Distribution of carotid sinus nerve afferent fibers to solitary tract nuclei of the cat using transganglionic transport of horseradish peroxidase

The distribution of carotid sinus nerve (CSN) afferent fibers to the solitary tract and its various nuclei has been determined using transganglionic transport of horseradish peroxidase (HRP). The ipsilateral solitary tract was labeled and, in addition, the heaviest labeling was seen dorsomedial within the medial solitary nucleus, as well as just lateral to the tractus solitarius in the lateral solitary nucleus. Bilateral labeling was present caudal to the obex in both the medial and commissural nuclei. The paucity of reaction product within the ventrolateral solitary nucleus supports previous neurophysiological studies suggesting carotid chemoreceptors do not make monosynaptic contacts with respiratory neurons within that nucleus.

Phrenic motoneurons in the cat: subpopulations and nature of respiratory drive potentials

1. Intracellular recordings were made from 78 phrenic motoneurons (PM) in anesthetized, paralyzed, artificially ventilated cats that were slightly hypercapnic. 2. Three subpopulations of PM (types A, B, and A/B) were identified on the basis of their membrane potential trajectories during expiration (E). Type A cells exhibited wholly linear trajectories. These were rapidly hyperpolarized at the onset of E followed by a slow ramp of increasing hyperpolarization observed in 51 of 59 type A cells. Types B (13 cells) and A/B (6 cells) had nonlinear trajectories in E. Type B cells approached their end-expiratory potential levels more slowly. 3. Measurements of axonal conduction velocity, expiratory phase input resistance, initial depolarization rate, and initial spike onset during inspiration revealed that type B cells had significantly slower axonal conduction velocities, higher input resistances, greater initial depolarization rates, and earlier initial spike onsets than type A cells. The properties of type A/B were intermediate between the other cell types. These results support the hypothesis that the PM pool is not homogeneous. 4. Active E-phase inhibition of all types of PM was directly demonstrated by reversal of the increasing hyperpolarizing wave to a depolarizing wave with hyperpolarizing current injection using a bridge circuit. Thus hyperpolarization of PM during E is not merely due to a central disfacilitation. 5. During hyperpolarizing current injection the inspiratory phase membrane potential trajectory of all PM became a ramp depolarization similar to that seen during control conditions in type A cells. These results support the conclusion that all cells within the PM pool are receiving a similar central excitatory synaptic input during inspiration. The rapid initial depolarization of type B and their concomitant early spike onset is a consequence in part of a rebound excitation from their expiratory phase inhibition as well as a higher input resistance, probably due to a smaller cell size. 6. Expiratory related neural activity was recorded within the phrenic motor nucleus. It is suggested that these expiratory related neural elements, based on the temporal pattern of their activity, may be responsible for the active inhibition of PM.

Properties of apneusis produced by reversible cold block of the rostral pons

Reversible cold block of the rostral pons was used to compare properties of normal and apneustic respiration in anesthetized, vagotomized, artificially ventilated cats. During apneusis we observed high frequency oscillations (HFO) in phrenic nerve activity which were reduced in frequency compared with those during a normal inspiration. Apneusis produced by mid-pontine transection or punctate pneumotaxic center (PC) lesion produced similar HFO changes. The minimal intensity of superior laryngeal nerve electrical stimulation needed to terminate a breath was higher early in an apneusis than at the same time during a normal breath. Later in apneusis the intensity required became constant and was approximately the same as that needed to end a normal inspiration at its natural termination. With intact vagi lung inflation produced a greater prolongation of expiration during apneustic respiration than during normal respiration. Apneustic type activity was observed in both phrenic and vagal inspiratory motoneurons. We suggest that: (1) HFO are generated without the PC, but the PC elevates the oscillation frequency; and (2) apneusis may result in part from a delayed activation of the normal inspiratory off-switch mechanism.

Neural regulation of respiration

During the last decade, we have seen our knowledge of the neural regulation of respiration progress from the concept of "respiratory centers" to that of densely packed groups of respiratory neurons located within specific medullary nuclei. By careful neurophysiological and histological studies employing intra- and extracellular recording, the sequence of processing of afferent information through these nuclei and the axonal projections from respiratory neurons have been determined. However, the site and the mechanism of respiratory rhythm generation remain unknown.

Lateralized phrenic nerve responses to stimulating respiratory afferents in the cat

We stimulated electrically pharyngeal branch of both glossopharyngeal nerves (PGLN), internal branch of superior laryngeal nerves (ISLN), and carotid sinus nerves (CSN) in anesthetized cats. We recorded simultaneously, averaged, and compared bilaterally evoked phrenic nerve (PHR) activity. Our objective was to demonstrate a short-latency evoked response in the PHR contralateral to the stimulus. Low-intensity stimulation of PGLN and ISLN during inspiration evoked a short-latency contralateral excitation with a latency of 5.2 ms +/- 0.2 SE (16 cats) for PGLN, and 3.8 ms +/- 0.1 SE (13 cats) for ISLN. This excitation could follow stimuli delivered at 100 Hz. Stimulation during expiration did not result in a lateralized excitation. The excitation is followed by bilateral inhibition. Neither strychnine nor picrotoxin prevented either the lateralized response or the inhibition, though strychnine diminished a delayed bilateral excitation following PGLN stimulation. This dalayed (latency 18.7 ms +/- 0.7 SE) bilateral excitation corresponds to the sniff reflex. CSN stimulation did not result in lateralized excitation. We suggest that the lateralized evoked response results from a gated paucisynaptic reflex pathway involving the PGLN and ISLN, ipsilateral inspiratory neurons, and contralateral PHR motoneurons.

Neural regulation of respiration

The main respiratory muscles are under both voluntary and involuntary (automatic) control. These two control systems come from separate sites in the CNS and have separate descending pathways; the final integration of these outputs occurs at segmental levels in the cord. Voluntary control arises from the motor and premotor cortex and descends in the cord in the corticospinal tract. Involuntary control is mediated by both rhythmic and nonrhythmic systems located in the brainstem. Recent studies have associated the classic respiratory centers with specific nuclei in the brainstem. The pneumotaxic center is located in the nucleus parabrachialis, and the medullary respiratory centers are located in the vicinity of the nucleus of the solitary tract (dorsal respiratory group) and the nuclei ambiguus and retroambigualis (ventral respiratory group). Most axons from the medullary nuclei cross in the medulla and descend in the ventral and lateral columns to segmental levels. The classic medullary respiratory centers described by Pitts have been shown to be the site of origin of tonically firing long reticulospinal axons that descend in the ventral and lateral columns. This system is thought to provide a nonrhythmic involuntary biasing of the membrane potential of respiratory motoneurons in the cord. The site of generation of eupnic breathing and the mechanism of rhythm generation remain unknown. However, recent studies indicate that reciprocal inhibition between populations of inspiratory and expiratory cells (bistable oscillator model) does not occur in the medulla; rather we suggest that inhibitory phasing of inspiratory cells generates inspiratory rhythm, and periodic inhibition of tonically active expiratory neurons results in respiratory rhythm. We suggest that the inhibitory phasing occurs in the dorsal respiratory group, which is also the site of integration of respiratory afferents. The main site of integration of the voluntary and involuntary control systems is the cord where the respiratory motoneuron output is determined by descending information from these systems, as well as with intrasegmental and intersegmental reflexes. Separation of the voluntary and involuntary control systems also occurs in man and discrete lesions may interrupt one system without significant alteration of the other.