An electrophysiological investigation of propriospinal inspiratory neurons in the upper cervical cord of the cat

Summary This study was performed in order to describe the location, axonal projection and possible synaptic action of the inspiratory neurons recently described in the upper cervical cord. In 26 cats anaesthetized with Nembutal, extracellular recordings were made from 224 cervical inspiratory units which were found near the lateral border of lamina VII and formed a column extending from the caudal end of the nucleus retroambigualis at the C₁ segment to the rostral half of the C₃ segment. Most of the units (approximately 85%) could be excited antidromically from the thoracic cord. Antidromic mapping showed collateral branches to the C₅ segment in the vicinity of the phrenic nucleus, occasionally crossing the midline. No synaptic connections with phrenic motoneurones could be revealed either by cross-correlation of the activity of the cervical units with the discharge of C₅ phrenic root, or by spike-triggered averaging (STA) of the post-synaptic noise recorded intracellularly from phrenic motoneurons. Extensive branching was found in the examined T₃–T₅ segments with arborizations near the ipsilateral intercostal motor nuclei and often extending across the midline. Cross-correlation experiments did not show clear monosynaptic connections to the inspiratory intercostal motoneurons. Intracellular recording from intercostal motoneurons and STA resulted in a few (2 out of 37) small, probably disynaptic, e.p.s.p.s. It is concluded that the upper cervical neurons are involved in the control of phrenic and intercostal motoneurons, probably through a disynaptic pathway involving segmental interneurons.     

Spinal connections of ventral-group bulbospinal inspiratory neurons studied with cross-correlation in the decerebrate rat

We examined the synaptic connections from ventral-group bulbospinal inspiratory neurons to upper cervical inspiratory neurons and phrenic and intercostal motoneurons in decerebrate rats using cross-correlation. Inspiratory neurons were recorded in the medulla (n=28) at the level of the obex and from the upper-cervical segments (C1 and C2) of the spinal cord (n=29) in 18 vagotomized, paralyzed, ventilated, and decerebrated rats. The neurons were identified by their inspiratory firing pattern and antidromic activation from the spinal cord at C7. Whole-nerve recordings were made using bipolar electrodes from the central cut ends of the C5 phrenic nerve and the external and internal intercostal nerves at various thoracic levels. Cross-correlation histograms were computed between these recordings to detect short time scale synchronizations indicative of synaptic connections. Cross-correlation histograms (n=20), computed between the activities of ventral-group bulbospinal inspiratory neurons and the phrenic nerve, all showed peaks (mean half-amplitude width±SD, 1.1±0.3 ms) at short latencies (mean latency±SD, 2.0±0.6 ms) suggestive of monosynaptic excitation. Cross-correlation histograms (n=33), computed between the activities of ventral-group bulbospinal inspiratory neurons and upper-cervical inspiratory neurons, displayed four (12%) peaks (mean halfamplitude width±SD, 0.9±0.1 ms) at short latencies(mean latency±SD, 1.8±0.6 ms) suggestive of monosynaptic excitation, and six (18%) peaks (mean half-amplitude width±SD, 1.4±0.4 ms) at latencies near zero suggestive of excitation fro m a common source. Cross-correlation histograms (n=34), computed between the activities of ventral-group bulbospinal inspiratory neurons and the internal and external intercostal nerves at various thoracic levels (T2-8), showed six (18%) peaks (mean half-amplitude width±SD, 2.5±0.5 ms) at short latency (mean latency±SD, 4.5±1.1 ms) suggestive of oligosynaptic connections. Cross-correlation histograms (n=42) computed between activities of intercostal nerves at various levels of the thoracic spinal cord showed central peaks suggestive of excitation from a common source. Although the size of the peaks decreased with segmental separation, the displacement of the peaks from time zero did not increase with segmental separation (mean displacement±SD, 0.6±0.6 ms) as would be expected if the common excitation resulted from a descending monosynaptic excitation by a source such as the ventral-groupbulbospinal inspiratory neurons. We conclude that all ventral-group bulbospinal inspiratory neurons make monosynaptic connections to phrenic motoneurons, a few make monosynaptic connections to upper-cervical inspiratory neurons, but connections to intercostal motoneurons are made via interneurons.     

Connections from upper cervical inspiratory neurons to phrenic and intercostal motoneurons studied with cross-correlation in the decerebrate rat

We examined the synaptic connections from upper cervical inspiratory neurons to phrenic and intercostal motoneurons in decerebrate rats using cross-correlation. Upper cervical inspiratory neurons (n=79) were recorded from the C1 and C2 segments of the spinal cord in 38 vagotomized, paralyzed, ventilated, and decerebrate rats. The neurons were identified by their inspiratory firing pattern and antidromic activation from the ipsilateral spinal cord at C7. Whole-nerve recordings were made using bipolar electrodes from the central cut ends of the C5 phrenic nerve and the external and internal intercostal nerves at various thoracic levels. Cross-correlation histograms were computed between these recordings to detect short time-scale synchronizations indicative of synaptic connections. The 55 cross-correlation histograms computed between the upper cervical inspiratory neurons and the ipsilateral phrenic nerve showed seven (13%) narrow peaks (mean half-amplitude width±SD, 1.09±0.15 ms) at short latencies (mean latency±SD, 1.29±0.26 ms) suggestive of monosynaptic excitation, and four (7%) broader peaks (mean half-amplitude width±SD, 1.50±0.17 ms) at short latencies (mean latency±SD, 1.40±0.24 ms) suggestive of oligosynaptic excitation. Another 14 (25%) cross-correlation histograms displayed a central broad peak (mean half-amplitude width±SD, 1.59±0.23 ms) suggestive of common activation. The eight cross-correlation histograms computed between the upper cervical inspiratory neurons and the contralateral phrenic nerve were featureless. The 77 cross-correlation histograms computed between the upper cervical inspiratory neurons and the internal and external intercostal nerves at various thoracic levels (T2–8) showed no peaks suggestive of synaptic connections. We conclude that some upper cervical inspiratory neurons make monosynaptic and paucisynaptic connections to phrenic motoneurons but not to intercostal motoneurons.     

Respiratory pre-motor control of hypoglossal motoneurons in the rat

The goal of this study was to determine the origin and transmission pathway of respiratory drive to hypoglossal motoneurons. First we recorded intracellularly from 28 antidromically activated inspiratory hypoglossal motoneurons (resting membrane potential, −50±3 mV), and found that injection of chloride ions had no discernible effect on the shape of their membrane potential trajectories. We concluded that the membrane potential trajectories of these hypoglossal motoneurons were determined primarily by inspiratory excitation. To determine the origin of this excitation we cross-correlated the extracellular discharge of medullary inspiratory neurons, including those in the hypoglossal motor nucleus, with the hypoglossal nerve discharge. We found 27 inspiratory neurons within the hypoglossal motor nucleus that were not antidromically activated from the ipsilateral hypoglossal nerve; their cross-correlograms featured either central peaks (1.7±0.2 ms) alone (n=14; 39%), or central peaks (1.3±0.2 ms) followed by troughs (1.3±0.1 ms) at short latencies (1.1±0.4 ms) (n=13; 36%), and suggest that these neurons are hypoglossal interneurons. We recorded from 238 inspiratory neurons throughout the rest of the medulla; the cross-correlograms of 19 neurons (8%), located mostly in the lateral tegmental field, displayed narrow half-amplitude peaks (1.0±0.1 ms) at short latencies (0.9±0.1 ms), which we interpreted as evidence for monosynaptic excitation of hypoglossal motoneurons.

We conclude that the respiratory control of hypoglossal motoneurons originates from inspiratory premotor neurons scattered throughout the lateral tegmental field and interneurons within the hypoglossal motor nucleus.     

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.     

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.     

Analysis of activity of motor units in the biceps brachii muscle after intercostal-musculocutaneous nerve transfer

We examined respiratory activity of motor units (MUs) in the internal intercostal nerves (IICNs)-transferred biceps brachii muscle (IC-biceps) in cats. MUs of IC-biceps showed respiratory discharges in inspiratory and expiratory phases, and these were enhanced by CO2 inhalation. Narrowing the airway also enhanced inspiratory and expiratory MUs activity. A mechanical load to the thorax immediately enhanced inspiratory MUs activity and weakened expiratory MUs activity. We analyzed the cross-correlation of MUs activity in interchondral muscle and IC-biceps to characterize the respiratory spinal descending inputs to motoneurons. We confirmed the short-term synchronization from interchondral muscles indicating divergence of a single respiratory presynaptic axon to thoracic motoneurons, but could not find synchronization from IC-biceps. The motor axonal conduction velocity (axonal CV) of IC-biceps MUs was lower than that of interchondral muscles. There was no correlation between the respiratory recruitment order of IC-biceps MUs and their axonal CV. These results indicate that IC-biceps shows the respiratory activities and afferent inputs from intercostal muscle spindles in the neighboring segments remain influential on activity of IC-biceps. In addition, the short-term synchronization from IC-biceps could not be found, suggesting that the intercostal nerve transfer alters the respiratory spinal descending inputs to thoracic motoneurons.     

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.     

Disynaptic excitation from the medial longitudinal fasciculus to lumbosacral motoneurons: modulation by repetitive activation,descending pathways,and locomotion

Summary Short-latency excitatory postsynaptic potentials (EPSPs) evoked by stimulation in the medial longitudinal fasciculus (MLF) were recorded intracellularly from motoneurons in the cat lumbosacral spinal cord. Monosynaptic and disynaptic EPSPs occurred in most flexor and extensor motoneurons studied. These EPSPs resulted from the activation of fast (> 100 m/s) descending axons from the MLF to the spinal cord. Several features distinguished monosynaptic and disynaptic MLF EPSPs. Disynaptic EPSPs exhibited temporal facilitation during short trains of stimulation, whereas monosynaptic EPSPs did not. Disynaptic EPSPs, but not monosynaptic EPSPs, were also facilitated by stimulation of the pyramidal tract and the mesencephalic locomotor region. However, disynaptic MLF EPSPs exhibited little or no facilitation when conditioned by short-latency cutaneous pathways. During fictive locomotion, the amplitude of disynaptic MLF EPSPs was modulated, with maximal amplitudes during the step cycle phase when the recorded motoneuron was active, resulting in reciprocal patterns of modulation of flexors and extensors. No comparable change was seen in the amplitude of monosynaptic MLF EPSPs during fictive stepping. These data suggest that the central pattern generator for locomotion modulates disynaptic MLF excitation at a premotoneuronal level in a phase-dependent manner. The effects of lesions made in the MLF and thoracic cord suggest that the interneurons in the disynaptic pathway from the MLF to motoneurons reside in the lumbosacral cord.     

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.     

Monosynaptic excitation of medullary inspiratory neurons by bulbospinal inspiratory neurons of the ventral respiratory group in the cat

Summary In Nembutal-anesthetized, immobilized, and artificially ventilated cats, we studied the connectivity of medullary collaterals of bulbospinal inspiratory (BS-I) neurons in the ventral respiratory group (VRG). BS-I neurons which projected to the contralateral spinal cord were isolated extracellularly and intracellular recordings were made from the respiratory neurons in the contralateral VRG. The intracellular membrane potentials were averaged using extracellular spikes of the BS-I neurons as triggers (spike-triggered averaging method). Fast-rising and short-lasting depolarizing potentials locked to the triggering spikes were obtained and shown to be unitary EPSPs induced monosynaptically by the medullary collaterals of BS-I neurons. A total of 137 pairs were analyzed and unitary EPSPs were found in 11 pairs. Four BS-I neurons and 7 inspiratory vagal motoneurons received EPSPs from the medullary collaterals of BS-I neurons. These findings suggest that 1) BS-I neurons in the VRG drive medullary motoneurons of accessory respiratory muscles and phrenic or intercostal motoneurons simultaneously, 2) BS-I neurons on both sides synchronize via the excitatory connections, and 3) the augmenting firing pattern of BS-I neurons might partly be produced by this reexcitatory connection within the population of BS-I neurons.     

Same spinal interneurons mediate reflex actions of group Ib and group II afferents and crossed reticulospinal actions

The aim of the study was to analyze interactions between neuronal networks mediating centrally initiated movements and reflex reactions evoked by peripheral afferents; specifically whether interneurons in pathways from group Ib afferents and from group II muscle afferents mediate actions of reticulospinal neurons on spinal motoneurons by contralaterally located commissural interneurons. To this end reticulospinal tract fibers were stimulated in the contralateral medial longitudinal fascicle (MLF) in chloralose-anesthetized cats in which the ipsilateral half of the spinal cord was transected rostral to the lumbosacral enlargement. In the majority of interneurons mediating reflex actions of group Ib and group II afferents, MLF stimuli evoked either excitatory or inhibitory postsynaptic potentials (EPSPs and IPSPs, respectively) or both EPSPs and IPSPs attributable to disynaptic actions by commissural interneurons. In addition, in some interneurons EPSPs were evoked at latencies compatible with monosynaptic actions of crossed axon collaterals of MLF fibers. Intracellular records from motoneurons demonstrated that both excitation and inhibition from group Ib and group II afferents are modulated by contralaterally descending reticulospinal neurons. The results lead to the conclusion that commissural interneurons activated by reticulospinal neurons affect motoneurons not only directly, but also by enhancing or weakening activation of premotor interneurons in pathways from group Ib and group II afferents. The results also show that both excitatory and inhibitory premotor interneurons are affected in this way and that commissural interneurons may assist in the selection of reflex actions of group Ib and group II afferents during centrally initiated movements.     

Integration of nonphaselocked exteroceptive information in the control of rhythmic flight in the locust

The integration of exteroceptive information in the flight control system of the locust was studied by determining the cellular basis of ocellar- (simple eye) mediated control of flight. Neural interactions that transform phase-independent sensory input into phase-specific motor output were characterized. Ocellar information about course deviations during flight was conveyed to the segmental thoracic ganglia by three pairs of large fast multimodal descending neurons. These made connections with thoracic motoneurons directly, via short-latency mono-or disynaptic pathways, and indirectly, via a population of intercalated thoracic interneurons. The synaptic potentials caused in the motoneurons by the direct pathway occurred at short latency and were adequate for summation with other types of sensory input. However, the strength of the synaptic effects of this pathway was weak compared with the central flight oscillator drive to the same motoneurons. In contrast, synaptic potentials evoked by the descending neurons in the thoracic interneurons were often large and brought these cells close to threshold. In turn, these interneurons always had stronger synaptic effects on postsynaptic flight motoneurons than did the descending neurons alone. We conclude that the indirect interneuronal pathway is more powerful in its effects on motoneurons than the direct pathway. Premotor thoracic interneurons, which received ocellar input appropriate for a role in correctional steering, were also rhythmically modulated during flight motor activity in phase with either depressor or elevator motoneurons. This phasic modulatory drive occurred in deafferented preparations, indicating that its source is the central oscillator for flight. Presentation of ocellar stimulation during flight motor activity showed that the central oscillatory modulation of the thoracic interneurons gated the transmission of sensory information through these interneurons. Ocellar-mediated postsynaptic potentials influenced the firing of thoracic interneurons only if they arrived during the proper phase of rhythmic drive. Thus the transmission of ocellar information from interneuron to motor neuron is possible only during appropriate phases of the flight cycle.     

Supraspinal monosynaptic excitation and inhibition of thoracic back motoneurons

Summary The effects of brain stem stimulation on thoracic back motoneurons were studied in cats anesthetized with pentobarbital. The population sampled consisted of the extensors interspinales (IS), longissimus dorsi (LD) and spinalis dorsi (SD), and of unidentified (UIC) motoneurons. The location of the motoneurons, between Th 1 and Th 10, at widely varying distances from the stimulating electrode permitted linear regression analysis of the descending neural influences.EPSPs evoked by MLF stimulation in all types of motoneurons were produced by a pathway with an average conduction velocity in the thoracic cord of 127 m/sec, and were monosynaptic. IPSPs were also produced by MLF stimulation. The IPSPs in IS and UIC motoneurons were monosynaptic and were produced by a pathway with an average conduction velocity of 69 m/sec.Stimulation of Deiters' nucleus evoked short latency EPSPs in many motoneurons. EPSPs in LD and UIC motoneurons were shown to be monosynaptic, although latency scatter and sample size made accurate determination of vestibulospinal conduction velocity impossible.Stimulation of the labyrinth evoked disynaptic EPSPs and IPSPs in many cells, as previously observed in neck motoneurons. IPSPs were frequently produced by stimulation of the contralateral labyrinth, probably by a pathway with a relay in the contralateral medial vestibular nucleus. Ipsilateral stimulation usually produced EPSPs. The excitatory pathway relays in Deiters' nucleus and, we suggest, in the descending vestibular nucleus.Supported in part by a research grant from the Public Health Service (NSO2619).     

Upper cervical inspiratory neurons in the rat: an electrophysiological and morphological study

Upper cervical inspiratory neurons form a distinct neuronal column located near the lateral edge of the intermediate grey matter in the rostral spinal segments. Previous studies conducted in cats have demonstrated synaptic inputs to these neurons from several respiratory related regions of the medulla, and long descending axonal projections mainly towards the motoneurons supplying the intercostal muscles. The aim of this study was to examine the electrophysiological and morphological properties of this propriospinal system in the rat. Extracellular recordings were made from 127 cervical inspiratory units, mainly in the C₁ and C₂ segments. Eighty-two percent could be antidromically activated from the C₇/C₈ border. No evidence of monosynaptic connection was obtained by cross-correlating the activity of some of these units with the discharge of the phrenic nerve. Intracellular recordings were made from seven neurons, three of which were labelled with biotinamide (neurobiotin). Long survival times after intracellular injections (up to 23 h) resulted in staining of axons for long distances, at least to the C₅ segment. Each of the three labelled axons issued only one short collateral which arborized in the region of the phrenic nucleus. These results demonstrate that upper cervical inspiratory neurons in the rat have features similar to those previously described in the cat, including only a limited projection to the phrenic nucleus. In addition, this study provides the first morphological identification of these neurons.     

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.     

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)     

Transneuronal tracing of neural pathways controlling activity of diaphragm motoneurons in the ferret

Previous studies have shown that neurons in addition to those in the medullary respiratory groups are involved in activating phrenic motoneurons during a number of behaviors, including vomiting and reaction to vestibular stimulation. However, the location of premotor inspiratory neurons outside of the main medullary respiratory groups is largely unknown, particularly in emetic species. In the present study, the transneuronal tracer pseudorabies virus was injected into the diaphragm of the ferret, and the locations of retrogradely-labeled motoneurons and transneuronally-labeled pre-motoneurons in the brainstem and cervical and thoracic spinal cord were mapped. Injections of a monosynaptic tracer, cholera toxin, were also made in order to verify the location of motoneurons innervating the diaphragm. Phrenic motoneurons identified with pseudorabies virus and cholera toxin were confined largely to the C₅–C₇ levels of spinal cord, and often gave rise to prominent polarized dendritic arbors that extended across the midline. At post-inoculation survival times ≥three days, transneuronally-labeled interneurons were located in the cervical and thoracic spinal cord and portions of the brainstem, including the midline pontomedullary reticular formation and the lateral medullary reticular formation. Double-labeling studies revealed that although the infected midline neurons were located in the proximity of serotonergic neurons, only a small number of the virus-containing cells were positive for serotonin.

These findings suggest that neurons in the midline of the medulla and pons influence the activity of phrenic motoneurons, perhaps during inspiratory behaviors unique to emetic animals (such as vomiting).     

Functional heterogeneity among neurons in the nucleus retroambiguus with lumbosacral projections in female cats

Nucleus retroambiguus (NRA), in the caudal medulla, projects to all spinal levels. One physiological role is abdominal pressure control, evidenced by projections to intercostal and abdominal motoneurons from expiratory bulbospinal neurons (EBSNs) within NRA. The roles of NRA projections to the lumbosacral cord are less certain, although those to limb motoneurons may relate to mating behavior and those to Onuf's nucleus (ON) to maintaining continence. To clarify this we physiologically characterized NRA projections to the lumbosacral cord. Extracellular recordings were made in NRA under anesthesia and paralysis in estrus cats. Administered CO(2) gave a strong respiratory drive. Antidromic unit responses were recorded to stimulation of the contralateral ventrolateral funiculus of L(6), L(7), or sacral segments and to microstimulation in the region of semimembranosus motor nucleus or ON. All units were found at sites showing expiratory discharges. Units that showed collisions between antidromic and spontaneous spikes (all in late expiration) were identified as EBSNs. These were common from the ventrolateral funiculus (VLF) of L(6) (42.5%) or L(7) (32.9%), but rare from the sacral VLF or the motor nuclei. Antidromic latencies revealed a subthreshold respiratory drive in some non-EBSNs. This group had lower conduction velocities than the EBSNs. The remainder, with a negligible respiratory drive, had even lower conduction velocities. A new population of NRA neurons has thus been defined. They are not active even with a strong respiratory drive, but may provide most of the synaptic input from NRA to lower lumbar and sacral segments and could subserve functions related to mating behavior.     

Patterns of spontaneous and reflexly-induced activity in phrenic and intercostal motoneurons

Summary The discharge frequencies of 35 single phrenic and 13 inspiratory intercostal motoneurons were recorded in anaesthetised paralysed cats. Chemical stimulation by asphyxia or hypercapnia increased the discharge frequency and number of motoneurons active within each inspiratory discharge without altering the general pattern of respiratory activity, but mechanical stimulation of the epipharynx and electrical stimulation of the pharyngeal branch of the glossopharyngeal nerve caused repetitive bursts of very high frequency (up to 400 impulses/ sec) in inspiratory motoneurons, with disruption of their normal phasic activity. The latency of the motoneuron response to electrical stimulation of the glossopharyngeal nerve ranged from 15–30 msec and varied with respiratory phase, being shorter during spontaneous inspiratory activity.Phrenic motoneurons were divided according to their order of recruitment during inspiratory activity, and the later recruited (high-threshold) units had significantly larger spike amplitudes than motoneurons which discharged throughout inspiration. High-threshold motoneurons also achieved higher maximum discharge frequencies in response to electrical stimulation of the glossopharyngeal nerve, and it is suggested that these properties are important in increasing the tension developed by respiratory muscles near the end of inspiration when there is greater elastic resistance to lung inflation.