Synaptic actions of individual vestibular neurones on cat neck motoneurones

1. Unitary synaptic potentials evoked by the activity of single vestibulocollic neurones were recorded by means of spike-triggered signal averaging in neck extensor motoneurones of decerebrate cats. Properties of the vestibulocollic neurones which produced the potentials were examined.2. Vestibulocollic neurones were first identified as projecting to the C3 grey matter by antidromic microstimulation within the C3 extensor motoneurone pool. The spontaneous or glutamate-driven activity of the vestibulocollic neurones was then used to trigger the averaging computer. In this way ten inhibitory and two excitatory neurones were identified (20% of neurones tested).3. Action potentials in local branches of vestibulocollic neurones were usually recorded in the vicinity of motoneurones. Mean orthodromic conduction time from the foot of the extracellular spike, recorded in the vestibular nuclei, that triggered the averager was 0.72 msec. Mean synaptic delay was 0.4 msec.4. I.p.s.p.s had a mean time to peak of 0.81 msec and were readily reversed by injection of hyperpolarizing current. These data, together with the shape indices of i.p.s.p.s indicate that they are generated proximally on motoneurones.5. All vestibulocollic neurones making synapses with motoneurones were monosynaptically driven by stimulation of the ipsilateral vestibular nerve. Four out of seven tested were inhibited by stimulation of the contralateral vestibular nerve (commissural inhibition).6. Two excitatory neurones were located in Deiters' nucleus or on the Deiters'-descending border. Inhibitory neurones were found relatively medially in the vestibular complex in the medial, descending and Deiters' nuclei.7. Vestibulocollic neurones acting on motoneurones were tested for axon branching to more caudal levels of the spinal cord with electrodes placed at C5-7. Both of the excitatory and two out of nine inhibitory neurones branched.     

On the control of myotomal motoneurones during “fictive swimming” in the lamprey spinal cord in vitro

Intracellular recordings have been made from myotomal motoneurones during “fictive swimming” in the in vitro preparation of the lamprey spinal cord, while monitoring the efferent burst activity in the ventral roots. The pattern of rhythmic activity in the motoneurones is described, as well as how synaptic inputs from the premotoneuronal level exert their control of motoneurone activity. (1) All motoneurones investigated displayed rhythmic, symmetric oscillations of their membrane potential during “fictive swimming”. The period of depolarization occurred in phase with the burst discharge in the ventral root containing the motoneurone axon. (2) About one-third of the cells fired bursts of action potentials during the depolarized phase, while the remaining motoneurones exhibited subthreshold oscillations. (3) Intracellular injection of chloride ions reversed the sign of the hyperpolarized phase, demonstrating phasic active inhibition of the motoneurones during rhythmicity. (4) The depolarized phase was unaffected after chloride injection, showing that the motoneurones also received phasic active excitation. (5) “Pre-triggered” averaging of the motoneurone recording (using the ventral root spikes from other motoneurones for triggering), revealed that some degree of synchronous excitation of several motoneurones occurred, suggesting common excitation from the same premotor-interneurones. It is concluded that the rhythmic oscillations of membrane potential in lamprey myotomal motoneurones during “fictive locomotion” depend on phasic excitation alternating with phasic active inhibition. The premotoneuronal mechanism responsible for this control may consist of reciprocally organized groups of excitatory and inhibitory interneurones.     

Mono- and disynaptic pathways from Forel's field H to dorsal neck motoneurones in the cat

Summary 1. We analysed the synaptic actions produced by Forel's field H (FFH) neurones on dorsal neck motoneurones and the pathways mediating the effects. 2. Stimulation of ipsilateral FFH induced negative field potentials of several hundred microvolts with the latency of about 1.1 ms in the medial ponto-medullary reticular formation, being largest in the ventral part of the nucleus reticularis pontis caudalis (NRPC), and in the dorsal part of the nucleus reticularis gigantocellularis (NRG). 3. Stimulation of ipsilateral FFH induced excitatory postsynaptic potentials (EPSPs) in 90% (47/52) and inhibitory postsynaptic potentials (IPSPs) in 19% (10/52) of the reticulospinal neurones (RSNs) in the NRPC and the NRG. Latencies of the EPSPs and IPSPs were 0.7–3.0 ms, the majority of which were in the monosynaptic range. The monosynaptic connexions were confirmed by spike triggered averarging technique both in excitatory (n=4) and inhibitory (n=2) pathways. 4. Single stimulation of FFH induced EPSPs at the segmental latencies of 0.3–1.0 ms in neck motoneurones, which were clearly in the monosynaptic range. Repetitive stimulation of FFH produced marked temporal facilitation of EPSPs in neck motoneurones. The facilitated components of the EPSPs had a little longer latencies and their amplitude reached several times as large as that evoked by single stimulation in all the tested motoneurones. These facilitated excitations are assumed to be mediated by RSNs in the NRPC and NRG, since RSNs were mono- and polysynaptically fired by stimulation of FFH and they were previously shown to directly project to neck moteneurones. 5. EPSPs were induced in 91% (82/91) of motoneurones supplying m. biventer cervicis and complexus (BCC; head elevator), 10% (3/29) of motoneurones supplying m. splenius (SPL; lateral head flexor). Eikewise, stimulation of FFH produced EMG responses in BCC muscles, while not in SPL muscle. Thus FFH neurones produce excitations preferentially in BCC motoneurones. 6. Systematic tracking in and around FFH revealed that the effective sites for evoking above effects were in FFH and extended caudally along their efferent axonal course. 7. These results suggested that FFH neurones connect with neck motoneurones (chiefly BCC, head elevator) mono-, diand/or polysynaptically and are mainly concerned with the control of vertical head movements.     

The activity of spinal commissural interneurons during fictive locomotion in the lamprey

Commissural interneurons in the lamprey coordinate activity of the hemisegmental oscillators to ensure proper left-right alternation during swimming. The activity of interneuronal axons at the ventral commissure was studied together with potential target motoneurons during fictive locomotion in the isolated lamprey spinal cord. To estimate the unperturbed activity of the interneurons, axonal recordings were chosen because soma recordings inevitably will affect the level of membrane depolarization and thereby spike initiation. Of 227 commissural axons recorded during locomotor activity, 14 produced inhibitory and 3 produced excitatory postsynaptic potentials (PSPs) in target motoneurons. The axons typically fired multiple spikes per locomotor cycle, with approximately 10 Hz sustained frequency. The average shortest spike interval in a burst corresponded to an instantaneous frequency of approximately 50 Hz for both the excitatory and inhibitory axons. The maximum number of spikes per locomotor cycle was inversely related to the locomotor frequency, in accordance with previous observations in the spinal hemicord preparation. In axons that fired multiple spikes per cycle, the mean interspike intervals were in the range in which the amplitude of the slow afterhyperpolarization (sAHP) is large, providing further support for the role of the sAHP in spike timing. One hundred ninety-five axons (86%) fired rhythmically during fictive locomotion, with preferred phase of firing distributed over either the segmental locomotor burst phase (40% of axons) or the transitional phase (between bursts; 60%). Thus in lamprey commissural interneurons, we found a broad distribution of firing rates and phases during fictive locomotion.     

Effects of groups of propriospinal interneurons on fictive swimming in the isolated spinal cord of the lamprey

Fictive swimming activity was induced in isolated spinal cords of adult lampreys Ichthyomyzon unicuspis and Petromyzon marinus by addition of D-glutamate or N-methyl-D,L-aspartate (NMA) to the bathing fluid. Propriospinal interneurons are defined as nerve cells within the spinal cord with projections longer than 1 segment. The hypothesis that propriospinal interneurons contribute to intersegmental coordination during fictive swimming was tested using electrical stimulation, extracellular recording, and separated compartments. Stimulation of the split caudal end of the spinal cord indirectly excited ascending propriospinal interneurons, which enhanced and entrained bursts in rostral contralateral ventral roots. Indirect electrical stimulation of descending propriospinal interneurons could delay and diminish bursts in caudal contralateral ventral roots. Extracellular recordings from the rostral and caudal split ends of the spinal cord sometimes showed spike activities in phase with contralateral or ipsilateral ventral roots. Inhibition of 1-3 segments by spot applications of glycine or gamma-aminobutyric acid (GABA) did not interrupt normal coordination or rostrocaudal phase lag. When a middle region of spinal cord was inhibited in a compartment with GABA or glycine, the caudal spinal cord could entrain the bursts in rostral ventral roots. In a few preparations the caudal region induced antiphasic bursts in previously silent rostral roots through the inhibited region. The maximum separation for caudal-upon-rostral antiphasic entrainment was approximately 20 segments in Ichthyomyzon and 36 segments in Petromyzon. Increased concentrations of an excitatory amino acid in a rostral compartment could produce descending entrainment of bursts in an adjacent caudal compartment at a higher frequency with rostrocaudal phase lag. The rostral-upon-caudal entrainment could still occur through spot applications of GABA or glycine but not through long inhibited regions. Two hypothetical groups of propriospinal interneurons are proposed for the coordination of swimming activities in the isolated spinal cords of adult lampreys. 1) Crossed, ascending interneurons may be excited in phase with nearby motoneurons and may excite and entrain rostral pattern generators on the opposite side. 2) Short, commissural interneurons may be excited in phase with nearby motoneurons and may inhibit contralateral generators.     

Synaptic potentials in motoneurons during fictive swimming in spinal Xenopus embryos

Embryos spinalized at the 3rd to 6th postotic myotome and immobilized in 10(-4) M tubocurarine can respond to a brief skin stimulus with motor root activity suitable for swimming. Embryos spinalized at the more caudal levels give shorter episodes of fictive swimming. We have previously described the synaptic inputs to motoneurons during fictive swimming in intact embryos (23). In the present paper we look to see if similar synaptic inputs are present in spinal embryos and are therefore spinal in origin. All motoneuron firing during fictive swimming is associated with a tonic depolarization that falls away slowly once firing stops, is increased by hyperpolarizing current, and is reduced by depolarizing current. A slow depolarizing potential evoked by lower levels of skin stimulation has similar properties and rate of fall. In 1-2 mM PDA, an excitatory amino acid antagonist, only a small remnant of the depolarization remains, and motoneuron firing stops. The NMDA antagonist 50 microM APV reduces the depolarization less but also blocks firing. Motoneurons fire one spike per swimming cycle, in phase with nearby motor root discharge. Spikes are preceded by a depolarizing prepotential. This increases with hyperpolarizing current, which can block the spike to reveal an underlying depolarizing potential. In phase with motor root discharge on the opposite side of the body, motoneurons receive a midcycle inhibitory postsynaptic potential, which increases with depolarizing current, decreases with hyperpolarizing current, and is blocked by 10(-6) M strychnine. Strychnine, 5 X 10(-7) M, leads first to broadening of motor root bursts then to loss of the alternating swimming pattern of activity, which is replaced by synchronous bursts on both sides of the body. We conclude that the synaptic inputs to motoneurons during fictive swimming in spinal embryos are very similar in properties and pharmacology to those in intact embryos. These inputs, including the tonic depolarization always associated with motoneuron firing during swimming, must be at least partly spinal in origin.     

Synaptic actions of single interneurones mediating reciprocal Ia inhibition of motoneurones

1. The investigation was aimed at defining the function of the interneurones which, according to indirect evidence, mediate the reciprocal Ia inhibition of motoneurones (Hultborn, Jankowska & Lindstrom, 1971 b) by studying their direct synaptic actions. These actions were tested by recording post-synaptic potentials in motoneurones following spike activity of single interneurones activated by iontophoretic application of glutamate. The interneurones were found to produce unitary monosynaptic IPSPs in those motoneurones in which disynaptic IPSPs are evoked by the group Ia afferents which monosynaptically excite the interneurones.2. Unitary IPSPs were found in more than 80% of the motoneurones impaled in the immediate vicinity of the axonal branches of the investigated Q interneurones in the PBSt motor nucleus. It is estimated that each interneurone might inhibit about every fifth PBSt motoneurone. The amplitudes of the unitary IPSPs ranged between 8 and 220 muV and were 10-200 times smaller than the maximal Ia IPSPs evoked in the same motoneurones.3. The synaptic delay in the generation of unitary IPSPs was measured in relation to the spike potentials recorded from the terminal branches of interneurones in the immediate vicinity of the impaled motoneurones. The synaptic delays ranged between 0.28 and 0.42 msec.4. From chloride reversal tests and an analysis of the time course of the unitary IPSPs it was concluded that the terminals of the investigated interneurones make synaptic contact predominantly on the soma and/or on the proximal parts of the dendrites of the motoneurones, their distribution being, however, not quite uniform.     

Afterhyperpolarization in neurones of the red nucleus

Summary Afterhyperpolarization (AHP) following single or short trains of spikes in rubrospinal neurones (RN neurones) of the cat has been studied with intracellular recording techniques. The AHP amplitude was potential dependent; it increased with depolarization and decreased with hyperpolarization and had an extrapolated reversal potential about 20 mV below resting membrane potential. The AHP was associated with an increase in the membrane conductance and it was concluded that the AHP is primarily caused by an increase in membrane conductance to potassium ions. The time course of the conductance change underlying the AHP was measured with short current pulses and calculated from the AHP voltage. The AHP following a single spike was conditioned at different interspike intervals by a preceding spike (or several spikes). In many RN neurones the AHP (conductance) following a spike added approximately linear to that generated by a preceding spike. In most cells, however, the AHP following a spike was instead depressed by a preceding spike. The summation of AHPs increased progressively, while the depression appeared to be already maximal with one preceding spike. The depression was then approximately constant for interspike intervals less than the AHP duration. It will be shown in a following paper that these properties of the AHP are reflected in the behaviour of the repetitive discharge evoked by constant current pulses in the same neurones.Dr. H. Hultborn was supported by the Japan Society for the Promotion of Science.     

Local synaptic circuits and epileptiform activity in slices of neocortex from children with intractable epilepsy.

1. Single and dual intracellular recordings were performed in neocortical slices obtained from tissue samples surgically removed from children (8 mo to 15 yr) for the treatment of intractable epilepsy. Electrical stimulation and glutamate microapplication were used to study local synaptic inputs to pyramidal cells. 2. In recordings with potassium-acetate electrodes, activation of presynaptic neocortical neurons with glutamate microdrops did not elicit a clear increase in postsynaptic potentials (PSPs) but did suppress current-evoked repetitive spike firing in recorded neurons. Bicuculline (10 microM) blocked this effect, suggesting it was caused by the activation of presynaptic gamma-aminobutyric acid (GABA) cells. In recordings with KCl electrodes, glutamate microdrops elicited an increase in the frequency and amplitude of depolarizing PSPs. Bicuculline (5-10 microM) blocked the glutamate-evoked PSPs, suggesting they were reversed GABAA-receptor-mediated inhibitory postsynaptic potentials (IPSPs). In one cell recorded with a KCl electrode (total n = 8), current-evoked spike trains elicited afterdischarges of reversed IPSPs, thus revealing a recurrent inhibitory circuit. Therefore local inhibitory synaptic circuits were robust and could be observed in tissue from patients as young as 11 mo. 3. In addition to short-latency (10-25 ms), monosynaptic excitatory postsynaptic potentials (EPSPs), electrical stimulation at low intensities sometimes elicited delayed EPSPs (20-60 ms). When GABAA-receptor-mediated synaptic inhibition was partially reduced in bicuculline (5-10 microM), electrical stimulation evoked large EPSPs at long and variable latencies (100-300 ms). Glutamate microapplication caused an increase in the frequency and amplitude of EPSPs; preliminary results suggest that glutamate microdrops were less likely to evoke EPSPs in tissue from younger patients (8-12 mo) than in slices from patients greater than 4 yr. Evidence for local excitatory synaptic circuits was thus found when synaptic inhibition was partially reduced. 4. After further reduction of inhibition in bicuculline (5-50 microM), electrical stimulation elicited epileptiform bursts. In pairs of simultaneously recorded neurons, bursts were generated synchronously from long-latency EPSPs (100-300 ms) in slices from patients as young as 8 mo. Reflected EPSPs at very long and variable latencies (500-1,100 ms) and repetitive epileptiform bursts could be evoked synchronously in pairs of cells. Glutamate activation of local presynaptic neurons elicited robust epileptiform events in recorded cells. This was seen in slices from patients as young as 16 mo. 5. These data provide physiological evidence for the presence of local inhibitory and excitatory synaptic circuits in human neocortex at least as early as 11 and 8 mo, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)     

Electrophysiological characteristics of immunochemically identified rat oxytocin and vasopressin neurones in vitro.

1. Intracellular recordings were made from supraoptic neurones in vitro from hypothalamic explants prepared from adult male rats. Neurones were injected with biotinylated markers, and of thirty-nine labelled neurones, nineteen were identified immunocytochemically as containing oxytocin-neurophysin and twenty as containing vasopressin-neurophysin. 2. Vasopressin and oxytocin neurones did not differ in their resting membrane potential, input resistance, membrane time constant, action potential height from threshold, action potential width at half-amplitude, and spike hyperpolarizing after-potential amplitude. Both cell types exhibited spike broadening during brief, evoked spike trains (6-8 spikes), but the degree of broadening was slightly greater for vasopressin neurones. When hyperpolarized below -75 mV, all but one neurone exhibited a transient outward rectification to depolarizing pulses, which delayed the occurrence of the first spike. 3. Both cell types exhibited a long after-hyperpolarizing potential (AHP) following brief spike trains evoked either with a square wave pulse or using 5 ms pulses in a train. There were no significant differences between cell types in the size of the AHP evoked with nine spikes, or in the time constant of its decay. The maximal AHP evoked by a 180 ms pulse was elicited by an average of twelve to thirteen spikes, and neither the size of this maximal AHP nor its time constant of decay were different for the two cell types. 4. In most oxytocin and vasopressin neurones the AHP, and concomitantly spike frequency adaptation, were markedly reduced by the bee venom apamin and by d-tubocurarine, known blockers of a Ca(2+)-mediated K+ conductance. However, a minority of neurones, of both cell types, were relatively resistant to both agents. 5. In untreated neurones, 55% of vasopressin neurones and 32% of oxytocin neurones exhibited a depolarizing after-potential (DAP) after individual spikes or, more commonly, after brief trains of spikes evoked with current pulses. For each neurone with a DAP, bursts of spikes could be evoked if the membrane potential was sufficiently depolarized such that the DAP reached spike threshold. In four out of five vasopressin neurones a DAP became evident only after pharmacological blockade of the AHP, whereas in six oxytocin neurones tested no such masking was found. 6. The firing patterns of neurones were examined at rest and after varying the membrane potential with continuous current injection. No identifying pattern was strictly associated with either cell type, and a substantial number of neurones were silent at rest.(ABSTRACT TRUNCATED AT 400 WORDS)     

Alterations of synaptic action in chromatolysed motoneurones of the cat

1. Monosynaptic EPSPs in lumbosacral motoneurones undergoing chromatolysis were studied by intracellular recording from 7 to 20 days after section of the appropriate ventral roots of the cat.2. The maximum monosynaptic EPSPs evoked in chromatolysed motoneurones by afferent volleys from the biceps-semitendinosus or the triceps surae muscles ranged from 1.0 to 9.5 mV in amplitude. The time-to-peak of these EPSPs was 1.7 msec on the average. These values were significantly smaller and longer, respectively, than the amplitude and the time-to-peak of monosynaptic EPSPs observed in normal motoneurones. The long time-to-peak of EPSPs in chromatolysed motoneurones could not be accounted for by asynchronous transmitter release.3. The mean number of unit EPSPs responding to a single afferent impulse (m) in chromatolysed motoneurones was comparable to that found in normal motoneurones.4. The amplitude of unit EPSPs estimated from the mean EPSP amplitude and the m value following stimulation of a single afferent fibre was significantly smaller in chromatolysed motoneurones than in normal motoneurones. This difference was attributed to a difference in synaptic location.5. The shape of monosynaptic EPSPs evoked in chromatolysed motoneurones by stimulation of single afferent fibres was analysed on the basis of Rall's compartment model. The analysis suggested that there is a lack of the excitatory synaptic input to the cell body in chromatolysed motoneurones.6. Similar alterations were also found in IPSPs. The degree of change in synaptic responses evoked by stimulation of various pathways appears to depend on the synaptic location.7. Following the study of the interaction of several inputs on the motoneurone and of their dependence on membrane potential, a tentative model of the synaptic distribution of different pathways is proposed.     

Mechanisms regulating the activity of facial nucleus motoneurones--2. Synaptic activation from the caudal trigeminal nucleus

Field and postsynaptic potentials of facial motoneurones evoked by stimulation of the caudal trigeminal nucleus were studied in cats by means of extra- and intracellular recording. Mono- and polysynaptic input onto facial motoneurones from the caudal trigeminal nucleus were shown. Four types of responses were distinguished: excitatory postsynaptic potentials generating a single action potential; a gradual shift of depolarization inducing multiple discharges; a rhythmic discharge of action potentials appearing at a low level of depolarization; excitatory postsynaptic potentials or a sequence of excitatory and inhibitory postsynaptic potentials. Multiple discharge was shown to appear as a result of effective summation of high frequency excitatory influences from efferent neurones of the caudal trigeminal nucleus projecting into the facial nucleus. Factors facilitating the development of gradual depolarization are: dendritic localization of synaptic terminals, dendritic origin of after-depolarizing processes and the high input resistance of the facial motoneurone membrane. It is thought that specific features of facial motoneurones and properties of afferent inputs are supposed to provide high sensitivity of neuronal organization of the facial nucleus to afferent signals as well as wide diversity in controlling its activity.     

Monosynaptic excitatory connexions of reticulospinal neurones in the nucleus reticularis pontis caudalis with dorsal neck motoneurones in the cat

Summary 1. Projections of reticulospinal neurones (RSNs) in the nucleus reticularis pontis caudalis (N.r.p.c.) to dorsal neck motoneurones supplying splenius (SPL, lateral head flexor) and biventer cervicis and complexus (BCC, head elevator) muscles were studied in the cat anaesthetized with pentobarbiturate or -chloralose. 2. Threshold mapping for evoking antidromic spikes revealed that most of RSNs tested projecting down to brachial segments but not to lumbar segments (C-RSNs) gave off collaterals to the gray matter of the upper spinal cord in C2–C3 segments. 3. Spike triggered averaging showed that negative field potentials were evoked after firing of a single C-RSN (single fibre focal synaptic potentials, FSPs) in the region of C2–C3 where large antidromic field potentials from nerves supplying SPL or BCC muscles were evoked. The single fibre FSPs ranged between 1 and 10 V in amplitude and had latencies between 0.7 and 1.2 ms from the onset of the triggering spike. In most cases, a presynaptic spike preceded the negative potential by 0.3 ms. These results indicated that C-RSNs project to the SPL or BCC motor nucleus. 4. Spike triggered averaging of postsynaptic potentials revealed EPSPs (single fibre EPSPs) in 36 dorsal neck motoneurones, predominantly in SPL (25) and less in BCC (11) motoneurones, evoked from 15 C-RSNs. The amplitude of the single fibre EPSPs ranged from 5 to 310 V, and had latencies of 0.8–2.0 ms from the onset of the triggering spikes of C-RSNs, or 0.3–0.5 ms from the presynaptic spike when recorded. The results indicated monosynaptic excitatory connexions of C-RSNs to dorsal neck motoneurones. 5. Single fibre EPSPs from a C-RSN were usually recorded from either BCC or SPL motoneurones but not from both types of motoneurones, when tested in many motoneurones. This showed that connexions of C-RSNs with dorsal neck motoneurones were muscle specific. 6. RSNs projecting down to the lumbar segment (L-RSN) also showed branching in C2–C3 segments. Excitatory monosynaptic connexion of L-RSNs with neck motoneurones were demonstrated by recording single fibre postsynaptic population potentials (p.s.p.p.s.) from the C2 ventral root perfused with sucrose. The probability of evoking monosynaptic single fibre p.s.p.p.s. was less (19%) than for C-RSNs (59%).     

Short-term synchronization of intercostal motoneurone activity.

1. The hypothesis is advanced that the joint occurrence of unitary excitatory post-synaptic potentials e.p.s.p.s) evoked in motoneurones by branches of common stem pre-synaptic fibres causes short-term synchronization of their discharge during the rising phases of the unitary e.p.s.p.s. 2. This hypothesis was tested using the pre- and post-stimulus time (PPST) histogram to detect synchronized firing among groups of intercostal motoneurones discharging in response to their natural synaptic drives. 3. Motor nerve action potentials were recorded monophasically from nerve filaments of the external intercostal muscles of anaesthetized, paralysed cats maintained on artificial ventilation. 4. Computer methods were used to measure peak spike amplitude, spike amplitude, spike interval and filament identification for simultaneous recordings from four filaments. The spike amplitude histograms were derived for each filament and groups of spikes were selected for analysis. 5. With spikes of one group designated as 'stimuli' (occurring at zero time) and those of a second as 'response' the PPST histogram was computed with different time bin widths. 6. With bin widths of 100 and 10 msec the central respiratory periodicity was apparent in the PPST histogram. With 1.0 msec bins the PPST histogram showed a narrow central peak extending to +/- 3.0 msec at its base. This 'short-term synchronization' supports the hypothesis of joint firing due to common presynaptic connectivity. 7. It was shown that detection of short-term synchronization was critically dependent on a sufficient quantity of data but that provided a simple criterion of adequate counts per bin in the PPST histogram was met, short-term synchronization could be detected between intercostal motoneurones of the same and adjacent segments.     

On the regulation of repetitive firing in lumbar motoneurones during fictive locomotion in the cat

Summary Repetitive firing of motoneurones was examined in decerebrate, unanaesthetised, paralysed cats in which fictive locomotion was induced by stimulation of the mesencephalic locomotor region. Repetitive firing produced by sustained intracellular current injection was compared with repetitive firing observed during fictive locomotion in 17 motoneurones. During similar interspike intervals, the afterhyperpolarisations (AHPs) during fictive locomotion were decreased in amplitude compared to the AHPs following action potentials produced by sustained depolarising current injections. Action potentials were evoked in 10 motoneurones by the injection of short duration pulses of depolarising current throughout the step cycles. When compared to the AHPs evoked at rest, the AHPs during fictive locomotion were reduced in amplitude at similar membrane potentials. The post-spike trajectories were also compared in different phases of the step cycle. The AHPs following these spikes were reduced in amplitude particularly in the depolarised phases of the step cycles. The frequency-current (f-I) relations of 7 motoneurones were examined in the presence and absence of fictive locomotion. Primary ranges of firing were observed in all cells in the absence of fictive locomotion. In most cells (6/7), however, there was no relation between the amount of current injected and the frequency of repetitive firing during fictive locomotion. In one cell, there was a large increase in the slope of the f-I relation. It is suggested that this increase in slope resulted from a reduction in the AHP conductance; furthermore, the usual elimination of the relation is consistent with the suggestions that the repetitive firing in motoneurones during fictive locomotion is not produced by somatic depolarisation alone, and that motoneurones do not behave as simple input-output devices during this behaviour. The correlation of firing level with increasing firing frequency which has previously been demonstrated during repetitive firing produced by afferent stimulation or by somatic current injection is not present during fictive locomotion. This lends further support to the suggestion that motoneurone repetitive firing during fictive locomotion is not produced or regulated by somatic depolarisation. It is suggested that although motoneurones possess the intrinsic ability to fire repetitively in response to somatic depolarisation, the nervous system need not rely on this ability in order to produce repetitive firing during motor acts. This capability to modify or bypass specific motoneuronal properties may lend the nervous system a high degree of control over its motor output.     

Floccular influence on excitatory relay neurones of vestibular reflexes of anterior semicircular canal origin in the cat

Floccular influence on excitatory vestibular reflex arcs of anterior semicircular canal origin was examined in the anaesthetized cat. Stimulation of the anterior semicircular canal nerve (ACN) evoked disynaptic excitatory postsynaptic potentials (EPSPs) in all sampled inferior oblique (IO), superior rectus (SR), and biventor cervicis (BIV) muscle motoneurones of the contralateral side. Conditioning stimulus to the flocculus depressed the amplitude of the EPSPs in both IO and SR motoneurones by 50% on the average but not in any BIV motoneurones. The excitatory vestibulo-ocular neurones identified by orthodromic and antidromic responses to stimulation of the ACN and the contralateral IO motoneurone pool, respectively, were classified as VOC (vestibulo-ocular neurones with axons descending to the cervical segment) or VO (vestibulo-ocular proper) neurones on the basis of whether or not they responded antidromically to stimulation of the spinal cord in the C1 segment. All of the VO neurones in the superior vestibular nucleus (n = 19) were inhibited from the flocculus while the activities of three-fourths of the VO neurones (36/48) in the other vestibular nuclei were not suppressed by floccular stimulation. In contrast, none of VOC neurones (n = 49) received floccular inhibition. Besides inhibition, floccular stimulation induced the antidromic or orthodromic responses in some VO and VOC neurones.     

Uncrossed actions of feline corticospinal tract neurones on hindlimb motoneurones evoked via ipsilaterally descending pathways

Despite numerous investigations on the corticospinal system there is only scant information on neuronal networks mediating actions of corticospinal neurones on ipsilateral motoneurones. We have previously demonstrated double crossed pathways through which pyramidal tract neurones can influence ipsilateral motoneurones, via contralaterally descending reticulospinal neurones and spinal commissural interneurones. The aim of the present study was to examine the effects of stimulation of pyramidal tract (PT) fibres mediated via ipsilaterally descending pathways and to find out which neurones relay these effects. This was done by using intracellular recordings from 96 lumbar motoneurones in deeply anaesthetized cats. To eliminate actions of fibres descending on the side contralateral to the location of the motoneurones, the spinal cords were hemisected on this side at a low-thoracic level. Stimuli that selectively activated ipsilateral PT fibres evoked EPSPs and/or IPSPs in 34/47 motoneurones tested. These PSPs were evoked at latencies indicating that the most direct coupling between PT neurones and motoneurones in uncrossed pathways is disynaptic. Occlusion and spatial facilitation between actions evoked by stimulation of ipsilateral PT and of reticulospinal tract fibres in the ipsilateral medial longitudinal fascicle (MLF) indicated that PT actions are mediated by reticulospinal neurones with axons in the MLF. However, after transection of the MLF in the caudal medulla, stimulation of the ipsilateral PT continued to evoke EPSPs and IPSPs with characteristics similar to when the MLF was intact (in 15/49 motoneurones) suggesting the existence of parallel disynaptic pathways via other relay neurones.     

Premotor interneurones contributing to actions of feline pyramidal tract neurones on ipsilateral hindlimb motoneurones

The aim of the study was to analyse the potential contribution of excitatory and inhibitory premotor interneurones in reflex pathways from muscle afferents to actions of pyramidal tract (PT) neurones on ipsilateral hindlimb motoneurones. Disynaptic EPSPs and IPSPs evoked in motoneurones in deeply anaesthetized cats by group Ia, Ib and II muscle afferents were found to be facilitated by stimulation of the ipsilateral, as well as of contralateral, PT. The ipsilateral actions were evoked by either uncrossed or double-crossed pathways. The results show that interneurones mediating reflex actions of muscle afferents may be activated strongly enough by PT stimulation to contribute to movements initiated by ipsilateral PT neurones and that PT actions relayed by them might be enhanced by muscle stretches and/or contractions. However, in some motoneurones disynaptic IPSPs and EPSPs evoked from group Ib or II afferents were depressed by PT stimulation. In order to analyse the basis of this depression, the transmitter content in terminals of 11 intracellularly labelled interneurones excited by PT stimulation was defined immunohistochemically and their axonal projections were reconstructed. The interneurones included 9 glycinergic and 2 glutamatergic neurones. All but one of these neurones were mono- or disynaptically excited by group I and/or II afferents. Several projected to motor nuclei and formed contacts with motoneurones. However, all had terminal projections to areas outside the motor nuclei. Therefore both inhibitory and excitatory interneurones could modulate responses of other premotor interneurones in parallel with direct actions on motoneurones.     

Phasic control of the transmission in the excitatory and inhibitory reflex pathways from cutaneous afferents to α-motoneurones during fictive locomotion in cats

In high spinal paralyzed cats the effect of cutaneous nerve stimulation on lumbar motoneurons was investigated during fictive locomotion. EPSPs evoked from the cutaneous afferents were generally larger during the active phase of the motoneurones, while IPSPs tended to increase during the reciprocal phase. In some cases EPSPs occurred during the active phase, while IPSPs dominated during the reciprocal phase. Apparently, the transmission in the excitatory and inhibitory segmental reflex pathways from cutaneous afferents to α-motoneurones depends on the phase of the step cycle, but there is no general phase dependent alternating switching between these two pathways.     

Two descending medullary inspiratory pathways to phrenic motoneurones

Synaptic connections of the medullary inspiratory neurones of the nucleus tractus solitarius (NTS) and nucleus retroambigualis (NRA) with phrenic motoneurones were studied using spike triggered averaging of the synaptic noise of phrenic motoneurones. More than 60% of NTS inspiratory neurones made monosynaptic connections with phrenic motoneurones, while similar connections between NRA and phrenic motoneurones could be shown in less than 7% of studied neuronal pairs. Relations between cross-correlations and observed synaptic connectivity of the phrenic motoneurones are also discussed.