M. Fallert

Medullary respiratory-related neurons with axonal connections to rostral pons and their function in termination of inspiration

In urethane-anaesthetized, paralyzed and artificially ventilated rabbits, medullary respiration-related neurons (RRU) were classified according to the phase relation of their burst discharge to phrenic nerve activity. Phase-bound inspiratory (I) or expiratory (E) neurons were discriminated from phase-spanning expiratory-inspiratory (EI) or inspiratory-expiratory (IE) units. Mechanisms of termination of inspiration by electrical stimulation of rostral pontine nuclei (Nc. parabrachialis medialis; Lc. coeruleus) were examined firstly to demonstrate whether RRU receive descending excitatory and inhibitory afferents as well as ascending efferents and secondly to analyse the time course of the neuronal pathways involved. Of 120 RRU, 38 neurons were demonstrated to receive pontine afferents. About 33% of all E neurons became orthodromically excited during rostral pons stimulation whereas 18.2% of all I cells became orthodromically inhibited. Some RRU were shown to project up to the rostral pons. 50% of these were of the phase-spanning IE type. The onset of inspiratory inhibition induced by rostral pons stimulation occurred 3.4 ms after the onset of single electrical pulse stimulation. Based on these results a neuronal model for a pontine mechanism terminating inspiration is proposed.

Inspiratory Inhibition and Rebound Activation Caused by Intermittent Bulbar and Afferent Vagal Stimulation in the Rabbit

SummaryIn anesthetized rabbits spirogram and diaphragmatic activity were examined during electrical stimulation, of regions of the medulla oblongata. One volley of repetitive stimuli per breath was applied. The volleys were triggered by the animals own respiration.1.Volleys of 120 msec duration, applied during inspiration, caused an immediate and transient inhibition of the diaphragmatic activity. After the end of the volley, the inspiration continued. An inspiratory rebound appeared: the tidal volume was increased; the inspiration was prolonged up to 124 msec. The respiratory rate decreased.2.Stimulation with 100–150 pulses p.s. and 0.5 msec pulse duration was optimal. With lower frequencies or shorter pulses the threshold was higher.3.The stimulation effects were still present after bilateral cervical vagotomy. Respiration was partially inactivated by artificial hyperventilation or by inducing a metabolic alkalosis, and was activated by an additional dead space. Neither of these manoeuvres abolished or enhanced the stimulation effects.4.The histological verification showed that inhibition and rebound activation could be elicited from a relatively widespread area in the medulla oblongata.5.With afferent vagal stimulation with similar volleys, the same effects were observed.6.Results suggest that the stimulus-induced inhibition either corresponds to the expiratory component of the Hering-Breuer reflex, or is caused by direct stimulation of inhibitory interneurones The inspiratory rebound may be due to a delay in the activation by the respiratory centre of the central inhibitory feedback mechanism.

Oxyphenonium als Methode zur selektiven efferenten Vagusblockierung in der Atmungsphysiologie

SummaryIn respiratory experiments it may be necessary to stimulate the vagus nerves without severing them distally to the stimulation electrodes, especially when vagal respiratory reflexes are to be elicited. Oxyphenonium, a parasympathetic blocking synthetic quaternary ammonium compound, was used to suppress the efferent vagal effects. In the anesthetized rabbit 20 μg/kg oxyphenonium had no primary influence upon heart rate and blood pressure, but bradycardia elicited byefferent vagal stimulation was abolished for 2 h. Oxyphenonium increased the spontaneous respiratory rate by 8% and volume per minute by 10%. The respiratory effects ofafferent vagal stimulation (Hering-Breuer reflex), however, remained unaltered.In artificial respiration with a positive-negative pressure pump (respirator) each forced deflation elicits a reflex inspiration. Only within certain limits can the rabbit be artificially ventilated with a frequency and an inflation volume different from its own respiratory frequency and tidal volume. In case of too fast ventilation or too small inflation volume the sequence of the inspirations becomes slower or faster than that of the respirator. After oxyphenonium the reflex inspirations were elicited synchronously to the respirator action at a rate which was 8% higher than in control conditions. This suggests that in the rabbit the drug acts as a bronchial relaxant.

Patterns of bulbar respiratory neurons during and after artificial hyperventilation

Single bulbar respiratory unit activity was recorded in urethane-anesthetized rabbits. During artificial ventilation, reflex contractions of the diaphragm were elicited by the forced deflations of the respirator. Out of 44 neurons, four I, one E and two EI units exhibited a shift of their discharge relative to the contractile phase of the diaphragm, compared to normal respiration. During hypocapnic apnea, some I, IE and E neurons became silent. Other cells belonging to all phase types, however, continued to fire tonically at a rate less than the peak rate in normal bursts. When rhythmic respiration resumed after termination of the apneic pause, five (I, IE and E) units exhibited a transient phase shift of their burst discharge compared to normal respiration. The results of simultaneous recordings of two neurons belonging to different phase types are compatible with the assumption that inspiratory units are periodically inhibited by inspiratory-expiratory cells.

Der hering-Breuer-Reflex bei künstlicher Beatmung des Kaninchens

Summary1.Anesthetized, non-curarized rabbits were artificially ventilated at such high rate and small inflation volume that the inspirations no longer closely followed upon the deflation phases of the respirator.2.In this condition the medulla oblongata was electrically stimulated. One volley of repetitive stimuli per cycle of the respirator (inflation and deflation) was applied, the volleys lasting 1/6th of one cycles duration. When certain points were stimulated the inspirations closely followed upon the combined artificial ventilation and bulbar stimulation.3.100 and 150 impulses per second proved to be optimal to elicit the inspirations, whereas 20 impulses per second were ineffective.4.The influence of bulbar stimulation with 100 impulses per second upon the spontaneous respiration was examined, the parameters being the same as in combined artificial ventilation and stimulation. In and expiratory effects occurred. When certain points were stimulated the inspirations closely followed upon the stimulus volleys.5.At some points inspirations could only be elicited by combined artificial ventilation and bulbar stimulation. At other points the threshold voltage was lower in combined ventilation and central stimulation than in stimulation alone. Results suggest that during certain phases artificial ventilation exerts an inspiration-facilitating influence upon the bulbar reticular formation.

Inspiratory inhibition and rebound activation elicited by intermittent electrical bulbar stimulation in various states of pulmonary afferent vagal excitation

SummaryIn anesthetized rabbits spirogram and diaphragmatic activity were examined during electrical stimulation of regions of the medulla oblongata. The volleys were triggered by the animals own respiration.1.One volley of 120 msec duration at 100 pulses p. s., applied during inspiration, caused an immediate and transient inhibition of the diaphragmatic activity. After the end of the volley an inspiratory rebound appeared: the tidal volume was increased and the inspiration was prolonged by some 150 msec. The respiratory rate decreased.2.Continuous low or high frequency electrical stimulation of pulmonary stretch afferents caused an inspiratory or an expiratory effect respectively. In both conditions the effects of additional intermittent bulbar stimulation remained essentially unaltered.3.By means of a specially designed spirometer both pulmonary collapse and marked lung distension were produced. At low lung volume collapse afferents were excited. Inspiratory inhibition and rebound activation upon central stimulation persisted throughout the whole range of lung volume investigated; the rebound increase in tidal volume consecutive to the stimulus volley, however, was minimal in extreme lung distension.4.Results suggest that the stimulation effects were the consequence of manipulation on intrinsic mechanisms of the bulbar respiratory centre.

Metabolic control of respiratory neuronal activity and the accompanying changes in breathing movements of the rabbit

SummaryThe property of the neuronal membrane to be permeable to metabolic modifiers of two regulatory enzymes has been utilized to manipulate the spike activity of inspiratory (I) and expiratory-inspiratory (EI) neurons of the bulbar respiratory centre. The neurons have been classified according to their response to lung distension or collapse (α- or β-type) and to hyperventilation (tonic firing denoted by “+”, cessation of activity by “−”). Using extracellular microelectrodes for single unit recording, the medulla oblongata was superfused with a metabolite-containing CSF. The various neuronal sub-types exhibited a differential activating or inhibitory response to one or several metabolic effectors. For example Iα+ units were activated by 5 mM glucose-6-phosphate (G-6-P) and 3.5 mM 3-phosphoglycerate (3-PGA), which both inhibited Iβ+ neurons, while 5 mM AMP inhibited Iα+ much more strongly than Iβ+ cells. The spike density of Iα− and Iβ− neurons was increased in the presence of 2.5 mM fructose-6-phosphate and 3.5–5 mM AMP, but became reduced by G-6-P. In contrast, 3 mM fructose-1,6-diphosphate and 5 mM 3-PGA activated the Iα− but inhibited the Iβ− neurons. The EIβ units were characteristically activated by 10 mM citrate, which inhibited all I-type neurons. Activations of the Iα and Iβ neurons led to an accelerated respiratory rate and a higher tidal volume, while the opposite was true for EIβ neurons. Intravenous injection of metabolites could not duplicate the striking effects under local applications.

A device to trigger automatically electrical stimulation at different states of in- or expiration on the background of variable lung inflation in the rabbit.

SummaryA device is described which permits to obtain a signal at the beginning of in- or expiration respectively. This signal may be used to trigger an electrical stimulator. To permit studies at different states of lung inflation the tracheal cannula is connected to a spirometer circuit. To produce constant positive or negative air pressure this bellows-type spirometer is loaded with exchangeable metal weights. The tracheal pressure is recorded by means of a pressure transducer, the output of which is taken as signal. The device allows accurate compensation of the DC-component of the transducer output resulting from spirometer operation at a maintained positive or negative pressure.

The bulbar respiratory centre in the rabbit

SummaryIn anesthetized rabbits, spirogram and diaphragmatic activity were examined during electrical stimulation of regions of the medulla oblongata. The stimulating volleys were triggered by the phase transitions of the animals own respiration.1.Each earlyinspiratory volley of 120 ms duration at 100 pulses per second caused an immediate and transient inhibition of the diaphragmatic activity. Respiration was slowed down due to prolongation of inspiration. The tidal volume increased above control. Stimuli delivered after 30–40% of a control inspiration had elapsed cut short this phase and entailed a shortening of the following expiration, too. Respiration was thus accelerated.2.Each earlyexpiratory volley caused an inspiratory twitch after a short latency. The respiratory rate was slightly increased due to shortening of expiration. The spirogram exhibited a distinct inspiratory effect (elevation of the end-inspiratory and end-expiratory levels). Stimuli delivered after 60–70% of a control expiration had elapsed slowed down respiration due to prolongation of inspiration but did not alter the end-expiratory level. The expiration remained almost unaltered. The effects were still observed while an artificial state of lung distension or collapse was maintained.3.Volleys ofincreasing duration were delivered, starting withonset of expiration. The initial respiratory acceleration (shortening of both phases) and elevation of the end-expiratory level, observed when short volleys were applied, changed into slowing down of respiration (prolongation of both phases) and a shift of the end-expiratory level towards active expirations when the duration of the volley was somewhat longer than a normal expiration. The end-inspiratory level remained slightly elevated.nResults suggest that during inspiration a progressively increasing inhibitory state is built up. During expiration, both an increasing inspiratory and an expiratory tendency are present as revealed by mixed inexpiratory stimulation effects.

Possible mechanism of pontine influence on output of medullary respiratory center in the rabbit

Inhalation of biologically inert particles has long been used for studying the physiological lung function of lung clearance of particles. Conducting airways and deep lung are cleared of deposited particles by different mechanisms, which are reflected by different clearance rates. To find out, whether magnetopneumography (MPG) can be used for measuring dust deposition and retention, the clearance of inhaled radioactive magnetite particles (SgFe304, act iv i ty median aerodynamic diameter 1.7 ,m) was determined by y-ray measurement techniques and by MPG. Rats were exposed to 23 mg magnetite per m 3 for 2 hours; animals were sacrificed up to 120 days after exposure. Excised lungs were air dried at an inflation pressure of 20 cm H20. Right and lef t lungs were measured separately, their radioactivity with a well scint i l lat ion detector and magnetically with a second order SQUID magnetometer after magnetizing with a 67 m Tesla f ield for 6 sec. The long term clearance of magnetite was found to be slightly faster when measured with MPG as compared to radioactive measurements (36 days vs. 42 days half l ives). Although the slopes of the clearance curves did not di f fer significantly, analysis of individual measurements of the lungs during the long term phase revealed a 30 percent lower lung burden when determined by MPG. Solubil ity studies wil l c lar i fy to what extent slow solubi l i ty processes of magnetic particles in the lung may be responsible for this. MPG after magnetization to several depths revealed an enhanced sensitivity to translocation of dust to subpleural or pleural structures during clearance. MPG offers a noninvasive measure of lung dust retention and a means of evaluating redistribution of dust during clearance. Department of Radiation Biology and Biophysics, Medical Center, Univ. of Rochester, Rochester, NY 14642