The bulbar respiratory centre in the rabbit. II. Responses of respiratory neurons to intermittent electrical bulbar stimulation during in- or expiration

The brain-stem projections of pulmonary stretch afferent neurones in cats and rabbits

1. Micro-electrode recordings were made from slowly adapting pulmonary stretch afferents within the nodose ganglia of cats and rabbits. Recordings sites were distributed throughout the ganglia. 2. The projections of these afferents to the medulla oblongata were studied by antidromic stimulation. 'Point' and 'Field' type depth--threshold curves were interpreted as corresponding to stimulation of the main afferent axons and its branches, respectively. Increases in antidromic latency in conjunction with 'field' contours was additional evidence in support of this interpretation. 3. In cats, most (six out of seven) afferents had extensive branches, and probably also terminations, within the medial subnucleus of the ipsilateral nucleus tractus solitarius (n.t.s.). Two of these, plus one other afferent, also had projections to the lateral and ventrolateral subnuclei. 4. In rabbits the projections of such afferents were similar, i.e. mainly to the medial subnucleus of the n.t.s. (eight out of eleven) but also extending into the nucleus alaris, and occasionally to lateral and ventrolateral subnuclei (two out of eleven) or to both regions (one out of eleven). 5. Branching of single afferents was seen to occur over up to 3 mm of the rostro-caudal extent of the intermediate region of the n.t.s. The significance of the observations is discussed.

Metabolic control of respiratory neuronal activity and the accompanying changey-expiratory neurons

Expiratory-related neurons have been classified according to their phase relation within the respiratory cycle, their response to lung distension and collapse (alpha- and beta-type), and to hyperventilation (tonic firing denoted by "+", cessation of activity by "-"). The dorsal surface of the medulla oblongata was superfused with a metabolite-containing CSF solution and the activity of expiratory (E) and inspiratory-expiratory (IE) neurons was extracellulary recorded. The neuronal sub-types established by their functional behaviour could equally be distinguished by their differential response to one or several metabolites. In contrast to inspiratory (I) neurons, Ealpha- Ebeta-, Ebeta- and IEbeta- neurons are inhibited by 3.5 mM AMP, but are activated by 10 mM citrate (with the exception of Ebeta+ units). Furthermore I cells are activated by ATP, while Ealpha and Ebeta units become inhibited. Vagotomy in some instances affected the response of some IEbeta units. An increase in spike density of IEbeta+ and Ealpha- cells is paralleled by a reduction of both the respiratory rate and the tidal volume, while a lower spike density in IEbeta+, IEbeta- and Ealpha- units is accompanied by increases in respiratory rate and tidal volume. In the case of Ebeta+ and Ebeta- cells lower activity is associated with an increased tidal volume. No metabolite-induced changes could be obtained with cardiovascular or unspecific reticular neurons.

Metabolic control of respiratory neuronal activity and the accompanying changes in breathing movements of the rabbit. III. Phase shifts in respiratory neurons induced by inflation and collapse of the lung, hyperventilation, or metabolic modifiers

Phase shifts between inspiratory-related and expiratory-related discharge patterns can be reversibly induced in respiratory neurons following volume changes of the lung, hypocapnic apnea as a result of hyperventilation, or superfusion with certain metabolic modifiers. Phase-spanning expiratory-inspiratory or inspiratory-expiratory discharges are frequently induced in those neurons which are activated either by pulmonary stretch receptors or collapse afferents. The same is true for regulatory effectors which activate key steps of the neuronal metabolism such as ADP, 3-phosphoglycerate, L-glutamine, fructose-6-phosphate and fructose-1,6-diphosphate. In contrast, inhibitory vagal inputs or superfusion with citrate, an inhibitory metabolic modifier, revert preexisting expiratory-inspiratory discharges into a phase-coupled inspiratory pattern. It is postulated that the respiratory neuronal networks represents a time-optimal control system which strives to adjust to a new equilibrium value in a minimum of time, following a given mechanical or chemical perturbation. Following the hypothesis advanced by Cohen (1974) that the phase-spanning units modulate the activity of the in-phase neurons, it is suggested that the additional recruitment of expiratory-inspiratory and inspiratory-expiratory units provides a measure of the quality of time-optimal control and hence a performance index of the system.

The bulbar respiratory centre in the rabbit. I. Changes of respiratory parameters caused by intermittent electrical bulbar stimulation during inspiration or expiration

In 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 animal's own respiration. 1. Each early inspiratory 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 early expiratory 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 of increasing duration were delivered, starting with onset 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. Results 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.