Role of the ventrolateral region of the nucleus of the tractus solitarius in processing respiratory afferent input from vagus and superior laryngeal nerves

Summary The role of respiratory neurons located within and adjacent to the region of the ventrolateral nucleus of the tractus solitarius (vlNTS) in processing respiratory related afferent input from the vagus and superior laryngeal nerves was examined. Responses in phrenic neural discharge to electrical stimulation of the cervical vagus or superior laryngeal nerve afferents were determined before and after lesioning the vlNTS region. Studies were conducted on anesthetized, vagotomized, paralyzed and artificially ventilated cats. Arrays of 2 to 4 tungsten microelectrodes were used to record neuronal activity and for lesioning. Constant current lesions were made in the vlNTS region where respiratory neuronal discharges were recorded. The region of the vlNTS was probed with the microelectrodes and lesions made until no further respiratory related neuronal discharge could be recorded. The size and placement of lesions was determined in subsequent microscopic examination of 50 m thick sections. Prior to making lesions, electrical stimulation of the superior laryngeal nerve (4–100 A, 10 Hz, 0.1 ms pulse duration) elicited a short latency increase in discharge of phrenic motoneurons, primarily contralateral to the stimulated nerve. This was followed by a bilateral decrease in phrenic nerve discharge and, at higher currents, a longer latency increase in discharge. Stimulation of the vagus nerve at intensities chosen to selectively activate pulmonary stretch receptor afferent fibers produced a stimulus (current) dependent shortening of inspiratory duration. Responses were compared between measurements made immediately before and immediately after each lesion so that changes in response efficacy due to lesions per se could be distinguished from other factors, such as slight changes in the level of anesthesia over the several hours necessary in some cases to complete the lesions. Neither uni- nor bi-lateral lesions altered the efficacy with which stimulation of the vagus nerve shortened inspiratory duration. The short latency excitation of the phrenic motoneurons due to stimulation of the superior laryngeal nerve was severely attenuated by unilateral lesions of the vlNTS region ipsilateral to the stimulated nerve. Neither the bilateral inhibition nor the longer latency excitation due to superior laryngeal nerve stimulation was reduced by uni- or bi-lateral lesions of the vlNTS region. These results demonstrate that extensive destruction of the region of the vlNTS: a) does not markedly affect the inspiratory terminating reflex associated with electrical stimulation of the vagus nerve in a current range selective for activation of pulmonary stretch receptor afferents, and b) abolishes the short-latency increase, but not the bilateral decrease or longer latency increase in phrenic motoneuronal discharge which follows stimulation of the superior laryngeal nerve. We conclude that respiratory neurons in the region of the vlNTS do not play an obligatory role in the respiratory phase transitions in this experimental preparation. Neurons in the vlNTS region may participate in other reflexes, such as the generation of augmented phrenic motoneuronal discharge in response to activation of certain superior laryngeal or vagus nerve afferents.     

Peripheral chemoreceptor inputs to medullary inspiratory and postinspiratory neurons of cats

The effect of peripheral chemoreceptor activation on inspiratory and postinspiratory medullary neurons was investigated using intracellular recording techniques. Peripheral chemoreceptors were activated by injecting CO₂ saturated 1 N bicarbonate solution into the lingual artery or by electrically stimulating the carotid sinus nerve. Injections of 20–300 l bicarbonate solution evoked changes in respiratory frequency and in peak phrenic nerve discharge. The membrane potential of inspiratory alpha neurons, whether bulbospinal or not and independent of their anatomic location, was decreased during inspiration. A sequence of compound excitatory and inhibitory effects were observed when the stimulus was given during the postinspiratory and expiratory phases of the respiratory cycle. Inspiratory beta- and late-inspiratory neurons, however, were inhibited by peripheral chemoreceptor activation. Postinspiratory neurons were strongly activated during postinspiration. Neither class of respiratory neurons were shown to receive direct synaptic inputs from the peripheral chemoreceptors as tested by electrical stimulation of the carotid sinus nerve and signal averaging of the respiratory neuron membrane potential. The experiments revealed differential influences of afferent chemoreceptor activity on various components of the respiratory network. We conclude that chemoreceptor afferents activate non-respiratory modulated medullary neurons which, in turn, activate or inhibit various neurons of the medullary respiratory control network. The responses of each type of respiratory neuron to chemoreceptors afferents may then be considered in the context of this direct interaction as well as the network interactions of the various cells.     

Evidence for a monosynaptic connection between slowly adapting pulmonary stretch receptor afferents and inspiratory beta neurones

The synaptic connection between slowly adapting pulmonary stretch receptor afferents and inspiratory neurones within a region ventral to the tractus solitarius was determined using intracellular recording and spike triggered averaging techniques.When the vagus nerve was stimulated at intensities eliciting a Hering-Breuer reflex, the difference in mean latency between centrally recorded action potentials of slowly adapting pulmonary stretch receptor afferents and e.p.s.p.s of inspiratory beta neurones was 0.2 ms. This difference is indicative of a monosynaptic connection.Extracellular single unit spikes of slowly adapting pulmonary stretch receptors recorded from the nodose ganglion were used to trigger the averaging of synaptic noise recorded from inspiratory neurones. A prominent wave of synaptic depolarization was observed in all inspiratory beta neurones even when a small number of sweeps were averaged. This depolarization was absent from inspiratory alpha neurones.The shape indices of these depolarizations are consistent with a monosynaptic connection between slowly adapting pulmonary stretch receptor afferents and inspiratory beta neurones. In addition, the data raise the possibility that this connection is multiple and distributed.     

Regularity in the generation of discharge patterns by primary and secondary muscle spindle afferents,as recorded under a ramp-and-hold stretch

The discharge frequency of primary (Ia) and secondary (II) muscle spindle afferents from the tibial anterior muscle of the cat were recorded under a rampand-hold stretch of the host muscle. The rate of ramp stretch and the prestretch of the muscle were varied systematically. The degree of stretch was kept constant. For a discharge pattern recorded at a ramp rate of 10 mm/s, the peak dynamic discharge, the maximum static value and the final static value were determined. These three discharge rate values were plotted against the maximum static value. In the resulting charts the II afferents presented themselves as a homogeneous group of spindle afferents, whereas the Ia fibers separated into three subgroups. The existence of three subpopulations of Ia fibers was verified by the method of Hald. Furthermore, it is shown that each subpopulation generated its discharge patterns in its own regularly systematic manner. It was concluded that, as one of the three Ia subpopulations exhibits much the same dynamic and static stretch properties as the II fibers, the encoder of this subpopulation must receive its receptor current from the sensory terminals of passive intrafusal chain fibers. The encoder of a second Ia subpopulation indicates its action potentials using the receptor current stemming from the bag₁ sensory terminals, these Ia fibers eliciting a slow adaptation component of a high magnitude which is assumed to be the consequence of a high level of creep in the passive intrafusal bag₁ fiber. The third Ia subpopulation initiates its action potential sequences by means of the receptor current stemming from the passive bag₂ fiber, producing behavior patterns that lie between those of the other two Ia subpopulations.     

Bilateral reflex effects on phrenic nerve activity in response to single-shock vagal stimulation

The bilateral reflex actions of vagus nerve afferent signals on phrenic efferent activity have been tested by unilateral graded single shock electrical stimulation. An early excitation (latency 3–5 msec) was more prominent in the phrenic nerve contralateral to the stimulated vagus. Spinal cord hemisection at C₃ eliminated both contralateral and ipsilateral responses: thus, both were mediated via descending tracts in the contralateral cord. A bilaterally symmetrical early inhibition (latency 8–12 msec) followed the early excitation. The electrical thresholds for evoking the early responses and the temperature for blocking these responses during graded vagal cooling were closely similar to the threshold and blocking temperature for pulmonary stretch receptor afferents. Higher stimulus strengths evoked a strong, bilaterally similar, late excitation (latency 12–20 msec) followed by a late inhibition. At very high stimulus strengths a third excitation (latency 25–30 msec) could appear. Sometimes these responses were followed by lowered phrenic activity for the remainder of inspiration. Single shock stimulation of the intact vagus nerve or of the peripheral end of the cut recurrent laryngeal nerve provoked. by the contraction of laryngeal muscles, a strong, short latency (12 msec) inhibition of phrenic activity mediated by superior laryngeal nerve afferents. The implications of these results with respect to the reflex pathways of the different responses and their possible integration in the central respiratory control mechanisms are discussed.     

Characterization of hindlimb muscle afferents involved in ventilatory effects observed in decerebrate and spinal preparations

Summary Neurogenic changes of phrenic activity have previously been observed during periodic passive motions of one hindlimb in decorticate, unanaesthetized and curarized rabbit preparations before and after high spinal transection (Palisses et al. 1988). In decerebrate and spinal preparations, we aimed to determine, through rhythmic electrical stimulation of hindlimb muscle nerves, which muscle afferents are involved in these effects. In decerebrate preparations, these electrical stimulations (trains of shocks at 80 Hz for 300 ms every second for 20 s) produced ventilatory effects when group I+II afferent fibres of either flexor or extensor nerves were stimulated together and more powerful changes as soon as group III fibres were recruited. Stimulation of group I fibres alone induced no such effects. When present, these changes in respiratory activity consisted of a maintained decrease of the respiratory period due to both inspiratory and expiratory time shortening; in addition, the amplitude of the phrenic bursts greatly increased at the onset of electrical stimulation. After spinal transection at C2 level and pharmacological activation by nialamide and DOPA, only short-lasting phrenic bursts developed spontaneously; the electrical stimulation of group II and mainly group III flexor afferent fibres induced large amplitude phrenic activity whereas the stimulation of the same extensor afferents was relatively ineffective. The activation of phrenic motoneurones during group III flexor afferent stimulation was closely linked to each 300 ms period of stimulation. While the phrenic effects obtained in the spinal preparations by natural and by electrical periodic stimulation are quite similar to each other, those produced in decerebrate preparations differ substantially. It is concluded that the regulation of phrenic activity in decerebrate and spinal rabbit preparations by hindlimb proprioceptive afferents involves different muscle receptors; perhaps joint proprioceptors for the medullary resetting and muscle receptors connected to group III afferent fibres for the spinal reflex activation of phrenic motoneurones.     

Release of ATP and glutamate in the nucleus tractus solitarii mediate pulmonary stretch receptor (Breuer-Hering) reflex pathway

The Breuer–Hering inflation reflex is initiated by activation of the slowly adapting pulmonary stretch receptor afferents (SARs), which monosynaptically activate second-order relay neurones in the dorsal medullary nucleus of the solitary tract (NTS). Here we demonstrate that during lung inflation SARs release both ATP and glutamate from their central terminals to activate these NTS neurones. In anaesthetized and artificially ventilated rats, ATP- and glutamate-selective microelectrode biosensors placed in the NTS detected rhythmic release of both transmitters phase-locked to lung inflation. This release of ATP and glutamate was independent of the centrally generated respiratory rhythm and could be reversibly abolished during the blockade of the afferent transmission in the vagus nerve by topical application of local anaesthetic. Microionophoretic application of ATP increased the activity of all tested NTS second-order relay neurones which receive monosynaptic inputs from the SARs. Unilateral microinjection of ATP into the NTS site where pulmonary stretch receptor afferents terminate produced central apnoea, mimicking the effect of lung inflation. Application of P2 and glutamate receptor antagonists (pyridoxal-5'-phosphate-6-azophenyl-2',4'-disulphonic acid, suramin and kynurenic acid) significantly decreased baseline lung inflation-induced firing of the second-order relay neurones. These data demonstrate that ATP and glutamate are released in the NTS from the central terminals of the lung stretch receptor afferents, activate the second-order relay neurones and hence mediate the key respiratory reflex – the Breuer–Hering inflation reflex.     

The lateral vestibular nucleus of the toadfish Opsanus tau: Ultrastructural and electrophysiological observations with special reference to electrotonic transmission

The lateral vestibular nucleus of the toadfish Opsanus tau was localized by means of axonal iontophoresis of Procion Yellow. The ultrastructure of the lateral vestibular nucleus neurons was then correlated with their electrophysiological properties. The lateral vestibular nucleus consists of neurons of various sizes which are distributed in small clusters over a heavily myelinated neuropil. The perikarya and main dendrites of the large and the small neurons are surrounded by a synaptic bed, which is separated from the neighboring neuropil by a layer of thin astrocytic processes. The synaptic bed contains three main classes of axon terminals, club endings, large and small terminals, the first being quite infrequent. All the large terminals as well as the occasionally observed club endings contain a pure population of rounded synaptic vesicles. In some of the small axon terminals there are also rounded vesicles; however, the majority contain flattened vesicles or a pleomorphic population. These data indicate that the small terminals originate from different afferent sources. The synaptic interfaces of the large boutons and of the club endings bear three types of junctional complexes: attachment plates, gap junctions and active zones. Those showing both gap junctions and active zones were designated as morphologically ‘mixed synapses’. Gap junctions, although in large number, have only been observed at the synaptic interfaces between terminals with rounded vesicles and the perikarya or the dendrite of the lateral vestibular nucleus neurons. Therefore electrotonic coupling would only be possible by way of presynaptic fibers. Some axons observed in the neuropil were found to establish gap junctional complexes with two different dendritec profiles and this observation is in favour of electrotonic coupling by way of presynaptic terminals.Field and intracellular potentials were recorded in the lateral vestibular nucleus. The field potential evoked by stimulation of the vestibular nerve consisted of an early positive-negative wave followed by a slow negativity, and that evoked by spinal cord stimulation was composed of an antidromic potential followed by a slow negative wave. Vestibulo-spinal neurons were identified by their antidromic spikes. In these cells, stimulation of the ipsilateral vestibular nerve evoked an excitatory postsynaptic potential with two components. The short delay of the first component of this excitatory postsynaptic potential and its ability to follow paired stimulation at close intervals without reduction of the second response suggest that it is transmitted electrotonically from primary vestibular afferent fibers. By contrast the latency of the second peak of the vestibular evoked excitatory postsynaptic potential and its sensitivity to high stimulus frequencies are compatible with monosynaptic chemically mediated transmission from primary vestibular afferents. Spinal stimulation evoked graded antidromic depolarizations in vestibulo-spinal neurons. The latency of these potentials was too short to allow for chemical transmission through afferents or recurrent collaterals and suggests electrotonic spread of antidromic activity from neighboring neurons. An important finding is that the graded antidromic depolarizations can initiate spikes; thus coupling between neurons in the lateral vestibular nucleus is sufficiently close that a cell can be excited by activity spread from neighboring cells. Similar graded depolarizations were recorded in identified primary vestibular afferents; their latencies and time course indicate that they were brought about by electrotonic spread of postsynaptic potentials and spikes to the impaled presynaptic fibers; this confirms the morphological evidence that coupling between lateral vestibular nucleus neurons occurs, at least in part, by way of presynaptic vestibular axons. As the spinal stimulus strength was increased, these graded depolarizations became large enough to initiate spikes which presumably propagate to the vestibular receptors. Thus antidromic invasion of the presynaptic terminals may provide negative feedback by preventing their re-excitation at short intervals after a synchronous discharge of an adequate number of postsynaptic cells. Excitatory inputs to the neurons of the lateral vestibular nucleus were identified from the spinal cord and from the contralateral vestibular nerve. Long latency excitatory postsynaptic potentials large enough to excite the cells were recorded following spinal stimulation; the threshold intensity for evoking them was consistently higher than that adequate to generate the graded antidromic depolarizations. Field potentials recorded after stimulation of the contra lateral vestibular nerve consisted of an initial positive negative wave followed by a slow negative wave. the stimulus intensity for evoking these potentials was the same or slightly above the threshold for those evoked in the lateral vestibular nucleus on the stimulated side. Also lateral vestibular nucleus neurons exhibited excitatory postsynaptic potentials large enough to excite the cells following stimulation of the contralateral vestibular nerve. but no inhibitory postsynaptic potentials were detected. This lack of commissural inhibition indicates a qualitative difference between the central organization of these cells in the toadfish and in mammals.The presence of neurons in the lateral vestibular nucleus which send their axons to the labyrinth was confirmed by their heavy staining with Procion Yellow following axonal iontophoresis. In a number of vestibular neurons. abruptly rising spikes were evoked at short latencies after adequate stimulation of the ipsilateral vestibular nerve. Graded stimuli applied to the vestibular nerve evoked graded short latency depolarizations as well as long latency excitatory postsynaptic potentials in these presumed efferent neurons to the labyrinth; the former could indicate electrotonic coupling of the efferent cells or electrotonic transmission from primary afferents, resulting in a short latency feedback loop.From these studies, the synaptic organization of the lateral vestibular nucleus neurons is compared with that of the Mauthner cells of teleosts, and the possibility of a dual mode of transmission, electrical and chemical, by primary vestibular afferents is discussed.     

Responses of ventral respiratory neurones in the rat to vagus stimulation and the functional division of expiration.

In anaesthetized rats, extracellular and intracellular recordings were taken from 106 respiratory neurones in the intermediate region of the nucleus ambiguus. We observed unprovoked shortening of expiratory time accompanied, in all classes of respiratory neurone, by the elimination of the changes in membrane potential that were characteristic of stage II expiration. The demonstration of the elimination of stage II expiration in both the rat and cat strongly supports the functional division of expiration into stage I expiration (post-inspiration) and stage II expiration. In order to identify the neurones in the rat that receive inputs from vagal afferents and modulate the central respiratory rhythm, we examined whether any respiratory neurones responded to stimulation of the vagus nerve. Some post-inspiratory and stage II expiratory neurones responded. The short latency (< 2 ms) of four of the responses indicates that some vagal afferents act on post-inspiratory neurones via a disynaptic pathway. While repetitive stimulation of the vagus nerve could inhibit the phrenic rhythm, it appears that most inspiratory neurones in the intermediate region of the nucleus ambiguous complex are not directly involved in integrating the information from vagal afferents with the central respiratory rhythm.     

Synaptic connections of central carotid sinus afferents in the nucleus of the tractus solitarius of the rat. I. An electron microscopic study

Summary A transganglionic transport technique was used to study the synaptic connections of the central carotid sinus afferents in the nucleus of the tractus solitarius of the rat by electron microscopy. The caudal part of the nucleus was profusely innervated. Labelled fibres extended to the contralateral nucleus, and to the ipsilateral dorsal motor nucleus of the vagus nerve, nucleus ambiguus, spinal nucleus of the trigeminal nerve and the area postrema. The labelled terminals were densely packed with clear, predominantly spherical vesicles about 50 nm in diameter and a few often swollen mitochondria. The terminals synapsed on dendrites of various calibres, spindle- or pear-shaped somal profiles with short axes lesser than 8 m, and axon terminals. In axo-axonal synapses, most labelled terminals appeared to be presynaptic. Frequently, profiles of labelled terminals were in direct apposition with one another. The latter may represent the morphological substrate of the interaction between baro- and chemoreceptor inputs in the nucleus of the tractus solitarius and warrants further study. The present results indicate that in addition to direct inputs, the carotid sinus afferents are able to influence second-order neurons in the nucleus of the tractus solitarius indirectly through presynaptic modulation.     

Control of quantal transmitter release at frog's motor nerve terminals

Motor terminals on the cutaneous pectoris muscle of the frog were depolarized by current pulses through the recording macro-pathch-clamp electrode and the resulting quantal release was measured (excitation blocked with TTX). Above a threshold release increased very steeply with depolarization until saturation was approached. The dependence of release on duration of depolarization was even steeper: doubling pulse duration often produced more than 100-fold release (early facilitation) Distributions of delays of quantal release after the depolarization pulse were determined for wide ranges of depolarizations and pulse durations. The shape of these distributions was little affected by large changes in average release; increasing the temperature from 0°C to 10°C about halved the time scale of the distributions. Lengthening the depolarization from 0.5 to 6 ms produced a latency shift: the distributions of delays were shifted by almost the increase in pulse duration. At 5–6 ms pulse duration a few releases occurred during the final millisecond of the pulse. It is suggested that the time course of the phasic release is not controlled by the time course of changes in intracellular calcium concentration, but by an activator which is produced about proportional to supra-threshold pulse amplitude and duration, and that this activator effects release with a cooperativity of 6–7. An additional depolarization produced repressor is responsible for the minimum delay.Supported by the Deutsche Forschungsgemeinschaft     

GABA-immunoreactivity in processes presynaptic to the terminals of afferents from a locust leg proprioceptor

Summary Individually labelled sensory neurons from the femoral chordotonal organ, a proprioceptor at the femoro-tibial joint of a locust hindleg, were analysed by intracellular recording, and by electron microscopical immunocytochemistry to reveal the arrangement of their input and output synapses and to determine whether the input synapses were GABAergic. Intracellular recordings from these sensory neurons show spikes superimposed on a barrage of synaptic potentials during movements of the femoro-tibial joint. These synaptic inputs can be mimicked by GABA. Input synapses are made onto the vesicle-containing terminals of afferents and are often closely associated with the output synapses. By contrast, the axons of the afferents in the neuropil have no vesicles and neither make nor receive synapses. The input synapses to the afferent terminals are made from processes typically a few microns in diameter, whereas the output synapses are made onto much smaller processes of only 0.1–0.2 m. Input synapses at which an afferent terminal is the only postsynaptic element are common. Where the synapse is dyadic the second postsynaptic element does not usually appear to be a chordotonal afferent. The output synapses from the afferent terminals are usually dyadic. At 78% of the input synapses, the presynaptic neurite showed immunoreactivity to a GABA antibody, supporting the physiological evidence that the presynaptic effects can be mediated by the release of GABA. The remaining (22%) immunonegative synapses are intermingled with those showing GABA immunoreactivity, but their putative transmitter is unknown. These morphological observations suggest that the presynaptic control of the chordotonal afferents is largely mediated by GABAergic neurons, but because other types of neuron also appear to be involved, presynaptic modulation may be more complex than has yet been revealed by the physiology.     

Activation of mu-opioid receptors in rat ventrolateral medulla selectively blocks baroreceptor reflexes while activation of delta opioid receptors blocks somato-sympathetic reflexes.

The effects of activation of mu and delta-opioid receptors in the rostral ventrolateral medulla (RVLM) on somato-sympathetic, baroreceptor and chemoreceptor reflexes, as well as respiratory rhythmicity in sympathetic nerves, were examined in urethane anaesthetized (1.1-1.2 g/kg) and artificially ventilated Sprague-Dawley rats.Microinjection of the delta-opioid receptor agonist [D-Pen(2,5)]-enkephalin (DPDPE; 8 mM, 50 nl) bilaterally into the RVLM potently inhibited the post-inspiratory-related burst discharges of lumbar sympathetic nerve activity (LSNA) but had only limited effects on splanchnic sympathetic nerve activity (SSNA) and phrenic nerve discharge. Injection of DPDPE into the RVLM strongly attenuated the somato-sympathetic reflex (approximately 50-80%) evoked in the lumbar sympathetic nerve and splanchnic sympathetic nerve by tibial nerve stimulation but had no effect on baroreceptor reflexes and chemoreceptor reflexes evoked by aortic nerve stimulation and brief hypoxia, respectively.Injection of the mu-opioid receptor agonist, [D-Ala(2),N-Me-Phe(4),Gly-ol(5)]-enkephalin (DAMGO; 4 mM, 50 nl), also elicited a greater inhibition of LSNA than SSNA accompanied by an abolition of phrenic nerve discharge. Injection of DAMGO inhibited the baroreceptor reflex without significant effect on either the somato-sympathetic or the chemoreceptor reflexes.We propose that opioid peptides diminish specific excitatory and inhibitory inputs to the presympathetic neurons in RVLM via distinct presynaptic receptor subclasses.     

Transmitter release from nerve terminals evoked by depolarization pulses contains a short phase of repression

The time course of quantal transmitter release after a depolarization pulse was measured at frog and crayfish motor nerve terminals. Test pulses were arranged to elicit low release, and the delay of first releases and the median of distributions of release times were defined for large (>2000 stimuli) samples. Small, subthreshold depolarizing postpulses were added directly after the test pulses. Such postpulses of 1 to 4 ms duration prolonged the delay of first releases and shifted the median of the time course of release by up to 3 ms (at 0°C) depending on the duration and on the amplitude of the post-pulses. These latency shifts, which have been observed after prolonged depolarizations by other authors, were statistically highly significant. The results of post-pulses were very similar at neuromuscular junctions of frog and crayfish. It is concluded that depolarization in addition to the promotion of release has a short repressing action on release which is partly responsible for synaptic delay.Supported by the Deutsche Forschungsgemeinschaft Preliminary reports of part of the present results have appeared Dudel 1985 a, b).     

Interaction between peripheral afferent activity and presynaptic inhibition of ia afferents in the cat

It has been demonstrated in man that the H-reflex is more depressed by presynaptic inhibition than the stretch reflex. Here we investigated this finding further in the alpha-chloralose-anesthetized cat. Soleus monosynaptic reflexes were evoked by electrical stimulation of the tibial nerve or by stretch of the triceps surae muscle. Conditioning stimulation of the posterior biceps and semitendinosus nerve (PBSt) produced a significantly stronger depression of the electrically than the mechanically evoked reflexes. The depression of the reflexes has been shown to be caused by presynaptic inhibition of triceps surae Ia afferents. We investigated the hypothesis that repetitive activation of peripheral afferents may reduce their sensitivity to presynaptic inhibition. In triceps surae motoneurones, we measured the effect of presynaptic inhibition on excitatory postsynaptic potentials (EPSPs) produced by repetitive activation of the peripheral afferents or by fast and slow muscle stretch. EPSPs evoked by single electrical stimulation of the tibial nerve or by fast muscle stretch were significantly depressed by PBSt stimulation. However, the last EPSP in a series of EPSPs evoked by a train of electrical stimuli (5-6 shocks, 150-200 Hz) was significantly less depressed by the conditioning stimulation than the first EPSP. In addition, the last part of the long-lasting EPSPs evoked by a slow muscle stretch was also less depressed than the first part. A single EPSP evoked by stimulation of the medial gastrocnemius nerve was less depressed when preceded by a train of stimuli applied to the same nerve than when the same train of stimuli was applied to a synergistic nerve. The decreased sensitivity of the test EPSP to presynaptic inhibition was maximal when it was evoked within 20 ms after the train of EPSPs. It was not observed at intervals longer than 30 ms. These findings suggest that afferent activity may decrease the efficiency of presynaptic inhibition. We propose that the described interaction between afferent nerve activity and presynaptic inhibition may partly explain why electrically and mechanically evoked reflexes are differently sensitive to presynaptic inhibition.     

Excitability and depolarization-release characteristics of excitatory nerve terminals in a tail muscle of spiny lobster

In the deep abdominal L₁-extensor muscle of the spiny lobster (Panulirus penicillatus) quantal excitatory postsynaptic currents (EPSCs) were recorded through macropatch-clamp electrodes. Release of transmitter quanta from terminals was also elicited by depolarizing current pulses given through the recording electrode. The majoritiy of terminals were excitable: on increasing the depolarization pulses, release was triggered at a threshold in an all-ornothing manner. If excitation was blocked by tetrodotoxin (TTX), release was graded with depolarization reaching the amplitude of the all-or-nothing response at pulse amplitudes several times higher than the former threshold level. Some inexcitable terminals were also found: in these, release was graded for increasing depolarization pulses, and TTX did not alter the depolarization-release relation. Among the other types of terminals studied with the same technique, the proportion of excitable terminals in this lobster tail muscle is higher than in the crayfish opener and lower than in the frog's cutaneous pectoris muscle. The contribution of the increase in intraterminal Ca concentration to the control of release was estimated using facilitation of a test EPSC as an indicator of Ca inflow during a preceding depolarization pulse. This facilitation was found to have a maximum at a certain pulse amplitude, , and to decline for larger depolarizations. Release, however, rose considerably for depolarizations larger than those effected at .It is concluded that, like in crayfish and frog motor terminals, release is controlled directly by depolarization in addition to the control by Ca-inflow. Supported by a grant of the Deutsche Forschungsgemeinschaft to J. Dudel and I. Parnas     

Afferent projections from forelimb muscles to the external and main cuneate nuclei in the cat

Summary Muscular and cutaneous afferents from distal forelimb distributed to the cuneate and external cuneate nuclei have been demonstrated in cat with the method of transganglionic transport of horseradish peroxidase. Injections of the same tracer were also done in ganglia C7 to T6 to demonstrate the afferents to these two nuclei. It is concluded that only muscle afferents terminate in the external cuneate nucleus. Afferents from paw and forearm occupy sequential territories in the medial part of the nucleus, which are only partly exclusive. Afferents from individual flexor muscles of forearm occupy distinct sites but their distributions overlap with those of forearm extensor muscles. In the external cuneate nucleus, the, distributions of afferents from individual muscles constitute integral parts of a segmental representation. In the cuneate nucleus, cutaneous afferents are located dorsally and terminate over cells of the clusters. Muscle afferents are distributed in ventral regions and are topographically arranged. They terminate over reticular regions.     

Cortically evoked pre-and postsynaptic inhibition of impulse transmission to the dorsal spinocerebellar tract

Summary Synaptic actions evoked from primary afferents and the sensorimotor cortex in neurones of the dorsal spinocerebellar tract were investigated: 1. Stimulation of the anterior lobe of the cerebellum produced a small IPSP in only one but not in the other six neurones examined. 2. IPSPs were induced not only from group I fibres (in 41% of group I neurones) but also from cutaneous and/or high threshold muscle afferents (in 37%). 3. Stimulation of the contralateral sensorimotor cortex evoked IPSPs in 80% of group I neurones. The IPSP had a latency of 10–15 msec and lasted for 40–100 msec. EPSPs were evoked from the cortex in a small number of neurones. 4. Effects from the cortex were compared with those from primary afferents in individual neurones. The cortical IPSPs were induced independently of whether the neurone received monosynaptic EPSP from extensor or flexor group I fibres. The cortical IPSPs (or EPSPs) occurred more frequently in neurones which exhibited polysynaptic IPSPs (or EPSPs) from primary afferents. 5. The few FRA neurones encountered were all excited from the cortex.Excitability measurements of primary afferent terminals in or near Clarke's column showed that a terminal depolarization is evoked from the cortex in group Ib but not in Ia afferents.The relative importance of post-and presynaptic inhibition of transmission to the DSCT is discussed.     

Effects of hip joint angle changes on intersegmental spinal coupling in human spinal cord injury

Pathological expression of movement and muscle tone in human upper motor neuron disorders has been partly associated with impaired modulation of spinal inhibitory mechanisms, such as reciprocal or presynaptic inhibition. In addition, input from specific afferent systems contributes significantly to spinal reflex circuits coupled with posture or locomotion. Accordingly, the objectives of this study were to identify the involved afferents and their relative contribution to soleus H-reflex modulation induced by changes in hip position, and to relate these effects with activity of spinal interneuronal circuits. Specifically, we investigated the actions of group I synergistic and antagonistic muscle afferents (e.g. common peroneal nerve, CPN; medial gastrocnemius, MG) and tactile plantar cutaneous afferents on the soleus H-reflex during controlled hip angle variations in 11 motor incomplete spinal cord injured (SCI) subjects. It has been postulated in healthy subjects that CPN stimulation evokes an inhibition on the soleus H-reflex at a conditioning test (C-T) interval of 2–4 ms. This short latency reflex depression is caused mainly by activation of the reciprocal Ia inhibitory pathway. At longer C-T intervals (beyond 30 ms) the soleus H-reflex is again depressed, and is generally accepted to be caused by presynaptic inhibition of soleus Ia afferents. Similarly, MG nerve stimulation depresses soleus H-reflex excitability at the C-T interval of 6 ms, involving the pathway of non-reciprocal group I inhibition, while excitation of plantar cutaneous afferents affects the activity of spinal reflex pathways in the extensors. In this study, soleus H-reflexes recorded alone or during CPN stimulation at either short (2, 3, 4 ms) or long (80, 100, 120 ms) C-T intervals, and MG nerve stimulation delivered at 6 ms were elicited via conventional methods and similar to those adopted in studies conducted in healthy subjects. Plantar skin conditioning stimulation was delivered through two surface electrodes placed on the metatarsals at different C-T intervals ranging from 3 to 90 ms. CPN stimulation at either short or long C-T intervals and MG nerve stimulation resulted in a significant facilitation of the soleus H-reflex, regardless of the hip angle tested. Plantar skin stimulation delivered with hip extended at 10° induced a bimodal facilitation reflex pattern, while with hip flexed (10°, 30°) the reflex facilitation increased with increments in the C-T interval. This study provides evidence that in human chronic SCI, classically key inhibitory reflex actions are switched to facilitatory, and that spinal processing of plantar cutaneous sensory input and actions of synergistic/antagonistic muscle afferents interact with hip proprioceptive input to facilitate soleus H-reflex excitability. These actions might be associated with the pathological expression of neural control of movement in individuals with SCI, and potentially could be considered in rehabilitation programs geared to restore sensorimotor function in these patients.     

Receptor potentials and electrical properties of nonspiking stretch-receptive neurons in the sand crab Emerita analoga (Anomura, Hippidae).

Receptor potentials and electrical properties of nonspiking stretch-receptive neurons in the sand crab Emerita analoga (Anomura, Hippidae). Four nonspiking, monopolar neurons with central somata and large peripheral dendrites constitute the sole innervation of the telson-uropod elastic strand stretch receptor in Emerita analoga. We characterized their responses to stretch and current injection, using two-electrode current clamp, in intact cells and in two types of isolated peripheral dendritic segments, one that included and one that excluded the dendritic termini (mechanosensory membrane). The membrane potentials of intact cells at rest (mean +/- SD: -57 +/- 4. 4 mV, n = 30), recorded in peripheral or neuropil processes, are similar to the membrane potentials of isolated dendritic segments and always less negative than membrane potentials of motoneurons and interneurons recorded in the same preparations. Ion substitution experiments indicate that the membrane potential is influenced strongly by Na+ conductance, probably localized in the mechanotransducing terminals within the elastic strand. The form of the receptor potential in response to ramp-hold-release stretch remains the same as stretch amplitude is varied and is not dependent on initial membrane potential (-70 to -30 mV) or recording site: initial depolarization (slope follows ramp of applied stretch), terminated by rapid, partial repolarization to a plateau (delayed depolarization) that is intermediate between the peak depolarization and the initial potential and sustained for the duration of the stretch. Responses to depolarizing current pulses are similar to stretch-evoked receptor potentials, except for small amplitude stimuli: an initial peak occurs only in response to stretch and probably reflects elastic recoil of the extracellular matrix surrounding the dendritic terminals. The rapid, partial repolarization depends on holding potential and is abolished by 4-aminopyridine (4-AP; 10 mM), implicating a fast-activating, fast-inactivating K+ conductance; TEA (60 mM) abolishes the remaining slow repolarization to the plateau. In intact cells, but not dendritic segments, regenerative depolarizations can arise in response to stretch or depolarizing current pulses; they are reduced by CdCl2 (10 microM) in the saline containing TEA and 4-AP and probably reflect current spread from Ca2+ influx at presynaptic terminals in the ganglion. We found no evidence for other voltage-activated conductances. Unlike morphologically similar "nonspiking" thoracic receptors of other species, E. analoga's nonspiking neurons are electrically compact and do not boost the analogue afferent signal by voltage-activated inward currents. The most prominent (only?) voltage-activated extra-ganglionic conductances are for potassium; by reducing the slope of the stretch-plateau depolarization curve, they extend each neuron's functional range to the full range of sensitivity of the receptor.