Proprioceptive modulation of hip flexor activity during the swing phase of locomotion in decerebrate cats

This study examined the influence of proprioceptive input from hip flexor muscles on the activity in hip flexors during the swing phase of walking in the decerebrate cat. One hindlimb was partially denervated to remove cutaneous input and afferent input from most other hindlimb muscles. Perturbations to hip movement were applied either by 1) manual resistance or assistance to swing or by 2) resistance to hip flexion using a device that blocked hip flexion but allowed leg extension. Electromyographic recordings were made from the iliopsoas (IP), sartorius, and medial gastrocnemius muscles. When the hip was manually assisted into flexion, there was a reduction in hip flexor burst activity. Conversely, when hip flexion was manually resisted or mechanically blocked during swing, the duration and amplitude of hip flexor activity was increased. We also found some specificity in the role of afferents from individual hip flexor muscles in the modulation of flexor burst activity. If the IP muscle was detached from its insertion, little change in the response to blocking flexion was observed. Specific activation of IP afferent fibers by stretching the muscle also did not greatly affect flexor activity. On the other hand, if conduction in the sartorius nerves was blocked, there was a diminished response to blocking hip flexion. The increase in duration of the flexor bursts still occurred, but this increase was consistently lower than that observed when the sartorius nerves were intact. From these results we propose that during swing, feedback from hip flexor muscle afferents, particularly those from the sartorius muscles, enhances flexor activity. In addition, if we delayed the onset of flexor activity in the contralateral hindlimb, blocking hip flexion often resulted in the prolongation of ipsilateral flexor activity for long periods of time, further revealing the reinforcing effects of flexor afferent feedback on flexor activity. This effect was not seen if conduction in the sartorius nerves was blocked. In conclusion, we have found that hip flexor activity during locomotion can be strongly modulated by modifying proprioceptive feedback from the hip flexor muscles.     

Motor functions in rat hindlimb muscles following neonatal sciatic nerve crush.

The sciatic nerve was crushed in the right hindlimb in newborn (3-8 h old) rats. Two to four months later, electromyographic activity was recorded from both the control and reinnervated ankle extensor muscles soleus or lateral gastrocnemius and from the ankle flexor muscle tibialis anterior. Tonic postural activity was present in the extensor muscles on both sides during quiet stance. The control flexor muscles were usually silent in this situation, but the reinnervated flexors exhibited abnormal sustained activity. During locomotion, the control extensors were activated during the stance phase and their mean burst made up 61.5% of the step cycle. The control tibialis anterior muscle fired only during the swing phase, with the burst lasting 18.1% of the step cycle. In the reinnervated extensor muscles, the mean burst duration was decreased (46% of the cycle) but the basic locomotor pattern was not impaired. The reinnervated tibialis muscle, however, was activated abnormally, with one appropriate flexor burst during the swing phase and an "extensor-like" burst during the stance phase of the step. Reflex responses to stretch were weak or absent on the operated side. Histological examination showed that the reinnervated soleus and tibialis muscles were almost devoid of muscle spindles. The motor unit mean firing rates in the reinnervated soleus (22 imp/s) and lateral gastrocnemius (45 imp/s) matched those of the control muscles (25 and 42 imp/s, respectively). In contrast to the phasic, high-frequency firing (52-80 imp/s) in the control tibialis, the reinnervated tibialis motor units fired at significantly lower rates (22-56 imp/s).(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Critical points in the forelimb fictive locomotor cycle and motor coordination: effects of phasic retractions and protractions of the shoulder in the cat

This study investigates the responses to phasic shoulder retractions or protractions given at different times in the fictive locomotor cycle of the forelimbs of decerebrate cats. Generally, the responses in flexor and extensor muscles acting at the shoulder or elbow were bilaterally coordinated according to a negative feedback scheme. Perturbations in the direction of the movements that would have taken place if the animal had not been paralyzed tended to shorten the duration of the burst of activity of the muscles active during that phase and vice versa in the opposite phase. Changes in response patterns took place around critical points corresponding to the critical points B-D described in the companion paper using tonic perturbations of the limb. Past point C, at 58% of the ipsilateral extensor burst, protractions no longer prolonged the burst and no longer delayed onset of the contralateral extensor. At point B, occurring at 41% of the contralateral extensor burst, ipsilateral protractions maximally shortened the ipsilateral flexor phase, advancing ipsilateral extensor onset (point D) to point C of the contralateral extensor burst. During a critical period from the end of the ipsilateral flexor (point D) until the contralateral flexor onset, retractions elicited two alternative responses. Either the contralateral extensor activity was abolished and the contralateral flexor turned on, or it persisted for another cycle. We argue that the critical points found here correspond to critical biomechanical events in real locomotion and may underlie a phase-dependent motor coordination.     

Afferent roles in hindlimb wipe-reflex trajectories: free-limb kinematics and motor patterns

The hindlimb wiping reflex of the frog is an example of a targeted trajectory that is organized at the spinal level. In this paper, we examine this reflex in 45 spinal frogs to test the importance of proprioceptive afferents in trajectory formation at the spinal level. We tested hindlimb to hindlimb wiping, in which the wiping or effector limb and the target limb move together. Loss of afferent feedback from the wiping limb was produced by cutting dorsal roots 7-9. This caused altered initial trajectory direction, increased ankle path curvature, knee-joint velocity reversals, and overshooting misses of the target limb. We established that these kinematic and motor-pattern changes were due mainly to the loss of ipsilateral muscular and joint afferents. Loss of cutaneous afferents alone did not alter the initial trajectory up to target limb contact. However, there were cutaneous effects in later motor-pattern phases after the wiping and target limb had made contact: The knee extension or whisk phase of wiping was often lost. Finally, there was a minor and nonspecific excitatory effect of phasic contralateral feedback in the motor-pattern changes after deafferentation. Specific muscle groups were altered as a result of proprioceptive loss. These muscles also showed configuration-based regulation during wiping. Biceps, semitendinosus, and sartorius (all contributing knee flexor torques) all were regulated in amplitude based on the initial position of the limb. These muscles contributed to an initial electromyographic (EMG) burst in the motor pattern. Rectus internus and semimembranosus (contributing hip extensor torques) were regulated in onset but not in the time of peak EMG or in termination of EMG based on initial position. These two muscles contributed to a second EMG burst in the motor pattern. After deafferentation the initial burst was reduced and more synchronous with the second burst, and the second burst often was broadened in duration. Ankle path curvature and its degree of change after loss of proprioception depended on the degree of joint staggering used by the frog (i.e., the relative phasing between knee and hip motion) and on the degree of motor-pattern change. We examined these variations in 31 frogs. Twenty percent (6/31) of frogs showed largely synchronous joint coordination and little effect of deafferentation on joint coordination, end-point path, or the underlying synchronous motor pattern. Eighty percent of frogs (25/31) showed some degree of staggered joint coordination and also strong effects of loss of afferents. Loss of afferents caused two major joint level changes in these frogs: collapse of joint phasing into synchronous joint motion and increased hip velocity. Fifty percent of frogs (16/31) showed joint-coordination changes of type (1) without type (2). This change was associated with reduction, loss, or collapse of phasing of the sartorius, semitendinosus and biceps (iliofibularis) in the initial EMG burst in the motor pattern. The remaining 30% (9/31) of frogs showed both joint-coordination changes 1 and 2. These changes were associated with both the knee flexor EMG changes seen in the other frogs and with additional increased activity of rectus internus and semimembranosus muscles. Our data show that multiple ipsilateral modalities all play some role in regulating muscle activity patterns in the wiping limb. Our data support a strong role of ipsilateral proprioception in the process of trajectory formation and specifically in the control of limb segment interactions during wiping by way of the regulation and coordination of muscle groups based on initial limb configuration.     

Rhythmic fluctuations of dorsal root potentials and antidromic discharges of primary afferents during fictive locomotion in the cat

1. This study examines rhythmical activity of primary afferents occurring during "fictive" locomotion in decorticate paralyzed cats. Oscillations of the dorsal root potential (DRP) at the frequency of the locomotor rhythm have been observed at the lumbosacral and cervical levels. In addition, rhythmic antidromic discharges of primary afferent units have been recorded from the proximal stumps of cut dorsal root filaments. A detailed study of the relationships between the DRP fluctuations, the antidromic discharges, and the locomotor activity monitored by recording extensor and flexor muscle nerves is presented. 2. Typical DRP recordings from both lumbosacral and cervical levels show two negative waves (N1 and N2) separated by positive troughs (P1 and P2) in each locomotor cycle. Linear regression analyses indicate that the first negative wave (which generally has the largest amplitude) is related to the flexor activity whereas the second is related to the extensor activity. The relative amplitude of the two negative waves may vary without apparent concomitant changes in the recorded flexor or extensor motor nerves. The positive troughs occur respectively close to the period of transition between flexor and extensor activities and between extensor and flexor activities. 3. DRPs of similar period and amplitude can be observed in different ipsilateral roots recorded simultaneously. The DRPs recorded bilaterally from the same segment have the same periodicity but are out-of-phase. Point-to-point variations of amplitude in bilaterally recorded roots are not correlated. This suggests that the polarization of primary afferents on one side is mainly related to the locomotor events on that side. DRPs have been recorded in cats spinalized at Th13 and injected with nialamide and l-DOPA. This suggests that although the supraspinal contribution may be important, at least part of the DRPs may result from locomotor activity within the spinal cord itself. 4. A salient finding in our experiments was that of rhythmic antidromic unit discharges in the proximal stump of cut dorsal root filaments. Of the 194 units recorded, 19% (37/194) discharged in distinct bursts occurring at fixed times in the locomotor cycle. The majority of the units discharged either one burst during the period of flexor or extensor activity or one burst during one of the two periods of transition. Three units discharged two bursts per locomotor cycle. The frequency of the antidromic discharges of some units in one limb were also found to be modulated by stimulation of the skin or passive manipulation of the limbs.(ABSTRACT TRUNCATED AT 400 WORDS)     

The organization of primary afferent depolarization in the isolated spinal cord of the frog

1. The organization of primary afferent depolarization (PAD) produced by excitation of peripheral sensory and motor nerves was studied in the frog cord isolated with hind limb nerves.2. Dorsal root potentials from sensory fibres (DR-DRPs) were evoked on stimulation of most sensory nerves, but were largest from cutaneous, joint and flexor muscle afferents. With single shock stimulation the largest cutaneous and joint afferent fibres gave DR-DRPs, but potentials from muscle nerves resulted from activation of sensory fibres with thresholds to electrical stimulation higher than 1.2-1.5 times the threshold of the most excitable fibres in the nerve. This suggests that PAD from muscle afferents is probably due to excitation of extrafusal receptors.3. Dorsal root potentials produced by antidromic activation of motor fibres (VR-DRPs) were larger from extensor muscles and smaller or absent from flexor muscles. The VR-DRPs were produced by activation of the lowest threshold motor fibres.4. Three types of interactions were found between test and conditioning DRPs from the same or different nerves. With maximal responses occlusion was usually pronounced. At submaximal levels linear summation occurred. Near threshold the conditioning stimulus frequently resulted in a large facilitation of the test DRP. All three types of interactions were found with two DR-DRPs, two VR-DRPs or one DR-DRP and one VR-DRP.5. The excitability of sensory nerve terminals from most peripheral nerves was increased during the DR-DRP. The magnitude of the excitability increase varied roughly with the magnitude of the DR-DRP evoked by the conditioning stimulus.6. There was a marked excitability increase of cutaneous and extensor muscle afferent terminals during the VR-DRP. Flexor muscle afferent terminals often showed no excitability changes to ventral root stimulation. In those experiments where afferent terminals from flexor muscles did show an excitability increase, the effects were smaller than those of cutaneous and extensor terminals.7. The VR-DRPs appear to reflect activity of a negative feed-back loop from extensor motoneurones on to sensory fibres from cutaneous and extensor muscles. This system may have a role in modulating the ballistic movement of the frog. DR-DRPs, on the contrary, are widespread in origin and distribution. PAD from sensory fibres may function to sharpen contrast between incoming afferent information.     

Effect of hindlimb unloading on two hindlimb muscles during treadmill locomotion in rats

The purpose of the study was to examine the pattern of electromyographic (EMG) activity of the rat soleus (SOL) and tibialis anterior (TA) muscles during treadmill locomotion at various speeds after 7 days of hindlimb unloading (HU). Raw EMG signals were processed to determine cycle duration, burst duration and mean EMG (burst surface divided by its duration). Cycle duration and SOL burst duration increased after HU (+7% and +5%, respectively) while TA burst duration decreased (?16%). After HU, the alternating pattern of activity between extensor and flexor muscles was maintained. Nevertheless, a co-activation of the two muscles was sometimes observed. The EMG pattern revealed no difference in the timing of the coordination between flexor and extensor muscles after HU. The delay between TA offset and SOL onset was increased (+12 ms), but this increase could be explained by the decrease in TA burst duration. Neither TA burst duration nor TA mean EMG were changed with increased treadmill speed, so that the flexor muscle activity was not related to speed of locomotion. These results would suggest that SOL activity is centrally programmed. Moreover, it is proposed that a decline in afferent feedback from SOL in rats which are suspended has an effect upon the locomotor pattern, leading to an hyperexcitability of SOL motoneurons and, via reciprocal inhibition, to a reduction in TA activity.     

Contribution of sensory feedback to the generation of extensor activity during walking in the decerebrate Cat

In this investigation we have estimated the afferent contribution to the generation of activity in the knee and ankle extensor muscles during walking in decerebrate cats by loading and unloading extensor muscles, and by unilateral deafferentation of a hind leg. The total contribution of afferent feedback to extensor burst generation was estimated by allowing one hind leg to step into a hole in the treadmill belt on which the animal was walking. In the absence of ground support the level of activity in knee and ankle extensor muscles was reduced to approximately 70% of normal. Activity in the ankle extensors could be restored during the "foot-in-hole" trials by selectively resisting extension at the ankle. Thus feedback from proprioceptors in the ankle extensor muscles probably makes a large contribution to burst generation in these muscles during weight-bearing steps. Similarly, feedback from proprioceptors in knee extensor appears to contribute substantially to the activation of knee extensor muscles because unloading and loading these muscles, by lifting and dropping the hindquarters, strongly reduced and increased, respectively, the level of activity in the knee extensors. This conclusion was supported by the finding that partial deafferentation of one hind leg by transection of the L4-L6 dorsal roots reduced the level of activity in the knee extensors by approximately 50%, but did not noticeably influence the activity in ankle extensor muscles. However, extending the deafferentation to include the L7-S2 dorsal roots decreased the ankle extensor activity. We conclude that afferent feedback contributes to more than one-half of the input to knee and ankle extensor motoneurons during the stance phase of walking in decerebrate cats. The continuous contribution of afferent feedback to the generation of extensor activity could function to automatically adjust the intensity of activity to meet external demands.     

Proprioceptive input resets central locomotor rhythm in the spinal cat

Summary The reflex regulation of stepping is an important factor in adapting the step cycle to changes in the environment. The present experiments have examined the influence of muscle proprioceptors on centrally generated rhythmic locomotor activity in decerebrate unanesthetized cats with a spinal transection at Th12. Fictive locomotion, recorded as alternating activity in hindlimb flexor and extensor nerves, was induced by administration of nialamide (a monoamine oxidase inhibitor) and L-DOPA. Brief electrical stimulation of group I afferents from knee and ankle extensors were effective in resetting fictive locomotion in a coordinated fashion. An extensor group I volley delivered during a flexor burst would abruptly terminate the flexor activity and initiate an extensor burst. The same stimulus given during an extensor burst prolonged the extensor activity while delaying the appearance of the following flexor burst. Intracellular recordings from motoneurones revealed that these actions were mediated at premotoneuronal levels resulting from a distribution of inhibition to centres generating flexor bursts and excitation of centres generating extensor bursts. These results indicate that extensor group I afferents have access to central rhythm generators and suggest that this may be of importance in the reflex regulation of stepping. Experiments utilizing natural stimulation of muscle receptors demonstrate that the group I input to the rhythm generators arises mainly from Golgi tendon organ Ib afferents. Thus an increased load of limb extensors during the stance phase would enhance and prolong extensor activity while simultaneously delaying the transition to the swing phase of the step cycle.     

Peripheral control of the cat's step cycle

Acute low spinal and curarized cats injected with noradrenergic agonists i.v. can elicit an efferent burst pattern which can be recorded in muscle nerve filaments and can be referred to as “fictive locomotion”. This study investigates the effect that feedback, arising from movements in the hip joint, can exert on the central network generating fictive locomotion. The central network is uncoupled from generating any active movements by curarization. The motor pattern could be entrained by applying sinusoidal hip movements, even when a very extensive denervation of the leg had been performed leaving only some of the muscles around the hip and the hip joint innervated. During flexion movements, efferents to different flexor muscles became active and during movements in the reverse direction (extension), efferents to extensors were active. With an increasing movement frequency the onsets of both flexor and extensor bursts were delayed in the movement cycle. The duration of the extensor bursts varied markedly with the movement cycle, whereas pure flexors changed less in burst duration. The frequency range within which the efferent burst activity was entrained in a strict 1:1 relation to the movement varied between 5 to 70% above and below the resting burst period. In preparations with a narrow 1:1 range, a “relative coordination” was encountered outside this range. The flexor burst duration was in these cases dependent on where in the hip movement cycle the bursts appeared.     

Peripheral control of the cat's step cycle. II. Entrainment of the central pattern generators for locomotion by sinusoidal hip movements during "fictive locomotion."

Acute low spinal and curarized cats injected with noradrenergic agonists i.v. can elicit an efferent burst pattern which can be recorded in muscle nerve filaments and can be referred to as "fictive locomotion". This study investigates the effect that feedback, arising from movements in the hip joint, can exert on the central network generating fictive locomotion. The central network is uncoupled from generating any active movements by curarization. The motor pattern could be entrained by applying sinusoidal hip movements, even when a very extensive denervation of the leg had been performed leaving only some of the muscles around the hip and the hip joint innervated. During flexion movements, efferents to different flexor muscles became active and during movements in the reverse direction (extension), efferents to extensors were active. With an increasing movement frequency the onsets of both flexor and extensor bursts were delayed in the movement cycle. The duration of the extensor bursts varied markedly with the movement cycle, whereas pure flexors changed less in burst duration. The frequency within which the efferent burst activity was entrained in a strict 1:1 relation to the movement varied between 5 to 70% above and below the resting burst period. In preparations with a narrow 1:1 range, a "relative coordination" was encountered outside this range. The flexor burst duration was in these cases dependent on where in the hip movement cycle the bursts appeared.     

Functional organization within the medullary reticular formation of the intact unanesthetized cat. III. Microstimulation during locomotion.

1. This article presents the results from stimulation in 21 loci within the medullary reticular formation (MRF; between 0.5 and 2.5 mm from the midline) and in 5 loci in the medial longitudinal fasciculus (MLF) of four intact, unanesthetized cats during locomotion. Stimulus trains (11 pulses, 0.2-ms duration, 330 Hz, stimulus strength 35 microA) were applied at those loci in each track at which the most widespread effects in each of the four limbs were obtained with the cat at rest. Electromyograms were recorded from flexor and extensor muscles of each limb. 2. As previously reported, stimulation with the cat at rest generally evoked brief, short-latency, twitch responses in both flexor and extensor muscles of more than one limb. In contrast, stimulation during locomotion evoked a more complex pattern of activity in which responses were normally evoked in one or other of the muscle pairs and incorporated into the locomotor pattern. 3. In the majority of sites, the stimulation evoked excitatory responses in the flexor muscles of each of the four limbs during that period of the step cycle in which each respective muscle was naturally active; stimulation in the stance phase of locomotion, although less effective, was also capable of producing responses in these muscles. All three ipsilateral extensor muscles studied [long and lateral heads of triceps and vastus lateralis (Tri, TriL, and VL, respectively)] were normally inhibited during their phase of muscle activity, although excitatory responses were occasionally seen. Responses in the contralateral (co) Tri were invariably excitatory and were largest during the period of muscle activity, whereas responses during the period of activity of the coVL were mixed, with both excitatory and inhibitory responses being seen from any one locus. 4. Excitatory responses were normally largest when stimulation was applied during the time that the muscle was active during the locomotor cycle. Responses evoked at times when the muscle was inactive were sometimes larger than those evoked with the animal at rest; such responses were most commonly seen in the hindlimb flexors and in the coVL. 5. In both flexors and extensors of each of the four limbs, the latency of the responses was greatest when the cat was at rest and least for stimuli given during the period of activity of the respective muscle. Average latencies during the period of muscle activity ranged from a minimum of 9.0 +/- 2.6 (SD) ms for inhibitory responses in the ipsilateral Tri and TriL to a maximum of 17.1 +/- 3.0 ms for the responses evoked in the ipsilateral semitendinosus.(ABSTRACT TRUNCATED AT 400 WORDS)     

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.     

Locomotor activities in the decerebrate bird without phasic afferent input

We examined whether forelimb and hindlimb phasic afferent input is a prerequisite for the production of avian locomotor patterns. We eliminated phasic afferent feedback through paralysis of a decerebrate animal. The term "fictive" has been used to describe the neural activity associated with spontaneous or evoked motor output during neuromuscular paralysis. We observed that a paralysed decerebrate bird is capable of producing similar locomotor activity patterns as an unparalysed preparation, regardless of whether the "fictive" locomotion is generated spontaneously, or in response to focal electrical and/or neurochemical stimulation of discrete brainstem locomotor regions. Not all aspects of "fictive" locomotor patterns were identical to the locomotion elicited prior to paralysis. The stimulus current threshold necessary to evoke hindlimb locomotion increased from 69 +/- 22 mu A (mean +/- S.D.) prior to paralysis to 185 +/- 87 mu A for "fictive" stepping. For wing activity, the threshold increased from 84 +/- 46 mu A during wing flapping to 228 +/- 148 mu A for "fictive" flight. In addition, the frequency of "fictive" efferent locomotor activity from the leg nerve (1.04 +/- 0.44 Hz) decreased relative to the frequency of leg activity prior to paralysis (1.55 +/- 0.70 Hz). Similarly, the frequency of wing activity decreased from 2.73 +/- 0.73 Hz before paralysis to 1.8 +/- 0.69 Hz after paralysis. Finally flexor burst duration remained constant during treadmill and "fictive" walking while the extensor burst duration was markedly increased during "fictive" walking. Thus, the relative contributions of leg flexor activity to the overall step cycle (burst proportion = burst duration/cycle duration) decreased during evoked "fictive" stepping, while the burst proportion of the leg extensor increased. Afferent feedback therefore appears to modulate leg extensor burst duration more than leg flexor duration. For the wings, the burst proportion of the major wing depressors remained constant before and after paralysis.     

Paw-shake responses with joint immobilization: EMG changes with atypical feedback

Summary To determine the effects of atypical motion-related feedback on motor patterns of the paw shake, EMG patterns of selected flexor and extensor muscles were recorded under four conditions of joint immobilization (hip and ankle alone, hip-knee, hip-knee-ankle) and compared to responses evoked in the freely-moving hindlimb of the chronic-spinal cat. With only the ankle joint casted, paw shaking was easily evoked by applying tape to the paw, and cyclic characteristics were not altered. However, under the three conditions with hip-joint immobilization (hip alone, hip-knee, hip-knee-ankle), responses were difficult to obtain, and if elicited, the number of cycles within a response decreased and cycle periods were prolonged. The temporal organization of consecutive cycles, however, was not altered by immobilization of any joint(s). Ankle (LG) and hip (GM) extensor activity was relatively unaffected by conditions of joint immobilization. In contrast, hip flexor (IP) and knee extensor (VL) bursts were often absent under all three conditions of hip-joint immoblization, and if present, VL burst durations decreased under the casted hip-knee-ankle condition, while the onset of IP activity occurred early in the cycle with prolonged bursts under casted ankle and casted hip-knee-ankle conditions. The coactivity of the knee extensor (VL) and ankle flexor (TA) was disrupted by conditions of hip-joint immobilization: VL onset was dissociated from TA onset and coincident with LG onset. These results suggest that motion-related feedback from the hip joint is particularly important in the initiation, cycle frequency, and the number of cycles of paw-shake responses. The presence of atypical motion-dependent feedback from the hip joint altered activity of knee and ankle anterior muscles, while motion-dependent feedback from the ankle joint changed activity of the anterior hip muscle. Moreover, the results suggest a differential control of posterior and anterior muscles of the hindlimb, consistent with paw-shake limb dynamics.     

Central locomotor program for the cat's hindlimb

The central program for activation of different hindlimb muscles during ‘fictive locomotion’ in immobilized thalamic cats, expressed in the time structure of motor discharges in nerves to different muscles, was investigated. For the majority of hindlimb muscles this program was quite simple and consisted of alternating appearance of bursts of discharges in the nerves to flexors and extensors. The level of activity within individual bursts of discharges in the nerves to flexors and extensors was changed in time in an unpredictable manner. Correlation analysis of activity which appeared in the same phase in the nerves to flexors (or, respectively, extensors) showed that the maximum dependence between two concurrent bursts was observed at zero time shift. These data support Brown's hypothesis about the existence in each part of the spinal cord of two half-centers operating alternately during locomotion—one for the flexors and another for the extensors—which activate the motoneurones of the corresponding muscles. Activity in the nerves to biarticular muscles (m. semitendinosus, m. posterior biceps, m. tenuissimus) and in the nerves to m. extensor digitorum brevis and m. peroneus tertius was of a more complex nature. The central program of activation of these muscles depended on the intensity of ‘fictive locomotion’. A suggestion was made that the motor nuclei of these muscles are connected with the half-centers in a more complex manner.     

Vestibulospinal and reticulospinal neuronal activity during locomotion in the intact cat. I. Walking on a level surface

To examine the function of descending brain stem pathways in the control of locomotion, we have characterized the discharge patterns of identified vestibulo- and reticulospinal neurons (VSNs and RSNs, respectively) recorded from the lateral vestibular nucleus (LVN) and the medullary reticular formation (MRF), during treadmill walking. Data during locomotion were obtained for 44 VSNs and for 63 RSNs. The discharge frequency of most VSNs (42/44) was phasically modulated in phase with the locomotor rhythm and the averaged peak discharge frequency ranged from 41 to 165 Hz (mean = 92.8 Hz). We identified three classes of VSNs based on their discharge pattern. Type A, or double peak, VSNs (20/44 neurons, 46%) showed two peaks and two troughs of activity in each step cycle. One of the peaks was time-locked to the activity of extensor muscles in the ipsilateral hindlimb while the other occurred anti-phase to this period of activity. Type B, or single pause, neurons (13/44 neurons, 30%) were characterized by a tonic or irregular discharge that was interrupted by a single pronounced and brief period of decreased activity that occurred just before the onset of swing in the ipsilateral hindlimb; some type B VSNs also exhibited a brief pulse of activity just preceding this decrease. Type C, or single peak, neurons (9/44 neurons, 23%) exhibited a single period of increased activity that, in most cells, was time-locked to the burst of activity of either extensor or flexor muscles of a single limb. The population of RSNs that we recorded included neurons that showed phasic activity related to the activity of flexor or extensor muscles [electromyographically (EMG) related, 26/63, 41%], those that were phasically active but whose activity was not time-locked to the activity of any of the recorded muscles (13/63, 21%) and those that were completely unrelated to locomotion (24/63, 38%). Most of the EMG-related RSNs showed one (15/26) or two (11/26) clear phasic bursts of activity that were temporally related to either flexor or extensor muscles. The discharge pattern of double-burst RSNs covaried with ipsilateral and contralateral flexor muscles. Peak averaged discharge activity in these EMG-related RSNs ranged from 4 to 98 Hz (mean = 35.2 Hz). We discuss the possibility that most VSNs regulate the overall activity of extensor muscles in the four limbs while RSNs provide a more specific signal that has the flexibility to modulate the activity of groups of flexor and extensor muscles, in either a single or in multiple limbs.     

Simultaneous control of two rhythmical behaviors. II. Hindlimb walking with paw-shake response in spinal cat

The simultaneous control of the hindlimb paw-shake response and hindlimb walking at slow treadmill speeds (0.2-0.4 m/s) was examined in adult cats spinalized at the T12 level, 3-6 mo earlier. Paw shaking was elicited by either 1) application of adhesive tape or 2) water to the right hindpaw. To assess intralimb and interlimb coordination of the combined behaviors, activity from selected flexor and extensor muscles at the hip, knee, and ankle was recorded, and the kinematics of these joints were determined from high-speed cinefilm. When paw shaking was combined with hindlimb walking, the response in the stimulated limb was initiated during swing (F phase) of the step cycle. The onset of knee extensor activity provided the transition from the flexor synergy of swing to the mixed synergy of paw shake. At the end of the paw shake, an extensor synergy initiated the E-1 phase of swing, and the resultant joint motion was in-phase extension at the hip, knee, and ankle to lower the paw for contact with the treadmill belt. During the rapid (81 ms) paw-shake cycles, knee extensor and ankle flexor muscles exhibited single, coactive bursts that were reciprocal with coactive hip and ankle extensor bursts. This mixed synergy was reflected in the limb coordination, as knee flexion coincided with ankle extension and knee flexion coincided with ankle extension. Phasing of hip motions was variable, reflecting the role of the proximal in stabilization during paw shake (16). Although the number of paw-shake cycles combined during swing varied greatly from 2 to 14, average cycle periods, burst durations, and intralimb synergies were similar to those previously reported for spinal cats tested under conditions in which the trunk was suspended and hindlimbs were pendent (23, 27). For step cycles during which a long paw-shake response of 8-14 cycles occurred, swing duration of the shaking limb increased by 1 s, and during this prolonged interval, the contralateral hindlimb completed two support steps. Stance duration of the support steps was also prolonged. This adjustment maximized the duration of paw-contact and minimized any period of nonsupport by the contralateral hindlimb during paw shake. Completion of the paw-shake response was followed by either an alternating, or a nonalternating, gait pattern on the recovery steps. One spinal cat combined locomotion with short two-cycle paw-shake responses, and because the shortened response was limited primarily to the time ordinarily devoted to swing, interlimb adjustments were slight.(ABSTRACT TRUNCATED AT 400 WORDS)     

Development of coordinated movement in chicks: I. Temporal analysis of hindlimb muscle synergies at embryonic days 9 and 10

In this study, we examined electromyographic activity for an ensemble of hindlimb muscles during spontaneous activity in chick embryos to advance understanding of early motor coordination and its relationship to later emerging behaviors. Four-channel recordings were obtained from 6 muscles in ovo at embryonic Days 9 and 10. Analyses indicated that when muscles are repetitively active, patterns during embryonic motility are distinct from those for other behaviors. For example, unlike the muscle patterns for locomotion, extensor muscles and flexor muscles are synchronously activated at 50% of the extensor cycle period. Furthermore, flexor and extensor bursts are similar in duration and show little correlation with extensor cycle period. Finally, our data suggest that the ensemble of muscles active can vary from cycle to cycle. This study provides the basis for future studies that will examine neural and biomechanical interactions underlying the development of coordinated movement.