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PubMed1955

Transmitter potentiality of homonymous and heteronymous monosynaptic reflex connections of individual motoneurons.

An assemblage of 110 individual tricipital motoneurons has been examined with the aim of determining those factors that predispose certain motoneurons to heteronymous response in post-tetanic potentiation. Motoneurons that respond most readily to homonymous volleys are not those that respond most readily to post-tetanically potentiated heteronymous volleys. Hence differences in presynaptic organization rather than differences in mean postsynaptic threshold determine differences in readiness for response. Every motoneuron exhibits a distinct asymmetry in transmitter potentiality of homonymous and heteronymous monosynaptic reflex connections. The range of transmitter potentialities is wide and that of heteronymous connections to some motoneurons is greater than that of homonymous connections to some other motoneurons.

Motor Neurons↗
PubMed2004

Immature developmental pattern of the monosynaptic reflex in isolated spinal cord of glial mutant taiep rats.

There is increasing evidence suggesting that glial cells play a crucial role in the formation and maturation of neural circuits. However, little is known about the effects of glial alterations on the establishment of functional circuitry in vivo during the development. The taiep rat, a long-lived neurological mutant characterized by early astrogliosis and demyelination affecting selectively the CNS, provides an interesting model to study the glia-neuron interaction in situ. In the present study, we evaluated the functional development of segmental neural circuits recording the monosynaptic reflex responses (MSR) in the isolated spinal cord of neonatal taiep rats. To evaluate the developmental changes during the first two postnatal weeks, we measured the latency of MSR, the magnitude of depression to paired pulses and the time course of post-tetanic recovery. During the early postnatal period, the MSR of control rats reduced their latency and decreased their sensitivity to depression, as a function of age. By contrast, the MSR of taiep rats failed to develop further from neonatal stage. Near the end of the second postnatal week, the MSR latencies were still prolonged, and the MSR showed a significantly stronger paired pulse depression, and higher post-tetanic recovery times than the age-matched controls. The lack of MSR maturation in taiep rats suggests an early alteration of functional mechanisms underlying the maturation of the spinal reflexes, probably due to the characteristic glial dysfunction(s) of this mutant.

Animals↗
PubMed1987

Hypoxia and monosynaptic reflexes in humans.

The recruitment curves of the monosynaptic Hoffmann (H) reflex and of the direct motor (M) excitation of alpha-motor fibers of the posterior popliteal nerve were studied in seven human subjects in normoxic and hypoxic conditions at sea level. The amplitude of the H and M responses were determined from the computerized full-wave rectified and integrated surface electromyographic (EMG) signal derived from bipolar surface electrodes placed over the soleus muscle. Hypoxic exposure [end-tidal O2 fraction (FETO2) = 0.066 +/- 0.003 and end-tidal CO2 fraction (FETCO2) = 0.0504 +/- 0.001 (SE)] did not affect the maximal M (Mmax) response but decreased significantly (7%) the maximal H (Hmax) response. The Hmax/Mmax ratio decreased from 0.60 to 0.53. Furthermore, by fitting the rising phase of the recruitment curves of the H and M responses vs. stimulus intensity with linear regressions, hypoxia was found to produce a significant decrease of similar magnitude (6%) in the threshold of both the H and M responses with no change in slope. Using a constant stimulus strength eliciting an H response of half the maximum (H50%) of the control conditions, hypoxia resulted in a 50% increase in the amplitude of the H response within 12 min. These results suggest that the effects of hypoxia on the nervous system consist of a direct depolarizing action on the peripheral alpha-fibers and 1A sensory fibers and of a central effect on supraspinal structures affecting the spinal alpha-motoneurons.

Adult↗
PubMed1994

Fluctuations of excitability in the monosynaptic reflex pathway to lumbar motoneurons in the cat.

1. It is well known that the amplitude of successive monosynaptic reflexes (MSR), elicited by afferent stimuli of constant strength, fluctuate from trial to trial. Previous evidence suggests that such excitability fluctuations within the motor pool can be introduced either pre- and/or postsynaptically. Using unanesthetized decerebrate or decerebrate/spinal cats, we attempted to evaluate the relative importance of pre- and postsynaptic mechanisms to MSR variability and the potential contribution of changes in the identities of responding motoneurons to such variability. 2. Comparisons between the MSR amplitude, measured in a severed ventral root, and the probability of firing of up to three individual motoneurons in fine filaments teased from the same root, confirmed that both correlated and uncorrelated fluctuations of motoneuron excitability are involved in MSR variability. Linear regression analysis from concurrent intracellular recordings from homonymous motoneurons showed that the MSR fluctuations were correlated with the variations in membrane potential baseline, as well as with the fluctuations in the monosynaptic excitatory postsynaptic potential peak amplitude. In all 11 cases tested, the former correlation was stronger than the latter. 3. Stimulation of the caudal cutaneous sural nerve (CCS) was used to alter the postsynaptic potential background on which triceps surae (GS) MSRs were generated. The interval chosen between CCS conditioning and the GS stimulation excluded the involvement of presynaptic inhibition. When conditioned by preceding CCS stimulation, GS population MSRs generally (8/9 cases tested) increased in amplitude without much change in their overall variance. However, the individual motoneurons that contributed to the population responses did show changes in both relative excitability and in the uncorrelated component of their response variance. About half of the concurrently recorded motoneurons (6/13) showed a decrease in relative excitability after CCS conditioning, 5/13 showed an increase, and 2/13 were unchanged. Comparison of unit and population responses indicated that the identities of the motoneurons that responded at any given level of population response were quite different with and without CCS conditioning. 4. High-frequency stimulation of Ia fibers was used to alter the state of presynaptic Group Ia-afferents that produced population MSRs. Post tetanic potentiation following high-frequency stimulation did not greatly alter the variance of population MSRs or ratio of correlated and uncorrelated fluctuations in MSR responses among individual motoneurons within the responding population. However, intratetanic depression and posttetanic potentiation of population MSRs were accompanied by marked shifts in individual motoneuron excitability relative to the population response, again indicated that changes in the identities of responding motoneurons contributes to population response fluctuations.(ABSTRACT TRUNCATED AT 400 WORDS)

Afferent Pathways↗
PubMed1980

Early phase of vincristine neuropathy in man. Electrophysiological evidence for a dying-back phenomenon, with transitory enhancement of spinal transmission of the monosynaptic reflex.

Ten patients with Hodgkin's disease were examined before and after each administration of vincristine sulfate (2 intravenous injections of 1.4 mg/m2 of body surface during the first week of each month for 3 months). The motor conduction velocity of the peroneal nerve, the conduction velocity in palmar sensory fibres of the median nerve, and the conduction velocity in the H reflex pathway remained unchanged. The amplitude of distal muscle (extensor digitorum brevis) and sensory nerve (median) potentials decreased, while the maximal response of more proximal muscles (soleus) was not significantly modified. The soleus T response quickly decreased, although at the same time the H response was increased in the days following administration of vincristine. Thus the T/H ratio seems to be the only convenient electrophysiological method of evaluating the functional impairment of primary afferent distal segments. These results show that vincristine induces a transitory excitability enhancement of the monosynaptic reflex. It is suggested that the drug may cause an increase in the firing rate in proximal segments of injured Ia fibres. Apart from this phenomenon the electrophysiological results lead to the conclusion that vincristine induces distal axonal degeneration, similar to that in other toxic neuropathies (e.g. acrylamide or n-hexane) where a dying-back process has been clearly demonstrated.

Adolescent↗
PubMed1980

Sleep, wakefulness and the monosynaptic reflex in fetal and newborn lambs.

1. Electrocorticogram (ECoG), electromyogram (EMG) of the lateral rectus and antigravity muscles (neck and masseter) and breathing activity (FB) were monitored in chronically prepared fetal sheep of 125-140 days gestation and in newborn lambs up to 11 days postnatal age. 2. Awake state (AW), non-rapid eye movement sleep (NREM) and rapid eye movement sleep (REM) were defined using standard criteria for ECoG, eye movements and postural muscle tone. 3. The percentage of time the fetuses spent in each state was: NREM sleep 53%, REM sleep 41.4%, and AW 5.6%. 4. Spontaneous intrauterine breathing activity occurred only during REM sleep, but 35% of REM sleep was not associated with FB. 5. Gasps (isolated deep inspirations) appeared occasionally throughout the recording and were not related to any specific sleep state or wakefulness. 6. In the fetus, the monosynaptic reflex (MSR) induced by direct electrical stimulation of the fibular nerve was enhanced by about 75% during REM sleep compared to NREM and AW. In the newborn lamb the adult pattern of suppression of MSR during REM sleep was not seen until several days after birth.

Animals↗
PubMed1999

Neurotrophin modulation of the monosynaptic reflex after peripheral nerve transection.

The effects of neurotrophin-3 (NT-3) and NT-4/5 on the function of axotomized group Ia afferents and motoneurons comprising the monosynaptic reflex pathway were investigated. The axotomized medial gastrocnemius (MG) nerve was provided with NT-3 or NT-4/5 for 8-35 d via an osmotic minipump attached to its central end at the time of axotomy. After this treatment, monosynaptic EPSPs were recorded intracellularly from MG or lateral gastrocnemius soleus (LGS) motoneurons in response to stimulation of the heteronymous nerve under pentobarbital anesthesia. Controls were preparations with axotomized nerves treated directly with vehicle; other axotomized controls were administered subcutaneous NT-3. Direct NT-3 administration (60 microgram/d) not only prevented the decline in EPSP amplitude from axotomized afferents (stimulate MG, record LGS) observed in axotomy controls but, after 5 weeks, led to EPSPs larger than those from intact afferents. These central changes were paralleled by recovery of group I afferent conduction velocity. Removal of NT-3 4-5 weeks after beginning treatment resulted in a decline of conduction velocity and EPSP amplitude within 1 week to values characteristic of axotomy. The increased synaptic efficacy after NT-3 treatment was associated with enhanced connectivity of single afferents to motoneurons. NT-4/5 induced modest recovery in group I afferent conduction velocity but not of the EPSPs they elicited. NT-3 or NT-4/5 had no effect on the properties of treated motoneurons or their monosynaptic EPSPs. We conclude that NT-3, and to a limited extent NT-4/5, promotes recovery of axotomized group Ia afferents but not axotomized motoneurons or the synapses on them.

Afferent Pathways↗
PubMed1997

Inhibition of spinal monosynaptic reflex in newborn rats by aurintricarboxylic acid.

The effect of aurintricarboxylic acid (ATA), an inhibitor of nuclease, on glutamatergic synaptic transmission was examined electrophysiologically in the isolated spinal cords of newborn rats. Monosynaptic reflex (MSR) was depressed about 20%, 50 min after exposure to 100 microM of ATA. Pretreatment with APV, a N-methyl-D-aspartate (NMDA) type receptor antagonist, depressed MSR by about 10%, but additional application of ATA did not affect the MSR further. In contrast, the remaining MSR following treatment with DNQX, a non-NMDA type receptor antagonist, in the Mg2+-free medium was almost completely inhibited by addition of ATA. ATA depressed NMDA- but not D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxalone propionic acid (AMPA)- or kainate-induced depolarization in the medium containing normal ionic composition. Thus it is concluded that the reduction of MSR by ATA is due to blockade of NMDA type but non-NMDA type glutamate receptors. The present study also confirmed the previous conclusion that Ia monosynaptic transmission in the spinal cord of the newborn rat is mediated by NMDA as well as non-NMDA type glutamate receptors.

Animals↗
PubMed1992

[Change in the background afferent influx to the spinal cord after transsecting the sciatic nerve and early enhancement of monosynaptic reflexes].

Background impulse activity (BIA) of separate fibres of dorsal roots of 5 lumbar segment of rats after one-sided cutting of the sciatic nerve which caused early enhancement of monosynaptic segmental reflexes (5 days after operation) has been recorded. It is revealed that afferent fibres with BIA are found more rarely on side of cutting than on the opposite side and frequency of background activity is lower than in the opposite root. It is concluded that development of spontaneous activity of the spinal ganglion neurons after cutting nerves does not ensure a higher BIA level in afferent fibres. The given phenomenon cannot be a reason of early enhancement of monosynaptic reflexes after the nerve cutting.

Afferent Pathways↗
PubMed1989

A physiological study of the monosynaptic reflex responses of cat spinal alpha-motoneurons after partial lumbosacral deafferentation.

In adult cats the whole S1 and rostral half of the L7 dorsal roots were cut on the left side of the spinal cord to produce a partial monosynaptic deafferentation of the ipsilateral alpha-motoneurons. Three, 6 or 12 weeks later, monosynaptic reflexes (MSRs) were recorded from the L6, L7 and S1 ventral roots or from various peripheral nerves during stimulation of the L6 and remaining parts of the L7 dorsal roots. Also, monosynaptic excitatory postsynaptic potentials (EPSPs) were recorded intracellularly in different types of medial gastrocnemius alpha-motoneurons of the L7 segment during stimulation of various hind limb muscle nerves. The right side with an identical acute deafferentation served as control. On the chronically lesioned side the MSRs were increased in size, also during post-tetanic potentiation. The monosynaptic EPSPs had increased amplitudes in all motoneuron types, but the relation in EPSP size between different motoneuron types as well as between different synergistic inputs remained largely unchanged. EPSP rise times were not changed, and aberrant monosynaptic connections from non-synergist muscles were not observed. It is concluded that the extent of reactive reflex changes may be related to both the number of vacant synaptic sites and the degree of functional synergism between the eliminated and remaining monosynaptic pathways. Possible underlying mechanisms are discussed.

Action Potentials↗
PubMed1977

The action of botulinum A neurotoxin on the inhibition by antidromic stimulation of the lumbar monosynaptic reflex.

1. Botulinum toxin type A was injected into gastrocnemius muscles of cats and 2 days later its action on the monosynaptic gastrocnemius reflex and on the recurrent inhibition was investigated. 2. In early local botulism, no significant change in the monosynaptic gastrocnemius reflex was observed. 3. The inhibition by antidromic stimulation of the monosynaptic gastrocnemius reflex was significantly reduced. 4. During the early phases of local botulism an action of botulinum toxin on parts of the soma membrane of the alpha-motoneuron is suggested.

Action Potentials↗
PubMed1993

Effect of intrathecal baclofen on the monosynaptic reflex in humans: evidence for a postsynaptic action.

Intrathecal baclofen is a very powerful antispastic agent. Its mechanism of action on the monosynaptic H-reflex in spinal patients was investigated. It could inhibit rapidly and profoundly monosynaptic reflexes in lower limbs, but did not modify Ia vibratory inhibition of the soleus H-reflex. To assess more precisely its effect on Ia afferents, an experimental paradigm using Ia heteronymous facilitation of the soleus H-reflex was used. Intrathecal baclofen did not modify the amount of monosynaptic facilitation of the soleus H-reflex brought about by stimulation of the femoral nerve. This demonstrates that the main part of the inhibitory effect of baclofen on the H-reflex in spinal patients is not due to a presynaptic effect, suggesting a postsynaptic site of action.

Adolescent↗
PubMed1990

Interaction of thyrotropin-releasing hormone and methysergide on the monosynaptic reflex in isolated mammalian spinal cord.

The interaction between thyrotropin-releasing hormone (TRH) and methysergide (MeSG) on reflex activity was examined in spinal cords from neonatal rats. MeSG depressed the monosynaptic reflex (MSR) by nearly 90% at 0.03 microM but had no effect on the dorsal root reflex at 0.003-3.0 microM. Neither spiperone, ketanserin, cyproheptadine nor ICS 205-930 (3-tropanyl-indole-3-carboxylate) depressed the MSR nor did they affect the potentiation elicited by TRH. TRH reversed the depression of the MSR by MeSG in a concentration-dependent manner without affecting the dorsal root reflex. MeSG-induced depression of the MSR was also reversed by 3,4-diaminopyridine which simultaneously increased the magnitude and duration of both reflexes. It appears that neither MeSG-induced depression nor TRH-induced potentiation of the MSR involves the spinal serotonergic system or blockade of K+ channels.

5,7-Dihydroxytryptamine↗
PubMed1986

Gradual changes in the recovery curve of monosynaptic reflex during postnatal development in kittens.

Conditioning-testing stimulus pulses were applied to soleus-gastrocnemius muscle nerves while reflexes were recorded from L7 ventral root in kittens (1-100 days). The rate of recovery of the monosynaptic test reflex response was monitored at different ages. These observations show that the very slow recovery rate seen in the newborns becomes much faster in the first 10 postnatal days, and it attains adult-like values in ca. 4 weeks after birth.

Age Factors↗
PubMed1993

Differential effects of footpad stimulation on the monosynaptic reflex in the spinalized cat.

Experiments were performed in 17 adult cats spinalized at T10-11. The effects of electrical stimulation of the central pad (CP) and toe pads (TP-2, 3, 4, 5) on the monosynaptic reflex (MR) in lumbar spinal segments were studied. The conditioning stimulation had different effects on the MRs of the various motoneurons except the posterior biceps and semitendinosus, depending on the footpad stimulated. Lateral gastrocnemius and soleus, and medial gastrocnemius (MG) MR were inhibited by footpad stimulation. The effect of TP-2 (medial footpad) stimulation on MG-MR was weak. The popliteus (Pop) and tibialis anterior muscles (TA) are inward and lateral rotators of the knee joint, respectively. Pop- and TA-MR were excited by stimulation of lateral and medial TPs, respectively. Plantaris MR was enhanced and inhibited by stimulation of the CP and TPs, respectively. There were remarkable differences in the effects of TP conditioning stimulation on the flexor digitorum longus, extensor digitorum longus, peroneus brevis and tertius, and peroneus longus-MRs, depending on the toe pads stimulated. These results suggest that afferent inputs from footpads modulate the activity of motoneurons, stabilize the foot and help maintain body balance.

Animals↗
PubMed1995

Homonymous and heteronymous monosynaptic reflexes in biceps brachii.

Using poststimulus time histograms, it has been reported that stimulation of the median nerve at the elbow produces a monosynaptic EPSP in voluntarily active single motoneurons of the human biceps brachii. The present study was undertaken to: (i) determine whether such stimulation could evoke a reproducible reflex response in biceps brachii; and (ii) establish the optimal conditions for eliciting the reflex under clinical conditions. Twelve normal subjects were studied. No reflex response was recordable when biceps brachii was relaxed. A reflex response with a mean latency of 14.0 ms (+/- 0.96 ms) could be recorded during a background voluntary contraction. The response was small (0.5-4.5% of the maximal M wave) but symmetrical, and could be obtained in all subjects. The responsible afferents appear to be rapidly conducting fibers from forearm flexor muscles and the latencies of the response were consistent with a monosynaptic reflex. Reflex amplitude increased with stimulus intensity and contraction strength. Stimulus rate did not affect amplitude significantly. It is concluded that a reproducible heteronymous monosynaptic reflex can be recorded from the contracting biceps brachii on stimulation of the median nerve at the elbow. Although smaller than the homonymous H reflex evoked by stimulation at Erb's point, it was technically easier to demonstrate that the EMG potential was of reflex origin (rather than part of an M wave). These reflexes should be of value in the assessment of the C-5/C-6 segments and the upper trunk of the brachial plexus.

Adult↗