Decrementing expiratory neurons of the Bötzinger complex

Summary In Nembutal-anesthetized, immobilized and artificially ventilated cats, decrementing expiratory (E-DEC) neurons which were excited by lung inflation were isolated in the vicinity of the Bötzinger complex. Then intracellular recordings were made from the respiratory neurons in the contralateral ventral respiratory group (VRG). The intracellular membrane potentials were averaged using extracellular spikes of the E-DEC neurons as triggers (spike-triggered averaging method). Hyperpolarizing potentials locked to the triggering spikes were obtained and they were shown to be unitary IPSPs since their polarity was reversed when averaged during passage of hyperpolarizing current. The latencies of antidromic activation of the E-DEC-neurons from the area of intracellular recordings were shorter by about 0.2 ms than those of unitary IPSPs. This showed that the connections were monosynaptic. A total of 47 pairs were analyzed and unitary IPSPs were found in 12 pairs. The E-DEC neurons inhibited both inspiratory and expiratory neurons, including bulbospinal inspiratory neurons, propriobulbar inspiratory neurons, and vagal motoneurons with expiratory activity. These inhibitory E-DEC neurons, receiving excitatory inputs from the stretch receptors of the lungs, presumably intervene in reflex loops such as the Hering-Breuer reflex and may make some contribution to normal breathing.Supported by grants-in-aid for science research nos. 60304044, 62570068 from the Japan Ministry of Education, Science and Culture     

Extensive monosynaptic inhibition of ventral respiratory group neurons by augmenting neurons in the Bötzinger complex in the cat

Summary Axonal projections and synaptic connectivity of expiratory B?tzinger neurons with an augmenting firing pattern (Bot-Aug neurons) to neurons in the ipsilateral ventral respiratory group (VRG) were studied in anaesthetized cats. Antidromic mapping revealed extensive axonal arborizations of Bot-Aug neurons (24 of 45) to the rostral or caudal VRG, with some having arbors in both regions. Of 234 pairs of neurons studied with intracellular recording and spike-triggered averaging, monosynaptic inhibitory postsynaptic potentials (IPSPs) were evoked in 49/221 VRG neurons by 38/98 Bot-Aug neurons. The highest incidence of monosynaptic inhibition was found in inspiratory bulbospinal neurons (10 of 23 tested). Evidence was also found for monosynaptic inhibition, by a separate group of Bot-Aug neurons, of expiratory bulbospinal neurons (12/58), while excitatory postsynaptic potentials (EPSPs) were identified in another two of these neurons. In addition, monosynaptic IPSPs were recorded from 13 of 53 identified laryngeal motoneurons, and from 14 of 100 respiratory propriobulbar neurons. Presumptive disynaptic IPSPs were recorded from 11 of the 221 VRG neurons. We conclude that Bot-Aug neurons exert widespread inhibition on all major neuron categories in the ipsilateral VRG, and should be regarded as an important element in shaping the spatiotemporal output pattern of both respiratory motoneurons and premotor neurons.     

Excitation and inhibition of medullary inspiratory neurons by two types of burst inspiratory neurons in the cat

In Nembutal-anesthetized and artificially ventilated cats, we studied the connectivity of burst inspiratory (I) neurons in the B?tzinger complex and the ventral respiratory group (VRG) with spike-triggered averaging methods. Burst I neurons exhibited tonic (I-TON) or decrementing (I-DEC) firing patterns. Spikes of I-TON neurons induced monosynaptic EPSPs in intracellularly recorded I neurons of both the VRG and the dorsal respiratory group (DRG). Spikes of I-DEC neurons induced monosynaptic inhibitory postsynaptic potentials (IPSPs) in both VRG and DRG I neurons.     

Inputs to intercostal motoneurons from ventrolateral medullary respiratory neurons in the cat

The investigation examined the synaptic input from medullary respiratory neurons in the nucleus retroambigualis (NRA) to external (EIM) and internal (IIM) intercostal motoneurons. Antidromic mapping revealed that 112/117 (96%) tested NRA units had axons descending into thoracic spinal cord with extensive arborizations at many thoracic segments, mainly contralaterally. The conduction velocities ranged from 10 to 105 m X s-1. The descending projections did not appear to be somatotopically arranged. Cross-correlation of the spike trains of NRA inspiratory units with the discharge of external intercostal nerves (performed usually with 4 contralateral nerves) showed significant narrow peaks only in 5 out of 40 averages. Of the 25 trigger units tested for the thoracic projection in this series of experiments, 24 were antidromically activated. Intracellular recordings were made from 52 IIMs [mean membrane potential 65.3 mV, central respiratory drive potentials (CRDPs) greater than 1 mV present in 23/52] and 53 EIM (mean membrane potential 54.3 mV, CRDPs in 31/53). During the depolarizing phase of the CRDPs, synaptic noise with frequent and apparently unitary EPSPs with amplitudes in excess of 1 mV was observed. Spike-triggered averages of synaptic noise were computed for 153 pairings between 137 NRA neurons and 105 contralateral intercostal motoneurons. Only four PSPs were revealed: two monosynaptic EPSPs between expiratory NRA units and IIMs and two probably disynaptic EPSPs between inspiratory NRA units and EIMs. When advancing the microelectrode down to the motoneuron pools, frequent recordings were made from interneurons with spontaneous respiratory discharge (inspiratory or expiratory) located dorsal and medial to the motor nuclei. The interneurons could be excited following stimulation of segmental afferents. It is concluded that monosynaptic connections between respiratory NRA neurons and intercostal motoneurons are rare (connectivity no more than approximately 4%). Segmental interneurons, interposed between the majority of descending respiratory axons and intercostal motoneurons, are likely to produce large unitary EPSPs and, thus, short-term synchronization in the discharge of intercostal motoneurons as observed by others.     

The role of dorsal respiratory group neurons studied with cross-correlation in the decerebrate rat

We examined the role of dorsal respiratory group (DRG) inspiratory neurons as transmitters of respiratory drive to phrenic and intercostal motoneurons and as relays of afferent information to ventral respiratory group (VRG) bulbospinal, inspiratory neurons. Attempts to antidromically activate 76 DRG neurons from the spinal cord at the C7 segment resulted in only 4 (5.3%) successes (3 contralateral, 1 ipsilateral). Cross-correlating DRG neuron discharge with that of the ipsilateral (56) and contralateral (20) phrenic nerve detected common activation peaks in 2 and 3 cases respectively, with no evidence for monosynaptic connections. Cross-correlating DRG neuron discharge with that of bulbospinal, inspiratory VRG neurons found some evidence for interaction. Peaks in 7 of 73 (10%) cross-correlation histograms were attributed to a monosynaptic excitation of DRG neurons by VRG neurons, although a common activation cannot be ruled out; troughs, some with an accompanying peak, in 9 (12.3%) histograms were interpreted as a combined excitation of the DRG neuron and delayed inhibition of the VRG neuron. In addition, 2 cross-correlation histograms showed peaks with latencies and half-amplitude widths consistent with a disynaptic excitation of a DRG neuron by a bulbospinal inspiratory VRG neuron. Cross-correlating the discharge of 57 pairs of DRG inspiratory neurons (6 contralateral) detected common activation peaks in 7 (12.3%) cases (none contralateral) and one case interpreted as evidence for a disynaptic excitation. These findings suggest that the role of the DRG inspiratory neurons in rats differs from that in cats, primarily because they do not act to transmit respiratory rhythmic drive directly to phrenic and intercostal motoneurons. The results offer some support for an excitation of DRG neurons by VRG inspiratory neurons, but no support for a role of DRG inspiratory neurons as mediators of afferent information transfer to VRG bulbospinal inspiratory neurons.     

Behavior of hypoglossal inspiratory premotor neurons during the carbachol-induced, REM sleep-like suppression of upper airway motoneurons

In individuals with compromised upper airway anatomy, genioglossus (GG), the main protruder muscle of the tongue, is an important upper airway dilator which helps prevent upper airway obstructions. During rapid eye movement (REM) sleep, both the tonic and inspiratory-modulated components of GG activity are suppressed in parallel with the characteristic postural atonia. We tested whether the REM sleep-related reduction in the respiratory activity of GG may, in part, result from a reduced inspiratory drive relayed to hypoglossal (XII) motoneurons from their premotor medullary inspiratory neurons. In 15 urethane-anesthetized, paralyzed, vagotomized and artificially ventilated rats, we recorded XII nerve activity and the extracellular activity of medullary inspiratory-modulated neurons antidromically activated with latencies of 0.8 ms +/- 0.3(SD) from within (n = 19) or adjacent to (n = 11) the XII nucleus. Carbachol (10-20 nl, 10 mM), a cholinergic agonist, was microinjected into the dorsomedial pons. Such injections trigger a REM sleep-like state in chronically instrumented, intact animals and, in anesthetized rats, produce respiratory and electrocortical changes similar to those of REM sleep. Following the injections, the respiratory component of XII nerve activity was depressed by 51 +/- 22%, while the mean inspiratory firing rate of the neurons decreased by only 7.4 +/- 13.8% (from 69 +/- 34 Hz to 65 +/- 37 Hz; P < 0.02; n = 30). The activity of ventral respiratory group (VRG) and reticular formation inspiratory neurons with axons within the XII nucleus was reduced by 10 +/- 14% (P < 0.005; n = 19), whereas the activity of neurons located near the VRG that had axons passing below the XII nucleus did not change (n = 5). Thus, the depressant effect of carbachol on medullary inspiratory neurons was slightly more pronounced in reticular formation and VRG cells premotor to XII motoneurons than in other medullary inspiratory cells. For all cells, the magnitude of the decrease of cell activity was not related to the magnitude of depression of XII nerve activity, the simultaneously occurring decrease in respiratory rate or the cell's control firing rate. Since the magnitude of this depressant effect on all cell types was disproportionately small when compared with the depression of XII nerve activity, the REM sleep-like decrease in GG activity must be mainly mediated by non-respiratory premotor pathways.     

Synaptic origin of the respiratory-modulated activity of laryngeal motoneurons

To determine the synaptic source of the respiratory-related activity of laryngeal motoneurons, spike-triggered averaging of the membrane potentials of laryngeal motoneurons was conducted using spikes of respiratory neurons located between the Bötzinger complex and the rostral ventral respiratory group as triggers in decerebrate, paralyzed cats. We identified one excitatory and two inhibitory sources for inspiratory laryngeal motoneurons, and two inhibitory sources for expiratory laryngeal motoneurons. In inspiratory laryngeal motoneurons, monosynaptic excitatory postsynaptic potentials were evoked by spikes of inspiratory neurons with augmenting firing patterns, and monosynaptic inhibitory postsynaptic potentials (IPSPs) were evoked by spikes of expiratory neurons with decrementing firing patterns and by spikes of inspiratory neurons with decrementing firing patterns. In expiratory laryngeal motoneurons, monosynaptic IPSPs were evoked by spikes of inspiratory neurons with decrementing firing patterns and by spikes of expiratory neurons with augmenting firing patterns. We conclude that various synaptic inputs from respiratory neurons contribute to shaping the respiratory-related trajectory of membrane potential of laryngeal motoneurons.     

Physiological properties of late inspiratory neurons and their possible involvement in inspiratory off-switching in cats

To assess the functional significance of late inspiratory (late-I) neurons in inspiratory off-switching (IOS), membrane potential and discharge properties were examined in vagotomized, decerebrate cats. During spontaneous IOS, late-I neurons displayed large membrane depolarization and associated discharge of action potentials that started in late inspiration, peaked at the end of inspiration, and ended during postinspiration. Depolarization was decreased by iontophoresis of dizocilpine and eliminated by tetrodotoxin. Stimulation of the vagus nerve or the nucleus parabrachialis medialis (NPBM) also evoked depolarization of late-I neurons and IOS. Waves of spontaneous chloride-dependent inhibitory postsynaptic potentials (IPSPs) preceded membrane depolarization during early inspiration and followed during postinspiration and stage 2 expiration of the respiratory cycle. Iontophoresed bicuculline depressed the IPSPs. Intravenous dizocilpine caused a greatly prolonged inspiratory discharge of the phrenic nerve (apneusis) and suppressed late-inspiratory depolarization as well as early-inspiratory IPSPs, resulting in a small constant depolarization throughout the apneusis. NPBM or vagal stimulation after dizocilpine produced small, stimulus-locked excitatory postsynaptic potentials (EPSPs) in late-I neurons. Neurobiotin-labeled late-I neurons revealed immunoreactivity for glutamic acid decarboxylase as well as N-methyl-D-aspartate (NMDA) receptors. These results suggest that late-I neurons are GABAergic inhibitory neurons, while the effects of bicuculline and dizocilpine indicate that they receive periodic waves of GABAergic IPSPs and glutamatergic EPSPs. The data lead to the conclusion that late-I neurons play an important inhibitory role in IOS. NMDA receptors are assumed to augment and/or synchronize late-inspiratory depolarization and discharge of late-I neurons, leading to GABA release and consequently off-switching of bulbar inspiratory neurons and phrenic motoneurons.     

Synaptic effects of intercostal tendon organs on membrane potentials of medullary respiratory neurons

1. Stimulation of intercostal muscle tendon organs or their afferent fibers reduces medullary inspiratory neuron activity, decreases motor output to inspiratory muscles, and increases the activity of expiratory laryngeal motoneurons. The present study examines the synaptic mechanisms underlying these changes to obtain information about medullary neurons that participate in the afferent limb of this reflex pathway. 2. Membrane potentials of medullary respiratory neurons were recorded in decerebrate paralyzed cats. Postsynaptic potentials (PSPs) elicited in these neurons by intercostal nerve stimulation (INS) were compared before and after intracellular iontophoresis of chloride ions. After chloride injection, the normal hyperpolarization caused by inhibitory (I) PSPs is "reversed" to depolarization. 3. In inspiratory neurons, reversal of IPSPs by chloride injection also reversed hyperpolarization produced by INS when applied during any portion of the respiratory cycle. This observation suggests that increased chloride conductance of the postsynaptic membrane mediated the inhibition. Further, it is very likely that the last-order interneuron in the afferent pathway must be excited by INS and alter inspiratory neuron activity via an inhibitory synapse. The linear relationship between the amplitude of the INS induced PSP and membrane potential of inspiratory neurons provided evidence that neurons in the afferent pathway are not respiratory modulated. 4. The membranes of expiratory vagal motoneurons and post-inspiratory neurons were depolarized by INS during all portions of the respiratory cycle before IPSP reversal. Reversal of IPSPs affected neither this depolarization of expiratory vagal motoneurons during stage I and II expiration nor that of post-inspiratory neurons during stage I expiration. Thus this depolarization probably resulted from synaptic excitation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Branching pattern and properties of vertical- and horizontal-related excitatory vestibuloocular neurons in the cat

1. The axonal trajectories of excitatory vestibuloocular neurons and their synaptic contacts with extraocular motoneurons were studied by means of spike-triggered signal averaging and microstimulation techniques. A majority of the excitatory neurons related to the vertical semicircular canals were located in the border of the descending and medial nuclei and the rostral half of the descending nucleus. 2. Individual vestibuloocular neurons activated by stimulation of the ampullary nerve of the anterior semicircular canal excited motoneurons within both the contralateral inferior oblique and contralateral superior rectus motoneuron pools. 3. Individual vestibuloocular neurons receiving input from the ampullary nerve of the posterior semicircular canal excited motoneurons in both the contralateral trochlear nucleus and contralateral inferior rectus motoneuron pools. The branching pattern of single vestibuloocular neurons activated by the anterior and posterior canals probably underlies conjugate eye movement during vertical head rotation. 4. Time to peak and shape indices of unitary excitatory postsynaptic potentials (EPSPs) suggested that the location of the synaptic contact of vestibuloocular neurons was on the soma or proximal dendrites of the target extraocular motoneurons. 5. In contrast, we did not find conclusive evidence that single vestibuloocular neurons receiving input from the horizontal semicircular canal give off axon collaterals to motoneurons innervating both the contralateral lateral rectus and the ipsilateral medial rectus muscles. Projection of horizontal vestibuloocular neurons to motoneurons supplying individual muscles might be useful for convergence during horizontal head movement.     

Synchronization of ventral-group, bulbospinal inspiratory neurons in the decerebrate rat

We examined the synaptic connections between ventral-group, bulbospinal inspiratory neurons in 27 vagotomized, paralyzed, ventilated, and decerebrated rats using cross-correlation and spike-triggered averaging of intracellular potentials. The neurons were recorded in the medulla about the level of the obex and identified by their inspiratory firing pattern and antidromic activation from the spinal cord at C7. Whole C5 phrenic nerve recordings were made using bipolar electrodes from the central cut ends of the nerve. Most (108/137, 79%) inspiratory neurons discharged only during inspiration but some (29/137, 21%) also discharged during early expiration. Their intracellular membrane potentials displayed a pattern of depolarization during inspiration, repolarization during early expiration, and hyperpolarization during late expiration. Intracellular chloride iontophoresis changed the inspiratory membrane potential trajectories from augmenting to decrementing in 11 of 19 neurons tested (58%), and demonstrated the presence of both early-decrementing and late-augmenting waves of inhibitory postsynaptic potentials during expiration in 11 of 19 neurons tested (58%). Cross-correlation histograms were computed between pairs of extracellularly recorded neurons to detect short time scale synchronizations indicative of synaptic connections (26 ipsilateral; 23 contralateral). While none of the cross-correlation histograms for contralateral pairs showed peaks, most (23, 88%) of those for ipsilateral pairs showed peaks (mean half-amplitude width ± SD = 1.3 ± 0.4 ms) at time zero suggestive of common activation. Some of the latter (6, 23%) showed troughs superimposed on the central peaks (mean half-amplitude width ± SD = 0.9 ± 0.2 ms) at short latencies (mean latency ± SD = 1.8 ± 1.9 ms) suggestive of inhibition; others (8, 31%) had asymmetrical central peaks and two had bilateral peaks suggesting more complex interconnections. Averages of intracellular membrane potentials of inspiratory neurons (n = 24), triggered by action potentials of a nearby extracellularly recorded inspiratory neuron, were computed to detect synchronized postsynaptic potentials. Over half (16, 67%) showed postsynaptic potentials (mean amplitude ± SD = 201 ± 176 μV; mean half-amplitude width ± SD = 2.3 ± 0.8 ms) confirming the cross-correlation findings of common excitation. We conclude that in decerebrated rats, ventral-group inspiratory neurons projecting to the C7 spinal segment share powerful, ipsilaterally distributed excitatory inputs which enhance their synchronous activity during inspiration. They also receive inhibition during inspiration and early-decrementing and late-augmenting inhibitory inputs during expiration. Received: 25 October 1996 / Accepted: 21 March 1997     

Control of abdominal muscles by brain stem respiratory neurons in the cat

Control of abdominal musculature by brain stem respiratory neurons was studied in decerebrate unanesthetized cats by determining 1) which brain stem respiratory neurons could be antidromically activated from the lumbar cord, from which the abdominal muscles receive part of their innervation, and 2) if lumbar-projecting respiratory neurons make monosynaptic connections with abdominal motoneurons. A total of 462 respiratory neurons, located between caudal C2 and the retrofacial nucleus (B?tzinger complex), were tested for antidromic activation from the upper lumbar cord. Fifty-eight percent of expiratory (E) neurons (70/121) in the caudal ventral respiratory group (VRG) between the obex and rostral C1 were antidromically activated from contralateral L1. Eight of these neurons were activated at low thresholds from lamina VIII and IX in the L1-2 gray matter. One-third (14/41) of the E neurons that projected to L1 could also be activated from L4-5. Almost all antidromic E neurons had an augmenting firing pattern. Ten scattered inspiratory (I) neurons projected to L1 but could not be activated from L4-5. No neurons that fired during both E and I phases (phase-spanning neurons) were antidromically activated from the lumbar cord. In order to test for possible monosynaptic connections between descending E neurons and abdominal motoneurons, cross-correlations were obtained between 27 VRG E neurons, which were antidromically activated from caudal L2 and contralateral L1 and L2 abdominal nerve activity (47 neuron-nerve combinations). Only two neurons showed a correlation with one of the two nerves tested. Although there is a large projection to the lumbar cord from expiratory neurons in the ventral respiratory group caudal to the obex, cross-correlation analyses suggest that strong monosynaptic connections between these neurons and abdominal motoneurons are scarce.     

Excitatory input from interneurons in the abducens nucleus to medial rectus motoneurons mediating conjugate horizontal nystagmus in the cat

Summary A class of interneurons in the cat abducens nucleus was identified by its antidromic activation from the contralateral ascending MLF, disynaptic activation from the contralateral vestibular nerve and type II response to rotation of the turntable. They were also activated antidromically from the contralateral oculomotor nucleus, the region of medial rectus motoneurons. Extracellular spikes of single interneurons, spontaneous or glutamate-driven, were used as triggers for perior post-spike averaging of three kinds of potentials. (1) The average of the extracellular field potentials within the contralateral oculomotor nucleus consisted of an early positive or positive-negative spike and a late, slow negative wave. The early spike was an action current caused by impulses along the axon of the interneuron. The late potential was the extracellular counterpart of unitary EPSPs. (2) The averaged membrane potential of contralateral medial rectus motoneurons revealed unitary EPSPs with monosynaptic latencies, evidence that interneurons were excitatory in nature. (3) The average of compound potentials of the contralateral medial rectus nerve showed a monosynaptic excitatory effect relevant to unitary EPSPs. This effect was observed with nearly all interneurons. All interneurons thus identified exhibited discharge patterns closely correlated with the activity of medial rectus motoneurons in both slow and quick phases of vestibular nystagmus. It was concluded that these interneurons controlled activities of contralateral medial rectus motoneurons associated with conjugate horizontal eye movements by their monosynaptic excitatory connections.     

Chemical activation of caudal medullary expiratory neurones alters the pattern of breathing in the cat.

1. The purpose of this work was to ascertain whether the activation of caudal expiratory neurones located in the caudal part of the ventral respiratory group (VRG) may affect the pattern of breathing via medullary axon collaterals. 2. We used microinjections of DL-homocysteic acid (DLH) to activate this population of neurones in pentobarbitone-anaesthetized, vagotomized, paralysed and artificially ventilated cats. Both phrenic and abdominal nerve activities were monitored; extracellular recordings from medullary and upper cervical cord respiratory neurones were performed. 3. DLH (160 mM) microinjected (10-30 nl for a total of 1.6-4.8 nmol) into the caudal VRG, into sites where expiratory activity was encountered, provoked an intense and sustained activation of the expiratory motor output associated with a corresponding period of silence in phrenic nerve activity. During the progressive decline of the activation of abdominal motoneurones, rhythmic inspiratory activity resumed, displaying a decrease in frequency and a marked reduction or the complete suppression of postinspiratory activity as its most consistent features. 4. Medullary and upper cervical cord inspiratory neurones exhibited inhibitory responses consistent with those observed in phrenic nerve activity, while expiratory neurones in the caudal VRG on the side contralateral to the injection showed excitation patterns similar to those of abdominal motoneurones. On the other hand, in correspondence to expiratory motor output activation, expiratory neurones of the Bötzinger complex displayed tonic discharges whose intensity was markedly lower than the peak level of control breaths. 5. Bilateral lignocaine blockades of neural transmission at C2-C3 affecting the expiratory and, to a varying extent, the inspiratory bulbospinal pathways as well as spinal cord transections at C2-C3 or C1-C2, did not suppress the inhibitory effect on inspiratory neurones of either the ipsi- or contralateral VRG in response to DLH microinjections into the caudal VRG. 6. The results show that neurones within the column of caudal VRG expiratory neurones promote inhibitory effects on phrenic nerve activity and resetting of the respiratory rhythm. We suggest that these effects are mediated by medullary bulbospinal expiratory neurones, which may, therefore, have a function in the control of breathing through medullary axon collaterals.     

Multimodal medullary neurons and correlational linkages of the respiratory network.

This study addresses the hypothesis that multiple sensory systems, each capable of reflexly altering breathing, jointly influence neurons of the brain stem respiratory network. Carotid chemoreceptors, baroreceptors, and foot pad nociceptors were stimulated sequentially in 33 Dial-urethan-anesthetized or decerebrate vagotomized adult cats. Neuronal impulses were monitored with microelectrode arrays in the rostral and caudal ventral respiratory group (VRG), nucleus tractus solitarius (NTS), and n. raphe obscurus. Efferent phrenic nerve activity was recorded. Spike trains of 889 neurons were analyzed with cycle-triggered histograms and tested for respiratory-modulated firing rates. Responses to stimulus protocols were assessed with peristimulus time and cumulative sum histograms. Cross-correlation analysis was used to test for nonrandom temporal relationships between spike trains. Spike-triggered averages of efferent phrenic activity and antidromic stimulation methods provided evidence for functional associations of bulbar neurons with phrenic motoneurons. Spike train cross-correlograms were calculated for 6,471 pairs of neurons. Significant correlogram features were detected for 425 pairs, including 189 primary central peaks or troughs, 156 offset peaks or troughs, and 80 pairs with multiple peaks and troughs. The results provide evidence that correlational medullary assemblies include neurons with overlapping memberships in groups responsive to different sets of sensory modalities. The data suggest and support several hypotheses concerning cooperative relationships that modulate the respiratory motor pattern. 1) Neurons responsive to a single tested modality promote or limit changes in firing rate of multimodal target neurons. 2) Multimodal neurons contribute to changes in firing rate of neurons responsive to a single tested modality. 3) Multimodal neurons may promote responses during stimulation of one modality and "limit" changes in firing rates during stimulation of another sensory modality. 4) Caudal VRG inspiratory neurons have inhibitory connections that provide negative feedback regulation of inspiratory drive and phase duration.     

Role of upper cervical inspiratory neurons studied by cross-correlation in the cat

Summary Axonal projections and synaptic connectivity of upper cervical inspiratory neurons (UCINs) were investigated in anaesthetised cats to clarify their role as propriospinal respiratory interneurons. Antidromic mapping showed axonal collaterals near phrenic and intercostal motonuclei. Of the UCINs tested, 37% had collaterals at T3-4; 55% had ipsilateral projections and 45% had contralateral projections. Ipsilateral or contralateral cross-correlations of the activity of pairs of UCINs (one on each side of the spinal cord) with the discharge of internal intercostal, external intercostal (T3-4) or phrenic nerves revealed similar features. Those with the internal intercostal and phrenic nerves were interpreted as evidence for shared or oligosynaptic excitation, those with the external intercostal nerve as shared excitation and inhibition. No evidence for monosynaptic connections was found. Monosynaptic connections could also not be demonstrated between inspiratory intercostal neurons located near (< 0.5 mm) the UCINs collateral arborizations in T3-4, examined by cross-correlation. Afferent feedback from internal intercostal nerves (T3-4) was investigated by cross-correlating nerve stimulation with UCINs activity. Ipsilateral and contralateral cross-correlograms had similar features, providing evidence for excitation in some cases and inhibition in others. Finally, cross-correlations between ipsilateral UCINs and cervical sympathetic nerves were featureless. The results suggest that the role of UCINs as part of a respiratory propriospinal control system analagous to forelimb motor control is untenable, although they may be part of an intercostal afferent feedback loop.     

Synaptic interactions between respiratory neurons during inspiratory on-switching evoked by vagal stimulation in decerebrate cats

To elucidate neuronal mechanisms underlying phase-switching from expiration to inspiration, or inspiratory on-switching (IonS), postsynaptic potentials (PSPs) of bulbar respiratory neurons together with phrenic nerve discharges were recorded during IonS evoked by vagal stimulation in decerebrate and vagotomized cats. A single shock stimulation of the vagus nerve applied at late-expiration developed an inspiratory discharge in the phrenic neurogram after a latency of 79+/-11 ms (n = 11). Preceding this evoked inspiratory discharge, a triphasic response was induced, consisting of an early silence (phase 1 silence), a transient burst discharge (phase 2 discharge) and a late pause (phase 3 pause). During phase 1 silence, IPSPs occurred in augmenting inspiratory (aug-I) and expiratory (E2) neurons, and EPSPs in postinspiratory (PI) neurons. During phase 2 discharge, EPSPs arose in aug-I neurons and IPSPs in PI and E2 neurons. These initial biphasic PSPs were comparable with those during inspiratory off-switching evoked by the same stimulation applied at late-inspiration. In both on- and off-switching, phase-transition in respiratory neuronal activities started to arise concomitantly with the phrenic phase 3 pause. These results suggest that vagal inputs initially produce a non-specific, biphasic response in bulbar respiratory neurons, which consecutively activates a more specific process connected to IonS.     

Direct excitatory and inhibitory synaptic inputs from the medial mesodiencephalic junction to motoneurons innervating extraocular oblique muscles in the cat

Summary This study investigated the nature of synaptic inputs from the Forel's field H (FFH) in the medial mesodiencephalic junction to inferior oblique (IO) motoneurons in the oculomotor nucleus and superior oblique (SO) motoneurons in the trochlear nucleus in anesthetized cats, using intracellular recording techniques. Stimulation of the FFH induced monosynaptic EPSPs in IO motoneurons on both sides. Paired stimulation of the ipsilateral FFH and contralateral vestibular nerve substantiated that the FFH-induced EPSPs were caused mainly by direct excitatory fibers from the FFH to IO motoneurons and partly by axon collaterals of excitatory neurons in the vestibular nuclei. Among parts of the FFH, the medial part was most effective for producing the EPSPs. Systematic tracking with the stimulating electrode in and around the FFH revealed that effective sites of stimulation inducing negative field potentials in the IO subdivision of the oculomotor nucleus, identified as extracellular counterparts of the EPSPs in IO motoneurons, were also located in the interstitial nucleus of Cajal, nearby reticular formation and posterior commissure, besides within and near the medial part of the FFH. Areas far rostral, dorsal and ventral to the FFH were ineffective. EPSP-IPSPs or EPSPs were mainly induced in SO motoneurons on both sides by FFH stimulation. Latencies of these EPSPs and IPSPs were close to those of the EPSPs in IO motoneurons, indicating their monosynaptic nature. Effective stimulation sites for inducing these synaptic potentials overlapped those for the EPSPs in IO motoneurons. Based on these results, it was suggested that excitatory and inhibitory premotor neurons directly controlling IO and SO motoneurons were located within and near the medial part of the FFH.     

Excitatory connections between neurons of the central cervical nucleus and vestibular neurons in the cat

 The central cervical nucleus (CCN) of the cat receives input from upper cervical muscle afferents, particularly primary spindle afferents. Its axons cross in the spinal cord, and while in the contralateral restiform body give off collaterals to the vestibular nuclei. In order to study the connections between CCN axons and vestibular neurons, we stimulated the area of the CCN in decerebrate cats while recording intra- or extracellularly from neurons in the contralateral vestibular nuclei. CCN stimulation evoked excitatory postsynaptic potentials (EPSPs) or extracellularly recorded firing in the lateral, medial and descending vestibular nuclei. The latency of EPSPs (mean 1.6 ms) was on average 0.4 ms longer than the latency of antidromic spikes evoked in the CCN by stimulation of the contralateral vestibular nuclei (mean 1.2 ms), demonstrating that the excitation was typically monosynaptic. The results provide further evidence that the CCN is an important excitatory relay between upper cervical muscle afferents and neurons in the contralateral vestibular nuclei. Received: 1 August 1996 / Accepted: 16 December 1996     

Direct inhibitory synaptic linkage of pontomedullary reticular burst neurons with abducens motoneurons in the cat

1. Unit spikes of burst neurons were extracellularly recorded in the pontomedullary reticular formation of the cat. These neurons were identified by their burst activity coincident with the quick inhibitory phase of the contralateral abducens nerve during vestibular nystagmus and their antidromic activation from the contralateral abducens nucleus. 2. When the extracellular field potentials in and near the abducens nucleus were triggered by spikes of a contralateral burs neuron, the averaged potential consisted of an early di- or triphasic spike and a late slow positive wave. The early spike was an action current caused by impulses conducting along the axon of the burst neuron. 3. The action potentials of a contralateral burst neuron. 3. The action potentials of a contralateral burst neuron were employed to trigger a post-spike average of the membrane potential of abducens motoneurons. Then unitary IPSPs with monosynaptic latencies were revealed. This provided direct evidence that the burst neurons are inhibitory in nature. The amplitudes of unitary IPSPs ranged from 18 to 220 mu V. Each inhibitory burst neuron branched widely in the abducens nucleus and was estimated to make inhibitory connections with approximately 60% of the motoneuron pool. 4. The post-spike average of compound potentials of the abducens nerve triggered by action potentials of contralateral single inhibitory burst neurons revealed inhibition of spike activity with latencies and time courses compatible with those of unitary IPSPs in motoneurons. The inhibition was observed with all inhibitory burst neurons tested.