Primary afferent depolarization of muscle afferents elicited by stimulation of joint afferents in cats with intact neuraxis and during reversible spinalization

Nociception induces a differential presynaptic modulation of the synaptic efficacy of nociceptive and proprioceptive joint afferents

A previous study has indicated that during the state of central sensitization induced by the intradermic injection of capsaicin, there is a gradual facilitation of the dorsal horn neuronal responses produced by stimulation of the high-threshold articular afferents that is counteracted by a concurrent increase of descending inhibitory actions. Since these changes occurred without significantly affecting the responses produced by stimulation of the low-threshold articular afferents, it was suggested that the capsaicin-induced descending inhibition included a preferential presynaptic modulation of the synaptic efficacy of the slow conducting nociceptive joint afferents (Ramírez-Morales et al., Exp Brain Res 237:1629-1641, 2019). The present study was aimed to investigate more directly the contribution of presynaptic mechanisms in this descending control. We found that in the barbiturate anesthetized cat, stimulation of the high-threshold myelinated afferents in the posterior articular nerve (PAN) produces primary afferent hyperpolarization (PAH) in the slow conducting (25-35 m/s) and primary afferent depolarization (PAD) in the fast conducting (40-50 m/s) articular fibers. During the state of central sensitization induced by capsaicin, there is a supraspinally mediated shift of the autogenic PAH to PAD that takes place in the slow conducting fibers, basically without affecting the autogenic PAD generated in the fast conducting afferents. It is suggested that the change of presynaptic facilitation to presynaptic inhibition induced by capsaicin on the slow articular afferents is part of an homeostatic process aimed to keep the nociceptive-induced neuronal activity within manageable limits while preserving the proprioceptive information required for proper control of movement.

Direct evidence for decreased presynaptic inhibition evoked by PBSt group I muscle afferents after chronic SCI and recovery with step-training in rats

KEY POINTS

Presynaptic inhibition is modulated by supraspinal centres and primary afferents in order to filter sensory information, adjust spinal reflex excitability, and ensure smooth movement. After spinal cord injury (SCI), the supraspinal control of primary afferent depolarization (PAD) interneurons is disengaged, suggesting an increased role for sensory afferents. While increased H-reflex excitability in spastic individuals indicates a possible decrease in presynaptic inhibition, it remains unclear whether a decrease in sensory-evoked PAD contributes to this effect. We investigated whether the PAD evoked by hindlimb afferents contributes to the change in presynaptic inhibition of the H-reflex in a decerebrated rat preparation. We found that chronic SCI decreases presynaptic inhibition of the plantar H-reflex through a reduction in PAD evoked by posterior biceps-semitendinosus (PBSt) muscle group I afferents. We further found that step-training restored presynaptic inhibition of the plantar H-reflex evoked by PBSt, suggesting the presence of activity-dependent plasticity of PAD pathways activated by flexor muscle group I afferents.

ABSTRACT

Spinal cord injury (SCI) results in the disruption of supraspinal control of spinal networks and an increase in the relative influence of afferent feedback to sublesional neural networks, both of which contribute to enhancing spinal reflex excitability. Hyperreflexia occurs in ∼75% of individuals with a chronic SCI and critically hinders functional recovery and quality of life. It is suggested that it results from an increase in motoneuronal excitability and a decrease in presynaptic and postsynaptic inhibitory mechanisms. In contrast, locomotor training decreases hyperreflexia by restoring presynaptic inhibition. Primary afferent depolarization (PAD) is a powerful presynaptic inhibitory mechanism that selectively gates primary afferent transmission to spinal neurons to adjust reflex excitability and ensure smooth movement. However, the effect of chronic SCI and step-training on the reorganization of presynaptic inhibition evoked by hindlimb afferents, and the contribution of PAD has never been demonstrated. The objective of this study is to directly measure changes in presynaptic inhibition through dorsal root potentials (DRPs) and its association with plantar H-reflex inhibition. We provide direct evidence that H-reflex hyperexcitability is associated with a decrease in transmission of PAD pathways activated by posterior biceps-semitendinosus (PBSt) afferents after chronic SCI. More precisely, we illustrate that the pattern of inhibition evoked by PBSt group I muscle afferents onto both L4-DRPs and plantar H-reflexes evoked by the distal tibial nerve is impaired after chronic SCI. These changes are not observed in step-trained animals, suggesting a role for activity-dependent plasticity to regulate PAD pathways activated by flexor muscle group I afferents.

Early pre- and postsynaptic decrease in glutamatergic and cholinergic signaling after spinalization is not modified when stimulating proprioceptive input to the ankle extensor α-motoneurons: Anatomical and neurochemical study

Alpha-motoneurons (MNs) innervating ankle extensor muscles show reduced peripheral inputs from Ia proprioceptive afferents and cholinergic afferents after chronic spinalization (SCT). That phenomenon is not observed on ankle flexor MNs, indicating a smaller vulnerability of the latter MNs circuit to SCT. Locomotor training of spinal rats which partially restored those inputs to extensor MNs tended to hyper innervate flexor MNs, disclosing a need for selective approaches. In rats with intact spinal cord 7-days of low-threshold proprioceptive stimulation of the tibial nerve enriched glutamatergic Ia and cholinergic innervation of lateral gastrocnemius (LG) MNs, suggesting usefulness of selective stimulation for restoration of inputs to extensor MNs after SCT. Accordingly, to examine its effectiveness after SCT, tibial nerves and soleus muscles were implanted bilaterally, and for MN identification fluorescence tracers to LG and tibialis anterior (TA) muscles were injected two weeks prior to spinalization. Stimulation of tibial nerve, controlled by H-reflex recorded in the soleus muscle, started on the third post-SCT day and continued for 7 days. Nine days post-SCT the number and volume of glutamatergic Ia and of cholinergic C-boutons on LG MNs was decreased, but stimulation affected neither of them. Postsynaptically, a threefold decrease of NMDAR NR1 subunit and thirtyfold decrease of M2 muscarinic receptor transcripts caused by SCT were not counteracted by stimulation, whereas a threefold decrease of AMPAR GluR2 subunit tended to deepen after stimulation. We conclude that LG MNs, supported with proprioceptive stimuli after SCT, do not transcribe the perceived cues into compensatory response at the transcriptional level in the early post-SCT period.

Descending inhibition selectively counteracts the capsaicin-induced facilitation of dorsal horn neurons activated by joint nociceptive afferents

Previous studies from our laboratory showed that in the anesthetized cat, the intradermal injection of capsaicin in the hindpaw facilitated the intraspinal field potentials (IFPs) evoked by stimulation of the intermediate and high-threshold myelinated fibers in the posterior articular nerve (PAN). The capsaicin-induced facilitation was significantly reduced 3-4 h after the injection, despite the persistence of hindpaw inflammation. Although this effect was attributed to an incremented descending inhibition acting on the spinal pathways, it was not clear if it was set in operation once the capsaicin-induced effects exceeded a certain threshold, or if it was continuously operating to keep the increased neuronal activation within manageable limits. To evaluate the changes in descending inhibition, we now examined the effects of successive reversible spinal blocks on the amplitude of the PAN IFPs evoked at different times after the intradermal injection of capsaicin. We found that after capsaicin the PAN IFPs recorded in laminae III-V by activation of high-threshold nociceptive Aδ myelinated fibers increased gradually during successive reversible spinal blocks, while the IFPs evoked by intermediate and low threshold proprioceptive Aβ afferents were only slightly affected. It is concluded that during the development of the central sensitization produced by capsaicin, there is a gradual increase of descending inhibition that tends to limit the nociceptive-induced facilitation, mainly by acting on the neuronal populations receiving inputs from the capsaicin-activated afferents without significantly affecting the information on joint angle transmitted by the low threshold afferents.

Assessment of the H-reflex at two contraction levels before and after fatigue

The aim of the present study was to compare the H-reflex evoked at rest and at 20% maximal voluntary contraction (MVC) prior to and after fatiguing the lateral gastrocnemius (LG). The maximal H-reflex and M-wave were recorded in the LG, and soleus (SOL). Electrical evoked potentials were delivered to the posterior tibial nerve when muscles were inactivated and at 20% MVC. After fatigue, the H_max /M_max ratio of the fatigued LG was increased for both contraction levels (rest and 20% MVC) and remained unaltered for non-fatigued SOL. Before fatigue, the H_max /M_max ratio of SOL was enhanced at rest compared with the H_max /M_max ratio at 20% MVC. No differences were observed for LG. Fatigue of a single muscle leads to increased spinal reflex activity of the homonymous muscle. Contrary to previous recommendations in the literature, there appears to be no benefit with regard to the H-reflex amplitude in evoking electrical potentials during constant voluntary contractions at 20% MVC compared with inactivated muscles. The observed difference in SOL prior to fatigue was most likely due to hyperpolarization of the muscle fiber membrane.

The effects of experimental knee pain on lower limb corticospinal and motor cortex excitability

Introduction

Notable weakness of the quadriceps muscles is typically observed as a consequence of knee joint arthritis, knee surgery and knee injury. This is partly due to ongoing neural inhibition that prevents the central nervous system from fully activating the quadriceps, a process known as arthrogenic muscle inhibition (AMI). To investigate the mechanisms underlying AMI, this study explored the effects of experimental knee pain on lower limb corticospinal and motor cortex excitability.

Methods

Twenty-four healthy volunteers participated in this study. In experiment 1, experimental knee pain was induced by the injection of hypertonic saline into the infrapatellar fat pad (n = 18). In experiment 2, isotonic saline was injected into the fat pad as a non-painful control (n = 8). Pain intensity was measured on a 10-cm electronic visual analogue scale. Transcranial magnetic stimulation and electromyography were used to measure lower limb motor-evoked potential amplitude and short-interval intracortical inhibition before and after the injection.

Results

The peak VAS score following hypertonic saline (5.0 ± 0.5 cm) was higher than after isotonic saline (p <0.001). Compared with baseline, there was a significant increase in vastus lateralis (p = 0.02) and vastus medialis motor-evoked potential amplitude (p = 0.02) during experimental knee pain that was not apparent during the control condition. Biceps femoris and tibialis anterior motor-evoked potential amplitude did not change following injection (all p >0.05). There was no change in short-interval intracortical inhibition measured from vastus lateralis following injection (both p >0.05).

Conclusions

Quadriceps corticospinal excitability increases during experimental knee pain, providing no evidence for a supraspinal contribution to quadriceps AMI.

Recovery of neuronal and network excitability after spinal cord injury and implications for spasticity

The state of areflexia and muscle weakness that immediately follows a spinal cord injury (SCI) is gradually replaced by the recovery of neuronal and network excitability, leading to both improvements in residual motor function and the development of spasticity. In this review we summarize recent animal and human studies that describe how motoneurons and their activation by sensory pathways become hyperexcitable to compensate for the reduction of functional activation of the spinal cord and the eventual impact on the muscle. Specifically, decreases in the inhibitory control of sensory transmission and increases in intrinsic motoneuron excitability are described. We present the idea that replacing lost patterned activation of the spinal cord by activating synaptic inputs via assisted movements, pharmacology or electrical stimulation may help to recover lost spinal inhibition. This may lead to a reduction of uncontrolled activation of the spinal cord and thus, improve its controlled activation by synaptic inputs to ultimately normalize circuit function. Increasing the excitation of the spinal cord with spared descending and/or peripheral inputs by facilitating movement, instead of suppressing it pharmacologically, may provide the best avenue to improve residual motor function and manage spasticity after SCI.

Removal of supraspinal input reveals a difference in the flexor and extensor monosynaptic reflex response to quipazine independent of motoneuron excitation

The purpose of this study was to determine if quipazine, a serotonergic agonist, differentially modulates flexor and extensor motor output. This was achieved by examining the monosynaptic reflex (MSR) of the tibial (extensor) and peroneal (flexor) nerves, by determining the basic and rhythmic properties of extensor and flexor motoneurons, and by recording extracellular Ia field potentials of the tibial and peroneal nerves in the in vivo adult decerebrate rat in both spinal intact and acute spinalized preparations. In the spinal intact preparation, the tibial and peroneal MSR amplitude significantly increased compared with baseline in response to quipazine, with no difference between nerves ( P < 0.05). In the spinalized preparation, the MSR was significantly increased in both the tibial and peroneal nerves with the latter increasing more than the former (5.7 vs. 3.6 times; P < 0.05). Intracellular motoneuron experiments demonstrated that rheobase decreased, while input resistance, afterhyperpolarization amplitude, and the firing rate at a given current injection increased in motoneurons following quipazine administration with no differences between extensor and flexor motoneurons. Both the tibial and peroneal nerve extracellular Ia field potentials increased with the peroneal demonstrating a significantly greater increase (7 vs. 38%; P < 0.05) following quipazine. It is concluded that in the spinal intact preparation quipazine does not have a differential effect on flexor or extensor motor output. However, in the acute spinalized preparation, quipazine preferentially affects the flexor MSR compared with the extensor MSR, likely due to the removal of a descending tonic inhibition on flexor Ia afferents.

Synchronous and asynchronous electrically evoked motor activities during wind-up stimulation are differentially modulated following an acute spinal transection

In this study, we used a novel technique to study reflex wind-up when the spinal cord is intact and following an acute spinal transection. Specifically, we evaluated reflex responses evoked by a series of 10 electrical pulses to the tibial or superficial peroneal nerves in 9 decerebrate adult cats, before and after an acute spinal transection. Electromyograms were recorded in four hindlimb muscles (lateral gastrocnemius, tibialis anterior, semitendinosus, and sartorius) to evaluate reflex amplitude, duration, and the temporal summation of reflex responses, so-called wind-up. We identified two distinct reflex responses evoked by electrical stimulation of the tibial or superficial peroneal nerves on the basis of their pattern of change following acute spinal transection, a short-latency (∼10 ms) compound action potential (CAP) that was followed by a burst of sustained activity (SA). Wind-up of CAP and SA amplitudes was clearly present when the spinal cord was intact but was drastically reduced after acute spinalization in some muscles. Moreover, CAP and SA reflex responses were differentially modified by the acute spinalization. When the effects of acute spinal transection were significant, CAP responses were increased after acute spinalization, whereas SA responses were reduced, suggesting that the two signals are regulated by different neuronal mechanisms. The present results provide the first assessment of reflex wind-up before and after an acute spinal transection in the same animals and indicate that different reflex components must be considered separately when evaluating changes in neuronal excitability following SCI.

Altered activation patterns by triceps surae stretch reflex pathways in acute and chronic spinal cord injury

Spinal reflexes are modified by spinal cord injury (SCI) due the loss of excitatory inputs from supraspinal structures and changes within the spinal cord. The stretch reflex is one of the simplest pathways of the central nervous system and was used presently to evaluate how inputs from primary and secondary muscle spindles interact with spinal circuits before and after spinal transection (i.e., spinalization) in 12 adult decerebrate cats. Seven cats were spinalized and allowed to recover for 1 mo (i.e., chronic spinal state), whereas 5 cats were evaluated before (i.e., intact state) and after acute spinalization (i.e., acute spinal state). Stretch reflexes were evoked by stretching the left triceps surae (TS) muscles. The force evoked by TS muscles was recorded along with the activity of several hindlimb muscles. Stretch reflexes were abolished in the acute spinal state due to an inability to activate TS muscles, such as soleus (Sol) and lateral gastrocnemius (LG). In chronic spinal cats, reflex force had partly recovered but Sol and LG activity remained considerably depressed, despite the fact that injecting clonidine could recruit these muscles during locomotor-like activity. In contrast, other muscles not recruited in the intact state, most notably semitendinosus and sartorius, were strongly activated by stretching TS muscles in chronic spinal cats. Therefore, stretch reflex pathways from TS muscles to multiple hindlimb muscles undergo functional reorganization following spinalization, both acute and chronic. Altered activation patterns by stretch reflex pathways could explain some sensorimotor deficits observed during locomotion and postural corrections after SCI.

Quadriceps arthrogenic muscle inhibition: neural mechanisms and treatment perspectives

OBJECTIVES

Arthritis, surgery, and traumatic injury of the knee joint are associated with long-lasting inability to fully activate the quadriceps muscle, a process known as arthrogenic muscle inhibition (AMI). The goal of this review is to provide a contemporary view of the neural mechanisms responsible for AMI as well as to highlight therapeutic interventions that may help clinicians overcome AMI.

METHODS

An extensive literature search of electronic databases was conducted including AMED, CINAHL, MEDLINE, OVID, SPORTDiscus, and Scopus.

RESULTS

While AMI is ubiquitous across knee joint pathologies, its severity may vary according to the degree of joint damage, time since injury, and knee joint angle. AMI is caused by a change in the discharge of articular sensory receptors due to factors such as swelling, inflammation, joint laxity, and damage to joint afferents. Spinal reflex pathways that likely contribute to AMI include the group I nonreciprocal (Ib) inhibitory pathway, the flexion reflex, and the gamma-loop. Preliminary evidence suggests that supraspinal pathways may also play an important role. Some of the most promising interventions to counter the effects of AMI include cryotherapy, transcutaneous electrical nerve stimulation, and neuromuscular electrical stimulation. Nonsteroidal anti-inflammatory drugs and intra-articular corticosteroids may also be effective when a strong inflammatory component is present with articular pathology.

CONCLUSIONS

AMI remains a significant barrier to effective rehabilitation in patients with arthritis and following knee injury and surgery. Gaining a better understanding of AMI's underlying mechanisms will allow the development of improved therapeutic strategies, enhancing the rehabilitation of patients with knee joint pathology.

Changes in synaptic effectiveness of myelinated joint afferents during capsaicin-induced inflammation of the footpad in the anesthetized cat

The present series of experiments was designed to examine, in the anesthetized cat, the extent to which the synaptic efficacy of knee joint afferents is modified during the state of central sensitization produced by the injection of capsaicin into the hindlimb plantar cushion. We found that the intradermic injection of capsaicin increased the N2 and N3 components of the focal potentials produced by stimulation of intermediate and high threshold myelinated fibers in the posterior articular nerve (PAN), respectively. This facilitation lasted several hours, had about the same time course as the paw inflammation and was more evident for the N2 and N3 potentials recorded within the intermediate zone in the L6 than in the L7 spinal segments. The capsaicin-induced facilitation of the N2 focal potentials, which are assumed to be generated by activation of fibers signaling joint position, suggests that nociception may affect the processing of proprioceptive and somato-sensory information and, probably also, movement. In addition, the increased effectiveness of these afferents could activate, besides neurons in the intermediate region, neurons located in the more superficial layers of the dorsal horn. As a consequence, normal joint movements could produce pain representing a secondary hyperalgesia. The capsaicin-induced increased efficacy of the PAN afferents producing the N3 focal potentials, together with the reduced post-activation depression that follows high frequency autogenetic stimulation of these afferents, could further contribute to the pain sensation from non-inflamed joints during skin inflammation in humans. The persistence, after capsaicin, of the inhibitory effects produced by stimulation of cutaneous nerves innervating non-inflamed skin regions may account for the reported reduction of the articular pain sensations produced by trans-cutaneous stimulation.

Tonic and phasic differential GABAergic inhibition of synaptic actions of joint afferents in the cat

The aim of this study was to examine the functional organization of the spinal neuronal networks activated by myelinated afferent fibers in the posterior articular nerve (PAN) of the anesthetized cat. Particular attention was given to the tonic and phasic GABAa inhibitory modulation of these networks. Changes in the synaptic effectiveness of the joint afferents were inferred from changes in the intraspinal focal potentials produced by electrical stimulation of the PAN. We found that conditioning stimulation of cutaneous nerves (sural, superficial peroneus and saphenous) and of the nucleus raphe magnus often inhibited, in a differential manner, the early and late components of the intraspinal focal potentials produced by stimulation of low and high threshold myelinated PAN afferents, respectively. The degree of the inhibition depended on the strength of both the conditioning and test stimuli and on the segmental level of recording. Conditioning stimulation of group I muscle afferents was less effective, but marked depression of the early and late focal potentials was produced by stimuli exceeding 5 xT. The i.v. injection of 1-2.5 mg/kg of picrotoxin, a GABAa blocker, had relatively minor effects on the early components of the PAN focal potentials, but was able to induce a significant increase of the late components. It also reduced the inhibitory effects of cutaneous and joint nerve conditioning on PAN focal responses. Conditioning autogenetic stimulation with high-frequency trains depressed the PAN focal potentials. The late components of the PAN responses remained depressed several minutes after discontinuing the conditioning train, even after picrotoxin administration. The present observations indicate that the neuronal networks activated by the low threshold PAN afferents show a relatively small post-activation depression and appear to be subjected to a minor tonic inhibitory GABAa control. In contrast, the pathways activated by stimulation of high threshold myelinated afferents have a strong post-activation depression and are subjected to a significant tonic GABAergic modulation. These contrasting features, together with the phasic differential GABAergic inhibition of the responses produced by stimulation of the different populations of joint afferents, may contribute to the preservation of the original information on joint position transmitted by large diameter joint afferents, in contrast with the tonic presynaptic inhibition exerted on the fine myelinated joint afferents, which may be involved in the adjustment of compensatory reactions to inflammation.

Patterns of primary afferent depolarization of segmental and ascending intraspinal collaterals of single joint afferents in the cat

We have examined in the anesthetized cat the threshold changes produced by sensory and supraspinal stimuli on intraspinal collaterals of single afferents from the posterior articular nerve (PAN). Forty-eight fibers were tested in the L3 segment, in or close to Clarke's column, and 70 fibers in the L6-L7 segments within the intermediate zone. Of these, 15 pairs of L3 and L6-L7 collaterals were from the same afferent. Antidromically activated fibers had conduction velocities between 23 and 74 m/s and peripheral thresholds between 1.1 and 4.7 times the threshold of the most excitable fibers (xT), most of them below 3 xT. PAN afferents were strongly depolarized by stimulation of muscle afferents and by cutaneous afferents, as well as by stimulation of the bulbar reticular formation and the midline raphe nuclei. Stimulation of muscle nerves (posterior biceps and semitendinosus, quadriceps) produced a larger PAD (primary afferent depolarization) in the L6-L7 than in the L3 terminations. Group II were more effective than group I muscle afferents. As with group I muscle afferents, the PAD elicited in PAN afferents by stimulation of muscle nerves could be inhibited by conditioning stimulation of cutaneous afferents. Stimulation of the cutaneous sural and superficial peroneal nerves increased the threshold of few terminations (i.e., produced primary afferent hyperpolarization, PAH) and reduced the threshold of many others, particularly of those tested in the L6-L7 segments. Yet, there was a substantial number of terminals where these conditioning stimuli had minor or no effects. Autogenetic stimulation of the PAN with trains of pulses increased the intraspinal threshold in 46% and reduced the threshold in 26% of fibers tested in the L6-L7 segments (no tests were made with trains of pulses on fibers ending in L3). These observations indicate that PAN afferents have a rather small autogenetic PAD, particularly if this is compared with the effects of heterogenetic stimulation. Therefore, the depression of the PAN intraspinal fields produced by autogenetic stimulation described by Rudomin et al. (Exp Brain Res DOI 10.1007/s00221-006-0600-x, 2006) may be ascribed to other mechanisms besides a GABAa PAD. It is suggested that the small or no autogenetic PAD displayed by the examined joint afferents prevents presynaptic filtering of their synaptic actions and preserves the original information generated in the periphery. This could be important for proper adjustment of limb position.

Tendon reflexes for predicting movement recovery after acute spinal cord injury in humans

OBJECTIVE

Use the tendon reflex to examine spinal cord excitability after acute spinal cord injury (SCI), relating excitability findings to prognosis.

METHODS

We conducted repeated measures of reflex responses to mechanical taps at the patellar and Achilles tendons of the lower limbs, and the wrist flexor tendons of the upper limbs in persons with acute SCI, beginning as early as the day of injury. The single largest EMG response (peak-to-peak) for each site was recorded. Subjects were compared based on level of injury and final neurologic status of lower limb motor function (i.e. absence of any voluntary recruitment in a lower limb muscle: motor-complete; voluntary recruitment in 1 or more lower-limb muscles: motor-incomplete).

RESULTS

We studied 229 subjects with acute SCI. Persons with injury to the cervical or thoracic spinal cord and who were (or became) motor-incomplete showed large tendon responses, even at the time of initial evaluation. In combination with larger tendon response amplitudes, the presence of the 'crossed-adductor' response to patellar tendon taps at the acute stage was highly predictive of functional motor recovery following SCI. In marked contrast, tendon responses were small (e.g. < 0.1 mV) or absent in persons with acute, motor-complete injury (and which remained motor-complete), and the crossed-adductor response was never seen. Reflex amplitudes and the incidence of the crossed-adductor response increased somewhat over time in persons with motor-complete SCI, but did not approach the values seen in motor-incomplete subjects.

CONCLUSIONS

Taken together, tendon response amplitude and reflex spread were sensitive and specific indicators of preserved supraspinal control over lower limb musculature in subjects with acute SCI. A simple algorithm using these outcome measures predicted a 'motor-complete' status with 100% accuracy, and a motor-incomplete status with accuracy exceeding 91%.

Tonic differential supraspinal modulation of PAD and PAH of segmental and ascending intraspinal collaterals of single group I muscle afferents in the cat spinal cord

We compared in the anesthetized cat the effects of reversible spinalization by cold block on primary afferent depolarization (PAD) and primary afferent hyperpolarization (PAH) elicited in pairs of intraspinal collaterals of single group I afferents from the gastrocnemius nerve, one of the pairs ending in the L3 segment, around the Clarke's column nuclei, and the other in the L6 segment within the intermediate zone. PAD in each collateral was estimated by independent computer-controlled measurement of the intraspinal current required to maintain a constant probability of antidromic firing. The results indicate that the segmental and ascending collaterals of individual afferents are subjected to a tonic PAD of descending origin affecting in a differential manner the excitatory and inhibitory actions of cutaneous and joint afferents on the pathways mediating the PAD of group I fibers. The PAD-mediating networks appear to function as distributed systems whose output will be determined by the balance of the segmental and supraspinal influences received at that moment. It is suggested that the descending differential modulation of PAD enables the intraspinal arborizations of the muscle afferents to function as dynamic systems, in which information transmitted to segmental reflex pathways and to Clarke's column neurons by common sources can be decoupled by sensory and descending inputs, and funneled to specific targets according to the motor tasks to be performed.

Pre- and post-synaptic control of motoneuron excitability in athletes

PURPOSE

The purpose of this study was to examine the efficacy of two spinal mechanisms in gating motoneuron excitability in power-trained athletes (N = 9), endurance-trained athletes (N = 9), and untrained subjects (N = 9).

METHODS

The dependent variable for each protocol was the peak-to-peak amplitude of the conditioned soleus Hoffmann reflex (H-reflex). Modulations of the test reflex amplitude were evaluated for each subject by using two experimental conditioning protocols: recurrent inhibition (RI) and paired-reflex depression (PRD). Also, to assess the effects of different levels of input on motoneuron excitability, two H-reflex stimulus intensities were used (10% and 30% of maximal motor response (M-max)). For each protocol, seven conditioned H-reflex trials were obtained from each subject during quiet stance. The RI protocol consisted of two reflex responses that were separated by 10 ms. The first was of the same intensity as the unconditioned trials. The second stimulus was of an intensity great enough to elicit a supramaximal motor response. The PRD protocol utilized two reflex stimuli of the same intensity separated by 80 ms. A group by intensity (3 x 2) analysis of variance was performed to determine group differences within each condition.

RESULTS

For both protocols, increases in stimulus intensity resulted in significantly greater inhibition. Significant differences were observed between the trained groups for both the RI and the PRD protocols. For the RI protocol, the endurance-trained athletes demonstrated significantly less RI than either the power-trained athletes or the untrained subjects. For the PRD protocol, the endurance-trained athletes demonstrated significantly greater PRD than either the power-trained athletes or the untrained subjects.

CONCLUSIONS

These observations indicate differential control of motoneuron excitability as a result of segmental reflex pathways among differently trained athletes.

Central control of information transmission through the intraspinal arborizations of sensory fibers examined 100 years after Ramón y Cajal

About 100 years ago, Santiago Ramón y Cajal reported that sensory fibers entering the spinal cord have ascending and descending branches, and that each of them sends collaterals to the gray matter where they have profuse ramifications. To him this was a fundamental discovery and proposed that the intraspinal branches of the sensory fibers were "centripetal conductors by which sensory excitation is propagated to the various neurons in the gray matter". In addition, he assumed that "conduction of excitation within the intraspinal arborizations of the afferent fibers would be proportional to the diameters of the conductors", and that excitation would preferentially flow through the coarsest branches. The invariability of some elementary reflexes such as the knee jerk would be the result of a long history of plastic adaptations and natural selection of the safest neuronal organizations. There is now evidence suggesting that in the adult cat, the intraspinal branches of sensory fibers are not hard wired routes that diverge excitation to spinal neurons in an invariable manner, but rather dynamic pathways where excitation flow can be centrally addressed to reach specific neuronal targets. This central control of information flow is achieved by means of specific sets of GABAergic interneurons that produce primary afferent depolarization (PAD) via axo-axonic synapses and reduce transmitter release (presynaptic inhibition). The PAD produced by single, or by small groups of GABAergic interneurons in group I muscle afferents, can remain confined to some sets of intraspinal arborizations of the afferent fibers and not spread to nearby collaterals. In muscle spindle afferents this local character of PAD allows cutaneous and descending inputs to differentially inhibit the PAD in segmental and ascending collaterals of individual fibers, which may be an effective way to decouple the information flow arising from common sensory inputs. This feature appears to play an important role in the selection of information flow in muscle spindles that occurs at the onset of voluntary contractions in humans.

Pain behavior and response properties of spinal dorsal horn neurons following experimental diabetic neuropathy in the rat: modulation by nitecapone, a COMT inhibitor with antioxidant properties

We attempted to characterize a spinal neuronal correlate of painful neuropathy induced by diabetes mellitus (DM). Pain behavior and response properties of spinal dorsal horn neurons were determined in rats with a streptozocin-induced DM. A catechol-O-methyltransferase inhibitor with potent antioxidant properties, nitecapone, was used in an attempt to attenuate neuropathic symptoms. Behaviorally DM induced mechanical hypersensitivity that was markedly attenuated by oral treatment with nitecapone. The antihyperalgesic effect of nitecapone was not reversed by naloxone, an opioid antagonist, or atipamezole, an alpha-2-adrenoceptor antagonist. Electrophysiological recordings performed in pentobarbitone-anesthetized animals revealed that the most distinct abnormality in response properties of spinal dorsal horn wide-dynamic range (WDR) neurons was the increase in their spontaneous activity observed in untreated but not in nitecapone-treated DM rats. Conditioning electrical stimulation and a lidocaine block of the rostroventromedial medulla (RVM) had a similar modulatory effect on evoked responses of spinal dorsal horn WDR neurons in all experimental groups. The response properties of spinal dorsal horn nociceptive-specific or low-threshold mechanoreceptive neurons were not markedly different between the experimental groups. The results indicate that increased spontaneous activity in spinal dorsal horn WDR neurons may be causally related to behaviorally observed mechanical hypersensitivity in DM. Attenuation of the increased spontaneous activity in WDR neurons may explain the antihyperalgesic effect by nitecapone, due to naloxone- and alpha-2-adrenoceptor-insensitive mechanisms. DM or nitecapone treatment did not produce significant changes in phasic or tonic descending pain regulation originating in the RVM.

Presynaptic excitability changes of group Ia fibres to muscle nociceptive stimulation in humans

Tonic muscle nociceptive discharge evoked chemically from the foot extensor digitorum brevis muscle in man produces a depression of Ia excitation and Ib inhibition of the soleus (Sol) motoneurones (Mns). The possibility that both these changes partly result from presynaptic inhibition of Ia fibres projecting to Sol Mns and to interneurones mediating group I non-reciprocal inhibition is tested. Convergence of Ia fibres on these interneurones was deduced from evidence that reducing the excitatory effect of the extensor Ia fibres (by potentiation of their presynaptic inhibition) resulted in Ib disinhibition. Nociceptive-induced potentiation of Ia presynaptic inhibition was deduced from the following congruent results obtained by two independent methods: (1) depression of heteronymous Ia monosynaptic facilitation of the quadriceps muscle to Sol Mns; (2) potentiation of presynaptic inhibition of Sol Ia fibres evoked by mechanical activation of the tibialis anterior primary spindle afferents. It is concluded that nociceptive volleys arising from dorsal foot muscles facilitate the activity of interneurones intercalated in pathways responsible for presynaptic inhibition of Sol Ia fibres. It is also proposed that the same Ia presynaptic inhibition depresses the excitability of interneurones mediating group I non-reciprocal inhibition, thus resulting in Ib disinhibition of Sol Mns.

Local control of information flow in segmental and ascending collaterals of single afferents

In the vertebrate spinal cord, the activation of GABA(gamma-amino-butyric acid)-releasing interneurons that synapse with intraspinal terminals of sensory fibres leading into the central nervous system (afferent fibres) produces primary afferent depolarization and presynaptic inhibition. It is not known to what extent these presynaptic mechanisms allow a selective control of information transmitted through specific sets of intraspinal branches of individual afferents. Here we study the local nature of the presynaptic control by measuring primary afferent depolarization simultaneously in two intraspinal collaterals of the same muscle spindle afferent. One of these collaterals ends at the L6-L7 segmental level in the intermediate nucleus, and the other ascends to segment L3 within Clarke's column, the site of origin of spinocerebellar neurons. Our results indicate that there are central mechanisms that are able to affect independently the synaptic effectiveness of segmental and ascending collaterals of individual muscle spindle afferents. Focal control of presynaptic inhibition thus allows the intraspinal branches of afferent fibres to function as a dynamic assembly that can be fractionated to convey information to selected neuronal targets. This may be a mechanism by which different spinal postsynaptic targets that are coupled by sensory input from a common source could be uncoupled.

Changes in PAD patterns of group I muscle afferents after a peripheral nerve crush

In the anesthetized cat we have analyzed the changes in primary afferent depolarization (PAD) evoked in single muscle spindle and tendon organ afferents at different times after their axons were crushed in the periphery and allowed to regenerate. Medial gastrocnemius (MG) afferents were depolarized by stimulation of group I fibers in the posterior biceps and semitendinosus nerve (PBSt), as soon as 2 weeks after crushing their axons in the periphery, in some cases before they could be activated by physiological stimulation of muscle receptors. Two to twelve weeks after crushing the MG nerve, stimulation of the PBSt produced PAD in all MG fibers reconnected with presumed muscle spindles and tendon organs. The mean amplitude of the PAD elicited in afferent fibers reconnected with muscle spindles was increased relative to values obtained from Ia fibers in intact (control) preparations, but remained essentially the same in fibers reconnected with tendon organs. Quite unexpectedly, we found that, between 2 and 12 weeks after crushing the MG nerve, stimulation of the bulbar reticular formation (RF) produced PAD in most afferent fibers reconnected with muscle spindle afferents. The mean amplitude of the PAD elicited in these fibers was significantly increased relative to the PAD elicited in muscle spindle afferents from intact preparations (from 0.08 +/- 0.4 to 0.47 +/- 0.34 mV). A substantial recovery was observed between 6 months and 2.5 years after the peripheral nerve injury. Stimulation of the sural (SU) nerve produced practically no PAD in muscle spindles from intact preparations, and this remained so in those afferents reconnected with muscle spindles impaled 2-12 weeks after the nerve crush. The mean amplitude of the PAD produced in afferent fibers reconnected with tendon organs by stimulation of the PBSt nerve and of the bulbar RF remained essentially the same as the PAD elicited in intact afferents. However, SU nerve stimulation produced a larger PAD in afferents reconnected with tendon organs 2-12 weeks after the nerve crush (mean PAD changed from 0.05 +/- 0.04 to 0.32 +/- 0.17 mV). The results obtained indicate that the PAD patterns of the afferent fibers reconnected with muscle spindle and tendon organ afferents are changed after crushing their axons in the periphery: stimulation of the bulbar RF appears to produce larger PAD in fibers reconnected with muscle spindles, and stimulation of cutaneous afferents produces larger PAD in fibers reconnected with tendon organs. It is suggested that these alterations in the patterns of PAD of muscle afferents result from central changes in the balance of excitatory and inhibitory influences acting on the segmental pathways mediating the PAD. Although the functional role of these changes has not been established, they may reflect compensatory changes aimed to adjust information arising from damaged afferents.

Presynaptic modulation of spinal reflexes

Recent evidence suggests that independent sets of interneurons mediate presynaptic inhibition of primary and secondary muscle spindles and of tendon organ afferents. There is also evidence that the information which flows through different intraspinal collaterals of a single muscle spindle or tendon organ afferent fiber is selectively affected by electrical stimulation of the motor cortex. These studies suggest that presynaptic inhibition plays an important role in the selection of the sensory signals required for the execution of a specific motor task.