Modulation of short latency cutaneous excitation in flexor and extensor motoneurons during fictive locomotion in the cat

Summary We examined modulation of transmission in short-latency, distal hindlimb cutaneous reflex pathways during fictive locomotion in 19 decerebrate cats. Fictive stepping was produced either by electrical stimulation of the mesencephalic locomotor region (MLR) or by administration of Nialamide and 1-DOPA to acutely spinalized animals. Postsynaptic potentials (PSPs) produced by electrical stimulation of low threshold afferents (< 2.5 times threshold) in the superficial peroneal (SP), sural, saphenous or medial plantar nerves were recorded intracellularly from various extensor (n = 28) and flexor (n = 24) motoneurons and averaged throughout the step cycle, together with voltage responses to intrasomatic constant current pulses (in order to monitor relative cell input resistance). Each motoneuron studied displayed rhythmic background oscillations in membrane potential and correlated variations in input resistance. The average input resistance of extensor motoneurons was lowest during mid-flexion, when the cells were relatively hyperpolarized and silent. Conversely, average input resistance of flexor motoneurons was highest during mid-flexion, when they were depolarized and active. The amplitude of the minimum-latency excitatory components of PSPs produced by cutaneous nerve stimulation were measured from computer averaged records representing six subdivisions of the fictive step cycle. Oligosynaptic EPSP components were consistently modulated only in the superficial peroneal responses in flexor motoneurons, which exhibited enhanced amplitude during the flexion phase. With the other skin nerves tested (sural, saphenous, and plantar), no consistent patterns of modulation were observed during fictive locomotion. We conclude that transmission through some, but not all, oligosynaptic excitatory cutaneous pathways is enhanced by premotoneuronal mechanisms during the flexion phase of fictive stepping in several cat hindlimb motor nuclei. The present results suggest that the patterns of interaction between the locomotor central pattern generator and excitatory cutaneous reflex pathways depend on the source of afferent input and on the identity of the target motoneuron population.     

Phasic modulation of short latency cutaneous excitation in flexor digitorum longus motoneurons during fictive locomotion

Summary We examined modulation of transmission of short-latency excitation produced by distal hindlimb cutaneous input, as well as fluctuations in motoneuron membrane potential and input resistance, in flexor digitorum longus (FDL) motoneurons during fictive locomotion. Fictive stepping was induced in unaesthetized, decerebrate cats either by repetitive stimulation of the mesencephalic locomotor region (MLR) or by administration of Nialamide and 1 DOPA after low spinal section. In the MLR preparations, brief depolarizing waves occurred in FDL cells during the early flexion phase of fictive stepping, immediately after cessation of activity in extensor muscles. In some FDL cells, plateau-like depolarizations also occurred during the extensor phase. Fictive stepping induced in acutely spinalized cats by administration of l-DOPA was slower and more variable; peak polarization in FDL motoneurons always occurred during the early flexion phase but there was usually no distinct depolarization during extension. In both types of preparation, the initial EPSP components in synaptic potentials (SP-EPSPs) produced by electrical stimulation of the cutaneous division of the superficial peroneal nerve (SP) were maximally facilitated during early flexion, coincident with the peak of background depolarization. This enhancement was manifested by an increase in the amplitude of initial SP-EPSP components or by decreased central latency of the initial EPSP components, or both. In most FDL motoneurons, input resistance decreased systematically during late flexion, coincident with relative membrane hyperpolarization. Correction of SP-EPSP amplitudes for changes in input resistance suggested that SP-EPSP facilitation persisted throughout the flexion phase These findings are discussed with reference to modulation of cutaneous reflexes during locomotion and the possibility that excitatory last-order interneurons in particular cutaneous reflex pathways may distribute excitatory drive from the central pattern generator for locomotion to FDL -motoneurons     

Peripheral and central control of flexor digitorum longus and flexor hallucis longus motoneurons: The synaptic basis of functional diversity

Summary The two long toe flexor muscles in the cat, flexor digitorum longus (FDL) and flexor hallucis longus (FHL), have essentially identical mechanical actions, yet are used very differently during locomotion (O'Donovan et al. 1982). We attempted to identify the origin of the synaptic drive responsible for this functional differentiation.The organization of peripheral and central synaptic drive to FDL and FHL motoneurons was examined using two basic paradigms. (1) In animals anesthetized with chloralose or after ischemic destruction of the brain, peripheral reflex circuits were studied by recording intracellular responses from -motoneurons produced by electrical stimulation of muscular and cutaneous nerves. (2) Fictive locomotion, the centrally generated rhythmic synaptic drive produced in paralyzed, decerebrate animals by stimulation of the mesencephalic locomotor region or intravenous injection of L-DOPA and Nialamide, was monitored by recording electro-neurograms from the central end of cut motor nerves.Despite their functional dissimilarity, FDL and FHL motoneurons received monosynaptic EPSPs from both FDL and FHL la afferents. Ipsilateral cutaneous afferents in the sural nerve and from the central plantar pad produced multiphasic PSPs which were not different in FDL and FHL cells. However afferents from the saphenous and superficial peroneal nerves did exert differential effects: the first component of the multiphasic PSP in most FDL cells was an EPSP, which was not present in most FHL cells. The central latency of this early EPSP in FDL motoneurons (0.8–1.5 ms) strongly suggests a disynaptic linkage. Cutaneous afferents from the ipsilateral forelimb produced IPSPs in most FHL cells but in only one of 18 FDL cells. Since some peripheral reflex circuits exerted differential effects on FDL and FHL cells, but others did not, the intracellular data did not demonstrate that the functional differences between FDL and FHL could be explained by differences in reflex organization.During fictive locomotion elicited by electrical or pharmacological stimulation, FHL motoneurons were coactive with ankle extensors during the extension phase of the fictive step cycle. In contrast, FDL motoneurons were most consistently activated in a brief burst at the onset of the flexion phase, showing much weaker and more variable coactivity with ankle extensors. These patterns were essentially identical to those reported for FDL and FHL motor pools during treadmill locomotion by O'Donovan et al. (1982).We conclude that the central pattern generator (CPG) for locomotion produces distinct and highly differentiated sets of instructions for FDL and FHL motoneurons. Peripheral and descending systems are important in initiating and biasing the activity of the CPG, but are not responsible for the intrinsic structure of the locomotor command signals.     

Modelling spinal circuitry involved in locomotor pattern generation: insights from the effects of afferent stimulation

A computational model of the mammalian spinal cord circuitry incorporating a two-level central pattern generator (CPG) with separate half-centre rhythm generator (RG) and pattern formation (PF) networks has been developed from observations obtained during fictive locomotion in decerebrate cats. Sensory afferents have been incorporated in the model to study the effects of afferent stimulation on locomotor phase switching and step cycle period and on the firing patterns of flexor and extensor motoneurones. Here we show that this CPG structure can be integrated with reflex circuits to reproduce the reorganization of group I reflex pathways occurring during locomotion. During the extensor phase of fictive locomotion, activation of extensor muscle group I afferents increases extensor motoneurone activity and prolongs the extensor phase. This extensor phase prolongation may occur with or without a resetting of the locomotor cycle, which (according to the model) depends on the degree to which sensory input affects the RG and PF circuits, respectively. The same stimulation delivered during flexion produces a temporary resetting to extension without changing the timing of following locomotor cycles. The model reproduces this behaviour by suggesting that this sensory input influences the PF network without affecting the RG. The model also suggests that the different effects of flexor muscle nerve afferent stimulation observed experimentally (phase prolongation versus resetting) result from opposing influences of flexor group I and II afferents on the PF and RG circuits controlling the activity of flexor and extensor motoneurones. The results of modelling provide insights into proprioceptive control of locomotion.     

Functionally complex muscles of the cat hindlimb

Similarities between the muscle synergies associated with the flexion reflex and locomotion in reduced preparations have suggested that spinal circuits subserving these two motor tasks might share common interneurons. To test this hypothesis in functionally complex muscles, we studied the interaction between low-threshold cutaneous afferents and the locomotor central pattern generator (CPG) during treadmill locomotion in awake, intact cats. Electrical stimuli were delivered via implanted nerve cuff electrodes at all phases of locomotion, and EMGs were recorded from fourteen intramuscular subregions in eight bifunctional thigh muscles (adductor femoris, biceps femoris, caudofemoralis, gracilis, semimembranosus, semitendinosus, tensor fasciae latae, and tenuissimus). In addition, the EMG patterns recorded during locomotion were compared with those recorded during two other centrally driven rhythmical behaviors, scratching and paw shaking, to determine whether the functional relationships among these intramuscular subregions were fixed or task dependent. Four of the five broad, bifunctional muscles studied (biceps femoris, gracilis, semimembranosus, and tensor fasciae latae) had functional subunits that could be differentially activated in one or more of the three movements studied; adductor femoris was consistently uniformly activated despite its distributed skeletal attachments. The pattern of recruitment of the intramuscular functional subunits was movement-specific. The locomotor CPG and cutaneous reflex pathways both similarly subdivided some bifunctional muscles, but not others, into intramuscular subregions. The results of the present study confirm that some combinations of muscle subregions and cutaneous nerves constitute simple reciprocal categories of flexors and extensors, as described originally by Sherrington (1910). "Typical" low threshold excitatory or inhibitory reflex responses were produced in muscles or muscle subregions that were recruited as "net" flexors of extensors, respectively. However, muscles with complex activation patterns during walking often had very individualized, complex reflex responses during locomotion that did not conform to the background locomotion synergies. All of the reflex responses observed were mediated by low threshold cutaneous afferents. These data indicate that there are multiple, low threshold, excitatory and inhibitory cutaneous reflex pathways that have highly specialized connections with flexor and extensor muscles and even their intramuscular subregions. It is also clear that the premotoneuronal circuits mediating these cutaneous reflex effects are not necessarily synonymous with those of the locomotor CPG. These two systems do interact powerfully, however, suggesting some convergence. The nature of the convergence between the CPG and the many independent subsets of spinal interneurons mediating cutaneous reflexes is specialized and muscle subregion-specific.     

The use of state-dependent modulation of spinal reflexes as a tool to investigate the organization of spinal interneurons

This review examines the proposition that state-dependent modulation of transmission through spinal reflex pathways can be used as an investigative tool to reveal details about the organization of spinal interneurons into functional circuits. The first set of examples includes the use of spinal and supraspinal lesions, as well as the administration of the drug l-dihydroxyphenylalanine (l-DOPA), to produce different, relatively stable ”states” of the central nervous system (CNS), revealing previously unsuspected spinal pathways activated by the flexor reflex afferents (FRA). The second set of examples deals with the use of fictive locomotion and scratching to investigate the organization of oligosynaptic excitatory and inhibitory reflex pathways from cutaneous and muscle afferents. As in the first set of examples, several hitherto unknown reflex pathways have been found only during the flexion or extension phases of rhythmic locomotion, which are regarded as different CNS states. Differences in the patterns of control can be used to infer the existence of distinct sets of reflex pathway interneurons that have remarkably precise input/output relations. Received: 4 February 1999 / Accepted: 19 April 1999     

Parallel reflex pathways from flexor muscle afferents evoking resetting and flexion enhancement during fictive locomotion and scratch in the cat

Reflex actions of muscle afferents in hindlimb flexor nerves were examined on ipsilateral motoneurone activity recorded in peripheral nerves during midbrain stimulation-evoked fictive locomotion and during fictive scratch in decerebrate cats. Trains of stimuli (15–30 shocks at 200 Hz) were delivered during the flexion phase at intensities sufficient to activate both group I and II afferents (5 times threshold, T ). In many preparations tibialis anterior (TA) nerve stimulation terminated ongoing flexion and reset the locomotor cycle to extension (19/31 experiments) while extensor digitorum longus (EDL) stimulation increased and prolonged the ongoing flexor phase activity (20/33 preparations). The effects of sartorius, iliopsoas and peroneus longus muscle afferent stimulation were qualitatively similar to those of EDL nerve. Resetting to extension was seen only with higher intensity stimulation (5 T ) while ongoing flexor activity was often enhanced at group I intensity (2 T ) stimulation. The effects of flexor nerve stimulation were qualitatively similar during fictive scratch. Reflex reversals were consistently observed in some fictive locomotor preparations. In those cases, EDL stimulation produced a resetting to extension and TA stimulation prolonged the ongoing flexion phase. Occasionally reflex reversals occurred spontaneously during only one of several stimulus presentations. The variable and opposite actions of flexor afferents on the locomotor step cycle indicate the existence of parallel spinal reflex pathways. A hypothetical organization of reflex pathways from flexor muscle afferents to the spinal pattern generator networks with competing actions of group I and group II afferents on the flexor and extensor portions of this central circuitry is proposed.     

An ascending spinal pathway transmitting a central rhythmic pattern to the magnocellular red nucleus in the cat

The activity of cells in the magnocellular red nucleus (RNm) was recorded extra and intracellularly in the curarized thalamic cat performing fictive locomotion. The locomoter episodes were detected from the rhythmic activity recorded in the motor nerves of the contralateral hindlimb. It was confirmed that, during fictive locomotion, a large proportion of the rubrospinal cells (56% in our sample) exhibit a rhythmic pattern of activity which is synchronized with the efferent spinal motor nerve activity. On the basis of the intracellular recordings it was established that phases of intense synaptic activity with mixed excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) are involved in this rhythmicity. After eliminating the cerebellar input to the RNm, it was observed that the cells still received intense excitatory and inhibitory inputs, resulting in a continuous modulation of their membrane potential, due to the occurrence of EPSPs and IPSPs. During fictive locomotor like activity and after elimination of the cerebellar afferents to the RNm, it was observed that the spontaneous PSPs in RNm cells (in the case of 45% of the cells) were organized in repetitive subthreshold bursts occurring in phase relationships with the activity recorded in the motor nerves. Some extracellularly recorded cells (12%) showed a rhythmic firing pattern. It is generally recognized that, in the thalamic cat preparation, the locomotor pattern observed in efferent nerves originates from the central pattern generator (CPG) of the spinal cord. It therefore seems likely that the rhythmicity observed here in the RNm may originate from the spinal CPG and be transmitted through the spino rubral pathway ascending in the ventral part of the cord. It is concluded that the spino rubral pathway may transmit both somatosensory information and corollary discharges relating to the activity of the spinal CPG for locomotion.     

Intra-axonal recordings of cutaneous primary afferents during fictive locomotion in the cat

1. Cutaneous primary afferents were recorded intracellularly during fictive locomotion in decorticated cats with the goal of improving our understanding of how locomotor networks might centrally control the transmission in cutaneous pathways at a presynaptic level. 2. Identified cutaneous axons from superficialis peroneal nerve (SP) or tibialis posterior nerve (TP) were recorded intracellularly together with the electroneurograms (ENGs) of representative flexor and extensor muscle nerves of the hindlimb as well as dorsal root potential from L6 or L7 (DRP). Fictive locomotion occurred spontaneously after decortication (n = 12) or was induced by stimulation of the mesencephalic locomotor region (MLR) (n = 6). 3. The results revealed that all cutaneous axons (82 units with resting potential greater than 45 mV) showed fluctuations of their membrane potential (greater than or equal to 0.5 mV) at the rhythm of the fictive locomotion. The characteristics of fluctuation patterns, common to all cutaneous units, consisted of two depolarization waves per cycle: one related to the flexor activity, the other related to the extensor activity. The flexor-related depolarization was followed by a sharp trough of membrane repolarization. The extensor-related depolarization usually overlapped partly with the flexor-depolarization of the following cycle. The relative size of each depolarization could vary among different afferents of the same nerve in the same animal. Hence, maximal depolarization could occur in different parts of the locomotor cycle, but, for the majority of units (82%), it occurred during the flexor activity. These results were similar for SP and TP units. 4. Twenty percent of the units were discharging with a constant or irregular frequency. Phasic antidromic discharges related to locomotor ENGs were rarely encountered (5/82 units). 5. Linear regression analysis of the temporal relationships between fluctuations of membrane potential of cutaneous axons and locomotor bursts over several cycles showed that the timing of presynaptic events in cutaneous afferents is related to the events of the locomotor output. However, the same type of analysis showed that the amplitude of axonal depolarizations and the amplitude of flexor and extensor locomotor bursts could vary independently. Tight temporal relationships were also found between the depolarizations recorded in cutaneous units and the fluctuations recorded at the dorsal root level (DRP). 6. Based on the assumption that the locomotor fluctuations of cutaneous membrane potential are mediated through the primary afferent depolarization (PAD) pathways associated with presynaptic inhibition, it is proposed that the central pattern generator for locomotion (CPG) could phasically control the efficacy of transmission of cutaneous pathways at a presynaptic level as part of the locomotor program.     

Stretch and H reflexes in triceps surae are similar during tonic and rhythmic contractions in high decerebrate cats

During locomotion in decerebrate and spinal cats the group Ia afferents from hind leg muscles are depolarized rhythmically. An earlier study concluded that this locomotor-related primary afferent depolarization (PAD) does not contribute to modulation of monosynaptic reflex pathways during locomotion. This finding indicated that the neural network generating the locomotor rhythm, the central pattern generator (CPG), does not presynaptically inhibit monosynaptic reflexes. In this investigation we tested this prediction in decerebrate cats by measuring the magnitude of reflexes evoked in ankle extensor muscles during periods of tonic contractions and during sequences of rhythmic contractions. The latter occurred when the animal was induced to walk on a treadmill. At the similar levels of activity in the soleus muscle there was no significant difference in the magnitude of the soleus H reflex in these two behavioral situations. Similar results were obtained for reflexes evoked by brief stretches of the soleus muscle. We also examined the reflexes evoked by ramp-and-hold stretches during periods of rhythmic and tonic activity of the isolated medial gastrocnemius (MG) muscle. At similar levels of background activity, the reflexes evoked in the MG muscle were the same during rhythmic and tonic contractions. Our failure to observe a reduction in the magnitude of H reflexes and stretch reflexes during rhythmic contractions, compared with reflexes evoked at the same level of background activity during tonic contractions, is consistent with the notion that the CPG for stepping does not presynaptically inhibit monosynaptic reflexes during the extension phase of locomotor activity. Our results indicate that presynaptic inhibition of the monosynaptic reflex associated with normal locomotion in cats or humans arises from sources other than the extensor burst generating system of the central pattern generator.     

5-HT prolongs ventral root bursting via presynaptic inhibition of synaptic activity during fictive locomotion in lamprey

Locomotor pattern generation is maintained by integration of the intrinsic properties of spinal central pattern generator (CPG) neurons in conjunction with synaptic activity of the neural network. In the lamprey, the spinal locomotor CPG is modulated by 5-HT. On a cellular level, 5-HT presynaptically inhibits synaptic transmission and postsynaptically inhibits a Ca2+-activated K+ current responsible for the slow afterhyperpolarization (sAHP) that follows action potentials in ventral horn neurons. To understand the contribution of these cellular mechanisms to the modulation of the spinal CPG, we have tested the effect of selective 5-HT analogues against fictive locomotion initiated by bath application of N-methyl-d-aspartate (NMDA). We found that the 5-HT1D agonist, L694-247, dramatically prolongs the frequency of ventral root bursting. Furthermore, we show that L694-247 presynaptically inhibits synaptic transmission without altering postsynaptic Ca2+-activated K+ currents. We also confirm that 5-HT inhibits synaptic transmission at concentrations that modulate locomotion. To examine the mechanism by which selective presynaptic inhibition modulates the frequency of fictive locomotion, we performed voltage- and current-clamp recordings of CPG neurons during locomotion. Our results show that 5-HT decreases glutamatergic synaptic drive within the locomotor CPG during fictive locomotion. Thus we conclude that presynaptic inhibition of neurotransmitter release contributes to 5-HT-mediated modulation of locomotor activity.     

Intracellular analysis of reflex pathways underlying the stumbling corrective reaction during fictive locomotion in the cat

In cat and humans, contact between an obstacle and the dorsum of the foot evokes the stumbling corrective reaction (reflex) that lifts the foot to avoid falling. This reflex can also be evoked by short trains of stimuli to the cutaneous superficial peroneal (SP) nerve in decerebrate cats during the flexion phase of fictive locomotion. Here we examine intracellular events in hindlimb motoneurons accompanying stumbling correction. SP stimulation delivered during the flexion phase excites knee flexor motoneurons at short latency [minimum excitatory postsynaptic potential (EPSP) latency 1.8 ms; mean 2.7 ms]. Although a similar short latency excitation occurs in ankle extensors (mean latency, 2.8 ms), recruitment is delayed until successive shocks in the stimulus train overcome the locomotor-related hyperpolarization of ankle extensors. In ankle flexor motoneurons, SP stimulation evokes an inhibition (mean latency, 2.7 ms) that briefly reduces or stops their firing during the flexion phase. There is a phase-dependent modulation of SP-evoked EPSP amplitude as well as latency during locomotion. However, the more obvious change in SP reflex pathways with the onset of fictive locomotion is the reduced inhibition of ankle extensor motoneurons and the increased inhibition of ankle flexors. These results show that the characteristic pattern of hindlimb motoneuron activation during SP nerve-evoked stumbling correction results from 1) di- and trisynaptic excitation of knee flexor and ankle extensor motoneurons; 2) increased inhibitory postsynaptic potentials in ankle flexors and a suppression of inhibition in extensors, 3) sculpting of the short-latency SP postsynaptic effects by motoneuron membrane potential, and 4) longer latency excitatory effects that are likely evoked by lumbar interneurons involved in the generation of fictive locomotion.     

Excitability changes of ankle extensor group Ia and Ib fibers during fictive locomotion in the cat

Summary The present study examines the modulation of gastrocnemius-soleus (GS) monosynaptic reflexes as well as the intraspinal threshold changes of GS group I primary afferent terminals ending in the intermediate and motor nuclei during fictive locomotion in high decerebrate cats. The amplitude of the monosynaptic reflexes (MSR's) evoked in the medial gastrocnemius by stimulation of the lateral gastrocnemius nerve was increased during the extensor (E) phase, decreased during the flexion (F) phase of the step cycle and remainded transiently increased after spontaneous episodes of fictive stepping. The intraspinal threshold of populations and of single group Ia GS afferent fibers ending in the motor pool, as well as of single Ia and Ib fibers ending in the intermediate nucleus, showed a sustained reduction during the episodes of fictive locomotion with superimposed cyclic changes in phase with the step cycle. During fictive walking and trotting the reduction of the intraspinal threshold of both Ia and Ib fiber terminals was maximal during the middle or late portion of the F-phase. During fictive gallop elicited by stimulation of the superficial peroneus nerve, the decrease in the intraspinal threshold of the Ia afferent fibers occurred however in phase with the activity of the GS motoneurons. During episodes of fictive locomotion slow, sustained negative DC potential shifts lasting tents of seconds, reflecting an increase in the extracellular potassium concentration were recorded at the base of the dorsal horn and in the intermediate nucleus. The present findings support the existence of tonic and phasic depolarization of the intraspinal terminals of GS group Ia and Ib primary afferents during spontaneous fictive locomotion. It is suggested that accumulation of potassium ions in the extracellular space contributes mainly to the sustained depolarization of group I fibers. The phasic depolarization would be mostly due to the activation of specific sets of interneurons and may, in the case of Ia fibers, contribute to the cyclic modulation of the MRS elicited during fictive locomotion.     

Functional characterization of dI6 interneurons in the neonatal mouse spinal cord

Our understanding of the neural control of locomotion has been greatly enhanced by the ability to identify and manipulate genetically defined populations of interneurons that comprise the locomotor central pattern generator (CPG). To date, the dI6 interneurons are one of the few populations that settle in the ventral region of the postnatal spinal cord that have not been investigated. In the present study, we utilized a novel transgenic mouse line to electrophysiologically characterize dI6 interneurons located close to the central canal and study their function during fictive locomotion. The majority of dI6 cells investigated were found to be rhythmically active during fictive locomotion and could be divided into two electrophysiologically distinct populations of interneurons. The first population fired rhythmic trains of action potentials that were loosely coupled to ventral root output and contained several intrinsic membrane properties of rhythm-generating neurons, raising the possibility that these cells may be involved in the generation of rhythmic activity in the locomotor CPG. The second population fired rhythmic trains of action potentials that were tightly coupled to ventral root output and lacked intrinsic oscillatory mechanisms, indicating that these neurons may be driven by a rhythm-generating network. Together these results indicate that dI6 neurons comprise an important component of the locomotor CPG that participate in multiple facets of motor behavior.     

Phasic control of the transmission in the excitatory and inhibitory reflex pathways from cutaneous afferents to α-motoneurones during fictive locomotion in cats

In high spinal paralyzed cats the effect of cutaneous nerve stimulation on lumbar motoneurons was investigated during fictive locomotion. EPSPs evoked from the cutaneous afferents were generally larger during the active phase of the motoneurones, while IPSPs tended to increase during the reciprocal phase. In some cases EPSPs occurred during the active phase, while IPSPs dominated during the reciprocal phase. Apparently, the transmission in the excitatory and inhibitory segmental reflex pathways from cutaneous afferents to α-motoneurones depends on the phase of the step cycle, but there is no general phase dependent alternating switching between these two pathways.     

Serotonin modulates the properties of ascending commissural interneurons in the neonatal mouse spinal cord

The interneuron populations that constitute the central pattern generator (CPG) for locomotion in the mammalian spinal cord are not well understood. We studied the properties of a set of commissural interneurons whose axons cross and ascend in the contralateral cord (aCINs) in the neonatal mouse. During N-methyl-D-aspartate (NMDA) and 5-HT-induced fictive locomotion, a majority of lumbar (L2) aCINs examined were rhythmically active; most of them fired in phase with the ipsilateral motoneuron pool, but some fired in phase with contralateral motoneurons. 5-HT plays a critical role in enabling the locomotor CPG to function. We found that 5-HT increased the excitability of aCINs by depolarizing the membrane potential, reducing the postspike afterhyperpolarization amplitude, broadening the action potential, and decreasing the action potential threshold. Serotonin had no significant effect on the input resistance and sag amplitude of aCINs. These results support the hypothesis that aCINs play important roles in coordinating left-right movements during fictive locomotion and thus may be component neurons in the locomotor CPG in neonatal mice.     

Asymmetric control of cycle period by the spinal locomotor rhythm generator in the adult cat

During walking, a change in speed is accomplished by varying the duration of the stance phase, while the swing phase remains relatively invariant. To determine if this asymmetry in the control of locomotor cycles is an inherent property of the spinal central pattern generator (CPG), we recorded episodes of fictive locomotion in decerebrate cats with or without a complete spinal transection (acute or chronic). During fictive locomotion, stance and swing phases typically correspond to extension and flexion phases, respectively. The extension and flexion phases were determined by measuring the duration of extensor and flexor bursts, respectively. In the vast majority of locomotor episodes, cycle period varied more with the extension phase. This was found without phasic sensory feedback, supraspinal structures, pharmacology or sustained stimulation. We conclude that the control of walking speed is governed by an asymmetry within the organization of the spinal CPG, which can be modified by extraneous factors.     

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.     

Flexor reflex afferents reset the step cycle during fictive locomotion in the cat

 The generation of locomotor-like spinal rhythms has been proposed to involve two neural centres with mutual reciprocal inhibition (Graham Brown’s ”half-centre” hypothesis). Much later a particular set of segmental flexor reflex pathways were described as being organized in accordance with this half-centre hypothesis. As these pathways became operative following injection of monoaminoxidase inhibitors and l-3,4-dihydroxyphenylalanine (l-dopa), i.e. under the same conditions under which a spontaneous locomotor activity may develop, it was assumed that these particular pathways and spinal rhythm generators involve the same neuronal networks. In order to give further evidence to this hypothesis, we investigated whether short trains to ”flexor reflex afferents” (FRA) reset the spinal locomotor rhythm, i.e. shorten or lengthen the stimulated cycle after which the regular rhythm is resumed with step cycles of the original duration. The experiments were performed in anaemically decapitated, high-spinal curarized cats. A steady locomotor rhythm was induced by injection of nialamide and l-dopa and the influence of electrical stimulation (trains of 50–1000 ms) of FRA (joint, cutaneous, and group II and III muscle afferents) onto this rhythm was tested. Stimulation of FRA induced a clear resetting of the locomotor rhythm, which was mainly characterized by a flexion reflex pattern: during the extension phase the extensor activity was interrupted and a flexion phase was initiated; during the late flexion phase mainly a prolongation of that phase with a variable change of the following extension phase was induced. In addition to this prevailing pattern, stimulation of some nerves (in particular nerves to more distal extensors and the sural nerve) could often prolong extension, when stimulated during the late extension, or terminate the flexor burst and initiate a new extension phase, when stimulated during the late flexion phase. This pattern is probably due to the concomitant stimulation of group I afferents in the case of the muscle nerves and to separate non-FRA pathways in the case of the sural nerve. The results demonstrate that the interneurones of the FRA pathways, which are operative during l-dopa-induced locomotion in spinal animals, can be considered as neuronal elements of the rhythm-generating network for locomotion. Received: 25 June 1996 / Accepted: 27 April 1998     

Toe flexor muscle spindle discharge and stretch modulation during locomotor activity in the decerebrate cat

In order to investigate the nature (i.e. static or dynamic) of fusimotor drive to the flexor hallucis longus (FHL) and flexor digitorum longus (FDL) muscles during locomotion we recorded Ia and group II muscle spindle afferent responses to sinusoidal stretch (0.25 and 1 mm amplitude, respectively, 4–5 Hz) in a decerebrate cat preparation. FHL Ia and group II afferents generally had increased discharge rates and decreased modulation to stretch throughout the step cycle, compared to rest, suggesting raised static γ drive at all locomotor phases. Although the modulation of Ia afferents was reduced during locomotion, most (13 of 18) showed a clear increasing trend during homonymous muscle activity (extension). This was consistent with phasic dynamic γ drive to FHL spindles linked with α drive. In agreement with previous reports, FHL gave a single burst of EMG activity during the step cycle while FDL α drive had two components. One was related to extension while the other comprised a brief burst around the end of this phase. Typically FDL Ia and group II afferents also had elevated firing rates and reduced modulation at all locomotor phases, again implicating static γ drive. Half the afferents (seven Ia, three group II) showed increased discharge during extension, suggesting phasic static γ drive. There was no γ drive associated with the late FDL α burst. In conclusion, the γ drives to FHL and FDL differed during locomotion. FHL, which has the α drive of a classic extensor, received γ drive that closely resembled other extensors. The γ drive of FDL, which exhibits both extensor and flexor α synergies, did not match either muscle type. These observations are compatible with the view that fusimotor drive varies in different muscles during locomotion according to the prevailing sensorimotor requirements.