Pattern of 'non-reciprocal' inhibition of motoneurones by impulses in group Ia muscle spindle afferents in the cat

Hindlimb motor responses evoked by microstimulation of the lumbar dorsal root ganglia during quiet standing

Somatosensory afferent pathways have been a target for neural prostheses that seek to restore sensory feedback from amputated limbs and to recruit muscles paralyzed by neurological injury. These pathways supply inputs to spinal reflex circuits that are necessary for coordinating muscle activity in the lower limb. The dorsal root ganglia (DRG) is a potential site for accessing sensory neurons because DRG microstimulation selectively recruits major nerve branches of the cat hindlimb. Previous DRG microstimulation experiments have been performed in anesthetized animals, but effects on muscle recruitment and behavior in awake animals have not been examined.

OBJECTIVE

The objective of the current study was to measure the effects of DRG microstimulation on evoking changes in hindlimb muscle activity during quiet standing.

APPROACH

In this study, 32-channel penetrating microelectrode arrays were implanted chronically in the left L6 and L7 DRG of four cats. During each week of testing, one DRG electrode was selected to deliver microstimulation pulse-trains during quiet standing. Electromyographic (EMG) signals were recorded from intramuscular electrodes in ten hindlimb muscles, and ground-reaction forces (GRF) were measured under the foot of the implanted limb.

MAIN RESULTS

DRG Microstimulation evoked a mix of excitatory and inhibitory responses across muscles. Response rates were highest when microstimulation was applied on the L7 array, producing more excitatory than inhibitory responses. Response rates for the L6 array were lower, and the composition of responses was more evenly balanced between excitation and inhibition. On approximately one third of testing weeks, microstimulation induced a transient unloading of the hindlimb as indicated by a decrease in GRF. Reciprocal inhibition at the knee was a prevalent response pattern across testing days which contributed to the unloading force on this subset of testing weeks.

SIGNIFICANCE

Results show that single-channel microstimulation in the lumbar DRG evokes stereotyped patterns of muscle recruitment in awake animals, demonstrating that even limited sensory input can elicit hindlimb behavior. These findings imply that DRG microstimulation may have utility in neural prosthetic applications aimed at restoring somatosensory feedback and promoting motor function after neurological injury.

Diversity of molecularly defined spinal interneurons engaged in mammalian locomotor pattern generation

Mapping the expression of transcription factors in the mouse spinal cord has identified ten progenitor domains, four of which are cardinal classes of molecularly defined, ventrally located interneurons that are integrated in the locomotor circuitry. This review focuses on the properties of these interneuronal populations and their contribution to hindlimb locomotor central pattern generation. Interneuronal populations are categorized based on their excitatory or inhibitory functions and their axonal projections as predictors of their role in locomotor rhythm generation and coordination. The synaptic connectivity and functions of these interneurons in the locomotor central pattern generators (CPGs) have been assessed by correlating their activity patterns with motor output responses to rhythmogenic neurochemicals and sensory and descending fibers stimulations as well as analyzing kinematic gait patterns in adult mice. The observed complex organization of interneurons in the locomotor CPG circuitry, some with seemingly similar physiological functions, reflects the intricate repertoire associated with mammalian motor control and is consistent with high transcriptional heterogeneity arising from cardinal interneuronal classes. This review discusses insights derived from recent studies to describe innovative approaches and limitations in experimental model systems and to identify missing links in current investigational enterprise.

The representation of egocentric space in the posterior parietal cortex

The posterior parietal cortex (PPC) is the most likely site where egocentric spatial relationships are represented in the brain. PPC cells receive visual, auditory, somaesthetic, and vestibular sensory inputs; oculomotor, head, limb, and body motor signals; and strong motivational projections from the limbic system. Their discharge increases not only when an animal moves towards a sensory target, but also when it directs its attention to it. PPC lesions have the opposite effect: sensory inattention and neglect. The PPC does not seem to contain a "map" of the location of objects in space but a distributed neural network for transforming one set of sensory vectors into other sensory reference frames or into various motor coordinate systems. Which set of transformation rules is used probably depends on attention, which selectively enhances the synapses needed for making a particular sensory comparison or aiming a particular movement.

An equilibrium-point model of electromyographic patterns during single-joint movements based on experimentally reconstructed control signals

The purpose of this work has been to develop a model of electromyographic (EMG) patterns during single-joint movements based on a version of the equilibrium-point hypothesis, a method for experimental reconstruction of the joint compliant characteristics, the dual-strategy hypothesis, and a kinematic model of movement trajectory. EMG patterns are considered emergent properties of hypothetical control patterns that are equally affected by the control signals and peripheral feedback reflecting actual movement trajectory. A computer model generated the EMG patterns based on simulated movement kinematics and hypothetical control signals derived from the reconstructed joint compliant characteristics. The model predictions have been compared to published recordings of movement kinematics and EMG patterns in a variety of movement conditions, including movements over different distances, at different speeds, against different-known inertial loads, and in conditions of possible unexpected decrease in the inertial load. Changes in task parameters within the model led to simulated EMG patterns qualitatively similar to the experimentally recorded EMG patterns. The model's predictive power compares it favourably to the existing models of the EMG patterns.

Does the nervous system depend on kinesthetic information to control natural limb movements?

This target article draws together two groups of experimental studies on the control of human movement through peripheral feedback and centrally generated signals of motor commands. First, during natural movement, feedback from muscle, joint, and cutaneous afferents changes; in human subjects these changes have reflex and kinesthetic consequences. Recent psychophysical and microneurographic evidence suggests that joint and even cutaneous afferents may have a proprioceptive role. Second, the role of centrally generated motor commands in the control of normal movements and movements following acute and chronic deafferentation is reviewed. There is increasing evidence that subjects can perceive their motor commands under various conditions, but that this is inadequate for normal movement; deficits in motor performance arise when the reliance on proprioceptive feedback is abolished either experimentally or because of pathology. During natural movement, the CNS appears to have access to functionally useful input from a range of peripheral receptors as well as from internally generated command signals. The unanswered questions that remain suggest a number of avenues for further research.

Implications of neural networks for how we think about brain function

Engineers use neural networks to control systems too complex for conventional engineering solutions. To examine the behavior of individual hidden units would defeat the purpose of this approach because it would be largely uninterpretable. Yet neurophysiologists spend their careers doing just that! Hidden units contain bits and scraps of signals that yield only arcane hints about network function and no information about how its individual units process signals. Most literature on single-unit recordings attests to this grim fact. On the other hand, knowing a system's function and describing it with elegant mathematics tell one very little about what to expect of interneuronal behavior. Examples of simple networks based on neurophysiology are taken from the oculomotor literature to suggest how single-unit interpretability might decrease with increasing task complexity. It is argued that trying to explain how any real neural network works on a cell-by-cell, reductionist basis is futile and we may have to be content with trying to understand the brain at higher levels of organization.

Changes in the Estimated Time Course of the Motoneuron Afterhyperpolarization Induced by Tendon Vibration

Group Ia afferents are activated vigorously with high-frequency tendon vibration and provide excitatory input to the agonist muscle and inhibitory input to the antagonist muscle group via inhibitory interneurons. The purpose of this experiment was to determine whether the afterhyperpolarization (AHP) time course in humans is altered in response to tendon vibration. The AHP time course is estimated using the interval death rate (IDR) analysis, a transform of the motor unit action potential train. Single motor units from tibialis anterior (TA) were recorded as subjects held low force dorsiflexor contractions for 600 s with and without vibration. The vibratory stimulus was superimposed on the low force contraction either to the tendon of the TA or the antagonist Achilles tendon. During TA tendon vibration, the time course of the AHP, as expressed by its time constant (τ), decreased from 35.5 ms in the previbration control condition to 31.3 ms during the vibration ( P = 0.003) and returned to 36.3 ms after the vibration was removed ( P = 0.002). The AHP τ during vibration of the antagonist Achilles tendon (38.6 ms) was greater than the previbration control condition (33.6 ms; P = 0.001). It is speculated that the reduction in AHP time constant with TA vibration may have resulted alone or in combination with a modulation of motoneuron gain, an alteration of persistent inward currents and/or the restructuring of synaptic noise. A decrease in firing probability, possibly reflecting Ia reciprocal inhibition, may have been responsible for the larger AHP time constant.

Functional subdivision of feline spinal interneurons in reflex pathways from group Ib and II muscle afferents; an update

A first step towards understanding the operation of a neural network is identification of the populations of neurons that contribute to it. Our aim here is to reassess the basis for subdivision of adult mammalian spinal interneurons that mediate reflex actions from tendon organs (group Ib afferents) and muscle spindle secondary endings (group II afferents) into separate populations. Re-examining the existing experimental data, we find no compelling reasons to consider intermediate zone interneurons with input from group Ib afferents to be distinct from those co-excited by group II afferents. Similar patterns of distributed input have been found in subpopulations that project ipsilaterally, contralaterally or bilaterally, and in both excitatory and inhibitory interneurons; differences in input from group I and II afferents to individual interneurons showed intra- rather than inter-population variation. Patterns of reflex actions evoked from group Ib and II afferents and task-dependent changes in these actions, e.g. during locomotion, may likewise be compatible with mediation by premotor interneurons integrating information from both group I and II afferents. Pathological changes after injuries of the central nervous system in humans and the lineage of different subclasses of embryonic interneurons may therefore be analyzed without need to consider subdivision of adult intermediate zone interneurons into subpopulations with group Ib or group II input. We propose renaming these neurons 'group I/II interneurons'.

Genetically defined inhibitory neurons in the mouse spinal cord dorsal horn: a possible source of rhythmic inhibition of motoneurons during fictive locomotion

To ensure alternation of flexor and extensor muscles during locomotion, the spinal locomotor network provides rhythmic inhibition to motoneurons. The source of this inhibition in mammals is incompletely defined. We have identified a population of GABAergic interneurons located in medial laminae V/VI that express green fluorescent protein (GFP) in glutamic acid decarboxylase-65::GFP transgenic mice. Immunohistochemical studies revealed GFP+ terminals in apposition to motoneuronal somata, neurons in Clarke's column, and in laminae V/VI where they apposed GFP+ interneurons, thus forming putative reciprocal connections. Whole-cell patch-clamp recordings from GFP+ interneurons in spinal cord slices revealed a range of electrophysiological profiles, including sag and postinhibitory rebound potentials. Most neurons fired tonically in response to depolarizing current injection. Calcium transients demonstrated by two-photon excitation microscopy in the hemisected spinal cord were recorded in response to low-threshold dorsal root stimulation, indicating these neurons receive primary afferent input. Following a locomotor task, the number of GFP+ neurons expressing Fos increased, indicating that these neurons are active during locomotion. During fictive locomotion induced in the hemisected spinal cord, two-photon excitation imaging demonstrated rhythmic calcium activity in these interneurons, which correlated with the termination of ventral root bursts. We suggest that these dorsomedial GABAergic interneurons are involved in spinal locomotor networks, and may provide direct rhythmic inhibitory input to motoneurons during locomotion.

Spinal interneurons providing input to the final common path during locomotion

As the nexus between the nervous system and the skeletomuscular system, motoneurons effect all behavior. As such, motoneuron activity must be well regulated so as to generate appropriately timed and graded muscular contractions. Accordingly, motoneurons receive a large number of both excitatory and inhibitory synaptic inputs from various peripheral and central sources. Many of these synaptic contacts arise from spinal interneurons, some of which belong to spinal networks responsible for the generation of locomotor activity. Although the complete definition of these networks remains elusive, it is known that the neural machinery necessary to generate the basic rhythm and pattern of locomotion is contained within the spinal cord. One approach to gaining insights into spinal locomotor networks is to describe those spinal interneurons that directly control the activity of motoneurons, so-called last-order interneurons. In this chapter, we briefly survey the different populations of last-order interneurons that have been identified using anatomical, physiological, and genetic methodologies. We discuss the possible roles of these identified last-order interneurons in generating locomotor activity, and in the process, identify particular criteria that may be useful in identifying putative last-order interneurons belonging to spinal locomotor networks.

Heterogeneity of V2-derived interneurons in the adult mouse spinal cord

Spinal neurons and networks that generate rhythmic locomotor activity remain incompletely defined, prompting the use of molecular biological strategies to label populations of neurons in the postnatal mouse. During spinal cord development, expression of Lhx3 in the absence of Isl1 specifies a V2 interneuronal fate. In this study, postnatal V2-derived interneurons were identified by yellow fluorescent protein (YFP) expression in the double-transgenic offspring of Lhx3Cre/+ x thy1-loxP-stop-loxP-YFP mice. While some motoneurons were labelled, several populations of interneurons predominantly located in lamina VII could also be distinguished. Small interneurons were located throughout the spinal cord whereas larger interneurons were concentrated in the lumbar enlargement. Some V2-derived interneurons were propriospinal, with axons that bifurcated in the lateral funiculus. V2-derived interneurons gave rise to populations of both excitatory and inhibitory interneurons in approximately equal proportions, as demonstrated by in situ hybridization with VGLUT2 mRNA. Immunohistochemical studies revealed YFP+ boutons throughout the spinal cord. Both glutamatergic and glycinergic YFP+ boutons were observed in lamina IX where many apposed motoneuron somata. GABAergic YFP+ boutons were also observed in lamina IX, and they did not form P-boutons. At P0, more than half of the YFP+ interneurons expressed Chx10 and thus were derived from the V2a subclass. In adult mice, there was an increase in Fos expression in V2-derived interneurons following locomotion, indicating that these neurons are active during this behaviour. The heterogeneity of V2-derived interneurons in adult mice indicates that physiologically distinct subpopulations, including last-order interneurons, arise from these embryonically defined neurons.

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.

Candidate interneurones mediating group I disynaptic EPSPs in extensor motoneurones during fictive locomotion in the cat

In the present study we sought to find interneurones responsible for the group I-evoked disynaptic excitation of hindlimb extensor motoneurones that occurs during fictive locomotion. Locomotion was produced by stimulation of the mesencephalic locomotor region (MLR) in decerebrate paralysed cats in which activation of ankle extensor group I afferents evoked a disynaptic excitation of motoneurones during the extension phase of fictive locomotion. Extracellular recordings were used to locate interneurones fulfilling all, or five of the six following criteria: (i) weak or no response to stimulation of extensor group I afferents in the absence of locomotion; (ii) strong group I activation during locomotion; (iii) group I activation at monosynaptic latencies; (iv) strong group I activation during only the extensor phase of locomotion; and (v) antidromic activation from the extensor motor nuclei; but (vi) no antidromic activation from rostral spinal segments. Candidate excitatory interneurones were located in mid to caudal parts of the L7 segments in areas where monosynaptic field potentials were evoked by group I afferents, within 2 mm of the stimulation site in the ventral horn from which they were antidromically activated. All were activated during extension by stimulation of group I afferents in extensor nerves. In the absence of peripheral nerve stimulation, six of the seven candidate excitatory interneurones were rhythmically active with maximal activation during the extension phase of fictive locomotion. Rhythmic activity during extension was also seen in five additional interneurones located near candidate interneurones but not activated by group I strength stimulation of the tested nerves. We suggest that the lumbosacral interneurones located in the intermediate laminae that can be activated by extensor group I afferents during the extension phase are a previously unknown population of interneurones, and may mediate group I-evoked disynaptic excitation of extensor motoneurones. Their rhythmic activity suggests that they also provide central excitatory drive to extensor motoneurones during locomotion.

Medullary reticulospinal tract mediating a generalized motor inhibition in cats: iii. functional organization of spinal interneurons in the lower lumbar segments

The previous report of intracellular recording of hindlimb motoneurons in decerebrate cats [ 511] has suggested that the following mechanisms are involved in a generalized motor inhibition induced by stimulating the medullary reticular formation. First, the motor inhibition, which was prominent in the late latency (30-80 ms), can be ascribed to the inhibitory effects in parallel to motoneurons and to interneuronal transmission in reflex pathways. Second, both a group of interneurons receiving inhibition from flexor reflex afferents and a group of Ib interneurons mediate the late inhibitory effects upon the motoneurons. To substantiate the above mechanisms of motor inhibition we examined the medullary stimulus effects upon intracellular (n=55) and extracellular (n=136) activity of spinal interneurons recorded from the lower lumbar segments (L6-L7). Single pulses or stimulus trains (1-3) pulses, with a duration of 0.2 ms and intensity of 20-50 microA) applied to the medullary nucleus reticularis gigantocellularis evoked a mixture of excitatory and inhibitory effects with early (<20 ms) and late (>30 ms) latencies. The medullary stimulation excited 55 interneurons (28.8%) with a late latency. Thirty-nine of the cells, which included 10 Ib interneurons, were inhibited by volleys in flexor reflex afferents (FRAs). These cells were mainly located in lamina VII of Rexed. On the other hand, the late inhibitory effects were observed in 67 interneurons (35.1%), which included cells mediating reciprocal Ia inhibition, non-reciprocal group I (Ib) inhibition, recurrent inhibition and flexion reflexes. Intracellular recording revealed that the late inhibitory effects were due to inhibitory postsynaptic potentials with a peak latency of about 50 ms and a duration of 50-60 ms. The inhibitory effects were attenuated by volleys in FRAs. Neither excitatory nor inhibitory effects with a late latency were observed in 69 (36.1%) cells which were located in the intermediate region and dorsal horn. These results suggest the presence of a functional organization of the spinal cord with respect to the production of the generalized motor inhibition. Lamina VII interneurons that receive inhibition from volleys in FRAs possibly mediate the postsynaptic inhibition from the medullary reticular formation in parallel to motoneurons and to interneurons in reflex pathways.

Medullary reticulospinal tract mediating the generalized motor inhibition in cats: parallel inhibitory mechanisms acting on motoneurons and on interneuronal transmission in reflex pathways

The present study was designed to elucidate the spinal interneuronal mechanisms of motor inhibition evoked by stimulating the medullary reticular formation. Two questions were addressed. First, whether there is a parallel motor inhibition to motoneurons and to interneurons in reflex pathways. Second, whether the inhibition is mediated by interneurons interposed in known reflex pathways. We recorded the intracellular activity of hindlimb motoneurons in decerebrate cats and examined the effects of medullary stimulation on these neurons and on interneuronal transmission in reflex pathways to them. Stimuli (three pulses at 10-60microA and 1-10ms intervals) delivered to the nucleus reticularis gigantocellularis evoked inhibitory postsynaptic potentials in alpha-motoneurons (n=147) and gamma-motoneurons (n=5) with both early and late latencies. The early inhibitory postsynaptic potentials were observed in 66.4% of the motoneurons and had a latency of 4.0-5.5ms with a segmental delay of more than 1.4ms. The late inhibitory postsynaptic potentials were observed in 98.0% of the motoneurons and had a latency of 30-35ms, with a peak latency of 50-60ms. Both types of inhibitory postsynaptic potentials were evoked through fibers descending in the ventrolateral quadrant. The inhibitory postsynaptic potentials were not influenced by recurrent inhibitory pathways, but both types were greatly attenuated by volleys in flexor reflex afferents. Conditioning medullary stimulation, which was subthreshold to evoke inhibitory postsynaptic potentials in the motoneurons, neither evoked primary afferent depolarization of dorsal roots nor reduced the input resistance of the motoneurons. However, the conditioning stimulation often facilitated non-reciprocal group I inhibitory pathways (Ib inhibitory pathways) to the motoneurons in early (<20ms) and late (30-80ms) periods. In contrast, it attenuated test postsynaptic potentials evoked through reciprocal Ia inhibitory pathways, and excitatory and inhibitory pathways from flexor reflex afferent and recurrent inhibitory pathways. The inhibitory effects were observed in both early and late periods. The present results provide new information about a parallel inhibitory process from the medullary reticular formation that produces a generalized motor inhibition by acting on alpha- and gamma-motoneurons, and on interneurons in reflex pathways. Interneurons receiving inhibition from flexor reflex afferents and a group of Ib interneurons may mediate the inhibitory effects upon motoneurons.

Short latency heteronymous excitatory and inhibitory reflexes between antagonist and heteronymous muscles of the human shoulder and upper limb

The aims were (i) to investigate heteronymous excitatory and inhibitory Group Ia reflexes linking agonist/antagonist muscle pairs acting at the shoulder and elbow; clavicular pectoralis major (Pmajor) and posterior deltoid (Pdeltoid); biceps brachii (Bi) and Tri brachii (Tri), and linking muscles acting at the elbow (Bi and Tri) with muscles acting at the shoulder (Pmajor and Pdeltoid). (ii) To test the hypothesis that the excitability of the reflexes would vary between different tasks in a functionally relevant manner. The study was performed on 45 adults. Reflexes were recorded in the surface EMG when the target muscle was contracting at 10% maximum voluntary contraction. Reflexes were recorded in Bi and Tri with the elbow joint in one of three positions: 105 degrees, 80 degrees, or 55 degrees from full extension. Group Ia reflexes were evoked using a small, brief tap to the tendon of the muscle being stimulated. Reflexes were recorded by cross-correlation of the surface EMG and pseudo-random series of taps. All subjects demonstrated short latency inhibition and excitation between agonist/antagonist muscle pairs; inhibition was significantly more frequent than excitation. Excitation and inhibition occurred with equal frequency between muscle pairs acting between elbow and shoulder. Minimum central delays for excitatory reflexes were 1 ms, consistent with monosynaptic projections and for inhibitory responses were 2 ms consistent with disynaptic linkage. Later excitatory and inhibitory reflexes with central delays of up to 15 ms also occurred. The probability of evoking excitation or inhibition in Tri or Bi changed with the different elbow positions.

Group I disynaptic excitation of cat hindlimb flexor and bifunctional motoneurones during fictive locomotion

The incidence of short latency excitation of motoneurones innervating flexor and bifunctional muscles evoked by group I intensity (<= 2 x threshold) electrical stimulation of hindlimb muscle nerves was investigated during fictive locomotion in decerebrate cats. Intracellular recordings were made from hindlimb motoneurones in which action potentials were blocked by intracellular diffusion of a lidocaine (lignocaine) derivative (QX-314) and fictive locomotion was evoked by electrical stimulation of the midbrain. Few motoneurones (16%) received group I-evoked oligosynaptic excitation in the absence of fictive locomotion. During fictive locomotion 39/44 (89%) motoneurones innervating ankle, knee or hip flexor muscles and 18/28 (64%) motoneurones innervating bifunctional muscles received group I-evoked oligosynaptic EPSPs. In flexor motoneurones, locomotor-dependent excitation was present in both step cycle phases but largest during flexion. In bifunctional motoneurones, EPSPs were often largest at the transition between flexion and extension phases. Activation of homonymous afferents most consistently evoked the largest locomotor-dependent excitation (amplitude up to 4.6 mV), but in some cases stimulation of heteronymous flexor or bifunctional muscle nerves evoked large EPSPs. EPSP amplitude became maximal as stimulation intensity was increased to about twice threshold. This suggests that tendon organ afferents can evoke group I EPSPs during locomotion. The EPSPs resulting from brief, small stretches of extensor digitorum longus tendons indicate that group Ia muscle spindle afferents can also evoke the group I excitation of flexors. Stimulation of extensor group I afferents did not result in excitation of flexor motoneurones. The mean latency of locomotor-dependent group I excitation in flexor and bifunctional motoneurones was 1.64 +/- 0.16 ms, indicating a path consisting of a single interneurone interposed between group I afferents and motoneurones innervating flexor and bifunctional muscles. This disynaptic excitation is analogous to that recorded in extensor motoneurones and evoked from extensor group I afferents during locomotion. Differences in the phase dependence and sources of group I excitation to flexor and extensor motoneurones during locomotion suggest the existence of separate groups of excitatory interneurones exciting flexor and extensor motoneurones. The wide distribution of group I disynaptic excitation in motoneurones innervating extensor, flexor and bifunctional muscles acting on hip, knee and ankle joints suggests that these pathways can play an important role in the reinforcement of ongoing locomotor activity throughout the limb.

Load-regulating mechanisms in gait and posture: comparative aspects

How is load sensed by receptors, and how is this sensory information used to guide locomotion? Many insights in this domain have evolved from comparative studies since it has been realized that basic principles concerning load sensing and regulation can be found in a wide variety of animals, both vertebrate and invertebrate. Feedback about load is not only derived from specific load receptors but also from other types of receptors that previously were thought to have other functions. In the central nervous system of many species, a convergence is found between specific and nonspecific load receptors. Furthermore, feedback from load receptors onto central circuits involved in the generation of rhythmic locomotor output is commonly found. During the stance phase, afferent activity from various load detectors can activate the extensor part in such circuits, thereby providing reinforcing force feedback. At the same time, the flexion is suppressed. The functional role of this arrangement is that activity in antigravity muscles is promoted while the onset of the next flexion is delayed as long as the limb is loaded. This type of reinforcing force feedback is present during gait but absent in the immoble resting animal.

Recurrent inhibition in humans

Methods have been developed to investigate recurrent inhibition (RI) in humans. A conditioning reflex discharge is used to evoke in motoneurones (MNs) supplying homonymous and synergistic muscles, an inhibition the characteristics of which are consistent with RI: it appears and increases with the conditioning motor discharge, has a short latency and a long duration, and is enhanced by an agonist of acetylcholine. As in the cat, homonymous RI exists in all explored motor nuclei of the limbs except those of the digits and the pattern of distribution of heteronymous RI closely matches that of monosynaptic Ia excitation. However, striking inter-species differences exist concerning the distribution of heteronymous RI since it is much more widely extended in the human lower limb than in the cat hindlimb, whereas it is more restricted in the upper limb than in the cat forelimb. Changes in transmission in the recurrent pathway have been investigated during various voluntary or postural contractions involving different (homonymous, synergistic, antagonistic) muscles and it has been found that the activation of Renshaw cells (RCs) by the voluntary motor discharge via recurrent collaterals was powerfully controlled by descending tracts: for example, during homonymous contraction, RI evoked by a given conditioning reflex discharge is much smaller during strong than during weak contraction, which suggests that the descending control of RCs might contribute to the regulation of muscle force. The finding that RC inhibition is more marked during phasic than during tonic contraction of similar force of the homonymous muscle is discussed in relation with the projections of RCs to Ia interneurones mediating reciprocal inhibition. Only in patients with progressive paraparesis is there evidence for decreased RI at rest which may contribute to the exaggeration of the passively-induced stretch reflex underlying spasticity. However, despite the seemingly normal RI at rest in most patients, the control of RCs during voluntary movements is disturbed in these patients, which probably contributes to their motor disability.

Neuronal basis of afferent-evoked enhancement of locomotor activity

Activation of ankle extensor group Ia muscle spindle or Ib tendon organ afferents during locomotion can prolong and enhance hindlimb extensor motoneuron activity. A growing body of evidence suggests that these group I evoked reflexes not only compensate for a changing environment but also help shape extensor activity during normal, unperturbed locomotion. In this paper we review four mechanisms that underlie the group I evoked enhancement of ipsilateral extensor activity during locomotion. The first three are pre-motoneuronal mechanisms that are part of group I reflex pathway reorganization during locomotion. They are (1) a suppression of group I evoked nonreciprocal inhibition, (2) a release from inhibition of excitatory interneurons in disynaptic pathways from group I afferents to extensor motoneurons, and (3) longer latency excitation evoked through extensor portions of the locomotor circuitry. The fourth factor contributing to group I evoked increases in motoneuron activity during locomotion is the increase in motoneuron excitability produced by postsynaptic changes in motoneuron membrane conductances. Most results to be discussed were obtained during locomotion in decerebrate cats in which fictive locomotion was evoked by stimulation of the midbrain following neuromuscular blockade.

Sensitivity of H-reflexes and stretch reflexes to presynaptic inhibition in humans

The sensitivity of soleus H-reflexes, T-reflexes, and short-latency stretch reflexes (M1) to presynaptic inhibition evoked by a weak tap applied to the biceps femoris tendon or stimulation of the common peroneal nerve (CPN) was compared in 17 healthy human subjects. The H-reflex was strongly depressed for a period lasting up to 300-400 ms (depression to 48 +/- 23%, mean +/- SD, of control at a conditioning test interval of 70 ms) by the biceps femoris tendon tap. In contrast, the short-latency soleus stretch reflex elicited by a quick passive dorsiflexion of the ankle joint was not depressed. The soleus T-reflex elicited by an Achilles tendon tap was only weakly depressed (92 +/- 8%). The H-reflex was also significantly more depressed than the T-reflex at long intervals (>15 ms) after stimulation of CPN (H-reflex 63 +/- 14%, T-reflex 91 +/- 13%; P < 0. 01). However, the short-latency (2 ms) disynaptic reciprocal Ia inhibition evoked by stimulation of CPN was equally strong for H- and T-reflexes (H-reflex 72 +/- 10%, T-reflex 67 +/- 13%; P = 0.07). Peaks in the poststimulus time histogram (PSTH) of the discharge probability of single soleus motor units (n = 53) elicited by an Achilles tendon tap had a longer duration than peaks evoked by electrical stimulation of the tibial nerve (on average 5.0 ms as compared with 2.7 ms). All parts of the electrically evoked peaks were depressed by the conditioning biceps femoris tendon tap (average depression to 55 +/- 27% of control; P < 0.001). A similar depression was observed for the initial 2 ms of the peaks evoked by the Achilles tendon tap (69 +/- 48%; P < 0.001), but the last 2 ms were not depressed. Conditioning stimulation of the CPN at long intervals (>15 ms) also depressed all parts of the electrically evoked PSTH peaks (n = 34; average 65%; P < 0.001) but had only a significant effect on the initial 2 ms of the peaks evoked by the Achilles tendon tap (85%; P < 0.001). We suggest that the different sensitivity of mechanically and electrically evoked reflexes to presynaptic inhibition is caused by a difference in the shape and composition of the excitatory postsynaptic potentials underlying the two reflexes. This difference may be explained by a different composition and/or temporal dispersion of the afferent volleys evoked by electrical and mechanical stimuli. We conclude that it is not straightforward to predict the modulation of stretch reflexes based on observations of H-reflex modulation.

Changes in Ib heteronymous inhibition to soleus motoneurones during cutaneous and muscle nociceptive stimulation in humans

The effects of selective tonic cutaneous and muscle pain stimulation on heteronymous Ib pathways from the gastrocnemius medialis to the soleus motoneurones were studied in five healthy human subjects. Tonic pain stimulation, monitored by a psychophysical method, was obtained by local injection of 60 mg levo-ascorbic acid (L-AS) in a volume of 0.3 ml. Nociceptive cutaneous and muscle stimulation of the dorsal foot produced opposite, long-lasting changes in heteronymous Ib pathways to soleus motoneurones: Ib facilitation during cutaneous stimulation was reversed by disinhibition during muscle stimulation. The time-course of these Ib changes strictly paralleled that of subjective pain sensation. On the contrary, when the same nociceptive stimuli were applied to the leg (at pretibial level), the differences between cutaneous and muscle pain disappeared and similar short-lasting phases of facilitation and inhibition of Ib activity were observed. It is concluded that tonic discharge of cutaneous and muscle nociceptive afferents arising from the foot have specific and opposite effects on Ib inhibitory pathways to ankle extensor motoneurones. These interactions between muscle and cutaneous nociceptive and Ib pathways may be used to change muscle synergies, thus contributing to the establishment of appropriate adaptive locomotor strategies during pain.

Assessing changes in presynaptic inhibition of Ia afferents during movement in humans

Different methods, based on different principles, have been proposed to estimate changes in presynaptic inhibition of Ia terminals (accompanied by primary afferent depolarization, (PAD)) during voluntary contraction in humans. (i) A discrepancy between the H-reflex amplitude, at an equal level of EMG activity, in two situations (e.g., walking and standing) may be taken as suggesting a different control of PAD interneurones in the two cases. (ii) A conditioning stimulation (vibration or electrical stimulation) is used to activate PAD interneurones and to evoke presynaptic inhibition of the afferent volley of the test reflex. The resulting long-lasting depression of the reflex depends on the excitability of PAD interneurones, but can be contaminated by long-lasting post-synaptic effects. (iii) The amount of reflex facilitation evoked by a purely monosynaptic Ia volley varies inversely with the on-going presynaptic inhibition of Ia afferents mediating the conditioning volley, and can be used to assess this on-going presynaptic inhibition. None of these methods can provide by itself unequivocal evidence for a change in presynaptic inhibition of Ia terminals, but reasonably reliable interpretations may be proposed when congruent results are obtained with different methods. Thus it has been shown that, during selective voluntary contraction, presynaptic inhibition is decreased on Ia afferents projecting on motoneurones of the contracting muscle and increased on Ia afferents projecting on motor nuclei not involved in the contraction.

Group I extensor afferents evoke disynaptic EPSPs in cat hindlimb extensor motorneurones during fictive locomotion

1. Intracellular recording from extensor motoneurones in paralysed decerebrate cats was used to examine the distribution of short-latency non-monosynaptic excitation by group I afferents during fictive locomotion produced by stimulation of the mesencephalic locomotor region (MLR). 2. During the extension but not the flexion phase of fictive locomotion, stimulation of ankle extensor nerves at 1.2-2.0 times threshold evoked excitatory postsynaptic potentials (EPSPs) in motoneurones innervating hip, knee and ankle extensors. Disynaptic EPSPs were also evoked by selective activation of group Ia muscle spindle afferents by muscle stretch. 3. The central latencies of these group I-evoked EPSPs (mean, 1.55 ms) suggest their mediation by a disynaptic pathway with a single interneurone interposed between extensor group I afferents and extensor motoneurones. Disynaptic EPSPs were also evoked during periods of spontaneous locomotion following the cessation of MLR stimulation. 4. Hip extensor motoneurones received disynaptic EPSPs during extension following stimulation of both homonymous and ankle extensor nerves. Stimulation of hip extensor nerves did not evoke disynaptic EPSPs in ankle extensor motoneurones. 5. The appearance of disynaptic EPSPs during extension appears to result from cyclic disinhibition of an unidentified population of excitatory spinal interneurones and not postsynaptic voltage-dependent conductances in motoneurones or phasic presynaptic inhibition of group I afferents during flexion. 6. The reorganization of group I reflexes during fictive locomotion includes the appearance of disynaptic excitation of hip, knee and ankle extensor motoneurones. This excitatory reflex is one of the mechanisms by which group I afferents can enhance extensor activity and increase force production during stance.

Cutaneous nociceptive facilitation of Ib heteronymous pathways to lower limb motoneurones in humans

The effects of tonic pain stimulation on heteronymous Ib pathways from the gastrocnemius medialis (GM) to the soleus (Sol) and to the quadriceps (Q) muscles were investigated in four healthy human subjects. Tonic pain stimulation was performed by subcutaneous injection of 0.5 mg levo-ascorbic acid or vitamin C (L-LAS) in a volume of 0.5 ml on the dorsal surface of the ipsilateral foot. The mean curve of L-AS-induced pain sensation showed a steep rising phase reaching maximum intensity at 2-3 min, followed by a slow decay phase lasting about 15-20 min. Between about 5 and 20 min after injection, there was evidence of pure pain stimulation due to chemical activation of free nerve endings. During this interval, significant potentiation of Ib inhibition from GM to both Sol and Q motoneurones was observed. The time-course of these Ib heteronymous changes paralleled that of subjective pain sensation. These findings demonstrate that nociceptive discharge modifies the gain of Ib heteronymous effects in humans. Since the man function of these Ib pathways is to coordinate activity of muscles operating at different joints, it is suggested that nociceptive input may change muscle synergies by selecting specific subpopulations of Ib interneurones, thus contributing to establish appropriate adaptive motor strategies.

Disynaptic group I excitation of synergist ankle extensor motoneurones during fictive locomotion in the cat

1. Intracellular recording from medial gastrocnemius (MG) motoneurones was used to examine postsynaptic potentials produced by electrical stimulation of the plantaris nerve at group I strength at rest and during fictive locomotion. Fictive locomotion was evoked by stimulation of the midbrain locomotor region (MLR) in decerebrate cats or in decerebrate, acute low-spinal cats by perineal stimulation following intravenous administration of clonidine and naloxone. 2. In both MLR and spinal fictive locomotor preparations, stimulation of plantaris nerve group I afferents at rest evoked short-latency (< 2 ms) IPSPs in MG motoneurones. During the extensor phase of MLR-evoked locomotion, the same stimulation produced short-latency (1.6-1.8 ms) EPSPs. Such latencies suggest mediation by one interneurone interposed between plantaris nerve group I afferents and MG motoneurones. Non-monosynaptic, short-latency excitation was not seen at rest nor during the flexion phase of the step cycle. 3. Group I EPSPs during the extensor phase of MLR-evoked locomotion were evoked by stimulation at intensities ranging from 1.4-2 times threshold (T). The effectiveness of stimulation intensities < 1.5 T suggests that activation of group II afferents is not required to evoke disynaptic excitation. Selective activation of group Ia afferents by stretches of the Achilles tendon also produced disynaptic EPSPs during extension. 4. During fictive locomotion in spinal animals pretreated with clonidine, short-latency group I EPSPs were not seen but group I IPSPs recorded at rest disappeared or were greatly attenuated. The failure of depolarizing current to reveal group I IPSPs suggests that fictive locomotion involves an inhibition of the inhibitory interneurones that operate at rest. In both clonidine-treated spinal and MLR preparations, trains of stimuli at group I strength evoked longer-latency and slowly rising potentials that were more prominent during the flexor phase of fictive locomotion. 5. These results show a reduction in short-latency group I inhibition of synergists in both MLR and clonidine-treated spinal preparations during fictive locomotion. In addition, activation of group I afferents evokes short-latency excitation of synergists during extension in the MLR preparation. Such excitatory reflexes activated by ankle extensor group Ia and Ib afferents may form an excitatory feedback system, reinforcing on-going extensor activity during the stance phase of the step cycle.

Ankle extensor group I afferents excite extensors throughout the hindlimb during fictive locomotion in the cat

1. The effects of stimulating hindlimb extensor nerves (100-200 ms trains, 100 Hz, < or = 2 times threshold) during the flexor and extensor phases of the locomotor step cycle were analysed in the decerebrate, paralysed cat during fictive locomotion evoked by stimulation of the mesencephalic locomotor region. 2. Stimulation during extension of either the medial gastrocnemius (MG), lateral gastrocnemius-soleus (LGS) or plantaris (Pl) nerves was equally effective in increasing the duration and amplitude of electroneurogram (ENG) activity recorded in ipsilateral ankle, knee and hip extensor nerves. Enhancement of extensor ENG activity could be evoked with near threshold stimulation intensity and appeared within 10-40 ms of the onset of ankle extensor nerve stimulation. Stimulation of anterior biceps during extension occasionally evoked a modest increase in the duration of activity of hip, knee and ankle extensors. Stimulation of quadriceps during extension enhanced the activity of proximal extensors and soleus, but inhibited other ankle extensors. 3. Selective activation of ankle extensor Ia spindle afferents by muscle stretch also enhanced ipsilateral extension. It is argued that both muscle spindle and tendon organ afferents can contribute to the increase in extensor nerve activity evoked by group I stimulation intensity during fictive locomotion. 4. During flexion, stimulation of either the MG, Pl or LGS nerves at group I strength terminated on-going activity in ipsilateral flexors and initiated a burst of activity in ipsilateral hip, knee and ankle extensors, i.e. reset the step cycle to extension. 5. Low strength stimulation of the mixed muscle and cutaneous nerve innervating the plantar aspect of the foot produced extension enhancement and resetting similar to that evoked by group I muscle afferent stimulation. Stimulation of the cutaneous nerve supplying the dorsal aspect of the foot during extension enhanced extensor activity, and during flexion, enhanced the activity of flexors. 6. The effects reported here during fictive locomotion may also occur during overground locomotion with natural activation of group I muscle spindle and tendon organ afferents. Extensor spindle and tendon organ afferents may thus serve as an excitatory reflex system helping to shape the amplitude, duration and timing of ipsilateral extensor activity. Increased or unexpected activation of group I ankle extensor afferents or plantar foot afferents during locomotion could also compensate for increased loading of the limb.

Distribution of recurrent inhibition in the human upper limb

The distribution of homonymous and heteronymous recurrent inhibition among the motor nuclei innervating the main muscles of the human upper limb has been investigated in 25 healthy subjects. Homonymous recurrent inhibition was studied with a specially designed electrophysiological method with paired H-reflexes, previously described by Bussel & Pierrot-Deseilligny (1977), combined with a pharmacological study using a cholinergic agonist, the L-acetylcarnitine (Rossi & Mazzocchio 1991). These methods were used to investigate the Flexor Carpi Radialis (FCR), Extensor Carpi Radialis (ECR), Opponens Pollicis (OP) and Abductor Digiti Minimi (ADM) motor nuclei. In the Deltoid, Triceps, Flexor Carpi Ulnaris (FCU) and Extensor Carpi Ulnaris (ECU) motor nuclei in which it was impossible to evoke clearly distinguishable H-reflexes, homonymous recurrent inhibition was studied with the PSTH technique: homonymous recurrent inhibition was found in Deltoid, Triceps, FCR, FCU, ECR, ECU motor nuclei but not in OP and ADM motor nuclei. Heteronymous recurrent inhibition was studied with the PSTH technique in the Deltoid, Biceps, Triceps, FCR, ECR, FCU, ECU, Flexor Digitorum Communis (FDC), Extensor Digitorum Communis (EDC) motor nuclei as well as those innervating the intrinsic muscles of the hand. The following results were obtained: (1) motor neurones innervating muscles acting at digits do not receive any heteronymous recurrent inhibition; (2) motor neurones innervating muscles acting at the wrist give recurrent inhibition to motor nuclei of proximal muscles but do not receive any recurrent projections from the latter; and (3) motor neurones innervating proximal muscles (acting at shoulder or elbow) are interconnected by recurrent inhibition and receive heteronymous recurrent projections from some wrist muscles but not from intrinsic hand muscles.

A relay for input from group II muscle afferents in sacral segments of the cat spinal cord

1. A neuronal relay for input from group II afferents of hindlimb muscle nerves has been found in the previously little explored sacral segments of the cat spinal cord. 2. Electrical stimulation of group II muscle afferents of a number of nerves evoked negative potentials on the surface (cord dorsum potentials) and population postsynaptic potentials (field potentials) within the sacral segments. The largest potentials were evoked by stimulation of the posterior biceps-semitendinosus and triceps surae nerves which evoke much smaller potentials in other segments. Group II afferents of other nerves, notably those which have their main relay within the middle lumbar segments, were much less effective. 3. The sites at which cord dorsum and field potentials evoked by group II muscle afferents were recorded varied in relation to the external topography of the L7-S2 spinal segments but were consistent in their location relative to the pudendal motor nucleus (Onuf's nucleus). Potentials evoked by group II afferents of the posterior biceps and semitendinosus nerves peaked at a level corresponding to the rostral half of Onuf's nucleus and potentials evoked by afferents of the gastrocnemius nerves peaked just rostral to this nucleus. The largest field potentials (of 0.5-1.0 mV) were recorded within the dorsal horn. Field potentials in the intermediate zone were much smaller (< 0.3 mV) and were seen less frequently. 4. Evidence was obtained that the dorsal horn field potentials are to a great extent evoked monosynaptically by the fast conducting fraction of group II muscle afferents: (i) they were evoked at short latencies (2.4-2.7 ms from the stimulus; 1.3-1.7 ms from group I components of afferent volleys and 0.5-0.7 ms from group II components of these volleys), (ii) the conduction times of impulses in the fastest conducting fraction of group II afferents, between the sacral segments (where these impulses were induced by intraspinal stimuli) and the peripheral nerves, were only about 0.5 ms shorter than the latencies of field potentials recorded at the site of intraspinal stimulation and evoked by stimulation of the same peripheral nerves and, (iii) the field potentials followed repetitive stimuli without temporal facilitation. 5. Negative cord dorsum and field potentials were also evoked by small stretches of the semitendinosus and triceps surae muscles. Although they were smaller than potentials evoked by electrical stimulation of sensory fibres and appeared at longer latencies, their presence is consistent with a contribution of muscle spindle afferents to the actions of group II muscle afferents within the sacral segments.(ABSTRACT TRUNCATED AT 400 WORDS)

Gating of transmission to motoneurones by stimuli applied in the locus coeruleus and raphe nuclei of the cat

1. Neuronal systems activated by stimulation in the region of the locus coeruleus/subcoeruleus (LC/SC) and raphe nuclei have previously been shown to depress transmission from group II muscle afferents in regions of the midlumbar spinal segments in which premotor interneurones are located. The aim of the present investigation was to determine the extent to which such depression is paralleled by depression of the reflex actions of group II afferents on motoneurones. 2. The effects of short trains of conditioning electrical stimuli applied within the LC/SC and raphe nuclei were examined on postsynaptic potentials (PSPs) evoked by group I and group II muscle afferents in hindlimb motoneurones. The effects were examined over a wide range of conditioning-test intervals but particular emphasis was placed on the effects produced at long intervals (> 100 ms) since such effects are more likely to be mediated by the descending noradrenergic and serotonergic neurones of the LC/SC and raphe nuclei which are of slow conduction velocity. In addition, conditioning stimuli alone evoked PSPs in motoneurones (with latencies of 7-15 ms and a duration of 50-80 ms) and effects evoked at short conditioning-test intervals might therefore have been secondary to changes in motoneurone membrane properties. 3. At conditioning-test intervals between 100 and 350 ms synaptic actions of group II origin were strongly and consistently depressed. Both EPSPs and IPSPs were affected, two-thirds of those tested being reduced in amplitude by 50% or more. A similar depression was exerted on PSPs evoked from the quadriceps and deep peroneal nerves mediated predominantly by interneurones located in the midlumbar segments and on PSPs evoked from the hamstring and triceps surae nerves mediated by interneurones located in more caudal segments. It is thus concluded that neuronal systems activated by stimuli applied in the LC/SC and raphe nuclei are capable of gating transmission in all those interneuronal pathways which mediate the reflex actions of group II afferents on motoneurones in anaesthetized animals.

Synaptic mechanisms acting on lumbar motoneurons during postural augmentation induced by serotonin injection into the rostral pontine reticular formation in decerebrate cats

Intrapontine microinjections of serotonin in acutely decerebrated cats resulted in the bilateral augmentation of the postural muscle tone of the hindlimbs. Optimal injection sites were located in the dorsomedial part of the rostral pontine reticular formation corresponding to the nucleus reticularis pontis oralis (NRPo). In this study, attempts were made to elucidate the cellular basis for the serotoninergically induced augmentation of postural muscle tone by recording the electromyographic (EMG) activity of hindlimb extensor muscles, the monosynaptic reflex responses evoked by electrical stimulation of group Ia muscle afferent fibres and the membrane potentials of hindlimb alpha-motoneurons (MNs). Serotonin injections resulted not only in the augmentation of the EMG activity of gastrocnemius soleus muscles, but also in the restoration of EMG suppression, which was induced by previous injection of carbachol into the NRPo. Extensor and flexor monosynaptic reflex responses were facilitated by serotonin injections into the NRPo. Such reflex facilitation was not induced by serotonin injections into the mesencephalic or the medullary reticular formation. Intrapontine serotonin injections resulted in membrane depolarization of extensor and flexor MNs with decreases in input resistance and rheobase. Spontaneous depolarizing synaptic potentials (EPSPs) increased in both frequency and amplitude. Peak voltage of Ia monosynaptic EPSPs also increased. Serotonin injections which followed carbachol injections resulted in membrane depolarization of MNs along with an increase in the frequency of spontaneous EPSPs and a decrease in carbachol-induced inhibitory postsynaptic potentials. Following pontine carbachol injections, antidromic and orthodromic responses in MNs were suppressed. Discharges of MNs evoked by intracellular current injections were also suppressed, but were restored following serotonin injections. These results indicate that postsynaptic excitation, presynaptic facilitation and disinhibition (withdrawal of postsynaptic inhibition) simultaneously act on the hindlimb MNs during serotonin-induced postural augmentation and restoration.

Pattern of projections of group I afferents from forearm muscles to motoneurones supplying biceps and triceps muscles in man

1) Two independent methods were used, in man, to assess the modifications of the excitability of biceps and triceps brachii motoneurone pools following the stimulation of group I afferents coming from muscles acting at the wrist: a) the modifications of the excitability of a motoneuronal population were studied using a reflex technique, b) the modifications of the excitability of an isolated motor unit were estimated using a post-stimulus time histogram (p.s.t.h.) method. 2) The activation of group I afferents contained in the median nerve, originating from wrist flexors and pronators, resulted in a strong, short-latency facilitation of the biceps brachii motoneurones. A similar effect was also evoked by stimulation of group I afferents in the radial nerve, distally to the branch supplying the brachio-radialis muscle. The latency of both median and radial-induced facilitations is compatible with a monosynaptic linkage. 3) The stimulation of group I afferents in the median or the radial nerves produced inhibition of triceps motoneurones, with a latency compatible with a disynaptic linkage. 4) The prolonged vibration of the tendon of the flexor carpi radialis (FCR) or of the extensor carpi radialis (ECR) raised the threshold for both the facilitation of biceps and the inhibition of triceps motoneurones. The same pattern of excitatory and inhibitory convergence could also be obtained when the electrical conditioning stimulus to the median or radial nerves was replaced by a tap applied to the tendons of FCR or ECR respectively. Both results suggest that the conditioning fibres were Ia fibres. 5) The pattern of distribution of Ia afferents from muscles acting at the wrist onto motoneurones of muscles acting at the elbow has been compared to that described in the cat and monkey. A comparison has also been made between Ia connections of muscles acting at different joints in the upper and lower limb in man. The differences are discussed in relation to the manipulating capacity of the hand.

Reciprocal inhibition following lesions of the spinal cord in man

1. Reciprocal inhibition was studied in normal subjects and patients with spinal cord lesions by stimulating the posterior tibial nerve below the threshold of the soleus alpha-motoneurone axons and recording the changes in firing probability of single tibialis anterior motor units activated by voluntary contraction. A short-latency (about 35 ms) period of decreased firing probability was assumed to represent reciprocal inhibition. 2. For a given stimulus intensity this inhibition was greater in patients with spinal lesions than in normal subjects. 3. The stimulus intensities at which soleus motoneurones and the Ia inhibitory interneurones were brought to threshold provided an estimate of the relative excitability of these two neural populations. In the patients with spinal lesions the Ia inhibitory interneurones were more excitable than soleus motoneurones, whereas in normal subjects the excitabilities were approximately equal. 4. Stimulation of the posterior tibial nerve below the threshold of alpha-motoneurone axons also resulted in a second period of inhibition with a latency of approximately 50 ms. This was less prominent in the patients with spinal cord lesions. 5. It is concluded that transmission through the pathways mediating reciprocal inhibition of flexor muscles during their voluntary contraction is enhanced following a spinal cord lesion in man but that a later inhibitory process is depressed.

Distribution of recurrent inhibition within a motor nucleus. I. Contribution from slow and fast motor units to the excitation of Renshaw cells

The relation between the size of a monosynaptic reflex (MSR) to triceps surae and the resulting Renshaw cell discharge was used to evaluate the contribution from slow and fast motor units to the excitation of Renshaw cells. It is, however, difficult to interpret these results in terms of excitation contributed by slow and fast motor units because of the following reasons. First, the size of the MSR recorded in ventral roots is not linearly related to the number of recruited motor units, since larger motor axons contribute more to the size of the MSR than smaller ones. Second, the number of spikes evoked in a Renshaw cell burst is not linearly related to the excitatory input because Renshaw cell discharge saturates in the case of large responses. The contribution of small, early-recruited motoneurones to Renshaw cell excitation is consequently overestimated. Procedures were introduced to deal with these problems. It is concluded that the last-recruited motor units (probably 'fast twitch, fast fatiguing') on average contribute four times as much excitation to Renshaw cells as the first recruited ('slow twitch') motor units.

Cutaneous control of group I pathways from ankle flexors to extensors in man

Reciprocal inhibition from the anterior tibial muscle onto antagonist motoneurones of the soleus muscle was studied in normal man under control conditions and after low intensity stimulation of cutaneous afferent fibres from the sole and dorsal region of the ipsilateral and contralateral foot. Ipsilateral cutaneous stimulation increased the reciprocal inhibition to the soleus motoneurones, without qualitative differences between the effect from the sole and that from the dorsal region of the foot. Stimulation of cutaneous afferent fibres from the contralateral foot produced the reverse effect, i.e., depression of the Ia reciprocal inhibition from the tibialis anterior to the soleus motoneurones. No effects could be observed when cutaneous areas other than those of the foot were stimulated. The effects of cutaneous stimulation on the reciprocal inhibition became evident only when this inhibition approached its maximum and, thus, they most strongly influenced its recovery phase. Since cutaneous stimulation does not modify the test reflex when given alone, it is likely that there must be convergence on common premotoneuronal interneurones. Indirect evaluation of central delay suggests that the cutaneous afferent fibres from the foot have oligosynaptic spinal connections with interneurones belonging to the group I pathways to the antagonists. Our findings furnish additional evidence that short-latency inhibition of soleus motoneurones after a single conditioning stimulation of group I afferents from the tibialis anterior muscle constitutes a true example of disynaptic Ia reciprocal inhibition in man.

Assessing changes in presynaptic inhibition of I a fibres: a study in man and the cat

1. A method to assess changes in presynaptic inhibition of I a afferent terminals in man is proposed. The soleus H reflex was facilitated by a heteronymous I a volley from quadriceps and the amount of reflex facilitation was used to estimate the size of the conditioning I a excitatory post-synaptic potential (e.p.s.p.). It is argued that the size of this e.p.s.p. as measured by the resulting amount of reflex facilitation reflects the amount of presynaptic inhibition on the corresponding I a fibres. A decrease in the reflex facilitation may then be ascribed to an increase in presynaptic inhibition of the I a fibres mediating the conditioning volley. 2. That the heteronymous I a facilitation from quadriceps to soleus is caused by a purely monosynaptic e.p.s.p. is a prerequisite for the validity of the method. Experimental evidence is therefore given in an Appendix that in man the earliest part (first 0.5 ms) of this heteronymous I a facilitation is mediated through a monosynaptic pathway. Evidence is also given that this earliest facilitation is not yet contaminated by any polysynaptic effects from I a or I b afferents. 3. The validity of the method was established in animal experiments in which presynaptic inhibition of I a fibres and post-synaptic events in motoneurones could be assessed by direct tests. It was found that the amount of test reflex facilitation produced by a conditioning I a volley was decreased when I a fibres were subjected to presynaptic inhibition but remained unchanged when the motoneurone pool in which the test reflex was elicited received pure post-synaptic inhibition. 4. In man, presynaptic inhibition of I a fibres was evoked by a short-lasting (three shocks at 200 Hz) vibration applied to the tibialis anterior tendon. Such a vibratory burst reduced the efficiency of the heteronymous I a volley in facilitating the soleus H reflex. By contrast, during a pure post-synaptic inhibition of soleus motoneurones the efficiency of the conditioning volley in facilitating the test reflex remained unchanged. It is therefore argued that the amount of heteronymous I a facilitation can indeed be used to assess the amount of ongoing presynaptic inhibition exerted onto heteronymous I a fibres from the quadriceps muscle to soleus motoneurones. 5. The short-lasting tibialis anterior vibration used here evoked a long-lasting (300-500 ms) depression of soleus and quadriceps H reflexes.(ABSTRACT TRUNCATED AT 400 WORDS)

Sources of input to interneurones mediating group I non-reciprocal inhibition of motoneurones in the cat

Intracellular recordings have been made from laminae V-VI interneurones interposed in pathways of non-reciprocal inhibition of motoneurones from group I afferents of triceps surae and/or plantaris. A comparison of actions of brief stretches of triceps surae and plantaris with actions of electrical stimulation of nerves of these muscles revealed that I a afferents influenced about a half of the interneurones excited by I b afferents. Electrical stimulation of seven muscles nerves, three cutaneous nerves, posterior knee joint and interosseous nerves, the red nucleus and the pyramidal tract, evoked excitatory post-synaptic potentials (e.p.s.p.s) in various proportions of interneurones. Most of the interneurones were excited monosynaptically, or both monosynaptically and disynaptically by group I afferents. This, together with a very small proportion of interneurones in which e.p.s.p.s were evoked only disynaptically, indicates that the same last-order interneurones may subserve di- and trisynaptically evoked inhibition of motoneurones. Other fibre systems excited these interneurones monosynaptically (interosseal, rubro- and corticospinal), disynaptically (cutaneous, rubro- and corticospinal) and polysynaptically. The coupling of the earliest e.p.s.p.s from group II and joint afferents could not be established, but was consistent with their mediation by only one or two interposed interneurones. Inhibition was evoked from all fibre systems with excitatory input to the same or to other interneurones of the sample, except from group II afferents, the effects of which were found in a much smaller number of cells and may not be fully representative.

Crossed actions of group I muscle afferents in the cat

Reflex actions evoked by electrical stimulation of contralateral quadriceps, hamstring and gastrocnemius-soleus muscle afferents were investigated using intracellular recording from motoneurones, in chloralose-anaesthetized, acute, low spinal cats. Contralateral group I afferents were found to evoke excitatory post-synaptic potentials only occasionally. Contralateral group II afferents evoked post-synaptic potentials (excitatory to extensor motoneurones and inhibitory to flexor motoneurones) from the quadriceps nerve, whereas contralateral group III afferents evoked post-synaptic potentials (with the same pattern as quadriceps group II) from all muscles tested. Contralateral group I afferents were found to facilitate Ia reciprocal inhibition, and both the excitatory and inhibitory reflex actions from Ib afferents. This is taken to indicate an action of these afferents on interneurones interposed in ipsilateral reflex pathways.