for drug application to the spinal cord of walking mice.

With the increasing availability of mutant mice that allow the conditional silencing of specific classes of interneurons in the spinal cord by drug application, a method for easily delivering drugs locally on spinal cord segments in adult animals has the potential for providing insights into the functioning of neuronal networks controlling walking. Here we describe a simple technique for this purpose. The drug is applied in high concentrations in a bath created with Vaseline walls around one to three segments of the spinal cord exposed under general anesthetic (isoflurane) combined with a strong, long-lasting analgesic (buprenorphine). After 20min of drug application the Vaseline and the drug is removed and skin closed. We first document that the surgery and analgesic have no obvious influences on the kinematics of hind leg movements after recovery from the anesthetic, and that the analgesic enhanced locomotor activity. We then describe the influence of applying a glycine-receptor antagonist onto the lumbar segments of the spinal cord to demonstrate that the method is effective in modifying the functioning of neuronal systems in the spinal cord. Combining this method with kinematic and electromyographic recording techniques allows the detailed investigation of the effects of drugs on the walking behavior in adult mice.

Descending command systems for the initiation of locomotion in mammals.

Neurons in the brainstem implicated in the initiation of locomotion include glutamatergic, noradrenergic (NA), dopaminergic (DA), and serotonergic (5-HT) neurons giving rise to descending tracts. Glutamate antagonists block mesencephalic locomotor region-induced and spontaneous locomotion, and glutamatergic agonists induce locomotion in spinal animals. NA and 5-HT inputs to the spinal cord originate in the brainstem, while the descending dopaminergic pathway originates in the hypothalamus. Agonists acting at NA, DA or 5-HT receptors facilitate or induce locomotion in spinal animals. 5-HT neurons located in the parapyramidal region (PPR) produce locomotion when stimulated in the isolated neonatal rat brainstem-spinal cord preparation, and they constitute the first anatomically discrete group of spinally-projecting neurons demonstrated to be involved in the initiation of locomotion in mammals. Neurons in the PPR are activated during treadmill locomotion in adult rats. Locomotion evoked from the PPR is mediated by 5-HT(7) and 5-HT(2A) receptors, and 5-HT(7) antagonists block locomotion in cat, rat and mouse preparations, but have little effect in mice lacking 5-HT(7) receptors. 5-HT induced activity in 5-HT(7) knockout mice is rhythmic, but coordination among flexor and extensor motor nuclei and left and right sides of the spinal cord is disrupted. In the adult wild-type mouse, 5-HT(7) receptor antagonists impair locomotion, producing patterns of activity resembling those induced by 5-HT in 5-HT(7) knockout mice. 5-HT(7) receptor antagonists have a reduced effect on locomotion in adult 5-HT(7) receptor knockout mice. We conclude that the PPR is the source of a descending 5-HT command pathway that activates the CPG via 5-HT(7) and 5-HT(2A) receptors. Further experiments are necessary to define the putative glutamatergic, DA, and NA command pathways.

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.

Contribution of sensory feedback to ongoing ankle extensor activity during the stance phase of walking.

Numerous investigations over the past 15 years have demonstrated that sensory feedback plays a critical role in establishing the timing and magnitude of muscle activity during walking. Here we review recent studies reporting that sensory feedback makes a substantial contribution to the activation of extensor motoneurons during the stance phase. Quantitative analysis of the effects of loading and unloading ankle extensor muscles during walking on a horizontal surface has shown that sensory feedback can increase the activity of ankle extensor muscles by up to 60%. There is currently some uncertainty about which sensory receptors are responsible for this enhancement of extensor activity, but likely candidates are the secondary spindle endings in the ankle extensors of humans and the Golgi tendon organs in the ankle extensors of humans and cats. Two important issues arise from the finding that sensory feedback from the leg regulates the magnitude of extensor activity. The first is the extent to which differences in the magnitude of activity in extensor muscles during different locomotor tasks can be directly attributed to changes in the magnitude of sensory signals, and the second is whether the enhancement of extensor activity is determined primarily by feedback from a specific group of receptors or from numerous groups of receptors distributed throughout the leg. Limitations of current experimental strategies prevent a straightforward empirical resolution of these issues. A potentially fruitful approach in the immediate future is to develop models of the known and hypothesized neuronal networks controlling motoneuronal activity, and use these simulations to control forward dynamic models of the musculo-skeletal system. These simulations would help understand how sensory signals are modified with a change in locomotor task and, in conjunction with physiological experiments, establish the extent to which these modifications can account for changes in the magnitude of motoneuronal activity.

Generating the walking gait: role of sensory feedback.

In the walking system of the cat, feedback from muscle proprioceptors establishes the timing of major phase transitions in the motor pattern, contributes to the production of burst activity, generates some features of the motor pattern, and is required for the adaptive modification of the motor pattern in response to alterations in leg mechanics. How proprioceptive signals are integrated into central neuronal networks has not been fully established, largely due to the absence of detailed information on the functional characteristics of central networks in the presence of phasic afferent signals. Nevertheless, it appears likely that afferent signals reorganize the functioning of central networks, and the concept that the generation of the motor pattern can be explained by afferent modulation of a hard-wired central pattern generator may be too simplistic.

Capabilities of a penetrating microelectrode array for recording single units in dorsal root ganglia of the cat.

The recording capability of a microelectrode array in the cat dorsal root ganglion (DRG) was studied in 11 acute experiments, 373 single, discriminable sensory units were recorded on 587 electrodes (0.64 units/electrode). Sensory action potentials as large as 1750 microV were obtained (mean=132 microV). These were comparable to literature reports of the best DRG extracellular recordings made with conventional electrodes. We were able simultaneously to activate and record over 50 discriminable, time-varying units from L6 and L7 DRGs during a cyclic ankle displacement. We also successfully recorded stable, phase dependent multiple sensory units with very little artifact or electromyographic (EMG) contamination during decerebrate walking. Thus, the array is capable of recording more effectively from more DRGs neurons than has been achieved by conventional recording techniques. The recording selectivity and stability of the array, coupled with the large number of neurons that can be recorded simultaneously, provide attractive features for better understanding sensorimotor control principles.

Sartorius muscle afferents influence the amplitude and timing of flexor activity in walking decerebrate cats.

Recent investigations have demonstrated that afferent signals from hindlimb flexor muscles can strongly influence flexor burst activity during walking and during fictive locomotion in decerebrate cats. We have reported previously that modifying afferent feedback from the sartorius (Sart) muscles by assisting or resisting hip flexion has a marked effect on the magnitude and duration of activity in iliopsoas (IP) as well as the sartorius muscles. The objective of the present investigation was to identify the afferents responsible for these effects by examining, in walking decerebrate cats, the influence of electrically stimulating sartorius afferents on burst activity in the IP and tibialis anterior (TA) muscles. Stimulation of the sartorius nerve at group I strength resulted in an increase in the duration of IP and TA bursts and an increase in the magnitude of IP bursts. The effect on burst durations was only observed at stimulus strengths of 1.6 T and higher. At lower stimulus strengths, there was a strong excitatory effect on IP bursts but no effect on TA bursts. Stimulation of the sartorius nerve at group II strength yielded variable results. When group II stimulation was delivered repeatedly during a walking sequence, the initial response was usually a strong inhibition of burst activity in IP and TA followed by a progressive reduction in inhibition and the emergence in IP of an excitatory response. This observation, together with findings of previous studies, suggests the existence of parallel excitatory and inhibitory pathways from sartorius group II afferents to flexor motoneurons. Taken together, these results support an earlier speculation that feedback from large afferents from the sartorius muscles has a strong influence on the generation of flexor burst activity in walking cats.

The role of proprioceptive feedback in the regulation and adaptation of locomotor activity.

Feedback from muscle afferents is essential for locomotion to be functional under changing external conditions. In this article, we review the role of afferent feedback in adapting locomotor activity to transient and more sustained changes in sensory input in reduced and walking cat preparations. Much of the work on muscle afferent regulation of locomotion has focused on the regulation of stance phase activity. Proprioceptive feedback from extensor muscles during the stance phase ensures that the leg does not go into swing when loaded and that the magnitude of extensor activity is adequate for support. Proprioceptive feedback from flexor muscles towards the end of the stance phase facilitates the initiation of the swing phase of walking. Evidence that muscle afferent feedback also contributes to the magnitude and duration of flexor activity during the swing phase has been demonstrated recently. The regulation of the magnitude and duration of extensor and flexor activity during locomotion is mediated by monosynaptic, disynaptic, and polysynaptic muscle afferent pathways in the spinal cord. In addition to allowing for rapid adaptation in motor output during walking, afferent feedback from muscle proprioceptors is also involved in longer-term adaptations in response to changes in the biomechanical or neuromuscular properties of the walking system.