Medullary expiratory neurons in the decerebrate rat: an intracellular study.

Intracellular recordings and labelings with horseradish peroxidase (HRP) of expiratory (E) neurons were performed in decerebrate, paralyzed, and ventilated rats. A total of 37 neurons were recorded, from which 4 cells and 1 axon were labeled. They were located in two regions of the ventrolateral medulla. One was in the rostral portion of the nucleus ambiguus just caudal to the facial nucleus, and the other in the nucleus retroambiguus at the level of the caudal medulla. These expiratory neurons had rhythmical changes in membrane potential similar to those reported in cat, i.e., a depolarization in the intervals between phrenic bursts which evolved in an augmenting (E-aug, n = 15), or bell-shaped or 'plateau' (E-all, n = 22) pattern until a rapid hyperpolarization at the start of inspiration. Both types were hyperpolarized during inspiration by chloride-dependent, inhibitory postsynaptic potentials (IPSPs) which were demonstrated in 17 neurons (10 E-aug and 7 E-all) from which reversal was obtained. Such IPSPs also existed during post-inspiration (stage I of expiration) in 4 of the 10 augmenting E neurons. They were identified by antidromic stimulation or HRP labeling, or both, as bulbospinal neurons (n = 2), cranial motoneurons (n = 4), or not antidromically activated (NAA) neurons (n = 31). All the identified bulbospinal neurons and the motoneurons exhibited an E-all pattern. The expiratory neurons of the caudal medulla had various projections as demonstrated with HRP labeling: one bulbospinal neuron with ipsilateral axon giving off intramedullary collaterals, and NAA neurons with rostral medullary projections or with axons crossing the midline.(ABSTRACT TRUNCATED AT 250 WORDS)     

Intracellular electrophysiological and morphological study of the medullary inspiratory neurons of the decerebrate rat.

Intracellular recordings and labelings with horseradish peroxidase (HRP) of inspiratory neurons were performed in decerebrate, paralyzed and ventilated rats. A total of 58 neurons were located within the ventrolateral medulla. They were identified as bulbospinal neurons (n = 15), cranial motoneurons (n = 9) and not antidromically activated (NAA) neurons (n = 34) by antidromic stimulation or HRP labeling, or both. These inspiratory neurons had rhythmical changes in membrane potentials similar to those reported in cats, i.e. an abrupt depolarization at the onset of phrenic discharge followed by trajectories of depolarization which evolved into augmenting I, bell-shaped I or decrementing I patterns until a rapid repolarization at the start of expiration. All types were hyperpolarized during expiration by chloride-dependent inhibitory postsynaptic potentials (IPSPs) which were demonstrated in 13 neurons from which the reversal was obtained. Such IPSPs were apparent in two waves throughout expiration, an early one in post-inspiration (stage I of expiration) and a late one in late expiration (stage II of expiration). These properties are also similar to those of feline inspiratory medullary neurons. Four labeled bulbospinal neurons had axonal collaterals which were ipsi- and contralateral to the site of their somata. Two of 6 labeled NAA neurons exhibited profuse axonal arborizations within various medullary nuclei. No medullary axonal collateral was seen from 6 labeled motoneurons. These results indicate that even though in the rat a single concentration of inspiratory neurons within the ventrolateral medulla has been demonstrated, there is no fundamental difference in the organization of the inspiratory neuronal network compared to that of the cat.     

Are the post-inspiratory neurons in the decerebrate rat cranial motoneurons or interneurons?

We examined the membrane potentials of 63 respiratory neurons in the ventrolateral medulla of decerebrate rats, whose trajectories had the characteristics of the post-inspiratory neurons, i.e. exhibiting hyperpolarization during inspiration, rapid depolarization at end-inspiration and progressive repolarization with a decrementing pattern during the intervals between phrenic bursts. Synaptic responses of 6 post-inspiratory neurons which were tested by stimulation of cervical vagus or superior laryngeal nerves were excitatory. Eleven of these 63 post-inspiratory neurons were labeled by intracellular injection of horseradish peroxidase (HRP). Ten of these 11 labeled neurons were motoneurons since their axons exited the medulla after joining the roots of cranial nerves. However, only one of these motoneurons was antidromically activated by stimulation of the ipsilateral cervical vagus nerve. We assumed that most of the post-inspiratory medullary neurons of the present study were motoneurons, but not interneurons, although antidromic invasion was not possible after stimulation of the cervical vagus and superior laryngeal nerves. Two post-inspiratory neurons of this sample had bulbospinal axons, which were revealed by antidromical activation of spinal cord and HRP labeling, respectively. The axon of the labeled bulbospinal neuron had axonal collaterals which were distributed within the region of the nucleus ambiguous of the ipsilateral medulla. The functional significance of this type of post-inspiratory neuron is discussed.     

Glutamic acid decarboxylase-immunoreactivity of bulbar respiratory neurons identified by intracellular recording and labeling in rats

To distinguish the GABAergic neuron in the ventral respiratory group (VRG) of rats, immunohistochemical staining of glutamic acid decarboxylase (GAD) was performed in neurons that had been individually identified by in vivo intracellular recording and labeling with neurobiotin. A total of five types of respiratory neurons were identified and labeled; augmenting inspiratory (aug-I, n=12), decrementing or early inspiratory (early-I, n=3), inspiration-expiration phase spanning or late inspiratory (late-I, n=3), decrementing expiratory or postinspiratory (PI, n=8), and augmenting or stage 2 expiratory (E2, n=3). In addition, expiration-inspiration phase-spanning or pre-inspiratory neurons (pre-I, n=2) were recorded, but not labeled. The membrane potential trajectory of each neuron type resembled that previously described in cat, suggesting a comparable neuronal organization between the two species. According to the axonal arborization, those labeled neurons were further classified as propriobulbar (6 aug-I, all early-I, all late-I, and 3 PI), bulbospinal (2 aug-I and all E2) and cranial-motor neurons (4 aug-I and 5 PI). GAD-immunoreactivity was consistently detected in the propriobulbar neurons, while it was not seen in cranial-motor and bulbospinal neurons. In addition, GAD-immunoreactive varicosities were found surrounding the somatic and dendritic surface of all labeled neurons. The present results illustrate that the propriobulbar types of early-I, aug-I, late-I and PI neurons are GABAergic inhibitory neurons and virtually all types of respiratory neurons receive GABAergic inputs in the rat's VRG.     

Distribution of medullary respiratory neurons in the rat

In Nembutal-anesthetized and spontaneously breathing rats, a total of 226 respiratory neurons were recorded in the medulla extending from the caudal end of the facial nucleus to 1 mm caudal to the obex. They were classified into inspiratory (I) and expiratory (E) neurons by their temporal relationships to diaphragm EMGs. One hundred and seventeen I and 108 E neurons were identified. I and E neurons were further classified into augmenting, decrementing, and other types based on their firing patterns. Almost all the respiratory neurons recorded were located around the nucleus ambiguus and the nucleus retroambigualis, corresponding to the ventral respiratory group (VRG) of the cat. On the other hand, only a few respiratory neurons were identified around the ventrolateral nucleus of the solitary tract, corresponding to the dorsal respiratory group of the cat. In the VRG, 3 subgroups were distinguished rostrocaudally. One group of E neurons was located ventrally to the rostral part of the nucleus ambiguus, presumably corresponding to the B?tzinger complex defined in the cat. Another group of E neurons extended caudally beyond the obex, from the caudal portion of the nucleus ambiguus through the nucleus retroambigualis. Between these two groups of E neurons, an assembly of predominantly I neurons existed in the vicinity of the nucleus ambiguus. These characteristics of distributions were basically similar to those of the VRG of the cat.     

Morphology of expiratory neurons of the Bötzinger complex: An HRP study in the cat

In anesthetized and artificially ventilated cats, the physiological and morphological properties of expiratory neurons or their axons of the B?tzinger complex (BOT) were studied using intracellular recording and intracellular HRP labeling techniques. Thirteen expiratory neurons (nine cell somata and four axons) were successfully stained. Four of them were motoneurons, having relatively large cell somata in the retrofacial nucleus (RFN) and axons without any collaterals inside the brainstem. All the motoneurons showed a plateau shape of depolarization potentials during the expiratory phase. Any of the other nine expiratory neurons exhibited augmenting type firing or membrane potential changes during the expiratory phase. In five out of nine augmenting neurons, cell somata were stained and located ventral to the RFN. In four, only axons were stained. The majority of the augmenting neurons had two major axonal branches: one traveling toward the contralateral side and the other descending ipsilaterally in the brainstem. The most striking feature of the axonal trajectory was that all of the stained augmenting expiratory neurons, including the axons, had collateral branches with synaptic boutons in the BOT area, thus indicating that BOT expiratory neurons interact with some respiratory neurons in the BOT area and its vicinity.     

Effects of nerve growth factor on electrical membrane properties of cultured dorsal root ganglia neurons from normal and trisomy 21 human fetuses

Trisomy 21 (Down syndrome) results in abnormalities of electrical membrane properties of cultured human fetal dorsal root ganglion (DRG) neurons; namely, faster rates of depolarization and repolarization of the action potential, and a shortened spike duration. A possible role of nerve growth factor (NGF) in the expression of abnormal electrical membrane properties fetal human DRG neurons from trisomy 21 subjects was examined. DRG neurons obtained from normal and trisomy 21 abortuses of 16–20 weeks gestation were cultured in the presence or absence of 40 nM 7S NGF. After 1 week in culture, action potentials were recorded using the whole cell patch-clamp technique, in current clamp mode. At the testing membrane potential, normal (diploid) neurons grown without NGF showed reduced maximal rates of depolarization (−41.3%) and of repolarization (−31.4%), a decreased spike amplitude (−14.2%) and a prolonged action potential (+49.2%), when compared to normal cells cultured with NGF. Trisomy 21 neurons showed similar changes, but had a greater relative decrease in the rates of action potential depolarization and repolarization. These changes were evident at different membrane potentials. Normal and trisomic DRG neurons cultured without NGF showed differences in action potential parameters similar to those previously described using NGF-supplemented culture medium. These data indicate that NGF can regulate electrical membrane properties in cultured human fetal DRG neurons, but apparently is not responsible for the abnormalities observed in trisomy 21 neurons.     

Intracellular activity of medullary respiratory neurons

Records of intracellular activity were obtained from respiratory neurons in the lateral reticular formation of the medulla. The membrane resistance of these neurons was generally higher than that of spinal cord motoneurons. This observation corresponds to the smaller size of neurons found in this region of the medulla. Only early expiratory neurons showed a declining frequency during the period when the underlying slow depolarization and apparent firing level were increasing. In other types, especially inspiratory neurons with an augmenting frequency, the slow membrane depolarization and frequency usually changed in parallel. Frequency adaptation to depolarizing pulses of constant current occurred in early expiratory neurons having the adapting discharge pattern during spontaneous respiration and in some cells that normally had a constant discharge frequency. Distinct postsynaptic potentials with regular firing patterns typical of respiratory neurons were seen in several cells. When they occurred during the cell's active phase, they were clearly excitatory and often triggered action potentials. Those occurring during the silent phase although not clearly hyperpolarizing were probably inhibitory and a manifestation of reciprocal innervation. It was concluded that adaptation limits discharge in early expiratory neurons, and probably in some inspiratory and expiratory neurons that have a constant discharge frequency, but not in inspiratory neurons with an augmenting discharge pattern. Since firing of all respiratory neurons was underlain by synaptic activity, it is likely that cells were brought to threshold by summation of excitatory input from cells simultaneously active.     

Neurotrophin-3 prevents mitochondrial dysfunction in sensory neurons of streptozotocin-diabetic rats

Sensory neurons from streptozotocin (STZ)-diabetic rats exhibit depolarization of mitochondria and the related induction of reactive oxygen species has been proposed to contribute to the etiology of sensory polyneuropathy in diabetes. There is deficient neurotrophin-3 (NT-3)-dependent neurotrophic support of sensory neurons in diabetes and treatment of STZ-diabetic rats with NT-3 prevents neuropathological alterations in peripheral nerve. Therefore, we hypothesized that loss of NT-3 may contribute to mitochondrial dysfunction in sensory neurons in diabetic sensory neuropathy. The specific aim of this study was to determine whether treatment of STZ-diabetic rats with systemic NT-3 could prevent depolarization of the mitochondrial inner membrane potential (Deltapsi(m)). In vitro studies with cultured DRG neurons from control rats revealed that treatment with 50 ng/ml NT-3 for 6 h enhanced the Deltapsi(m), e.g., a higher polarized membrane potential, compared to untreated neurons (P < 0.05). Studies on DRG sensory neurons from control vs. STZ-diabetic rats demonstrated that NT-3 therapy prevented the diabetes-induced depolarization of Deltapsi(m) (P < 0.05) in parallel with normalization of diabetes-dependent deficits in sensory nerve conduction velocity. Furthermore, alterations in mitochondrial function in vitro and in vivo correlated with the level of activation/expression of Akt in DRG neurons.     

Reversible effects of hypoxia on neurons in mouse dorsal root ganglia in vitro

Mouse dorsal root ganglia (DRG) were isolated and maintained in a tissue chamber. Membrane potential of ‘A-type’ neurons was recorded with intracellular electrodes. When the supply of oxygen was reduced, cells depolarized by a few mV and then maintained a stable membrane potential or partially repolarized. During depolarization the action potential was reduced in amplitude and the hyperpolarizing afterpotential was depressed. Reoxygenation within 15–88 min was followed by a brief period of hyperpolarization and then complete recovery. In about 60% of the cells, invasion of the cell soma by impulses triggered by dorsal root (DR) stimulation failed during hypoxia while action potentials could still be evoked by stimulation of the peripheral nerve and by direct intracellular stimuli. Conduction from DR into the peripheral nerve stump was unchanged indicating that the blockade of DR-evoked impulse conduction occurred at the bifurcation of the axon. Results with paired pulse stimulation indicated that impulses passing the axon bifurcation leave a long lasting ( 25 ms) post-spike subnormal period. In DRG cells treated with tetraethylammonium (TEA) the calcium-mediated ‘shoulder’ of the action potential was curtailed during oxygen withdrawal. In contrast to CNS neurons, DRG cells did not show early hypoxic hyperpolarization, nor the delayed hypoxic spreading depression-like depolarization. The findings support the suggestion that the reversible depression of synaptic potentials in the CNS during the early phase of hypoxia is caused by a combination of conduction failure at axon branch points and curtailment of voltage calcium currents of presynaptic terminals, both effects resulting in reduced transmitter output.     

The identification of brainstem neurones projecting to thoracic respiratory motoneurones in the cat as demonstrated by retrograde transport of HRP

Brainstem neurones which project to the immediate vicinity of the spinal motoneurones which supply the intercostal and abdominal respiratory muscles were identified by means of the retrograde transport of horseradish peroxidase (HRP). A combined electrophysiological and histological technique was used in which recording of phasic inspiratory or expiratory motoneurone activity within upper (T3-T4) or lower (T8-T9) thoracic segments was followed by the ion-tophoretic injection of HRP at these recording sites. HRP labelled cells were concentrated in those brainstem regions known to contain phasic respiratory neurones, namely the ventrolateral nucleus of the solitary tract (vl-NTS) or dorsal respiratory group (DRG), the ambiguus complex or ventral respiratory group (VRG) and the parabrachial pontine (PB) nuclei. In 18 cats, 248 cells were labelled in these three respiratory regions of the brainstem while 668 were much more diffusely distributed in other regions of the medulla and pons. The ipsilateral and contralateral contributions within the respiratory regions were respectively; 23%:77% (DRG), 33%:67% (VRG), 95%:5% (PB). These results are considered in the general context of previous electrophysiological and histological findings, but also with particular reference to a related study of the projections from brainstem neurones to the phrenic nucleus [32].     

Respiratory neurons in the medulla of the rabbit: distribution, discharge patterns and spinal projections

To determine distribution, discharge patterns and the spinal projections of medullary respiratory neurons (RNs), a systematic mapping of 806 RNs was made in the medulla of anesthetized rabbits. In disagreement with previous reports that there are no discrete medullary respiratory neuronal groups in rabbits, two neuronal groups were identified: (1) dorsal respiratory group (DRG), associated with the nucleus tractus solitarius; and (2) ventral respiratory group (VRG), associated with the nucleus ambiguus compact formation. The density of RNs in the DRG was much lower than that in the VRG. In the VRG, 3 subdivisions of RN populations were found: predominantly expiratory neurons in the caudal and the rostral parts, and mainly inspiratory neurons in the intermediate region. Nine distinct types of RNs were classified on the basis of firing patterns. Nearly all types were found in both the DRG and each VRG subdivision. Antidromic mapping of 64 VRG neurons revealed that 67% projected to the spinal cord. Expiratory bulbospinal neurons in the rostral subdivision of the VRG projected only to the cervical cord (mainly ipsilaterally). Most neurons of the intermediate and caudal subdivisions of the VRG (74%) appeared to project either contralaterally or ipsilaterally below T. The axonal conduction velocity was 40-50 m/s by two-point determinations. We conclude that respiratory neuronal groups in the medulla of the rabbit are generally similar to those of the cat. Nearly equal proportions of bulbospinal RNs projected to the ipsilateral vs contralateral spinal cord.     

Neurophysiological abnormalities in cultured dorsal root ganglion neurons from the trisomy-16 mouse fetus, a model for Down syndrome

The trisomy-16 mouse is considered to be a model of human trisomy-21 (Down syndrome). We have examined the electrical membrane properties of cultured dorsal root ganglion (DRG) neurons from normal and trisomy-16 fetuses. Trisomy-16 neurons had significantly accelerated rates of action potential depolarization and repolarization compared to diploid neurons, resulting in decreased spike duration. These changes match those reported in human trisomy-21 DRG neurons. Such abnormalities may contribute to the mental retardation characteristic of Down syndrome.     

Subthreshold Oscillations Induced By Spinal Nerve Injury In Dissociated Muscle And Cutaneous Afferents Of Mouse DRG

Whole cell patch-clamp recordings were obtained from dissociated mouse lumbar dorsal root ganglion (DRG) neurons. Recordings were made from control neurons and neurons axotomized by transection of the corresponding spinal nerve 1-2 days prior to dissociation. Medium to large muscle and cutaneous afferent neurons were identified by retrograde transport of True Blue or Fluoro-Gold injected into the corresponding peripheral tissue. Action potentials were classified as non-inflected spikes (A(0)) and inflected spikes (A(inf)). High-frequency, low-amplitude subthreshold membrane potential oscillations were observed in 8% of control A(0) neurons, but their incidence increased to 31% in the nerve injury group. Fifty percent of axotomized muscle afferent A(0) cells displayed oscillations, while 26% of axotomized cutaneous afferents exhibited oscillations. Lower-frequency oscillations were also observed in a small fraction (4%) of A(inf) neurons on strong depolarization. Their numbers were increased after the nerve injury, but the difference was not statistically significant. The oscillations often triggered burst firing in distinct patterns of action potential activity. These results indicate that injury-induced membrane oscillations of DRG neurons, previously observed in whole DRG of rats, are present in dissociated DRG neurons of the adult mouse. Moreover, these observations indicate that both muscle and cutaneous afferents in the A(beta) size range give rise to injury-induced membrane oscillations, with muscle afferents being more prone to develop oscillations.     

Diminished retrograde transport causes axonal dystrophy in the nucleus gracilis

Summary To examine a possible cause of axonal dystrophy in the nucleus gracilis, dorsal root ganglion (DRG) neurons of rats were investigated by means of electron-microscopic autoradiography and horseradish peroxidase (HRP) tracing method. Following injections of tritiated amino acids into the L6 and S1 DRG, labeling was observed on the initial and halfway developed dystrophic terminals in the ipsilateral gracile nucleus. However, no grains or few, if any, were found on the well developed huge dystrophic endings. Compared with the thoracic and upper lumbar DRG, a decrease in velocity and amount of retrograde HRP transport was demonstrated in the lower lumbar and sacrococcygeal DRG neurons, especially of large cell diameter, irrespective of age of rats. These findings led us to conclude that the axonal dystrophy reflects a state of an anterograde overtransport of the axoplasm caused by a diminished retrograde transport which is specific to lower lumbar and sacrococcygeal DRG large neurons.     

Neurogenesis of subpopulations of rat lumbar dorsal root ganglion neurons including neurons projecting to the dorsal column nuclei

The time of birth of subpopulations of dorsal root ganglion (DRG) neurons was studied with immunohistochemistry for 5-bromodeoxyuridine (BrdU). Pregnant rats were injected with BrdU i.p. to label the neurons on one of the embryonic days (E) E11-E16. When they were adults, the rats were given injections of Fluoro-Gold (FG) into the gracile nucleus to identify DRG neurons projecting to this structure. Following a 5 day survival period, the animals were perfused with aldehyde fixative. Sections from the L3-L5 DRGs were processed for BrdU immunohistochemistry followed by either immunostaining for the antineurofilament antibody RT97, as marker of the light neuronal subpopulation, or histochemical staining for the B4 isolectin from Griffonia simplicifolia I, as marker of the small dark subpopulation. The results indicated that the DRG neurons were generated between E12 and E16. The RT97⁺ neurons were generated on E12–E15, with a peak at E13. FG⁺ neurons, the majority of which were RT97⁺, were also generated on E12–E15. The B4⁺ neurons were generated on E13–E16, with a peak around E14. The overall pattern of neurogenesis of the DRG neurons showed that the RT97⁺ neurons were produced prior to the B4⁺ neurons. These findings are in agreement with earlier observations that the large DRG neurons are generated earlier than the small dark neurons. Our findings also suggest the existence of a third neuronal subpopulation that might be produced at the latest period of DRG neurogenesis at E15–E16. © 1996 Wiley-Liss, Inc.     

Aging and ethanol alter neuronal electric membrane properties

The effect of age of donor mouse and acute ethanol exposure on the electrical membrane properties (EMP) of 825 freshly dissociated dorsal root ganglion (DRG) neurons was investigated. Both age and ethanol exerted a number of significant linear and independent effects. Age effects included a 52% increase in action potential overshoot, a 50% increase in a measure of afterhyperpolarization duration, a 31% increase in action potential duration due to decreased rate of repolarization and depolarization, and decreased electrical excitability. Also, specific membrane resistance increased and specific membrane capacitance decreased while the membrane time constant was not significantly affected by age. These EMP alterations with age for freshly dissociated DRG neurons are consistent with those reported previously for cultured DRG neurons and hence further support the hypothesis that the EMP of neurons in situ change significantly with age. Acute ethanol exposure caused relatively few EMP alterations including decreased electrical excitability, specific membrane capacitance, time constant, and overshoot; specific membrane resistance was increased. These changes are similar to those seen previously and are consistent with a membrane expansion effect of ethanol.     

Morphology of inspiratory neurons located in the ventrolateral nucleus of the tractus solitarius of the cat

The morphology of 11 dorsal respiratory group (DRG) inspiratory neurons located in the ventrolateral nucleus of the solitary tract (vl-NTS) was studied using the technique of intracellular labeling with the enzyme horseradish peroxidase (HRP). Six of these cells were cut in the transverse plane and had a mean somal diameter of 30.4 m?m, while five others sectioned in the horizontal plane had a mean of 38.2 m?m. These neurons produced an average of 6.2 primary dendrites (range: 4–10), many of which projected rostrally or caudally up to 1.0 mm from the cell bodies. These dendrites were oriented along the longitudinal axis; they ran parallel and ventral to the tractus solitarius. In general, all dendrites possessed numerous spines and appendages. Many axons could be traced for considerable distances within the medulla (in one instance, up to 8 mm). These axons were last discerned in the contralateral ventral medulla rostral to the level of their cell bodies. The axons of three neurons bifurcated in the ipsilateral medulla; one branch remained ipsilateral and projected caudally, while the other crossed the midline. A small number of counterstained cells of size similar to or larger than the HRP-stained neurons formed a column that constituted the vl-NTS. Based upon our observations of stained and counterstained cells, we conclude that the inspiratory neurons of the vl-NTS are few in number and represent a morphologically homogeneous population. The primary orientation of the dendritic arbors of vl-NTS inspiratory neurons appears to optimize the surface area available to receive synaptic contacts from sensory afferents emerging from the tractus solitarius.     

Brainstem projections to the phrenic nucleus: A HRP study in the cat

Brainstem neurones which project to the phrenic nucleus were identified using retrogradely transported horseradish peroxidase (HRP) as a marker. Following iontophoretic injection of HRP into the phrenic nucleus, labelled cells were encountered throughout large areas of the medulla and pons, but occurred with characteristic high densities in those regions known to contain phasic respiratory neurones: namely, the ventrolateral solitary tract nucleus (vl-NTS), known as the dorsal respiratory group (DRG), the ambiguus complex or ventral respiratory group (VRG) and the parabrachial pontine nuclei (BCM-KF). In 12 cats a total of 1540 cells was identified within these regions, the relative contralateral and ipsilateral contributions were respectively 72%:28% (vl-NTS), 65%:35% for the ambiguus complex, and 5%:95% (BCM-KF). In addition, labelled cells, predominantly ipsilateral, were observed in the pontine and medullary reticular formation and the vestibular nuclei. The labelled cells of the DRG had round, oval or triangular perikarya. Their mean soma diameter was 18.3 micrometers. The HRP-positive cells of the VRG had slightly larger somas (mean 21.2 micrometers) and they were fusiform and triangular. The neurones labelled in the BCM-KF nuclei were more heterogeneous with a mean soma size of 14.9 micrometers. The bilateral projections to the phrenic nucleus from the DRG and the VRG, and the predominantly ipsilateral projection from the BCM-KF are discussed in relation to current electrophysiological and autoradiographic findings.     

Medullary respiratory neurons in the guinea pig: localization and firing patterns

The location and firing patterns of medullary respiratory neurons have been described in a small number of species. The cat has been the most widely studied species, but some potentially important differences have recently been noted in others. A more complete survey of species is required to determine the significance of these differences. We describe the location and firing patterns of respiratory neurons in the medulla of anesthetized, paralyzed and mechanically ventilated adult guinea pigs. Extracellular single-unit recordings were made from the medulla, their phase relationship with phrenic nerve activity used to define them as respiratory and their location marked with fast green. Respiratory units were concentrated ventrolateral to the nucleus tractus solitarius (NTS) and within and surrounding the nucleus ambiguus (NA), corresponding to the dorsal respiratory group (DRG) and ventral respiratory group (VRG) of the cat, respectively. Most DRG respiratory units were inspiratory, while the VRG contained equal numbers of inspiratory and expiratory units. The DRG and VRG both contained early, late and constant-frequency inspiratory and expiratory units. In general, these findings are similar to those in other mammalian species examined, consistent with these basic aspects of the respiratory network being highly conserved.