Activity Of Renshaw Cells Research Articles

Monosynaptic Rabies Virus Reveals Premotor Network Organization and Synaptic Specificity of Cholinergic Partition Cells

Movement is the behavioral output of neuronal activity in the spinal cord. Motor neurons are grouped into motor neuron pools, the functional units innervating individual muscles. Here we establish an anatomical rabies virus-based connectivity assay in early postnatal mice. We employ it to study the connectivity scheme of premotor neurons, the neuronal cohorts monosynaptically connected to motor neurons, unveiling three aspects of organization. First, motor neuron pools are connected to segmentally widely distributed yet stereotypic interneuron populations, differing for pools innervating functionally distinct muscles. Second, depending on subpopulation identity, interneurons take on local or segmentally distributed positions. Third, cholinergic partition cells involved in the regulation of motor neuron excitability segregate into ipsilaterally and bilaterally projecting populations, the latter exhibiting preferential connections to functionally equivalent motor neuron pools bilaterally. Our study visualizes the widespread yet precise nature of the connectivity matrix for premotor interneurons and reveals exquisite synaptic specificity for bilaterally projecting cholinergic partition cells.

Multiple Modes of Amplification of Synaptic Inhibition to Motoneurons by Persistent Inward Currents

The ability of inhibitory synaptic inputs to dampen the excitability of motoneurons is augmented when persistent inward currents (PICs) are activated. This amplification could be due to an increase in the driving potential of inhibitory synapses or the deactivation of the channels underlying PICs. Our goal was to determine which mechanism leads to the amplification of inhibitory inputs by PICs. To reach this goal, we measured inhibitory postsynaptic currents (IPSCs) in decerebrate cats during somatic voltage-clamp steps. These IPSCs were generated by tonic activation of Renshaw cells. The IPSCs exhibited a rapid rise and a slower decay to a plateau level. Activation of PICs always led to an increase in the peak of the IPSC, but the amount of decay after the peak of the IPSC was inversely related to the size of the IPSC. These results were replicated in simulations based on compartmental models of motoneurons incorporating distributions of Renshaw cell synapses based on anatomical observations, and L-type calcium channels distributed as 100-microm-long hot spots centered 100 to 400 microm away from the soma. For smaller IPSCs, amplification by PICs was due to an increase in the driving force of the inhibitory synaptic current. For larger IPSCs, amplification was caused by deactivation of the channels underlying PICs leading to a lesser decay of the IPSCs. As a result of this change in the time course of the IPSC, deactivation of the channels underlying PICs leads to a greater amplification of the total inhibitory synaptic current.

Activity of Renshaw cells during locomotor-like rhythmic activity in the isolated spinal cord of neonatal mice.

In the present study, we examine the activity patterns of and synaptic inputs to Renshaw cells (RCs) during fictive locomotion in the newborn mouse using visually guided recordings from GABAergic cells expressing glutamic acid decarboxylase 67-green fluorescent protein (GFP). Among the GFP-positive neurons in the lumbar ventral horn, RCs were uniquely identified as receiving ventral root-evoked short-latency EPSPs that were markedly reduced in amplitude by nicotinic receptor blockers mecamylamine or tubocurarine. During locomotor-like rhythmic activity evoked by bath application of 5-HT and NMDA, 50% of the recorded RCs fired in-phase with the ipsilateral L2 flexor-related rhythm, whereas the rest fired in the extensor phase. Each population of RCs fired throughout the corresponding locomotor phase. All RCs received both excitatory and inhibitory synaptic inputs during the locomotor-like rhythmic activity. Blocking nicotinic receptors with mecamylamine markedly reduced the rhythmic excitatory drive, indicating that these rhythmic inputs originate mainly from motor neurons (MNs). Inhibitory synaptic inputs persisted in the presence of the nicotinic blocker. Part of this inhibitory drive and remaining excitatory drive could be from commissural interneurons because the present study also shows that RCs receive direct crossed inhibitory and excitatory synaptic inputs. However, rhythmic synaptic inputs in RCs were also observed in hemicord preparations in the presence of mecamylamine. These results show that, during locomotor activity, RC firing properties are modulated not only by MNs but also by the ipsilateral and contralateral locomotor networks.

Synaptic connections and rhythmic activity of Renshaw cells in GAD67-EGFP knock-in mouse

Synaptic connections and rhythmic activity of Renshaw cells in GAD67-EGFP knock-in mouse

Relative contribution of Ia inhibitory interneurones to inhibition of feline contralateral motoneurones evoked via commissural interneurones

The aim of the study was to examine to what extent the crossed inhibition of hindlimb lumbar alpha-motoneurones is evoked via interneurones that mediate reciprocal inhibition between flexors and extensors (Ia inhibitory interneurones), and to what extent via other spinal interneurones. The crossed inhibition was evoked by reticulospinal and vestibulospinal tract fibres, stimulated in the contralateral medullary longitudinal fascicle and the lateral vestibular nucleus, respectively, or by group II muscle afferents in the contralateral quadriceps nerve. The components of the IPSPs recorded in motoneurones that were mediated by Ia inhibitory interneurones were identified by their depression following activation of Renshaw cells. Trisynaptic components of IPSPs of reticulospinal and vestibulospinal origin, and polysynaptic (but not trisynaptic) components of IPSPs from group II afferents were found to be depressed in the majority of the motoneurones, while disynaptic components, those due to direct actions of inhibitory commissural interneurones, were not depressed. These results indicate that the coordination of left and right hindlimb movements based on crossed inhibition from reticulospinal and vestibulospinal neurones, depends on the degree of activation of Ia inhibitory interneurones by muscle spindle afferents and on their inhibition by Renshaw cells. Our results also indicate that Ia inhibitory interneurones do not operate as last-order inhibitory interneurones in crossed trisynaptic pathways from group II afferents, even though they mediate inhibition evoked by interneurones in shared polysynaptic pathways of crossed flexor and extensor reflexes coactivated by group II and other high-threshold muscle, skin and joint afferents.

Evidence for recurrent inhibition of reciprocal inhibition from soleus to tibialis anterior in man.

Reciprocal inhibition between ankle flexors and extensors has been the subject of numerous studies in Man. They have demonstrated that this reciprocal inhibition is in all likelihood caused by a disynaptic pathway at least partly fed by Ia afferents. It is thus generally agreed that this reciprocally organized inhibition between ankle flexors and extensors in Man is similar to the reciprocal Ia inhibition described in the cat. This conclusion has, however, been challenged, when Jankowska and McCrea described in the cat a non-reciprocal group I inhibition involving interneurones co-excited by Ia and Ib afferents and mediating inhibition to both antagonistic and non-antagonistic motoneurones. The only way to distinguish between reciprocal Ia inhibition and non-reciprocal group I inhibition is to test if the inhibition is blocked by recurrent inhibition, since only Ia interneurones are inhibited by recurrent inhibition. In the present study, reciprocal inhibition from soleus to tibialis anterior was thus investigated following activation of soleus-coupled Renshaw cells in normal human subjects. It was found that reciprocal inhibition induced in tibialis anterior motoneurones by the activation of soleus group I afferents is deeply depressed by activation of soleus-coupled Renshaw cells. This finding provides the missing data to identify disynaptic inhibition between antagonistic ankle muscles as a reciprocal Ia inhibition.

Involvement of spinal recurrent inhibition in spasticity. Further insight into the regulation of Renshaw cell activity.

Changes in the excitability of the soleus H-reflex are were studied after oral administration of L-acetylcarnitine, a cholinomimetic substance, in eight healthy control subjects and 23 spastic patients presenting with slowly progressive paraparesis (n = 10), a cord lesion (n = 9) and a cerebral lesion (n = 4). Changes in the amount of recurrent inhibition of soleus motor neurons at rest were also estimated in order to assess the level of activity of Renshaw cells before and after L-acetylcarnitine administration. Recurrent inhibition elicited by a conditioning reflex discharge (H1) was assessed by a subsequent test reflex (H'). Four patients lacked an H' reflex. In approximately 50% of the remaining patients, recurrent inhibition was normal, while in the other half there was evidence of reduced or absent inhibitory activity at rest. Pooling the data relative to the effect of L-acetylcarnitine on the H-reflex in relation to the strength of recurrent inhibition disclosed that the ratio of peak-to-peak amplitude values of the maximum H reflex to maximum M wave responses (Hmax:Mmax) was reduced in all the cases in which the recurrent inhibition at rest was normal, while such a reduction was never observed in the patients in whom recurrent inhibition was found to be decreased at rest. In the former cases, the size of the H' reflex evoked by the same conditioning H1 discharge was further depressed after L-acetylcarnitine, pointing to a potentiating effect of the drug on Renshaw cells; in the latter cases no such effect was seen. A significant decrease in the mean Hmax:Mmax ratio after L-acetylcarnitine intake was also seen in the healthy control subjects. Possible changes in the amount of presynaptic inhibition on Ia terminals on soleus motor neurons after L-acetylcarnitine were ruled out. It is proposed that the differential effect of the drug on the H-reflex excitability is directly related to the level of Renshaw cell activity, a reduction of which probably follows a lesion interrupting reticulo-spinal pathways with tonic facilitatory influences on Renshaw cells. These findings support the hypothesis that Renshaw cell excitability is set via cortico-reticulo-spinal systems.

Recurrent inhibition between motor nuclei innervating opposing wrist muscles in the human upper limb.

1. Effects of conditioning motor volleys of increasing amplitude on antagonistic motor nuclei were tested at the wrist and elbow level in man. 2. The modifications of biceps, triceps, flexor carpi radialis (FCR) and extensor carpi radialis (ECR) motoneurone excitability were tested both in post-stimulus time histogram (PSTH) and rectified averaged EMG experiments. 3. Conditioning motor volleys were evoked in biceps, triceps, FCR and ECR muscles by electrical stimuli applied to the corresponding nerves. The intensity of the conditioning stimuli was systematically varied from an intensity just below motor threshold to one giving rise to a motor response equal to half of the maximal direct motor response (Mmax). 4. The effect of the injection of a cholinergist agonist, L-acetylcarnitine (L-Ac), on antagonistic conditioning motor volleys was tested for each motor nucleus. 5. Results obtained at the wrist and elbow level were strikingly different. Antagonistic motor volleys resulted in an early and long-lasting inhibition enhanced by L-Ac injection in wrist motor nuclei, while no modification was observed in elbow motor nuclei. 6. The characteristics of the early and long-lasting inhibition evoked in wrist motor nuclei by antagonistic motor volleys suggest that they are due to the activation of Renshaw cells by the conditioning motor volleys. 7. It is therefore concluded that these experiments provide further evidence that the interconnections between motoneurones, interneurones mediating reciprocal inhibition and Renshaw cells at the wrist level are different from those described elsewhere. Indeed, it has already been shown that the interneurones mediating reciprocal inhibition between FCR and ECR are not inhibited by Renshaw cells, and the results presented here suggest that FCR-coupled Renshaw cells inhibit ECR motoneurones and vice versa.

Changes in Spinal Recurrent Inhibition in Patients During the Immediate Post-Stroke Period

Functional abnormalities of segmental spinal cord neurons likely contribute to the patho genesis of spasticity following stroke. Identification of dysfunctional circuit elements responsible for spasticity could lead to rational pharmacotherapy for this set of motor dis orders. The purpose of this study was to clarify the role of the spinal recurrent inhibito ry pathway in the development of spasticity: to seek clinico-electrophysiological corre lations between objective signs signifying increased spinal excitability and the level of recurrent inhibition acting on spinal segmental circuitry. The study population was lim ited to patients with recent motor strokes, and testing was done longitudinally during the development of spasticity. Recurrent inhibition was approximated by the conditioned H' response of Bussel and Pierrot-Desseiligny. Ratios of amplitudes of soleus H-reflex to conditioned H' responses were computed for patients who were tested at least twice dur ing the early post-stroke period. On initial testing, there were absent H' responses in the affected limbs of the patients, compared to healthy controls, indicating a supranor mal level of activity of Renshaw cells. In two patients, serial testing revealed a progres sive and sustained increase of the H' response amplitude in the affected limb coincident with development of hyperreflexia. Therefore, in at least some patients, decreased recur rent inhibition paralleling clinical detection of spasticity may indicate a pathogenetic of spasticity. If this trend is supported in larger studies, these data provide a rationale for specific antispastic pharmacological therapy targeted to the Renshaw cell early in the post-stroke period. This preliminary study demonstrates the importance of a longitudi nal'testing design in studies seeking to understand pathophysiological mechanisms of spasticity.

Heteronymous recurrent inhibition from gastrocnemius muscle to soleus motoneurones in humans

Presence of heteronymous recurrent inhibition in motoneurones (Mns) innervating the soleus muscle (Sol) was investigated following electrical stimulation of the nerve of gastrocnemius medialis muscle (GM). Sub-threshold electrical stimulation for alpha Mns produced short-lasting inhibition of the Sol, reflecting non-reciprocal group I inhibition. After increasing the intensity of stimulation above the motor threshold, a short-latency, long-lasting inhibition appeared superimposed on the group I inhibition. Its amount increased with the size of the conditioning motor response and after acute administration of l-acetylcarnitine. It is concluded that this long-lasting inhibition of the Sol Mns is due to the heteronymous activity of the GM-coupled Renshaw cells.

The effect of nicotine on recurrent inhibition in the spinal cord.

Recurrent inhibition via Renshaw cells provides a mechanism by which spinal and supraspinal centers exert control over movement. The conditioned H-reflex technique of Pierrot-Deseilligny and Bussel permits noninvasive assessment of recurrent inhibitory pathways. We employed this technique to investigate changes in Renshaw cell activity due to nicotine (a potent CNS cholinergic agonist that excites Renshaw cells in animals) contained in inhaled tobacco smoke. In 10 normal subjects, cigarette smoking caused a large, rapid drop in the conditioned H-response amplitude, implying increased activation of Renshaw cells. The time course of the change in conditioned H-response amplitude closely approximated the known pharmacokinetics of inhaled nicotine. Nicotine administered via chewing gum had a much slower and less dramatic effect, probably due to the slower rise in blood levels with this mode of administration. Increased activity in Renshaw cells may contribute to spasticity in spinal cord-injured patients, raising the possibility that cigarette smoking could cause further increases in tone in such patients.

Evoked activity of spinal neurons in the early period after sciatic nerve division

The character of dorsal horn motoneurons and interneurons evoked by stimulation of the dorsal root, and activity of Renshaw cells in response to stimulation of the ventral root were studied in albino rats in the lower lumbar segments of the spinal cord 5 days after sciatic nerve division. A significant increase in the mean amplitude of excitatory postsynaptic potentials of motoneurons was observed on the side of division of the nerve. No significant change in membrane potential and in the threshold of appearance of the action potential of these motoneurons took place. The mean number of action potentials and the duration of discharge of the Renshaw cells and dorsal horn interneurons likewise were not significantly changed.

Reciprocal Ia inhibition between elbow flexors and extensors in the human.

1. Reciprocal inhibition between elbow flexor and extensor muscles (biceps and triceps brachii) has been investigated in nine healthy subjects. Two techniques were used to assess changes in motoneurone excitability after stimulation of antagonist muscle afferents: (1) monosynaptic reflexes elicited by a mechanical stimulation of the distal muscle tendon (tendon tap); (2) post-stimulus time histograms (PSTH) of voluntarily activated motor units. 2. Electrical stimulation of the antagonist muscle nerve produced a short-latency and short-lasting inhibition of the flexor and extensor motoneurones. The amount of this inhibition was found to be similar in both motor nuclei. 3. The inhibition could be evoked with conditioning electrical stimuli as low as 0.7 x motor threshold (MT) or by very weak tendon taps applied to the antagonist tendon. In the former case the threshold of this inhibition was found to be consistently increased after raising the threshold of Ia afferent fibres by a long-lasting muscle vibration. Since a contribution from cutaneous afferent fibres was ruled out, it is concluded that this inhibition was Ia in origin. 4. Post-stimulus time histograms of voluntarily activated triceps and biceps motor units were made following electrical stimulation of homonymous and antagonist muscle afferents. This enabled an estimate of the central synaptic delay of the inhibitory process. An average central delay of 0.94 ms in excess of that of monosynaptic facilitation was found, thus suggesting that the inhibitory process could be mediated by only one interneurone. 5. A conditioning reflex discharge elicited in the antagonist muscle by a tendon tap depressed or suppressed this inhibition. This depression was maximal when the reflex discharge was elicited 10-20 ms before the conditioning stimulus for the inhibition and never lasted more than 30 ms. It is argued that the only mechanism compatible with such a depression is the inhibitory activity of Renshaw cells acting on the pathway mediating reciprocal inhibition. 6. We conclude that group Ia afferent fibres from elbow extensor and flexor muscles project monosynaptically onto Ia inhibitory interneurones to mediate disynaptic reciprocal inhibition of antagonist motoneurones.

Enhancement of recurrent inhibition by intravenous administration of L-acetylcarnitine in spastic patients.

The recurrent inhibition of the soleus alpha-motoneurons at rest, evaluated by a specially designed method of paired H reflexes, was estimated in 10 patients with spastic paraparesis. In three of these patients, Renshaw cell activity produced inhibition of the corresponding alpha-motor neurons comparable to that obtained in normal subjects, while the inhibition was reduced in five and absent in two. The effects of intravenously administered L-acetylcarnitine on the activity of Renshaw cells were studied in these patients. In all patients except the two with no evidence of recurrent inhibition, L-acetylcarnitine was found to significantly increase the amount of recurrent inhibition. Its effect became evident at about 30 minutes, reached a maximum around 50 minutes and vanished about 70 minutes from the onset of administration. A significant correlation was found between Renshaw cell activity and the ability of the drug to increase it. L-acetylcarnitine appeared to act specifically by enhancing recurrent inhibition, since no variation in the excitability of the monosynaptic reflex arc was observed.

Effect of nucleus raphe magnus stimulation on recurrent inhibition of the monosynaptic reflex in the cat

Experiments were performed on 8 cats anesthetized with urethan-chloralose. The effects of nucleus raphe magnus (NRM) conditioning stimulation on recurrent inhibition of posterior biceps semitendinosus nerve (PBSt) monosynaptic reflex elicited by electric stimulation of a part of L7 ventral root were investigated in the cat. It was found that (1) the PBSt monosynaptic reflex was facilitated by NRM conditioning stimulation at 30, 50 and 80 ms conditioning-test stimulus intervals, but that (2) the inhibited monosynaptic reflex by recurrent inhibition was further inhibited by NRM conditioning stimulation at the same conditioning-test stimulus intervals, and that (3) the activity of Renshaw cells as recorded by glass microelectrode was enhanced by NRM conditioning stimulation. These facts indicated that recurrent inhibition pathways are enhanced by NRM conditioning stimulation.

Primary afferent depolarization in the rat spinal cord is mediated by pathways utilising NMDA and non-NMDA receptors

In the present experiments the dorsal root-evoked dorsal root potential (DR-DRP) has been measured in vitro from a mature rat sacrococcygeal preparation. The DR-DRP is an index of presynaptic inhibition since it represents the depolarization of primary afferent terminals by γ-aminobutyric acid (GABA) released synaptically from interneurones. The present study shows that the synaptic excitation of the GABAergic interneurons contains a large component resistant to the selective N- methyl- d-aspartate (NMDA) receptor antagonists 2-amino-5-phosphonopentanoate (AP5) (100 μM) and 3((+)-2-carboxypiperazin-4-yl)propyl-1-phosphonate (CPP) 20–100 μM. This non-NMDA receptor mediated component reflected in the DR-DRP was depressed markedly by the non-selective excitatory amino acid receptor antagonists kynurenate (1–2 mM) and 6-cyano-2,3-dihydroxy-7-nitro-quinoxaline (CNQX) (10–20 μM). Because previous reports show non-cholinergic activation of Renshaw cells to be blocked by NMDA receptor antagonists the present observations suggest that pre- and postsynaptic inhibition in the spinal cord are mediated by different types of excitatory amino acid receptor.

Renshaw cells are inactive during motor inhibition elicited by the pontine microinjection of carbachol.

The present study was undertaken to determine whether the postsynaptic inhibition of motoneurons that occurs following the pontine microinjection of carbachol in the decerebrate cat is due to the activity of Renshaw cells. Thirty-two out of 37 Renshaw cells (86%) were spontaneously active prior to the administration of carbachol, whereas only 2 out of 13 Renshaw cells (15%) discharged during carbachol-induced motor inhibition. In addition, discrete inhibitory synaptic potentials were observed in 33% of the Renshaw cells from which intracellular recordings were obtained after carbachol administration, indicating that these cells were actively inhibited. The finding that a population of Renshaw cells, which inhibit motoneurons, were themselves inhibited during a period of profound motoneuron inhibition was quite unexpected. These results support the conclusion that Renshaw cells are not the inhibitory interneurons that are responsible for the powerful inhibition of motoneurons that occurs following the pontine microinjection of carbachol.

Evidence for renshaw cell-motoneuron decoupling during tonic vestibular stimulation in man

The influence of static head-body tilts in the sagittal plane on the activity of Renshaw cells coupled to the soleus extensor α-motoneurons was studied in eight human subjects. Head-body rotation was carried out using a tilting seat and its effect was evaluated at 80° (normal sitting position) and at 40° of backward inclination (noseup). Renshaw cell activity was assessed through a specially designed method of paired H-reflexes first described by Bussel and Pierrot-Deseilligny. α-Motoneuron excitability was also independently studied by mapping a reference H-reflex amplitude as a function of static head-body displacements. In almost all subjects Renshaw cell activity was increased at 40° backward inclination with respect to control values at 80°. These changes were attributed to the tonic labyrinthine reflexes capable of decoupling Renshaw cell activity from their motoneurons when the body was tilted backward from the upright position. We discuss the hypothetical functional modalities of the recurrent inhibitory circuit during postural adjustments elicited by labyrinthine input.

Central drive on Renshaw cells coupled with phrenic motoneurons

In anesthesized spontaneously breathing cats (C4–C5 deafferentation), recurrent inhibition of phrenic motoneurons was analyzed by studying either recurrent IPSPs in phrenic motoneurons, or Renshaw cell discharges evoked by C5 phrenic nerve stimulation. Of 90 intracellularly recorded phrenic motoneurons, 7 motoneurons showed evoked recurrent IPSPs with stimulation of C5 phrenic axons subthreshold for eliciting antidromic activation of the motoneuron from which intracellular recording was done. These IPSPs could be reversed by imposed hyperpolarization of the motoneuron, and were of greater amplitude during inspiration than during expiration. Within the phrenic nucleus, interneurons were classified as Renshaw cells if they responded to C5 phrenic axon stimulation with a typical high frequency burst of potentials. Reactivity of these Renshaw cells was related to the respiratory cycle, number of spikes in the burst being greater during inspiration than during expiration. Injection of a nicotinic cholinergic blocker (mecamylamine) decreased responses of Renshaw cells but the respiratory modulation was still present. Some Renshaw cells (18/33) were spontaneously active during inspiration. Their activity was generally maximal during the last third of inspiration. Since: (1) spontaneous activity of Renshaw cells is related to the respiratory drive; (2) persists after C7 spinal transection and after mecamylamine poisoning of the axonal recurrent pathway; and (3) might appear before sustained phrenic activity, the assumption of a central respiratory drive impinging on the Renshaw cells has to be retained.

Spontaneous respiratory activity of phrenic and intercostal Renshaw cells

Activity of Renshaw cells evoked by electrical stimulation of either phrenic or internal intercostal axons was extracellularly recorded in anaesthetized spontaneously breathing cats. The response of all the studied units to antidromic invasion of the corresponding motoneurones was related to the respiratory cycle and some units displayed spontaneous respiratory activity. Recurrent IPSPs were recorded on phrenic and intercostal motoneurones.