Disruption of fore- and hindlimb coordination during overground locomotion in cats with bilateral serial hemisection of the spinal cord

In order to investigate if inter-limb propriospinal reflexes participate in coordination of locomotive movements of fore- and hindlimbs, we examined the relations between fore- and hindlimbs during overground locomotion of adult cats with spinal cord lesions. In a group of cats (T-T preparations), the spinal cord was hemisected first at around Th12 and then at intervals of 37-126 days contralaterally at mid-thoracic level, propriospinal tracts being mostly severed in this group. In a second group of cats (C-T preparations), which received hemisections first at around C2 and then at intervals of 21-73 days at mid-thoracic level, propriospinal tracts were left intact at least on one side of the spinal cord. Control observations were also made in intact cats and those with single hemisections at C2 or Th12, or with double unilateral hemisections at Th6 and Th12. Thus, it was found that in both T-T and C-T preparations, step length of the forelimbs was shortened significantly, whereas that of the hindlimbs was significantly lengthened. Furthermore, phase relations between the fore- and hindlimbs were completely lost in these preparations, suggesting that the stepping generator for the forelimbs operates independently of that for the hindlimbs. In other single-hemisected or unilaterally double-hemisected preparations, by contrast, no such changes were observed. The close similarity of the results in T-T and C-T preparations, in spite of different degrees of impairment of propriospinal tracts in them, leads to a conclusion that inter-limb propriospinal reflexes play little role in coordination of locomotive movements of fore- and hindlimbs.     

Crossed commissural pathways in the spinal hindlimb enlargement are not necessary for right left hindlimb alternation during turtle swimming

We examined the coordination between right and left hindlimbs during voluntary forward swimming in adult red-eared turtles, before and after midsagittal section of the spinal cord hindlimb enlargement (segments D8-S2) or the enlargement plus the first preenlargement segment (D7-S2). Our purpose was to assess the role of crossed commissural axons in these segments for right-left hindlimb alternation during voluntary locomotion. Midsagittal splitting severed commissural fibers and separated the right and left halves of the posterior spinal cord. Adult turtles (n = 9) were held by a band clamp around the shell in a water-filled tank while digital video of forward swimming was recorded from below and computer analyzed with motion analysis software. In a subset of these animals (n = 5), we also recorded electromyograms from hip extensor and/or hip flexor muscles on both sides. Surprisingly, splitting spinal segments D8-S2 or D7-S2 did not affect the strength of out-of-phase coordination between right and left hindlimbs, although hindlimb movement amplitudes were reduced compared with presurgical controls. These results show that commissural axons in the hindlimb enlargement and preenlargement cord are not necessary for right-left hindlimb alternation during voluntary swimming. We suggest that alternating propriospinal drive from the right and left sides of the forelimb enlargement maintains the out-of-phase coordination of right and left hindlimbs in the bisected-cord preparation. Our data support the hypothesis that descending propriospinal (forelimb-hindlimb) and crossed commissural (hindlimb-hindlimb) spinal cord pathways function together as redundant mechanisms to sustain right-left hindlimb alternation during turtle locomotion.     

Different patterns of fore-hindlimb coordination during overground locomotion in cats with ventral and lateral spinal lesions

The effect of large, low thoracic (T10–T11), partial spinal lesions involving the ventral quadrants of the spinal cord and, to a different extent, the dorsolateral funiculi, on fore-hindlimb coordination was examined in cats walking overground at moderate speeds (40–100 cm/s). Three different forms of impairment of fore-hindlimb coordination depending on the extent of the lesions, were observed. Lesions sparing the dorsolateral or the ventral funiculus on one side preserved the equality of the fore- and hindlimb locomotor rhythms but changed the coupling between the movements of both girdles as compared to intact animals. Larger lesions in which, in addition to the ventral quadrants of the spinal cord, also major parts of the dorsolateral funiculi were destroyed elicited episodes of rhythm oscillations in both girdles, which appeared at the background of a small difference in these rhythms. Lesions destroying almost the whole spinal cord induced a permanent difference (about 200 ms) in the step cycle duration of the fore- and the hindlimbs. However, even in these animals some remnant form of fore-hindlimb coordination was found. The results suggest that dorsolateral funiculi play a major role in preserving the equality of rhythms in the foreand the hindlimbs, while lesions of the ventral quadrants change the coupling between limbs.     

Recovery of hindlimb motor functions after spinal cord transection is enhanced by grafts of the embryonic raphe nuclei

In this study, a piece of embryonic tissue from the raphe nucleus was transplanted into the spinal cord below the lesion 1 month after transection. Two months later the recovery of hindlimb motor function in rats which had received a transplant of neural tissue (ST rats) was much better than in spinal control animals without the graft (SC rats). Analysis of the electromyographic (EMG) activity showed that the timing of muscle activity during locomotor-like movement of hindlimbs in ST rats was more regular than in SC rats. In SC rats the relationships between EMG burst duration (soleus, tibialis anterior) and step cycle duration were significantly altered. The restoration of hindlimb motor function of ST rats was also reflected in the better interlimb coordination during locomotor-like hindlimb movements. The results of several behavioural tests demonstrated that the responses to stimulation of various receptors, such as tactile or proprioceptive, in ST rats were more complex than in SC rats. Additionally, unlike in SC animals, in ST rats long-lasting spontaneous episodes of air stepping movement of hindlimbs accompanied by a relatively high amplitude of EMG activity were obtained. These results confirm that grafted embryonic raphe nuclei which contain serotoninergic cells are likely to increase the excitability of neuronal circuitry in the injured spinal cord. Moreover, transplantation of embryonic raphe nuclei encourages the recovery of hindlimb motor function in adult rats even when the grafting is carried out several weeks after spinal cord injury.     

The rat lumbosacral spinal cord adapts to robotic loading applied during stance

Load-related afferent information modifies the magnitude and timing of hindlimb muscle activity during stepping in decerebrate animals and spinal cord-injured humans and animals, suggesting that the spinal cord mediates load-related locomotor responses. In this study, we found that stepping on a treadmill by adult rats that received complete, midthoracic spinal cord transections as neonates could be altered by loading the hindlimbs using a pair of small robotic arms. The robotic arms applied a downward force to the lower shanks of the hindlimbs during the stance phase and measured the position of the lower shank during stepping. No external force was applied during the swing phase of the step. When applied bilaterally, this stance force field perturbed the hindlimb trajectories so that the ankle position was shifted downward during stance. In response to this perturbation, both the stance and step cycle durations decreased. During swing, the hindlimb initially accelerated toward the normal, unperturbed swing trajectory and then tracked the normal trajectory. Bilateral loading increased the magnitude of the medial gastrocnemius electromyographic (EMG) burst during stance and increased the amplitude of the semitendinosus and rectus femoris EMG bursts. When the force field was applied unilaterally, stance duration decreased in the loaded hindlimb, while swing duration was decreased in the contralateral hindlimb, thereby preserving interlimb coordination. These results demonstrate the feasibility of using robotic devices to mechanically modulate afferent input to the injured spinal cord during weight-supported locomotion. In addition, these results indicate that the lumbosacral spinal cord responds to load-related input applied to the lower shank during stance by modifying step timing and muscle activation patterns, while preserving normal swing kinematics and interlimb coordination.     

Complete spinal cord transection at different postnatal ages: recovery of motor coordination correlated with spinal cord catecholamines

Summary The ability of rats that are cordotomized at different times between postnatal day (PN) 0-28, to recover four-limb motor coordination, varies as a function of the time of cordotomy. The rats were evaluated at 37 independent observers for four-limb coordination, scored on a scale of 10 (best) to 0 (worst). The rank order of recovery from best to worst is: PN7>PNO>PN14> PN21>PN28. The hindlimbs are active only when they receive proprioceptive sensation from contact with a surface. They appear completely paralyzed when, for example, the rats are challenged to climb an inclined surface of spaced metal bars (Fig. 4). The content of both dopamine (DA) and norepinephrine (NE) in the adult spinal cord rostral to the transection, also varied as a function of transection time. DA was present in the lumbar (that is, caudal to the transection) region of the cord in the PNO, PN7 and PN14 groups, with the highest concentration in the PN7 group. NE was not present in the lumbar region in any of the experimental groups. It is concluded that rats can recover a substantial degree of four limb motor activity after cordotomy, provided the cord is transected before the fourteenth postnatal day. Moreover, this recovery of motor coordination, apparently correlates closely with the presence of DA in the lumbar region of the cord. Whether there is a causal relationships between recovery of motor coordination and the content of DA in the lumbar cord is not known.     

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.     

Walking of cats on a grid: performance of locomotor task in spinal intact and hemisected cats.

We have investigated interlimb coordination during walking by observing locomotion of cats on a grid. Control cats made no error in forelimb placement on the grid, and landed their hindlimbs on the rung where each ipsilateral forelimb left moments earlier. This suggests that locomotor command signals were modified by grid information including its size and direction and affected the pattern generators of both forelimbs and hindlimbs. Hemisected cats, made both at high cervical and lower thoracic, showed deteriorated placement of the hindlimbs suggesting that interlimb coordination is carried out mainly by descending pathways from the brainstem such as ventrolateral fasciculus and dorsolateral fasciculus. Interlimb reflex pathways may play a limited role.     

Sensory regulation of quadrupedal locomotion: a top-down or bottom-up control system?

Propriospinal pathways are thought to be critical for quadrupedal coordination by coupling cervical and lumbar central pattern generators (CPGs). However, the mechanisms involved in relaying information between girdles remain largely unexplored. Using an in vitro spinal cord preparation in neonatal rats, Juvin and colleagues (Juvin et al. 2012) have recently shown sensory inputs from the hindlimbs have greater influence on forelimb CPGs than forelimb sensory inputs on hindlimb CPGs, in other words, a bottom-up control system. However, results from decerebrate cats suggest a top-down control system. It may be that both bottom-up and top-down control systems exist and that the dominance of one over the other is task or context dependent. As such, the role of sensory inputs in controlling quadrupedal coordination before and after injury requires further investigation.     

Analyses of treadmill locomotion in adult spinal dogs

The locomotion of the hindlimbs of two adult female spinal dogs, who were able to walk steadily on their hindlimbs 10 months after transection of the spinal cord (T10), and of two normal dogs was analyzed on a treadmill by means of high-speed cinematography and electromyography. With increase in walking speed, the duration of the step cycle was shortened by reduction of the duration of the stance phase, and the stride length was extended mainly by elongation of the transfer distance during the swing phase in both normal and spinal dogs. The patterns of muscle discharges of the hindlimbs in spinal dogs were similar to those in normal dogs. With increase in walking speed, reductions in the burst duration of the extensors were observed in both normal and spinal dogs. These results indicate that spinal dogs can adjust their locomotion speed in the same manner as normal dogs; this supports the theory that a central pattern generator regulating the locomotive activities of the hindlimbs exists in the spinal cord below the transection site.     

Interlimb postural coordination in the standing cat

The dorsal-side-up body posture in standing quadrupeds is maintained by coordinated activity of four limbs. We studied this coordination in the cat standing on the platform periodically tilted in the frontal plane. By suspending different body parts, we unloaded one, two, or three limbs. The activity of selected extensor muscles and the contact forces under the limbs were recorded. With all four limbs on the platform, extensors of the fore- and hindlimbs increased their activity in parallel during ipsilateral downward tilt. With two forelimbs on the platform, this muscular pattern persisted in the forelimbs and in the suspended hindlimbs. With two hindlimbs on the platform, the muscular pattern persisted only in the hindlimbs, but not in the suspended forelimbs. These results suggest that coordination between the two girdles is based primarily on the influences of the forelimbs upon the hindlimbs. However, these influences do not necessarily determine the responses to tilt in the hindlimbs. This was demonstrated by antiphase tilting of the fore- and hindquarters. Under these conditions, the extensors of the fore- and hindlimbs appeared uncoupled and modulated in antiphase, suggesting an independent control of posture in the fore- and hindquarters. With only one limb supporting the shoulder or hip girdle, a muscular pattern with normal phasing was observed in both limbs of that girdle. This finding suggests that reflex mechanisms of an individual limb generate only a part of postural corrections; another part is produced on the basis of crossed influences.     

Hindlimb stepping movements in complete spinal rats induced by epidural spinal cord stimulation

The locomotor ability of the spinal cord of adult rats deprived of brain control was tested by epidural spinal cord stimulation. The studies were performed on six rats that had a complete spinal cord transection (T7-T9) and epidural electrode implantations 2-3 weeks before testing was initiated. The stimulating epidural electrodes were implanted at the T12-L6 spinal segments. Epidural electrical stimulation of the dorsal surface of the spinal cord at frequencies between 1 and 50 Hz and intensities between 1 and 10 V without any pharmacological facilitation was used. Stimulation at each of the lumbar spinal cord segments elicited some rhythmic activity in the hindlimbs. However, stimulation at most segmental levels usually evoked activity in only one leg and was maintained for short periods of time (< 10s). Bilateral hindlimb locomotor activity was evoked most often with epidural stimulation at 40-50 Hz applied at the L2 segment. A necessary condition for initiation of locomotor activity was providing a specific amount (at least 5%) of body weight support. Therefore, the rat spinal cord isolated from brain control is capable of producing bilateral stepping patterns induced most readily by epidural stimulation applied at the L2 spinal segment. Furthermore, the induced stepping patterns were dependent on sensory feedback associated with weight bearing.     

Characteristics of H- and M-waves recorded from rat forelimbs

The Hoffman reflex (H-reflex) is a useful tool for studying the functional aspects of the spinal cord without anesthesia and/or damage to the body. H-reflex studies are performed mainly in the hindlimbs. The purpose of the present study was to evaluate the characteristics of the H-reflex in the forelimbs and hindlimbs in rats anesthetized with ketamine–HCl. H- and M-waves were recorded from the interosseous muscles after electrical stimulation of the n. lateral plantar of the hindlimb and n. medialis of the forelimb. Hmax/Mmax values were significantly smaller in the forelimbs than in the hindlimbs. Furthermore, paired-pulse attenuation tended to be stronger in the forelimbs than in the hindlimbs. These findings suggest that control by descending and/or propriospinal pathways is stronger in the forelimbs than in the hindlimbs in rats.     

Location of spinal cord pathways that control hindlimb movement amplitude and interlimb coordination during voluntary swimming in turtles

We performed mechanical lesions of the midbody (D2–D3; second to third postcervical spinal segments) spinal cord in otherwise intact turtles to locate spinal cord pathways that 1) activate and control the amplitude of voluntary hindlimb swimming movements and 2) coordinate hindlimb swimming with the movement of other limbs. Pre- and postlesion turtles were held by a band clamp around the carapace just beneath the water surface in a clear Plexiglas tank and videotaped from below so that kinematic measurements could be made of voluntary forward swimming with motion analysis software. Movements of the forelimbs (wrists) and hindlimbs (knees and ankles) were tracked relative to stationary reference points on the plastron to obtain bilateral measurements of hip and forelimb angles as functions of time along with foot trajectories. We measured changes in limb movement amplitude, cycle period, and interlimb phase before and after spinal lesions. Our results indicate that locomotor command signals that activate and regulate the amplitude of voluntary hindlimb swimming travel primarily in the dorsolateral funiculus (DLF) at the D2–D3 level and cross over to drive contralateral hindlimb movements. This suggests that electrical stimulation of the D3 DLF, which was previously shown to evoke swimming movements in the contralateral hindlimb of low-spinal turtles, activated the same locomotor command pathways that the animal uses during voluntary behavior. We also show that forelimb–hindlimb coordination is maintained by longitudinal spinal pathways that are largely confined to the ventrolateral funiculus (VLF) and mediate phase coupling of ipsilateral limbs, presumably by interenlargement propriospinal fibers.     

Metachronal coupling between spinal neuronal networks during locomotor activity in newborn rat

In the present study, we investigate spinal cord neuronal network interactions in the neonatal rat during locomotion. The behavioural and physiological relevance of metachronally propagated locomotor activity were inferred from kinematic, anatomical and in vitro electrophysiological data. Kinematic analysis of freely behaving animals indicated that there is a rhythmic sequential change in trunk curvature during the step cycle. The motoneurons innervating back and tail muscles were identified along the spinal cord using retrograde labelling. Systematic multiple recordings from ventral roots were made to determine the precise intrinsic pattern of coordination in the isolated spinal cord. During locomotor-like activity, rhythmic ventral root motor bursts propagate caudo-rostrally in the sacral and the thoracic spinal cord regions. Plotting the latency as a function of the cycle period revealed that the system adapts the intersegmental latency to the ongoing motor period in order to maintain a constant phase relationship along the spinal axis. The thoracic, lumbar and sacral regions were capable of generating right and left alternating motor bursts when isolated. Longitudinal sections of the spinal cord revealed that both the bilateral antiphase pattern observed for the sacral region with respect to the lumbar segment 2 as well as the intersegmental phase lag were due to cross-cord connections. Together, these results provide physiological evidence that the dynamic changes observed in trunk bending during locomotion are determined by the intrinsic organization of spinal cord networks and their longitudinal and transverse interactions. Similarities between this organization, and that of locomotor pattern generation in more primitive vertebrates, suggest that the circuits responsible for metachronal propagation of motor patterns during locomotion are highly conserved.     

On the central generation of locomotion in the low spinal cat

A central network of neurones in the spinal cord has been shown to produce a rhythmic motor output similar to locomotion after suppression of all afferent inflow. The experiments were performed mainly in acute spinal cats (th. 12), which had received DOPA i.v. and the monoamine oxidase inhibitor Nialamide. In some preparations all dorsal roots supplying the spinal cord were transected, in others phasic afferent activity was suppressed by curarization. The activity was recorded as neurograms from nerve filaments or as electromyograms. It is concluded that: 1. alternating activity between flexors and extensors of foot, ankel, knee, and hip of one limb can still occur 2. the duration of the flexor discharges vary less with the cycle duration than the extensor discharges 3. different flexor muscles may retain individual patterns 4. the activity at different joints can be dissociated 5. there is at least one network for each limb. 6. the coordination between the two hindlimbs can be alternating as in walking or be more closely spaced as in galloping 7. alternating activity in the ankle remains even when only segments L6, L7 and S1 are intact.     

Bilateral influences from the motor cortex on lumbar ventral horn interneurons

Microelectrode recordings were made of discharges of ventral horn interneurons (VHIN) in segments L_6–7 of the spinal cord during bilateral stimulation of the motor cortex in cats. The overwhelming majority of VHIN was shown to be activated by influences from the contralateral cortex, and about half of them also by ipsilateral influences. Clear correlation was established between convergence of afferent and cortical influences: Neurons with inputs from ipsilateral afferents only were activated, irrespective of their other characteristics, by descending influences from the contralateral cortex only, whereas bilateral cortical influences converged on cells with bilateral afferent connections. It is suggested that VHIN with bilateral segmental and supraspinal connections are important integrative elements in the mechanism of bilateral coordination of motor responses of different degrees of complexity.Translated from Fiziologicheskii Zhurnal SSSR imeni I. M. Sechenova, Vol. 65, No. 8, pp. 1172–1180, August, 1979.     

Metamorphosis-induced changes in the coupling of spinal thoraco-lumbar motor outputs during swimming in Xenopus laevis

Anuran metamorphosis includes a complete remodeling of the animal's biomechanical apparatus, requiring a corresponding functional reorganization of underlying central neural circuitry. This involves changes that must occur in the coordination between the motor outputs of different spinal segments to harmonize locomotor and postural functions as the limbs grow and the tail regresses. In premetamorphic Xenopus laevis tadpoles, axial motor output drives rostrocaudally propagating segmental myotomal contractions that generate propulsive body undulations. During metamorphosis, the anterior axial musculature of the tadpole progressively evolves into dorsal muscles in the postmetamorphic froglet in which some of these back muscles lose their implicit locomotor function to serve exclusively in postural control in the adult. To understand how locomotor and postural systems interact during locomotion in juvenile Xenopus, we have investigated the coordination between postural back and hindlimb muscle activity during free forward swimming. Axial/dorsal muscles, which contract in bilateral alternation during undulatory swimming in premetamorphic tadpoles, change their left-right coordination to become activated in phase with bilaterally synchronous hindlimb extensions in locomoting juveniles. Based on in vitro electrophysiological experiments as well as specific spinal lesions in vivo, a spinal cord region was delimited in which propriospinal interactions are directly responsible for the coordination between leg and back muscle contractions. Our findings therefore indicate that dynamic postural adjustments during adult Xenopus locomotion are mediated by local intraspinal pathways through which the lumbar generator for hindlimb propulsive kicking provides caudorostral commands to thoracic spinal circuitry controlling the dorsal trunk musculature.     

Development of locomotor mechanisms in the frog

Tadpoles swim by undulations of the body and tail, whereas frogs locomote by alternate (stepping) and synchronous (frog-kick) movements of the hindlimbs. The development of interlimb coordination was studied by recording the activity of hindlimb motoneurons from the left and right ninth ventral roots of the isolated central nervous system (CNS). Results showed that mechanisms responsible for interlimb coordination of stepping are functional when the hindlimb is still composed of undifferentiated mesenchyme and before the lateral motor column has stabilized (stage III). The early appearance of coordinated activation of hindlimb motoneurons suggests that innervation of appropriate target muscles is not a prerequisite for normal development of circuits that mediate interlimb coordination of stepping. Synchronous activation of left and right hindlimb motoneurons (fictive frog kicks) appeared later in development (stage XIV). Throughout larval development 1:1 frequency coupling between both alternating and synchronous bursts of hindlimb motoneurons and bursts of primary motoneurons (those innervating axial muscles) was found. Recordings of peripheral nerve activity showed that motoneurons innervating antagonistic muscles of the thigh burst in antiphase. This intralimb coordination was present at stage X, a foot paddle stage that was the earliest stage in which the peripheral nerves were successfully dissected. That the neural activity of the isolated nervous system described above indeed underlies coordinated locomotor movements of the hindlimbs was shown by single-frame videotape analysis of hindlimb movements produced by the otherwise isolated CNS. The stepping movements displayed by those preparations were consistent with patterns of electrophysiological burst activity recorded from the ventral roots and peripheral nerves. The ontogenetic sequence in which the different patterns of electrophysiological activity emerged is the same as that of the corresponding behaviors in the intact tadpole. Although there were developmental changes in the reliability with which coordinated activity in the ventral roots and peripheral nerves was observed, each mode of coordination remained qualitatively unchanged from its earliest appearance through metamorphosis. These results show that mechanisms underlying locomotor coordination of the hindlimbs develop very early in larval ontogeny of the frog and can function when isolated from the periphery.     

Somatic afferent regulation of plasma corticosterone in anesthetized rats.

Effects of cutaneous stimulation on plasma corticosterone were examined in adult male Wistar rats anesthetized with pentobarbital. Under the resting condition, plasma corticosterone measured every 15 min between 1430 and 1630 h revealed no significant circadian fluctuations. Nociceptive mechanical stimulation of bilateral hindpaws by pinching for 10 min significantly increased plasma corticosterone for the following 1 h, whereas innocuous mechanical stimulation of bilateral hindlimbs by brushing for 10 min produced no significant change in plasma corticosterone. These results indicate that somatic sensory information from skin can influence secretion of corticosterone from the adrenal cortex after emotional factors are eliminated by anesthetizing the subjects.