Comparison between spino-bulbo-spinal and propriospinal reflexes in thalamic cats during stepping

To analyze changes in the excitability of both the spinal cord and brainstem in thalamic cats stepping on a moving treadmill, we examined the cutaneous propriospinal (PSR) and spino-bulbo-spinal (SBS) reflex responses in 20 adult cats. Tracheal cannulation, spinal transection at the T10 segment, and decerebration at the stereotaxic A12 level were performed under ether anesthesia. Immediately after decerebration, the ether was withdrawn. The head was fixed in a stereotaxic device, the T2 spinal process clamped to a metal frame, and the lumbar region suspended by a hammock, with bilateral forelimb contact on the floor of a treadmill. Electrical stimulation was applied to the superficial radial nerve with a cuff electrode, and two reflex responses (PSR and SBS) were recorded from the biceps brachii muscle in the same forelimb. Shortly before the appearance of forelimb stepping, both PSR and SBS reflex responses were elevated in amplitude. During forelimb stepping, the amplitudes of PSR and SBS reflex responses fluctuated depending on the phase of the step cycle. The PSR response was enhanced in the early phase of the swing, whereas the SBS response was elevated during a wider period from the beginning of the stance to the middle of the swing. The SBS response was completely absent in the late phase of the swing. This period corresponded to the transfer from flexion to extension and the appearance of the EMG of the triceps brachii muscle of the same forelimb. The fluctuation of the SBS response during stepping may be produced at the brainstem level, and not the spinal cord level, because the PSR response was enhanced only during narrow periods. The generation of locomotion thus seems to result in an enhancement of excitability of reflex pathways in the spinal cord and particularly in the brainstem.     

Selective activation of human soleus or gastrocnemius in reflex responses during walking and running

Summary Phase-dependent reflex modulation was studied by recording the electromyographic (EMG) responses in soleus (SOL) and gastrocnemius medialis (GM) to a 20 ms train of 5 electrical pulses, applied to the sural or tibial nerve at the ankle, in 14 volunteers walking or running on a treadmill. Although both the spontaneous activity and the reflex responses were usually similar for both muscles, instances were identified in which separate control was evident. During walking (4 km/h), activity in SOL started earlier in the stance phase than GM activity. Correspondingly, the amplitude of the reflex responses was larger in SOL than in GM in early stance, both ipsi- and contralateral to the side of stimulation. In some cases, the same stimulus could elicit contralaterally a suppression of GM in synchrony with a facilitation of SOL. These crossed extensor reflexes had a low threshold (1.2 × T) and a latency ranging from 72 to 105 ms. During running (8 km/h or more), responses were seen selectively in GM instead, without concomitant responses in SOL. Such responses had a latency ranging from 82 to 158 ms and they appeared during the first extension phase, at the end of the swing phase. In addition, selective GM responses, with latencies above 200 ms, were seen near the transition from stance to swing during running. These instances of separate reflex control of SOL and GM were correlated with step cycle periods during which the motoneurones of either one of these muscles received more spontaneous activation than the other. Nevertheless, it is argued that premotoneuronal gating must also be involved since the increased amplitude of the crossed SOL responses (in early stance) and of GM responses (at end swing) was not strictly linked to an elevated amount of spontaneous activity during these parts of the step cycle as compared to other parts.     

A kinematic and electromyographic study of cutaneous reflexes evoked from the forelimb of unrestrained walking cats

A kinematic and electromyographic (EMG) analysis was undertaken of the responses evoked in the forelimb of the cat by either mechanical obstruction of the forelimb during the swing phase of locomotion or by electrical stimulation of low-threshold cutaneous afferents during both swing and stance. Mechanical obstruction of the forelimb with a stiff metal rod evoked a complex response that allowed the cat to smoothly negotiate the obstacle without undue disruption of the overall locomotor rhythm. The initial movements were a flexion of the shoulder, together with a locking of the elbow joint, and a dorsiflexion of the wrist, which caused the limb to withdraw from the obstacle. They were followed by an extension of the shoulder, a flexion of the elbow, and a ventroflexion of the wrist, which together brought the limb forward and above the obstacle. The associated and complex pattern of short- and long-latency EMG responses was shown to be related to different aspects of the movement. At the shoulder there was a strong activation of flexor muscles; these responses were of long duration (greater than or equal to 100 ms) and generally lasted throughout the period of shoulder flexion. At the elbow, both flexor and extensor muscles were activated at short latency (9-13 ms). In flexors, this was followed by a cessation and subsequently an augmentation and prolongation of their activity. Dorsiflexors of both the wrist and digits were activated at short latency (10-12 ms) and remained active throughout the period of dorsiflexion of these joints. An injection of a local anesthetic into the area of skin contacted by the metal rod reduced or abolished all of the reflex responses, which suggests that the integrity of cutaneous reflex pathways is essential for the elaboration of these responses. Electrical stimulation of a cutaneous nerve innervating the distal forelimb (the superficial radial nerve) resulted in qualitatively similar, although weaker, responses to those obtained with the mechanical stimulation. Terminal experiments confirmed that these responses were mediated by low-threshold cutaneous afferents. Electrical stimulation also evoked short-latency excitatory responses (10-12 ms) in extensor muscles of the elbow. Generally, the largest reflex effects were obtained during the period of swing for flexor, extensor, and bifunctional muscles. During stance the stimulus was normally ineffective in exciting flexor muscles and in extensors evoked a short-latency inhibition, which was frequently followed by an increase in activity.(ABSTRACT TRUNCATED AT 400 WORDS)     

Short latency crossed inhibitory reflex actions evoked from cutaneous afferents

Although stimulation of cutaneous limb afferents has been shown to evoke crossed extension reflexes in unanaesthetised decerebrate or spinalised animals, here we show that stimulation of cutaneous nerves evokes crossed inhibition rather than excitation of contralateral extensor motoneurones in anaesthetised, spinal cord intact cats. Single pulse stimuli delivered to the saphenous, sural or superficial peroneal nerves evoked IPSPs in a high proportion of contralateral motoneurones including those of knee and ankle extensors. These IPSPs had thresholds of less than 1.5 times the threshold of the most excitable fibres and so large myelinated afferents contributed to them. The relative latencies of IPSPs evoked by stimulation of the contralateral superficial peroneal and sural nerves were longer than those evoked via ipsilateral pathways by approximately 1 ms, suggesting that there are at least three synaptic relays in the crossed reflexes. The IPSPs evoked by stimulation of both ipsilateral and contralateral saphenous nerves had minimal latencies suggesting at least three synaptic delays. Like IPSPs evoked by group II afferents, the frequencies of occurrence of crossed IPSPs evoked by stimulation of cutaneous afferents were significantly reduced after spinal transection and the IPSPs recorded after spinalisation were significantly smaller. These findings are consistent with the recent proposal that dorsal horn neurones, which receive input from cutaneous afferents and contact premotor commissural interneurones may mediate the crossed inhibition.     

Roll tilt reflexes after vestibulospinal tract lesions

Summary The effects of lesions of the vestibulospinal tracts on vestibular reflexes evoked by roll tilt in forelimb and neck extensors were examined in decerebrate cats. Sectioning the medial longitudinal fasciculus, which contains the medial vestibulospinal tract, had no major effect on the phase of the reflex, although some gain was usually lost at high stimulus frequencies. Spinal lesions at C2–C3, both cord hemisections and more restrictive lesions which cut the lateral vestibulospinal tract, produced two major effects on the forelimb. Background EMG activity was usually abolished in the triceps ipsilateral to the lesion, with partial loss of activity in the opposite limb. The tilt reflex response in the ipsilateral limb appeared normal, although it was usually necessary to raise the background excitability of the preparation by administering L-Dopa in order to observe the reflex. In contrast, the response in the contralateral limb showed a phase reversal of 180 deg at low stimulus frequencies, implying that the reflex in intact cats receives a crossed otolith-spinal input. Responses in the neck extensors splenius and biventer, recorded from compartments caudal to the spinal lesion, were relatively unaffected.Partially supported by NASA grant NSG-2380 and PHS grants NS02619 and RR07065Recipient of NIH Postdoctoral Fellowship NS06128Supported in part by NIH Research Service Award 7524     

Ascending spinal tracts of the spino-bulbo-spinal reflex in cats.

Twenty-six chloralosed cats were employed in order to determine spinal ascending pathways of the spino-bulbo-spinal (SBS) reflex evoked by stimulation of the sural nerve. 1. Partial spinal transection of the dorsal part of the lateral funiculus abolished the SBS reflex ipsilateral to sural nerve stimulation. 2. By recording spinal cord potentials in response to sural nerve stimulation two pathways were established in the dorsolateral funiculus as the spinal ascending tracts of the SBS reflex; one is the direct pathway to the bulbar reticular-formation (direct spino-reticular tract) and the other one (indirect spino-reticular tract) is the relayed by the lateral cervical nucleus. Direct stimulation of the dorsolateral funiculus at the lumbar level elicited the SBS reflex. 3. Short-latency unit discharges were recorded from axons of the direct spino-reticular tract by sural nerve stimulation. These axons were discharged antidromically by stimulation of the bulbar reticular formation. 4. Intracellular recordings from the neurons of the lateral cervical nucleus revealed that spike potentials, riding on EPSPs, were induced by sural nerve stimulation and antidromic firings were obtained by stimulation of the bulbar reticular formation. 5. Neurons originating the spino-reticular tract, direct and indirect, were located in the Rexed V-VII laminae in the lower lumbar segments. They were fired monosynaptically by sural nerve stimulation and antidromically by stimulating the dorsolateral funiculus of the lumbar segments. Among them, some were activated antidromically by stimulating the bulbar reticular formation.     

Adaptation of cutaneous stumble correction when tripping is part of the locomotor environment

We recently showed that cutaneous reflexes evoked by stimulating the superficial peroneal (SP; innervates foot dorsum) nerve are modulated according to the level of postural threat. Context-related modulation was observed mainly in contralateral (c) responses but not in the ipsilateral responses. This lack of effect on ipsilateral (i) cutaneous reflexes might have been caused by the general nature of the whole body perturbation. We therefore hypothesized that context-relevant mechanical perturbations applied to the dorsum of the foot by an instrumented rod at early swing during walking would produce differences in ipsilateral cutaneous reflex amplitudes, consistent with the functional relevance of the SP nerve in stumble correction responses. Subjects walked on a motorized treadmill under four conditions: 1) normal, 2) normal with mechanical perturbations at the foot dorsum, 3) arms crossed, and 4) arms crossed with mechanical perturbations at the foot dorsum. Electrical stimulation of the SP nerve was delivered at five phases of the step cycle, and cutaneous reflexes were compared between all conditions for each phase of the step cycle. Reflex responses were generally found to be modulated in amplitude during walking conditions in which mechanical perturbations were delivered, particularly in ipsilateral tibialis anterior (iTA), which showed a marked reduction in inhibition. The results indicated cutaneous reflexes in iTA and contralateral medial gastrocnemius (cMG) were influenced by the threat of a trip, induced by applying mechanical perturbations to the foot dorsum during walking. This task-related gating of cutaneous reflexes was not generalized to all muscles, thus suggesting a functional role in the maintenance of stability during locomotion.     

Sensorimotor cortical representation in the rat and the role of the cortex in the production of sensory myoclonic jerks.

1. After administration of 1,2-dihydroxybenzene (catechol) to anaesthetized rats, rabbits and cats, a reflex jerk consisting of three distinct components was evoked in the limb muscles by peripheral stimulation. The second component of the jerk in the forelimb muscles of all three animals was specifically abolished by lesions confined to the contralateral forelimb sensorimotor cortex. 2. These lesions had no effect on the second response in either the hind limbs or in the forelimb ipsilateral to the lesion. The first and third responses were also unaffected. 3. Lesions in the cat hind-limb cortex abolished the contralateral hind-limb second response, but not the ipsilateral hind-limb or forelimb response. 4. In the rat and rabbit, unilateral hind-limb sensorimotor lesions were ineffective in completely abolishing the second response in the contralateral hind-leg muscles, and in addition, reduced the probability of occurrence of the response in the ipsilateral hind leg. Bilateral lesions abolished the response. 5. Re-investigation of the sensory and motor representation of the hind limb in the rat cortex revealed that this is bilateral in nature. Short-latency cortical responses (ca. 7-0 msec) could be evoked in one cortex by stimulation of either hind paw. The geometric centre of the cortical area from which these responses could be recorded was identical for each hind paw. 6. After catechol injection, stimulation of the cortical surface with single anodal shocks of threshold strength produced responses at similar latency (ca. 8-0 msec) in both hind limbs. 7. The behaviour of the second response after cortical lesions corresponds closely with the pattern of the somatosensorimotor cortical representation. The latency of the response is such as to allow its production by a long-loop cortical reflex, and this possibility is discussed.     

The bilateral reflex control of the trapezius muscle in humans

Abstract. Electromyographic recordings were made from the trapezius muscle in 18 healthy subjects using surface electrodes placed bilaterally. A mechanical tap applied to the insertion of the lower fibres of trapezius evoked a reflex at 11.9 +/- 1.4 ms (mean plus minus SD) in the ipsilateral trapezius muscle. In addition and surprisingly, short latency, facilitatory reflexes were also seen on the contralateral side at a latency of 14.3 +/- 1.7 ms. Electrical stimulation of the afferent nerve to trapezius, the cervical nerve of C3/4, also evoked short latency, facilitatory reflexes from both the ipsilateral and contralateral muscles. Since this nerve carries the Ia fibres from trapezius, this reflex would appear to be the equivalent of the H reflex seen in the soleus muscle. The latency of this reflex was 10.9 +/- 1.4 ms ipsilaterally and 11.9 +/- 1.5 ms contralaterally, the difference being only 1.0 +/- 0.7 ms. Voluntary activity of the ipsilateral trapezius increased the amplitude of both mechanically and electrically evoked ipsilateral and crossed reflexes, and vibration of the ipsilateral trapezius decreased the amplitude of both reflexes. These results together suggest that the earliest parts of both ipsilateral and crossed reflexes are monosynaptic in origin. If the crossed reflex is, in part, due to a common Ia presynaptic input, correlation of bilateral trapezius activity should produce a peak in the cross-correlogram. Indeed, cross-correlation of the bilateral trapezius activity during elevation of the arms produced a short duration peak. Moreover, this was significantly larger when compared to that constructed from activity of the upper and lower fibres of trapezius from one side. These results imply that muscle spindle afferents from the ipsilateral trapezius monosynaptically activate motoneurones contralaterally.     

Long spinal and pyramidal actions on hindlimb motoneurons of the marsupial brush-tailed possum, Trichosurus vulpecula.

The existence of descending propriospinal reflex linkages between forelimbs and hindlimbs has been established in the brush-tailed possum (Trichosurus vulpecula). In animals under chloralose anesthesia and with intact brain stem, forelimb volleys evoked facilitation of flexor and extensor monosynaptic reflexes of both hindlimbs, more pronounced on the ipsilateral side. Powerful inhibition of briefer latency and restricted to ipsilateral flexor digitorum longus (FDL) motoneurons was also brought about by forelimb volleys; at latencies exceeding 20-30 ms, FDL inhibition was usually replaced by facilitation. Distinctness of the two long spinal actions was shown by differences in forelimb receptive fields and in threshold of the executant afferent fibers. The field for reflex inhibition was located distally in the forepaw region, that for facilitation being wider, including deep as well as superficial tissues. Threshold of afferent fibers evoking inhibition was lower than that for facilitation. The descending long spinal actions were compared with those set up by repetitive stimulation of the motor cortex contralateral to the test hindlimb reflexes. In agreement with previous work, strong facilitation of most flexor or extensor motoneurons was produced, including those of quadriceps and ankle flexors, as well as gastrocnemius and hamstring motor nuclei; inhibition consistently appeared only in the FDL motoneuron pool. Weak and inconstant inhibitory action was occasionally observed in other motor pools. Pyramidal tract section abolished the cortical inhibition of FDL, but had little effect on facilitation; both long spinal actions were unchanged. Pyramid-sparing brain stem section greatly reduced both cortical and long spinal facilitatory action, but had little or no effect on FDL inhibition from either source. Interaction experiments demonstrated facilitation of weak inhibitory actions on FDL motoneurons of forelimb and cortical stimulation when elicited together, suggesting a sharing by the two inputs of common interneuronal elements. The observation is consistent with the notion that the long propriospinal system responsible for FDL inhibition from the forepaw might provide the pathway for pyramidal inhibition of the same group of motoneurons.     

Reflexes induced by nerve stimulation in walking cats with implanted cuff electrodes

Summary Neural cuffs, implanted around various hindlimb nerves (sural, common peroneal, posterior tibial), were used to deliver brief stimulus trains to unrestrained cats walking on a treadmill. The resulting perturbations of the step cycle were evaluated by analyzing the EMG bursts recorded from the ankle extensors and by high speed cinematography. It was found that relatively weak stimulation (1.4 to 2 X T) of the posterior tibial nerve was very effective in eliciting a prolongation of the flexion phase provided the stimuli were applied just prior to the expected onset of the ankle extensor EMG burst. This ipsilateral hyperflexion was correlated with a prolongation of the contralateral extension. The same stimuli applied during stance usually evoked a yielding of the stimulated leg and a prolongation of the ongoing contralateral stance. In addition to these flexor and extensor reflex effects, it was found that low threshold stimulation of the sural and common peroneal nerves resulted in a powerful reflex activation of the ankle extensors. In contrast, stimulation of the gastrocnemius-soleus nerve (a muscle nerve) produced no discernible behavioral effects, even for stimuli at 3 X T, indicating that the observed reflexes are probably mediated by cutaneous afferents. The results were largely confirmed in experiments using the same cuffs implanted in spontaneously walking premammillary cats.     

The vestibulospinal tract: Crossed and uncrossed effects on hindlimb motoneurones in the cat

Effects produced by stimulation of Deiters' nucleus on contralateral hindlimb motoneurones were investigated in the cat with intracellular and monosynaptic reflex recording. Polysynaptic PSPs were evoked in all species of motoneurone examined. The minimal synaptic linkage was found to be disynaptic in the excitatory path and prisynaptic in the inhibitory path. Experiments with various lesions to the spinal funiculus and those with mapping of the stimulated sites in the brain stem showed that the effects were evoked almost exclusively via the lateral vestibulospinal tract of the stimulated side. The crossed effects were excitatory in extensor motoneurones of all species studied except for hip extensors (ABSm) in which EPSPs and/or IPSPs were evoked. The effects on flexors, by contrast, were not simple and consisted of EPSPs, IPSPs or a mixture of both. The difference in effects depended, though not entirely, on the species of motor nucleus. Between the excitatory and inhibitory effects on knee flexor (PBSt) motoneurones the former predominated under chloralose and the latter under pentobarbital anaesthesia. By recording PSPs evoked from the two vestibulospinal tracts in the same motoneurone, the convergence pattern and the magnitude of effects from the two tracts were studied. The interneuronal organization of the crossed vestibulo-motoneuronel pathway and its functional significance are discussed.     

The effects of morphine administered intrathecally on the somatosympathetic reflex discharges in anesthetized cats

The effect of intrathecally applied morphine or naloxone on the sympathetic A and C reflexes evoked by stimulation of somatic myelinated A and unmyelinated C afferent nerves was investigated in anesthetized cats. Intrathecal application of morphine in the lumbar spinal region depressed the C reflex elicited by stimulation of a hindlimb nerve (superficial peroneal, SP) without affecting the C reflex elicited by stimulation of a forelimb nerve (radial, RA). Also, intrathecal application of naloxone in the lumbar spinal region was found to reverse the intravenous morphine-induced depression of the C reflex elicited by stimulation of the SP nerve. The present results suggest that selective depression of morphine on the somatosympathetic C reflex is mediated at the spinal cord level.     

Context-dependent modulation of interlimb cutaneous reflexes in arm muscles as a function of stability threat during walking

Cutaneous reflexes evoked in the muscles of the arms with electrical stimulation of nerves of the foot ("interlimb reflexes") are observed during walking. These reflexes have been suggested to coordinate the actions of the legs and arms when walking is disturbed. Recently, we showed that cutaneous reflexes evoked in the leg muscles after stimulation at the foot are modulated according to the level of postural threat during walking. We hypothesized that the amplitude of interlimb cutaneous reflexes would similarly be modulated when subjects walk in unstable environments. Subjects walked on a treadmill under four walking conditions: 1) normal; 2) normal with unpredictable anterior-posterior (AP) perturbations; 3) arms crossed; and 4) arms crossed with unpredictable AP perturbations. Interlimb reflexes evoked from electrical stimulation of the right superficial peroneal or sural nerves were recorded bilaterally, at four points of the step cycle. These reflexes were compared between conditions in which the arms were moving in a similar manner: 1) normal versus AP walking and 2) arms crossed versus arms crossed with AP perturbations. Differences in reflex amplitudes between arms-crossed conditions were observed in most upper limb muscles when subjects were perturbed while walking compared with undisturbed walking. This effect was less apparent when the arms were swinging freely. The results indicate that the strength of interlimb connections is influenced by the level of postural threat (i.e., the context of the behavior), thereby suggesting that these reflexes serve a functional link between the legs and arms during locomotion.     

Midbrain stimulation inhibits the micturition,defecation and rhythmic straining reflexes elicited by activation of sacral vesical and rectal afferents in the dog

Summary Inhibition of the micturition, defecation and rhythmic straining reflexes by midbrain stimulation was compared with the inhibition of the jaw-opening reflex caused by tooth pulp stimulation in decerebrate dogs. All of the reflexes were inhibited by stimulation of the dorsal and ventral periaqueductal gray, dorsal raphe nucleus and central tegmental field with similar threshold intensities. After a hemisection of the spinal cord at the C2 segment, the midbrain stimulation still suppressed the micturition reflex as well as field potential changes which were evoked by stimulation of the pelvic nerve and recorded from the lateral funiculus just caudal to the hemisection, but did not influence the discharges of the vesical branch of the pelvic nerve which were elicited by stimulation of the lateral funiculus just rostral to the hemisection. The results suggest that stimulation of the neural elements in the 4 midbrain areas depresses the ascending activities from vesical and colorectal afferents of the pelvic nerve at the spinal level, and consequently inhibits the pelvic nerve reflexes. Systemic methysergide suppressed midbrain inhibition of the jaw-opening reflex, but did not affect the midbrain inhibition of the pelvic nerve reflexes. Systemic naloxone did not influence midbrain inhibition of the pelvic nerve reflexes or the jaw-opening reflex, but enhanced the micturition and rhythmic straining reflexes. Possible roles of the midbrain inhibition of the pelvic nerve reflexes are discussed.     

Interlimb coordination in cat locomotion investigated with perturbation

Summary During locomotion of decerebrate and awake walking cats, perturbation (mechanical tap) was applied to the paw dorsum of the left forelimb (LF), and the responses of both forelimbs were recorded cinematographically and electromyographically (EMG). When the tap was applied during the LF stance phase, the duration of the ongoing LF stance was shortened by 10%; in the right forelimb (RF), the duration of the concomitant swing was shortened by 32%. A tap during the LF swing phase prolonged the duration of the ongoing LF swing phase and the concomitant RF stance phase by 55 and 15%, respectively. Analysis of RF joint angle excursions showed that the shortening of the RF swing phase was related mainly to acceleration of extension movement in the late swing phase; the prolongation of the RF stance phase was related to prolonged extension movement in the late stance phase. While EMG activities were relevant to these limb movements, a notable observation was that, by tapping the LF during the LF stance phase, EMG activity in the RF extensor started well before onset of the elbow extension movement to place down the limb; without the tap, the extensor activity started shortly after onset of the extension. Closely related to changes in phase durations of each forelimb, the period of bisupport phase where both forelimbs were in stance, was retained for more than 40% of that of unperturbed steps, even when the RF or LF made the first touchdown after the tap. The rostrocaudal level at RF touchdown after the tap was comparable to unperturbed steps. These findings on interlimb relation suggest that neural control ensures coordinated movements between symmetric limbs during locomotion.Supported by grant no. 557033 from the Japanese Ministry of Education, Science, and Culture     

Stumbling corrective reaction during fictive locomotion in the cat

An obstacle contacting the dorsal surface of a cat's hind foot during the swing phase of locomotion evokes a reflex (the stumbling corrective reaction) that lifts the foot and extends the ankle to avoid falling. We show that the same sequence of ipsilateral hindlimb motoneuron activity can be evoked in decerebrate cats during fictive locomotion. As recorded in the peripheral nerves, twice threshold intensity stimulation of the cutaneous superficial peroneal (SP) nerve during the flexion phase produced a very brief excitation of ankle flexors (e.g., tibialis anterior and peroneus longus) that was followed by an inhibition for the duration of the stimulus train (10-25 shocks, 200 Hz). Extensor digitorum longus was always, and hip flexor (sartorius) activity was sometimes, inhibited during SP stimulation. At the same time, knee flexor and the normally quiescent ankle extensor motoneurons were recruited (mean latencies 4 and 16 ms) with SP stimulation during fictive stumbling correction. After the stimulus train, ankle extensor activity fell silent, and there was an excitation of hip, knee, and ankle flexors. The ongoing flexion phase was often prolonged. Hip extensors were also recruited in some fictive stumbling trials. Only the SP nerve was effective in evoking stumbling correction. Delivered during extension, SP stimulus trains increased ongoing extensor motoneuron activity as well as increasing ipsilateral hip, knee, and ankle hindlimb flexor activity in the subsequent step cycle. The fictive stumbling corrective reflex seems functionally similar to that evoked in intact, awake animals and involves a fixed pattern of short-latency reflexes as well as actions evoked through the lumbar circuitry responsible for the generation of rhythmic alternating locomotion.     

Modulation of heteronymous reflexes from ankle dorsiflexors to hamstring muscles during human walking

In 16 human subjects, stimulation of the common peroneal nerve (CPN) was applied during walking and standing. The effect of the stimulation was evaluated from the rectified and averaged biceps femoris (BF) electromyographic (EMG) activity. In the swing phase of walking, the CPN stimulation evoked a suppression in the BF EMG in 12 of the subjects. In the early stance phase, the suppression was replaced by facilitation at a similar latency in 9 of the subjects. Of the other 3 subjects, in whom a suppression was observed during swing, a decrease in the suppression was observed in the stance phase in two of them. During a voluntary co-contraction of BF and tibialis anterior while standing, a suppression similar to that observed in the swing phase was observed. The thresholds of the suppression and facilitation were identical, suggesting that afferents of similar diameter were responsible. Cutaneous stimuli, which mimicked the sensation evoked by the CPN stimulation, but without activation of muscle afferents, did not produce similar effects in the BF EMG activity. It is suggested that the observed response and reflex reversal may reflect opening of an excitatory group I pathway in the early stance phase of walking with a concomitant shut-down of heteronymous group I inhibition.     

Recurrent inhibition of the bladder C fibre reflex in the cat and its response to naloxone

Recurrent inhibition of the bladder C fibre reflex was studied in adult female cats anaesthetized with α-chloralose. Test reflexes were evoked by electrical stimulation of bladder Aδ and C afferents in the right pelvic nerve and were recorded from the proximal end of a small ipsilateral pelvic nerve branch, transected close to the bladder. Such test reflexes were consistently depressed by repetitive electrical stimulation of the contralateral bladder pelvic nerve (20 Hz, 20 s) at intensities sufficient to recruit axons of bladder preganglionic neurones. The inhibition could be evoked after transection of the left dorsal roots S1–S4 and the sympathetic supply to the bladder but was abolished by transection of the pelvic nerve central to the site of stimulation. Hence, it most likely involved central recurrent collaterals of antidromically activated bladder preganglionic neurones. The reflex suppression was quite considerable – maximal C fibre reflexes were reduced to a group mean of 25% (± 9% confidence interval) of their control size. The effect had a slow onset, requiring a few seconds of conditioning stimulation to be revealed, and was very long lasting (minutes). Naloxone (0.01–0.5 mg kg⁻¹ i.v. ) abolished the recurrent inhibition of both the C fibre and Aδ bladder reflexes, while inhibition from afferents in the dorsal clitoris nerve remained unchanged. It is concluded that the segmental bladder C fibre reflex and the spino-ponto-spinal Aδ micturition reflex are both targets of recurrent inhibition from bladder parasympathetic preganglionic neurones and that the effect involves an enkephalinergic mechanism.     

Phase-dependent transmission in the excitatory propriospinal reflex pathway from forelimb afferents to lumbar motoneurones during fictive locomotion

In high spinal paralyzed cats the effect of forelimb nerve stimulation on hindlimb motoneurones was investigated during fictive locomotion, which was induced by injection of nialamide and L-DOPA. The EPSPs which were evoked by forelimb nerve stimulation in almost all species of hindlimb motoneurones showed a distinct dependence on the phase of the step cycle. In motoneurones to extensor they were only observed during the extension phase, in those to flexors only during the flexion phase. It is assumed that the transmission in the descending propriospinal excitatory reflex pathway is cyclically modulated at the lumbar level.