Postsynaptic Potentials In Motoneurons Research Articles

Bumetanide increases postsynaptic inhibition after chronic SCI and decreases presynaptic inhibition with step-training.

Current anti-spastic medication significantly compromises motor recovery after spinal cord injury (SCI), indicating a critical need for alternative interventions. Because a shift in chloride homeostasis decreases spinal inhibition and contributes to hyperreflexia after SCI, we investigated the effect of bumetanide, an FDA-approved sodium-potassium-chloride intruder (NKCC1) antagonist, on presynaptic and postsynaptic inhibition. We compared its effect with step-training as it is known to improve spinal inhibition by restoring chloride homeostasis. In SCI rats, a prolonged bumetanide treatment increased postynaptic inhibition but not presynaptic inhibition of the plantar H-reflex evoked by posterior biceps and semitendinosus (PBSt) group I afferents. By using in vivo intracellular recordings of motoneurons, we further show that a prolonged bumetanide increased postsynaptic inhibition by hyperpolarizing the reversal potential for inhibitory postsynaptic potentials (IPSPs) after SCI. However, in step-trained SCI rats an acute delivery of bumetanide decreased presynaptic inhibition of the H-reflex, but not postsynaptic inhibition. These results suggest that bumetanide might be a viable option to improve postsynaptic inhibition after SCI, but it also decreases the recovery of presynaptic inhibition with step-training. We discuss whether the effects of bumetanide are mediated by NKCC1 or by off-target effects. KEY POINTS: After spinal cord injury (SCI), chloride homeostasis is dysregulated over time in parallel with the decrease in presynaptic inhibition of Ia afferents and postsynaptic inhibition of motoneurons, and the development of spasticity. While step-training counteracts these effects, it cannot always be implemented in the clinic because of comorbidities. An alternative intervention is to use pharmacological strategies to decrease spasticity without hindering the recovery of motor function with step-training. Here we found that, after SCI, a prolonged bumetanide (an FDA-approved antagonist of the sodium-potassium-chloride intruder, NKCC1) treatment increases postsynaptic inhibition of the H-reflex, and it hyperpolarizes the reversal potential for inhibitory postsynaptic potentials in motoneurons. However, in step-trained SCI, an acute delivery of bumetanide decreases presynaptic inhibition of the H-reflex, but not postsynaptic inhibition. Our results suggest that bumetanide has the potential to decrease spastic symptoms related to a decrease in postsynaptic but not presynaptic inhibition after SCI.

Prolonged time course of population excitatory postsynaptic potentials in motoneurons of chronic stroke survivors.

Hyperexcitability of spinal motoneurons may contribute to muscular hypertonia after hemispheric stroke. The origins of this hyperexcitability are not clear, but we hypothesized that prolongation of the Ia excitatory postsynaptic potential (EPSP) in spastic motoneurons may be one potential mechanism, by enabling more effective temporal summation of Ia EPSPs, making action potential initiation easier. Thus, the purpose of this study is to quantify the time course of putative EPSPs in spinal motoneurons of chronic stroke survivors. To estimate the EPSP time course, a pair of low-intensity electrical stimuli was delivered sequentially to the median nerve in seven hemispheric stroke survivors and in six intact individuals, to induce an H-reflex response from the flexor carpi radialis muscle. H-reflex response probability was then used to quantify the time course of the underlying EPSPs in the motoneuron pool. A population EPSP estimate was then derived, based on the probability of evoking an H-reflex from the second test stimulus in the absence of a reflex response to the first conditioning stimulus. Our experimental results showed that in six of seven hemispheric stroke survivors, the apparent rate of decay of the population EPSP was markedly slower in spastic compared with contralateral (stroke) and intact motoneuron pools. There was no significant difference in EPSP time course between the contralateral side of stroke survivors and control subject muscles. We propose that one potential mechanism for hyperexcitability of spastic motoneurons in chronic stroke survivors may be associated with this prolongation of the Ia EPSP time course. Our subthreshold double-stimulation approach could provide a noninvasive tool for quantifying the time course of EPSPs in both healthy and pathological conditions. NEW & NOTEWORTHY Spastic motoneurons in stroke survivors showed a prolonged Ia excitatory postsynaptic potential (EPSP) time course compared with contralateral and intact motoneurons, suggesting that one potential mechanism for hyperexcitability of spastic motoneurons in chronic stroke survivors may be associated with this prolongation of the Ia EPSP time course.

Neuromodulatory Inputs to Motoneurons Contribute to the Loss of Independent Joint Control in Chronic Moderate to Severe Hemiparetic Stroke.

In chronic hemiparetic stroke, increased shoulder abductor activity causes involuntary increases in elbow, wrist, and finger flexor activation, an abnormal muscle coactivation pattern known as the flexion synergy. Recent evidence suggests that flexion synergy expression may reflect recruitment of contralesional cortico-reticulospinal motor pathways following damage to the ipsilesional corticospinal tract. However, because reticulospinal motor pathways produce relatively weak post-synaptic potentials in motoneurons, it is unknown how preferential use of these pathways could lead to robust muscle activation. Here, we hypothesize that the descending neuromodulatory component of the ponto-medullary reticular formation, which uses the monoaminergic neurotransmitters norepinephrine and serotonin, serves as a gain control mechanism to facilitate motoneuron responses to reticulospinal motor commands. Thus, inhibition of the neuromodulatory component would reduce flexion synergy expression by disfacilitating spinal motoneurons. To test this hypothesis, we conducted a pre-clinical study utilizing two targeted neuropharmacological probes and inert placebo in a cohort of 16 individuals with chronic hemiparetic stroke. Test compounds included Tizanidine (TIZ), a noradrenergic α2 agonist and imidazoline ligand selected for its ability to reduce descending noradrenergic drive, and Isradipine, a dihyropyridine calcium-channel antagonist selected for its ability to post-synaptically mitigate a portion of the excitatory effects of monoamines on motoneurons. We used a previously validated robotic measure to quantify flexion synergy expression. We found that Tizanidine significantly reduced expression of the flexion synergy. A predominantly spinal action for this effect is unlikely because Tizanidine is an agonist acting on a baseline of spinal noradrenergic drive that is likely to be pathologically enhanced post-stroke due to increased reliance on cortico-reticulospinal motor pathways. Although spinal actions of TIZ cannot be excluded, particularly from Group II pathways, our finding is consistent with a supraspinal action of Tizanidine to reduce descending noradrenergic drive and disfacilitate motoneurons. The effects of Isradipine were not different from placebo, likely related to poor central bioavailability. These results support the hypothesis that the descending monoaminergic component of the ponto-medullary reticular formation plays a key role in flexion synergy expression in chronic hemiparetic stroke. These results may provide the basis for new therapeutic strategies to complement physical rehabilitation.

Firing characteristics of deep dorsal horn neurons after acute spinal transection during administration of agonists for 5-HT1B/1D and NMDA receptors.

Spinal cord injury (SCI) results in a loss of serotonin (5-HT) to the spinal cord and a loss of inhibition to deep dorsal horn (DDH) neurons, which produces an exaggerated excitatory drive to motoneurons. The mechanism of this excitatory drive could involve the DDH neurons triggering long excitatory postsynaptic potentials in motoneurons, which may ultimately drive muscle spasms. Modifying the activity of DDH neurons with drugs such as NMDA or the 5-HT1B/1D receptor agonist zolmitriptan could have a large effect on motoneuron activity and, therefore, on muscle spasms. In this study, we characterize the firing properties of DDH neurons after acute spinal transection in adult mice during administration of zolmitriptan and NMDA, using the in vitro sacral cord preparation and extracellular electrophysiology. DDH neurons can be categorized into three major types with distinct evoked and spontaneous firing characteristics: burst (bursting), simple (single spiking), and tonic (spontaneously tonic firing) neurons. The burst neurons likely contribute to muscle spasm mechanisms because of their bursting behavior. Only the burst neurons show significant changes in their firing characteristics during zolmitriptan and NMDA administration. Zolmitriptan suppresses the burst neurons by reducing their evoked spikes, burst duration, and spontaneous firing rate. Conversely, NMDA facilitates them by enhancing their burst duration and spontaneous firing rate. These results suggest that zolmitriptan may exert its antispastic effect on the burst neurons via activation of 5-HT1B/1D receptors, whereas activation of NMDA receptors may facilitate the burst neurons in contributing to muscle spasm mechanisms following SCI.

Estimating the time course of population excitatory postsynaptic potentials in motoneurons of spastic stroke survivors.

Hyperexcitable motoneurons are likely to contribute to muscle hypertonia after a stroke injury; however, the origins of this hyperexcitability are not clear. One possibility is that the effective duration of the Ia excitatory postsynaptic potential (EPSP) is prolonged, increasing the potential for temporal summation of EPSPs, making action potential initiation easier. Accordingly, the purpose of this study was to quantify the time course of EPSPs in motoneurons of stroke survivors. The experimental protocol, which was based on parameters derived from simulation, involved sequential subthreshold electrical stimuli delivered to the median nerve of hemispheric stroke survivors. The resulting H-reflex responses were recorded in the flexor carpi radialis muscle. H-reflex response probability was then used to quantify the time course of the underlying EPSPs in the motoneuron pool. A population EPSP was estimated based on the probability of evoking an H reflex from the second electrical stimulus in the absence of a reflex response to the first stimulus. The accuracy of this time-course estimate was quantified using a computer simulation that explored a range of feasible EPSP parameters. Our experimental results showed that in all five hemispheric stroke survivors the rate of decay of the population EPSP was consistently slower in spastic compared with the contralateral motoneuron pools. We propose that one potential mechanism for hyperexcitability of motoneurons in spastic stroke survivors may be linked to this prolongation of the Ia EPSP time course. Our subthreshold double-stimulation approach also provides a noninvasive tool for quantifying the time course of EPSPs in both healthy and pathological conditions.

Effect of inhibitory amino acid antagonists on postsynaptic potentials of motoneurons of the frog Rana ridibunda

On preparations of the isolated spinal cord of the frog Rana ridibunda at intracellular recording from lumbar motoneurons, it is shown that response to the 10 mM GABA application decreased selectively by 40.7 ± 23.7% (n = 6) as a result of the spinal cord treatment with bicuculline (100–150 µM), while response to the Gly application decreased selectively by 50.7 ± 17.8% (n = 10) after the spinal cord treatment with strychnine (5–10 μM). Both strychnine and bicuculline produced potentiation of EPSP by amplitude and duration as well as paroxysmal depolarizational shifts (PDS). Strychnine produced more effectively the potentiation, while bicuculline—PDS. The inhibitory Gly effect decreased significantly the DR and RF EPSP (a decrease of amplitude and duration) as a result of the spinal cord treatment with strychnine (5–10 μM), but not with bicuculline. The inhibitory GABA effect on the DR and RF EPSP decreased as a result of the spinal cord treatment with bicuculline only in a half of the studied motoneurons and to the lesser degree than the inhibitory Gly effect on the same EPSP at the strychnine treatment. Based of the obtained data, it is suggested that the inhibitory effects on the excitatory inputs of the motoneuron in the frog are expressed weaker than in mammals and are related predominantly to the GABA and Gly effects on receptors of interneurons. It is suggested that GABA specifically acts mostly on GABAA receptors, whereas Gly—on Gly receptors, although there is some part of cross-inhibition.

5-HT Modulation of Identified Segmental Premotor Interneurons in the Lamprey Spinal Cord

Ipsilaterally projecting spinal excitatory interneurons (EINs) generate the hemisegmental rhythmic locomotor activity in lamprey, while the commissural interneurons ensure proper left-right alternation. 5-HT is a potent modulator of the locomotor rhythm and is endogenously released from the spinal cord during fictive locomotion. The effect of 5-HT was investigated for three segmental premotor interneuron types: EINs, commissural excitatory and commissural inhibitory interneurons. All three types of interneurons produced chemical postsynaptic potentials in motoneurons, but only those from EINs had an electrical component. The effect of 5-HT was studied on the slow afterhyperpolarization, involved in spike frequency regulation, and on the segmental synaptic transmission to motoneurons. 5-HT induced a reduction in the slow afterhyperpolarization and a depression of synaptic transmission in all three types of segmental interneurons. Thus 5-HT is a very potent modulator of membrane properties and synaptic transmission of last-order segmental premotor interneurons. Such modulation of locomotor network interneurons can partially account for the observed effects of 5-HT on the swimming pattern in lamprey.

Effects of GABA and Glycine on Postsynaptic Potentials of Motoneurons of the Frog Rana ridibunda

On a preparation of isolated spinal cord of the frog Rana ridibunda, using intracellular recording from lumbar motoneurons, there were compared effects of GABA and Gly on membrane potentials, membrane resistance, and EPSP evoked by activation of dorsal root fibers or of brainstem reticular formation. All parameters were compared in the same cell. At the same concentration (10 mM), Gly evoked membrane depolarization by 20–50% greater than GABA. Response of application of a mixture of GABA and Gly was by 20% lower than the arithmetic sum of responses to the GABA application and the Gly application. The late components and the semiwidth both of complex and of simpler DR and RF EPSP decreased significantly (to 95%) on the background of application of GABA and Gly. The late components of the both EPSP were inhibited stronger by GABA than by Gly. The early mono- and disynaptic EPSP components were inhibited to the essentially lesser degree than the late components. Gly always inhibited the early DR EPSP more markedly than GABA did. In the greater part of motoneurons the early RF EPSP were stronger inhibited by Gly, in their smaller part, by GABA. In many motoneurons the early components did not decrease at all. The motoneuron membrane resistance decreased under effect of GABA and Gly to the approximately equal extent (by 10–30%). Not in all neurons there was revealed the correspondence between a decrease of the membrane resistance (RM) and a decrease of the early EPSP components. Based on the obtained data, it is suggested that the inhibitory influences in frogs is mediated by both Gly- and GABA-receptors. On the membrane of motoneuron the inhibition is mediated to the relatively greater degree by Gly-receptors, whereas on the membrane of interneurons, by GABA-receptors. During inhibition of DR EPSP the predominance of Gly-receptors is observed in the greater number of motoneurons than during inhibition of RF EPSP. Between individual motoneurons there are significant quantitative differences of all parameters.

Estimation of postsynaptic potentials in rat hypoglossal motoneurones: insights for human work.

Classical techniques for estimating postsynaptic potentials in motoneurones include spike-triggered averages of rectified surface and multiunit electromyographic recordings (SEMG and MU-EMG), as well as the compilation of peristimulus time histograms (PSTH) based on the discharge of single motor units (SMU). These techniques rely on the probability of spike occurrence in relation to the stimulus and can be contaminated by count- and synchronization-related errors, arising from post-spike refractoriness and the discharge statistics of motoneurones. On the other hand, since these probability-based techniques are easy to use and require only inexpensive equipment, it is very likely that they will continue to be used in clinical and laboratory settings for the foreseeable future. One aim of the present study was to develop a modification of these probability-based analyses in order to provide a better estimate of the initial phase of postsynaptic potentials. An additional aim was to combine probability-based analyses with frequency-based analyses to provide a more reliable estimate of later phases of postsynaptic potentials. To achieve these aims, we have injected simple as well as complex current transients into regularly discharging hypoglossal motoneurones recorded in vitro from rat brainstem slices. We examined the discharge output of these cells using both probability- and frequency-based analyses to identify which of the two represented the profile of the postsynaptic potential more closely. This protocol was designed to obtain PSTHs of the responses of single motor units to repeated application of the same afferent input. We have also simulated multiunit responses to afferent input by replacing the times of spike occurrence in individual trials with a representation of either an intramuscular or surface-recording single motor unit waveform and summing many of these trials to obtain either a simulated SEMG or MU-EMG. We found that in a regularly discharging motoneurone, the rising phase of an EPSP moves the occurrence of spikes forward and hence induces a substantial peak in all probability-based records. This peak is followed immediately by a period of reduced activity ('silent period') due to the phase advancement of spikes that were to occur at this period. Similarly, the falling phase of an IPSP delays spikes so that they occur during the rising phase of the IPSP. During the delay, the probability-based analyses display gaps and during the occurrence of the delayed spikes they generate peaks. We found that all the probability-based analyses (SEMG, MU-EMG and PSTH) can be made useful for illustrating the underlying initial PSP by a special use of the cumulative sum (CUSUM) calculation. We have illustrated that, in most cases, the CUSUM of probability-based analyses can overcome the delay- or advance-related (i.e. the count-related) errors of the classical methods associated with the first PSP only. The probability-based records also induce secondary and tertiary peaks and troughs due to synchronization of the spikes in relation to the stimulus (i.e. the synchronization-related errors) by the first PSP to occur at fixed times from the stimulus. Special CUSUM analyses cannot overcome these synchronization-related errors. Frequency-based analysis (PSFreq) of individual and summed trials gave comparable and often better indications of the underlying PSPs than the probability-based analyses. When used in combination, these analyses compliment each other so that a more accurate estimation of the underlying PSP is possible. Since the correct identification of the connections in the central nervous system is of utmost importance in order to understand the operation of the system, we suggest that as well as the using the special CUSUM approach on probability-based records, researchers should seriously consider the use of frequency-based analyses in their indirect estimation of stimulus-induced compound synaptic potentials in human motoneurones.

Neuronal pathways from low-threshold muscle and cutaneous afferents innervating tail to trunk muscle motoneurons in the cat.

We studied neuronal pathways from low-threshold muscle (group I, II) and cutaneous afferents (group A(alpha)beta) innervating the tail to motoneurons innervating trunk muscles (m. iliocostalis lumborum and m. obliquus externus abdominus) in 18 spinalized cats. Stimulation of group I muscle afferents produced excitatory postsynaptic potentials or excitatory postsynaptic potentials followed by inhibitory postsynaptic potentials in all motoneurons innervating the m. iliocostalis lumborum which showed effects (32%), and predominantly inhibitory postsynaptic potentials in motoneurons innervating the m. obliquus externus abdominus (47%). Stimulation of group I+II afferents produced significant increases of the incidence of motoneurons showing postsynaptic potentials (the notoneurons innervating the m. iliocostalis lumborum, 87%; the motoneurons innervating the m. obliquus externus abdominus, 82%). The effects of low threshold cutaneous afferents were bilateral, predominantly producing inhibitory postsynaptic potentials in motoneurons innervating both muscles. These results suggest that neuronal pathways from muscle afferents to back muscle motoneurons mainly increase the stiffness of the trunk to maintain its stability, while those to abdominal muscles help to extend the dorsal column by decreasing their activities. The results also indicate that neuronal pathways from cutaneous afferents to trunk motoneurons functionallY disconnect the tail from the trunk.

Potentiation of postsynaptic potentials in motoneurons of the frog,Rana ridibunda, by gallamine

The potentiating effect of gallamine on postsynaptic potentials was studied intracellularly on the preparation of the isolated spinal cord of the frog,Rana ridibunda. The amplitude both of afferentPSP (stimulation of DR) and of descendingPSP (stimulation of LC and VC) increased to 200–600% of the initial value. The responses reached 20 mV, and a convulsive effect developed. Both early (mono- and disynaptic) and late components were considerably potentiated. New components, absent prior to the gallamine application, appeared. Responses to a shortterm application of glutamate were potentiated much less than PSP in the same cell. NMDAresponses were not potentiated. Brain treatment with EAA blockers showed that potentiation and convulsive effect might appear under conditions of blockage of NMDA receptors but not under conditions of blockage of non-NMDA receptors. Gallamine eliminated inhibition evoked by the agents that block EAA. After gallamine application the duration of antidromic and orthodromic AP did not change substantially, while after-depolarization increased considerably, which resulted in generation of 2-3 AP in response to a single stimulus. A competitive blocker of cholinergic transmission, d-tubocurarine, did not potentiate PSP but increased synaptic noise. Mechanisms of gallamine action are discussed.

Tachykininergic slow depolarization of motoneurones evoked by descending fibres in the neonatal rat spinal cord.

1. In the isolated spinal cord of the neonatal rat, repetitive electrical stimulation of the upper cervical region elicited a prolonged depolarization of lumbar motoneurones (L3-5) lasting 1-2 min, which was recorded extracellularly from ventral roots, or intracellularly. 2. This depolarizing response was markedly depressed by the excitatory amino acid receptor antagonists D-(-)-2-amino-5-phosphonovaleric acid (D-APV, 30 microM) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM). The remaining response was further depressed by a 5-hydroxytryptamine (5-HT) receptor antagonist, ketanserin (3 microM). 3. In the presence of these antagonists, a small part of the depolarizing response of slow time course remained, and this response was partially blocked by the tachykinin NK1 receptor antagonists GR71251 (0.3-5 microM) and RP67580 (0.3-1 microM). In contrast, RP68651 (0.3-1 microM), the inactive enantiomer of RP67580, had no effect on the depolarizing response. 4. The slow depolarizing response in the presence of D-APV, CNQX and ketanserin was markedly potentiated by a peptidase inhibitor, thiorphan (1 microM). 5. This descending fibre-evoked slow depolarization became smaller after prolonged treatment (5-7 h) with 5,7-dihydroxytryptamine (10 microM), a neurotoxin for 5-HT neurones. Under such conditions, the effects of thiorphan and GR71251 on the slow depolarization were virtually absent. 6. Under the action of D-APV, CNQX and ketanserin, applications of tachykinins, substance P and neurokinin A produced depolarizing responses of lumbar motoneurones, and the responses were depressed by GR71251 and potentiated by thiorphan. 7. These results suggest that tachykinins contained in serotonergic fibres serve as neurotransmitters mediating the descending fibre-evoked slow excitatory postsynaptic potentials in motoneurones.

Rescue of neuronal function by cross-regeneration of cutaneous afferents into muscle in cats.

1. This study investigates the relation between the peripheral innervation of low-threshold cutaneous afferents and the postsynaptic potentials elicited by electrical stimulation of those afferents. 2. In cats deeply anesthetized with pentobarbital sodium, cord dorsum potentials (CDPs) and postsynaptic potentials (PSPs) in spinal motoneurons were elicited by stimulation of the caudal cutaneous sural nerve (CCS), the lateral cutaneous sural nerve (LCS), and the medial gastrocnemius (MG) muscle nerve. We tested 1) unoperated cats, and cats in which CCS has been 2) chronically axotomized and ligated, 3) cut and self-reunited, 4) cut and cross-united with LCS, or 5) cut and cross-united with the MG. Terminal experiments were performed 3-36 mo after initial surgery. 3. In cats in which the CCS had been self-reunited or cross-united distally with LCS, tactile stimulation of the hairy skin normally innervated by the distal nerve activated afferents in the CCS central to the coaptation, indicating that former CCS afferents had regenerated into native or foreign skin, respectively. 4. In cats in which the CCS had been cross-united distally with the MG, both stretch and contraction of the MG muscle activated the former CCS afferents. 5. In unoperated cats, CDPs elicited by stimulation of CCS and of LCS exhibited a low-threshold N1 wave and a higher-threshold N2 wave. These waves were greatly delayed and appeared to merge after chronic axotomy of CCS. Regeneration of CCS into itself, into LCS, or into MG restored the normal latencies and configurations of these potentials. 6. In unoperated cats, stimulation of CCS, of LCS, and of MG each produced PSPs of characteristic configurations in the various subpopulations of motoneurons of the triceps surae. CDPs and PSPs elicited by the CCS cross-regenerated into LCS or MG were typical of those generated by the normal CCS, i.e., there was no evidence of respecification of central synaptic connections to bring accord between center and periphery after cross-regeneration. 7. Chronic axotomy of CCS increased the latencies of PSPs from CCS; regeneration of CCS into the CCS, LCS, or MG restored normal latencies. 8. In summary, CDPs and PSPs in motoneurons from the cutaneous sensory nerve CCS are altered by chronic axotomy of CCS, thus indicating their target dependency. These alterations are restored by regeneration of CCS into not only a native or foreign cutaneous target, but also into skeletal muscle. We conclude that muscle as well as skin is capable of providing trophic support for cutaneous afferents.

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

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

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.

Characterization of postsynaptic potentials evoked by sural nerve stimulation in hindlimg motoneurons from acute and chronic spinal cats

The purpose of this study was to characterize the changes in postsynaptic potentials recorted in ankle extensor motoneurons resulting from activation of the sural nerve spinal cord transection in the adult cat. Eight acute and nine chronic animals were spinalized at T 12. Intracellular recordings from motoneurons innervating the triceps surae were performed. Sural nerve stimulations evoked complex synaptic potentials consisting of early and late components in all motoneurons. Early excitatory and inhibitory postsynaptic potentials (PSPs), as well as long latency excitatory postsnnaptic potentials were recorded and averaged for assessment of PSP amplitude and duration. Early PSPs, both excitatory and inhibitory, were significantly larger in the motoneurons of cats spinalized 4–6 months earlier. Central latency of excitatory potentials were similar in the two samples of motoneurons, but the central latency associated with the initial inhibitory PSP was significantly shorter in the recordings from motoneurons of chronic spinal cats. In most recordings, an additional inhibitory PSP followed the initial excitatory PSP in motoneurons, and this secoddary inhibitory PSP was similar in peak amplitude and duration in both samples of motoneurons. Also, a long latency excitatory PSP was recorded in a large percentage of motoneurons from both samples. This potential was typically of greater amplitude and longer duration in the motonuurons from chronic animals, when compared to recordings from acute animals. Although changes in amplitude and duration of PSP activity could be documented, there was no marked alteration in the frequency of occurrence of each PSP pattern recorded from the two preparations. This suggests that the synaptic pathways mediating the sural nerve reflexes have not qualitatively changed in the chronic spinal animals. The changes in amplitude and durations of the PSPs in the chronic spatial cat indicate, however, the quantitative changes have occurred. The quantitative changes have probably occurred in the interneuronal networks activated by cutaneous nerve (sural) stimulation, since it was shown that only minor changes in motoneuron membrane properties could be recorded in these same chronic spinal animals.

Morphological and physiological studies of development of the monosynaptic reflex pathway in the rat lumbar spinal cord.

1. The developmental process of the monosynaptic reflex pathway was investigated morphologically and electrophysiologically in isolated lumbar spinal cords of new-born and fetal rats. 2. Dorsal root fibres were stained with horseradish peroxidase in the fourth lumbar (L4) segment at different ages ranging from embryonic day (E) 15.5 to post-natal day (P) 0. At E15.5, several collaterals issued from axons in the dorsal funiculus and reached the dorsal part of the dorsal horn. At E16.5, the number of collaterals entering the grey matter increased. Also, a group of collaterals extended ventralwards forming a bundle, and reached the intermediate region. At E17.5, a small number of collaterals reached the motor nuclei. The number of collaterals entering the motor nuclei increased almost linearly with age: 0 at E15.5 and at E16.5, 27 at E17.5, 184 at E18.5, 432 at E19.5 and 746 at P0. 3. The tips of collaterals and their branches had growth cones, boutons (round or oval varicosities) or other varicosities. The mean number of branches with these structures per collateral in the motor nuclei was 1.2 at E17.5, 2.5 at E18.5, 3.6 at E19.5 and 5.8 at E20.5. 4. The percentage of collaterals having growth cones in the motor nuclei was 75% at E17.5, 70% at E18.5, 38% at E19.5 and 15% at E20.5. 5. The mean number of boutons per collateral in the motor nuclei was 0.6 at E17.5, 3.2 at E18.5, 4.9 at E19.5 and 10.7 at E20.5. This increase with age was caused by both branching of collaterals and the increase in the number of boutons of the en passant type. The estimated total number of boutons in the motor nuclei of the L4 segment steeply increased after E17.5: 16 at E17.5, ca. 600 at E18.5, ca. 2000 at E19.5 and ca. 8000 at E20.5-P0. 6. Stimulation of the L4 dorsal root evoked a reflex response in the L4 ventral root, recorded as the ventral root potential, in two out of nine preparations at E15.5 and in all preparations at and after E16.5. The onset of the ventral root potential indicated the onset of excitatory post-synaptic potentials in motoneurones. The segmental latency of the ventral root potentials was markedly shortened between E17.5 and E18.5 (from 12.5 to 7.2 ms) and essentially unchanged at the later stages. 7. The magnitude of monosynaptic reflex responses in the L4 segment gradually increased with age during prenatal stages, becoming maximal at P2-3 and then decreased at the following stages (P4-P8).(ABSTRACT TRUNCATED AT 400 WORDS)

Post-synaptic actions of midlumbar interneurones on motoneurones of hind-limb muscles in the cat.

1. The hypothesis that interneurones in the 4th lumbar segment (L4) are interposed between group I and group II afferents and hind-limb motoneurones has been tested. Action potentials of single interneurones were induced by ionophoretically applied homocysteate and recorded in parallel with post-synaptic potentials in motoneurones; the latter were recorded from motor axons in the ventral root of the first sacral segment as population potentials, using the sucrose gap technique. 2. The action potentials of twenty-four L4 interneurones were found to be followed by either e.p.s.p.s. or i.p.s.p.s in motoneurones. The latencies of the majority of these p.s.p.s were consistent with monosynaptically evoked excitation or inhibition of motoneurones since they exceeded the latencies of antidromic activation of the interneurones from the S1 motor nuclei by only a fraction of a millisecond. 3. The dominant input to both the excitatory and the inhibitory interneurones was from group II muscle afferents, in particular from the quadriceps nerve. The latencies of excitation of the interneurones by these afferents indicated a monosynaptic coupling between them. The same interneurones were co-excited by group I and cutaneous afferents and by descending fibres. 4. We conclude that not only excitation but also inhibition of hind-limb motoneurones from group II afferents may be mediated disynaptically and that interneurones in the 4th lumbar segment contribute to both.

Postsynaptic potentials in motoneurons caused by spinal cord stimulation in humans

Epidural spinal cord stimulation (SCS) causes facilitation and/or inhibition of spinal motoneurons many segments below the level of the cathode. On the assumption that a spinal cord pathway is activated by the cathode the conduction velocity of the fiber system responsible for the facilitation of soleus motoneurons was estimated to be between 21.5 and 27 m/sec. It is clear that SCS in man can generate postsynaptic potentials in spinal neurons and thus is capable of modifying transmission in spinal segmental pathways.

Role of different types of fibers of the infraorbital nerve in synaptic activation of masseter and digastric motoneurons

Postsynaptic potentials of motoneurons of the masseter and digastric muscles evoked by stimulation of the infraorbital nerve with a strength of between 1 and 10 thresholds were investigated in cats anesthetized with a mixture of chloralose and pentobarbital. Depending on their ability to be activated by low-threshold afferents of this nerve, motoneurons of the masseter were divided into two groups. Stimuli with a strength of 1.2–2.5 times above threshold for the most excitable fibers of the infraorbital nerve evoked short-latency EPSPs in the motoneurons of the first group; a further increase in stimulus strength (3–9 thresholds) led to the appearance of IPSPs with latent periods of 2.8–3.5 msec. Motoneurons of the second group responded to stimulation of the infraorbital nerve with a strength of 3–9 thresholds by IPSPs whose latent periods varied from 6 to 8 msec. Stimuli below 3 thresholds in strength evoked no responses in these motoneurons. Stimulation of the infraorbital nerve with pulses of between 1 and 2 thresholds in strength evoked EPSPs in digastric motoneurons, but an increase in the strength of stimulation led to action potential generation. The presence of many excitatory and inhibitory inputs formed by afferent fibers of different types evidently provides a basis for functional diversity of jaw-opening and jaw-closing reflexes.