Quantitative relation of Renshaw cell discharges to monosynaptic reflex height

Integration of Convergent Sensorimotor Inputs Within Spinal Reflex Circuits in Healthy Adults

The output from motor neuron pools is influenced by the integration of synaptic inputs originating from descending corticomotor and spinal reflex pathways. In this study, using paired non-invasive brain and peripheral nerve stimulation, we investigated how descending corticomotor pathways influence the physiologic recruitment order of the soleus Hoffmann (H-) reflex. Eleven neurologically unimpaired adults (9 females; mean age 25 ± 3 years) completed an assessment of transcranial magnetic stimulation (TMS)-conditioning of the soleus H-reflex over a range of peripheral nerve stimulation (PNS) intensities. Unconditioned H-reflex recruitment curves were obtained by delivering PNS pulses to the posterior tibial nerve. Subsequently, TMS-conditioned H-reflex recruitment curves were obtained by pairing PNS with subthreshold TMS at short (−1.5 ms) and long (+10 ms) intervals. We evaluated unconditioned and TMS-conditioned H-reflex amplitudes along the ascending limb, peak, and descending limb of the H-reflex recruitment curve. Our results revealed that, for long-interval facilitation, TMS-conditioned H-reflex amplitudes were significantly larger than unconditioned H-reflex amplitudes along the ascending limb and peak of the H-reflex recruitment curve. Additionally, significantly lower PNS intensities were needed to elicit peak H-reflex amplitude (Hmax) for long-interval facilitation compared to unconditioned. These findings suggest that the influence of descending corticomotor pathways, particularly those mediating long-interval facilitation, contribute to changing the recruitment gain of the motor neuron pool, and can inform future methodological protocols for TMS-conditioning of H-reflexes. By characterizing and inducing short-term plasticity in circuitry mediating short- and long-interval TMS-conditioning of H-reflex amplitudes, future studies can investigate supraspinal and spinal circuit contributions to abnormal motor control, as well as develop novel therapeutic targets for neuromodulation.

Beyond muscular effects: depression of spinal recurrent inhibition after botulinum neurotoxin A

The natural target of the botulinum neurototoxin type A (BoNT-A) is the neuromuscular junction. When injected into a muscle, BoNT-A is internalized by motoneurone terminals where it functions as an endopeptidase, cleaving protein components of the synaptic machinery responsible for vesicle docking and exocytosis. As a result, BoNT-A induces a characteristic flaccid paralysis of the affected muscle. In animal models, BoNT-A applied in the periphery can also influence central activity via retrograde transport and transcytosis. An analogous direct central effect in humans is still debated. The present study was designed to address whether BoNT-A modifies the activity of the spinal recurrent inhibitory pathways, when injected at muscular level, in humans. To avoid methodological bias, the recurrent inhibition from an injected muscle (soleus) was investigated on an untreated muscle (quadriceps), and stimulation parameters (producing recurrent inhibition) were monitored on a third non-injected muscle but innervated by the same nerve as the soleus (flexor digitorum brevis). The experiments were performed on 14 post-stroke patients exhibiting spasticity in ankle plantarflexors, candidates for BoNT-A. One month after BoNT-A, the level of recurrent inhibition was depressed. It is suggested that the depression of recurrent inhibition was induced by BoNT-A, injected peripherally, through axonal transport and blockade of the cholinergic synapse between motoneurone recurrent collaterals and Renshaw cells.

Changes in transmission in the pathway of heteronymous spinal recurrent inhibition from soleus to quadriceps motor neurons during movement in man

H reflexes were induced in the human quadriceps muscle by electrical stimulation of the femoral nerve. The reflexes were conditioned by prior stimulation of the inferior soleus nerve. The conditioning stimulus produced an inhibition of long duration (>20 ms). The threshold of this inhibition was at zero soleus motor discharge and the inhibition scaled with soleus motor discharge. It was concluded that the inhibition was a heteronymous recurrent inhibition of quadriceps motor neurons mediated by Renshaw cells which had been activated by soleus motor neuron discharge. This recurrent inhibition declined during voluntary tonic contraction of the quadriceps, falling to zero at around one-third of maximum voluntary contraction. Antagonist contraction and weak co-contraction of the quadriceps and its antagonists did not lead to any significant change in recurrent inhibition. It is concluded that motor commands descending from the brain reduce heteronymous recurrent inhibition during isolated quadriceps muscle contraction, but to a much lesser extent during co-contraction. No evidence was obtained for any descending facilitation of heteronymous recurrent inhibition.

Decorrelating actions of Renshaw interneurons on the firing of spinal motoneurons within a motor nucleus: a simulation study

A simulation of spinal motoneurons and Renshaw cells was constructed to examine possible functions of recurrent inhibition. Recurrent inhibitory feedback via Renshaw cells is known to be weak. In our model, consistent with this, motoneuron firing was only reduced by a few pulses per second. Our initial hypothesis was that Renshaw cells would suppress synchronous firings of motoneurons caused by shared, dynamic inputs. Each motoneuron received an identical pattern of noise in its input. Synchrony coefficients were defined as the average motoneuron population firing relative to the activity of selected reference motoneurons; positive coefficients resulted if the motoneuron population was particularly active at the same time the reference motoneuron was active. With or without recurrent inhibition, the motoneuron pools tended to show little if any synchronization. Recurrent inhibition was expected to reduce the synchrony even further. Instead, it reduced the variance of the synchrony coefficients, without a comparable effect on the average. This suggests-surprisingly-that both positive and negative correlations between motoneurons are suppressed by recurrent inhibition. In short, recurrent inhibition may operate as a negative feedback mechanism to decorrelate motoneurons linked by common inputs. A consequence of this decorrelation is the suppression of spectral activity that apparently arises from correlated motoneuron firings due to common excitatory drive. Without recurrent inhibition, the power spectrum of the total motoneuron pool firings showed a peak at a frequency corresponding to the largest measured firing rates of motoneurons in the pool. Recurrent inhibition either reduced or abolished this peak, presumably by minimizing the likelihood of correlated firing among pool elements. Renshaw cells may act to diminish physiological tremor, by removing oscillatory components from aggregate motoneuron activity. Recurrent inhibition also improved coherence between the aggregate motoneuron output and the common drive, at frequencies above the frequency of the "synchronous" peak. Sensitivity analyses demonstrated that the spectral effect became stronger as the duration of inhibitory synaptic conductance was shortened with either the magnitude or the spatial extent of the inhibitory conductances increased to maintain constant net inhibition. Overall, Renshaw inhibition appears to be a powerful way to adjust the dynamic behavior of a neuron population with minimal impact on its static gain.

Sodium bicarbonate ingestion and its effects on anaerobic exercise of various durations

Four groups of male subjects participated in anaerobic testing on a Repco EX10 cycle ergometer to determine the effectiveness of sodium bicarbonate (0.3 g kg-1 body mass) as an ergogenic aid during exercise of 10, 30, 120 and 240 s duration. Blood was collected 90 min prior to ingestion of sodium bicarbonate (NaHCO3), after ingestion of NaHCO3 and immediately post-exercise from a heated (43-46 degrees C) fingertip and analysed immediately post-collection for pH, base excess, bicarbonate and lactate. The total work undertaken (kJ) and peak power achieved during the tests were also obtained via a Repco Work Monitor Unit. Blood bicarbonate levels were again increased above the control and placebo conditions (P < 0.001) and blood lactate levels were also increased following the bicarbonate trials. The pH levels fell significantly (P < 0.05) below the control and placebo conditions in all trials. The results indicate that NaHCO3 at this dosage has no ergogenic benefit for work of either 10 or 30 s duration, even though blood bicarbonate levels were significantly increased (P < 0.05) following ingestion of NaHCO3. For work periods of 120 and 240 s, performance was significantly increased (P < 0.05) above the control and placebo conditions following NaHCO3 ingestion.

Bicarbonate ingestion: effects of dosage on 60 s cycle ergometry

Nine healthy male subjects who were all participating in athletic events volunteered to take part in this study, the aim of which was to determine whether there are specific dosages of sodium bicarbonate (HCO3-) that are useful as an ergogenic aid as far as anaerobic performance times are concerned. A control, placebo (CaCO3 500 mg kg-1) and five dosages of bicarbonate (100, 200, 300, 400 and 500 mg kg-1) were used. The anaerobic test consisted of pedalling a Repco Exertech cycle ergometer for 1 min during which total work (kJ) and peak power (W) were measured. The subjects completed more work in the 200 (P < 0.05), 300, 400 and 500 mg kg-1 (P < 0.005) trials with most work being undertaken in the 300 mg kg-1 trial (41.9 kJ min-1). Peak power was not significantly different from the control until the 300 mg kg-1 dose, and there were no further changes from this with increasing doses of HCO3-. The highest level of peak power achieved was 1295 +/- 72.8 W at the 300 mg kg-1 dosage. Blood pH indicated that after ingestion of all but the 100 mg kg-1 dose, a state of alkalosis was achieved (P < 0.005), and this was also indicated by changes in base excess. Bicarbonate levels increased post-ingestion in all but the 100 mg kg-1 dose, with these changes reflecting the changes that occurred in the work output. Blood lactate (BLa) levels increased post-exercise (P < 0.0001).(ABSTRACT TRUNCATED AT 250 WORDS)

Sodium citrate ingestion and its effects on maximal anaerobic exercise of different durations

The effects of an alkalising agent were studied in ten subjects who participated in anaerobic testing on a cycle ergometer to determine the effectiveness of sodium citrate (0.5 g.kg-1 body mass) as an ergogenic aid during exercise of 10-s, 30-s, 120-s and 240-s duration. Blood was collected prior to, after ingestion of sodium citrate (NaHCO3), and postexercise, from a heated (43-46 degrees C) fingertip and analysed immediately postcollection for pH, partial pressure of oxygen and carbon dioxide, base excess and blood bicarbonate. Total work undertaken (kJ) and peak power (W) achieved during the tests was also obtained via a work monitor unit. The results indicated that a dose of 0.5 g.kg-1 body mass sodium citrate had no ergogenic benefit for exercise of either 10-s or 30-s duration. Blood bicarbonate concentrations, however, were significantly increased (P less than 0.05) following ingestion of the citrate during these trials. Exercise periods of 120 s and 240 s were significantly increased (P less than 0.05) above the control and placebo conditions following sodium citrate ingestion. Blood bicarbonate concentrations were again increased above control and placebo conditions and blood lactate concentrations were also increased following the citrate trials. The pH decreased significantly (P less than 0.05) in all trials below the control and placebo conditions. On the basis of the exercise undertaken in this study we would suggest that a dose of 0.5 g.kg-1 body mass of sodium citrate could improve anaerobic exercise performance of 120-s and 240-s duration.

Anaerobic work and power output during cycle ergometer exercise: effects of bicarbonate loading

Eight trained male cyclists who competed regularly in track races, were studied under control, alkalotic (NaHCO3) and placebo (CaCO3) conditions in a laboratory setting to study the effect of orally induced metabolic alkalosis on 60 s anaerobic work and power output on a bicycle ergometer. Basal, pre- and post-exercise blood samples in the three conditions were analysed for pH, pCO2, pO2, bicarbonate, base excess and lactate. All blood gas measurements were within normal limits at basal levels. There were significant differences in the amount of work produced, and in the maximal power output produced by the cyclists in the experimental condition when compared to the control and placebo conditions (P less than 0.01). The post-exercise pH decreased in all three conditions (P less than 0.05) and post-exercise pCO2 increased significantly in the alkalosis trial (P less than 0.01). In the alkalotic condition, the pre-exercise base excess and HCO3- levels were both higher (P less than 0.05) than the basal levels, suggesting that the bicarbonate ingestion had a significant increase in the buffering ability of the blood. Post-exercise lactate levels were significantly higher (P less than 0.05) after the alkalotic trial when compared to the other two conditions, immediately post-exercise and for the next 3 min. Post-exercise lactate levels were higher than basal or pre-exercise levels (P less than 0.001). This was true immediately post-exercise and for the next 5 min. The results of this study suggest that NaHCO3 is an effective ergogenic aid when used for typically anaerobic exercise as used in this experiment. We feel that this ergogenic property is probably due to the accelerated efflux of H+ ions from the muscle tissue due to increased extracellular bicarbonate buffering.

Relationship of noradrenergic locus coeruleus neurones to vestibulospinal reflexes

The electrical activity of presumably noradrenergic locus coeruleus (LC) neurones was recorded in decerebrate cats during roll tilt of the animal at 0.15 Hz, +/- 10 degrees, leading to sinusoidal labyrinth stimulation. Among the tested units, some of which projected to the lumbosacral spinal cord, 56.7% responded to animal tilt. Most of these neurones were activated during side-up and depressed during side-down tilt of the animal, while a smaller proportion of units showed the opposite response pattern. This predominant response pattern of LC neurones and coeruleospinal (CS) neurones to animal tilt was opposite in activation polarity to that of vestibulospinal (VS) neurones projecting to the same segments of the spinal cord. Both the VS and the CS neurones exert a direct excitatory influence on ipsilateral limb extensor motoneurones. However, VS neurones excite corresponding Renshaw (R) cells, though due to activation of limb extensor motoneurones and their recurrent collaterals, the CS neurones may inhibit them. It appears, therefore, that during side-down animal tilt, the motoneurones innervating the ipsilateral limb extensors are excited by the increased discharge of VS neurones, while the corresponding R-cells are disinhibited due to the reduced discharge of CS neurones. The functional coupling between ipsilateral limb extensor motoneurones and the corresponding R-cells would then increase, just at the time in which these motoneurones are driven by the excitatory VS volleys, thus limiting the response gain of limb extensors to labyrinth stimulation. This hypothesis is supported by two facts: (1) R-cells linked with limb extensor motoneurones discharge during side-down tilt, thus firing in phase with the excitatory VS volleys, and (2) functional inactivation of the noradrenergic LC neurones increases the gain of the vestibulospinal reflexes acting on limb extensors.

Distribution of recurrent inhibition within a motor nucleus. II. Amount of recurrent inhibition in motoneurones to fast and slow units

The maximal recurrent inhibition was studied by intracellular recording from 43 triceps surae motoneurones (tentatively type-identified by the biophysical properties of the neurones) and from 67 medial gastrocnemius motoneurones (type-identified by the muscle unit properties; fast fatiguing, FF; fatigue resistant, FR and slow, S). Maximal homonymous recurrent IPSP (RIPSP) and input resistance (RN) were measured at 'resting' membrane potential and close to firing threshold. The 'synaptic current' at the peak of the RIPSP was estimated (RIPSP/RN). At 'resting' membrane potential the RIPSPs increased in the order FF less than FR less than S (0.9, 1.4, and 2.5 mV, respectively). This order was preserved when the 'current' (RIPSP/RN) rather than voltage was considered, although the overall range was much reduced. When investigated close to firing threshold there were no significant differences in synaptic 'current'. This apparent paradox may be explained by systematic differences in firing threshold between motor unit types; it increased in the order S less than FR less than FF (4.6, 8.9 and 13.5 mV, respectively).

Induced metabolic alkalosis and its effects on 400-m racing time

Six trained male athletes who competed regularly in 400 metre races, were studied under control, alkalotic (NaHCO3) and placebo (CaCO3) conditions to study the effect of induced metabolic alkalosis on 400 m racing time. Pre and post exercise blood samples in the three conditions were analysed for pH, bicarbonate and base excess. Following ingestion of NaHCO3, pre-exercise pH, bicarbonate and base excess levels were significantly higher than either control or placebo conditions. In the alkalotic condition the subjects ran significantly (p less than 0.005) faster (1.52 s) than either the control of placebo conditions. The post-exercise pH, bicarbonate and base excess levels were all lower in the alkalotic condition than in the others. The results suggest that NaH-CO3 can be used as an effective ergogenic aid and support the speculation that the increased extracellular buffering afforded by NaHCO3 ingestion facilitated efflux of H+ from the working tissues, thus decreasing intracellular pH and hence offsetting fatigue.

Responses of Renshaw cells coupled with hindlimb extensor motoneurons to sinusoidal stimulation of labyrinth receptors in the decerebrate cat

Contraction of ipsilateral limb extensors during side-down roll tilt of the head, leading to selective stimulation of labyrinth receptors, is attributed to an increased discharge of excitatory vestibulospinal (VS) neurons (alpha-responses) and a decreased discharge of medullary inhibitory reticulospinal (RS) neurons (beta-responses), both of which act on ipsilateral extensor motoneurons. Experiments were performed in decerebrate cats, with the de-efferented gastrocnemius-soleus (GS) muscle fixed at a constant length, to find out whether Renshaw (R) cells linked with GS motoneurons responded to labyrinth stimulation elicited by head rotation, while the neck had been bilaterally deafferented. We hoped in this way to clarify the role and the mechanism by which these inhibitory interneurons act on limb extensor motoneurons during the vestibular reflexes. 72.7% of the R-cells, disynaptically excited by group I volleys elicited by single shock stimulation of the GS nerve, weakly responded to head rotation at frequencies of 0.026-0.15 Hz and at a peak amplitude of 10 degrees. For the frequency of head rotation of 0.026 Hz, +/- 10 degrees C, most of the GS R-cells increased their firing rate during side-down head displacement (alpha-responses); some responses were related to head position, but others showed some phase lead or lag with respect to head position. The gain of the first harmonic of these unit responses was very low and corresponded on the average to 0.084 +/- 0.062, S.D. imp./s/deg, while the sensitivity corresponded to 2.14 +/- 2.35, S.D.%/deg (base frequency, 6.85 +/- 5.97, S.D. imp./s). These responses were attributed to the activity of VS neurons, the increased discharge of which during side-down head rotation exerts a weak excitatory influence on a limited number of GS motoneurons and, through their recurrent collaterals, on the related R-cells. The modulation of the firing rate of R-cells coupled with the GS motoneurons increased linearly by increasing the peak amplitude of displacement from 5 degrees to 20 degrees at the frequency of 0.026 Hz, so that the response gain remained almost unchanged. An increase in frequency of head rotation from 0.026 to 0.32 Hz at a fixed amplitude of 10 degrees, thus changing the maximal angular acceleration from 0.26 degrees/s2 to 41.7 degrees/s2, reversed the response pattern of R-cells reported above. The resulting beta-responses, which also showed some phase lead or lag with respect to head position, were attributed to vestibular activation of RS neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Dynamic properties of Renshaw cells: equivalence of responses to step changes in recruitment and discharge frequency of motor axons

In decerebrate cats, the dynamic responses of Renshaw cells to step changes in input were determined separately both for changes in the number of alpha-axons excited and for changes in the frequency at which they were stimulated. Together, these two input variables to the Renshaw cells describe the level of activity in the motor output from the spinal cord. In either case, the dynamic responses of the interneurons depend only on their static activity before and after an input step occurs, but are otherwise indistinguishable. This favors the interpretation that the two input variables are equivalent under dynamic conditions, i.e., Renshaw cells respond to total motor output.

Recurrent inhibition and afterhyperpolarization following motoneuronal discharge in the cat

1. The relation between the size of a monosynaptic reflex (varying from the smallest values to the maximal motor response) and the output from Renshaw cells was investigated. This relation was extremely variable from one Renshaw cell to another. However, a linear relation between the reflex size and the early discharge emerged when the responses of all the Renshaw cells were averaged or when the summed activity of a pool of Renshaw cells was estimated by recording the recurrent inhibition in their target motoneurones. It was concluded that the lowest threshold motoneurones were efficient in producing recurrent inhibition. 2. In motoneurones, recorded intracellularly, the size of the depression caused by the afterhyperpolarization was compared to the maximum autogenetic recurrent inhibition. Under the particular experimental conditions used to mimic human experiments (Hultborn & Pierrot-Deseilligny, 1979a), it was found that recurrent inhibition had the same order of magnitude as the depression caused by afterhyperpolarization. 3. The additional depression caused by the summation of afterhyperpolarizations of two consecutive spikes was measured. It was shown that a summation of importance equal to the maximum autogenic recurrent inhibition required a mean interspike interval of 25 msec.

The relative dependence of the activity of Renshaw cells on recurrent pathways during contraction of the triceps muscle

Renshaw cell activity was recorded simultaneously with motoneuronal unit discharge during vibration and tetanic stimulation of triceps muscles in decerebrated cats. The experiments confirm that, in this preparation, the motoneurones are the main source of Renshaw cell firing during muscle stretch and vibration and when motoneuronal discharge was induced through the gamma loop. However they also show that a discharge of Renshaw cells, monosynaptically coupled with triceps motoneurones through their recurrent collaterals, could be elicited during contraction of the muscle at the time when the discharge of these motoneurones had been silenced. The recording of the stretch receptors and motoneuronal unit discharge during stretch, vibration, and ventral root stimulation gave evidence of the contribution of the withdrawal of excitation by primary endings to the occurrence of the silent period during tetanic contraction of the muscle. The measurements of the critical firing level in motoneuronal units responding reflexly to held stretch and vibration of the muscles, and silencing their discharge during muscle shortening, showed that these cells are amongst the lowest ranking in the pool For these reasons, these data suggest that Renshaw cell firing during vibration and tetanic contraction of the muscle cannot be attributed only to the alpha motoneurone excitation by the Ia fibres.

Neurochemical changes in the cat's spinal cord due to orthodromic tetanic stimuli. I: phospholipids

Posttetanic potentiation of monosynaptic reflexes has been used as a paradigm for neuronal plasticity. The explanation for this phenomenon is an increased responsiveness of the synaptic junctions. This would basically require chemical changes of the nervous structures involved. The ventral horn area of the spinal cord was therefore analyzed neurochemically. The determination of the phospholipids revealed an alteration of their composition. Sphingomyelin, phosphatidylcholine and phosphatidylinositide/serine behaved differently, whereas phosphatidylethanolamine and the total phospholipid content remained unchanged.

Lactic acid permeation rate in working gastrocnemii of dogs during metabolic alkalosis and acidosis

In isolated, blood perfused, supramaximally stimulated, isotonically working gastrocnemii of dogs lactic acid (LA) output and O2-consumption (V O2) were measured according to the Fick principle. Simultaneously concentration of muscle tissue was determined at rest and at different times during exercise. In one series of experiments metabolic alkalosis was induced by infusions of THAM of Na bicarbonate. As a result arterial pH increased to about 7.5 and standard [HCO3-1] to 31-35 mmol per 1. In another group of experiments metabolic acidosis was induced by HCl infusions. In these experiments pH decreased to 7.0-7.1 and standard [HO301] to 8-11 mmol per 1. During the first 3-4 min after the onset of exercise LA concentration of muscle tissue rose to 18-19 mumol per g wet weight in both series of experiments. During acidosis the highest average values for LA release from the muscle were about 1.1 mumoles per g per minute. During alkalosis LA permeation rate was nearly three times as high. As a consequence of increased rate of permeation, LA concentration of muscle tissue decreased more rapidly in alkalosis than in acidosis. In both series of experiments work per time and VO2 were practically equal during the first 5-6 min of exercise. Thereafter work per time and VO2 decreased more rapidly in acidosis than in alkalosis, a result which probably is due to higher LA concentration in muscle at this time in acidosis. It is concluded that LA permeation rate across muscle cell membrane is increased by high extracellular HCO3- concentration in combination with low H+ activity and vice versa.