Relationship between activity of respiratory center and EEG

Respiratory-associated rhythmic firing of midbrain neurons is modulated by vagal input

We recorded phrenic nerve activities and single unit firings of mesencephalic neurons in 19 decerebrate, paralyzed and ventilated cats, in which the spinal cord had been transected at C7-T1 and carotid sinus nerves cut but vagus nerves left intact. After we had found neurons with respiratory-associated rhythmic activity, we tested the effect of changing pulmonary vagal input by (1) stopping and restarting the ventilator; (2) changing the ventilator's tidal volume; (3) progressively cooling the vagus nerves to 6-7 degrees C; and (4) vagal section. All methods of testing yielded results that showed that vagal input, probably from pulmonary stretch receptors, tonically inhibits the respiratory-associated firing of the mesencephalic neurons by a direct mechanism that is independent of a vagal effect on medullary respiratory drive. We have suggested that these neurons are involved in the mechanism that conveys information about respiration to the cortex where it may be interpreted as the sensation of dyspnea. If so, movement and increased expansion of the lungs can be expected to lessen the sensation.

Respiratory-associated thalamic activity is related to level of respiratory drive

We recorded phrenic nerve activity and thalamic single unit firing in unanesthetized, suprathalamically decerebrated, paralyzed and ventilated cats, in which vagi and carotid sinus nerves (CSN) had been ablated. Seventy-six (14%) of 545 neurons in regions of the thalamus related to the ascending reticular system, which had been tonically firing at low respiratory drives, developed rhythmic increases of firing associated with each respiration when drive had been increased by CSN stimulation or hypercapnia. The increases of neuronal firing occurred in late inspiration/post-inspiration but sometimes lasted into expiration; the magnitude of change was graded according to the magnitude of respiratory activity. Thalamic neurons also fired with a rhythm related to ventilator-induced chest expansion, some units showing both the respiratory-associated and the ventilator-related rhythms. Simultaneously recorded mesencephalic and thalamic neurons developed similar rhythms when drive was increased. We suggest that these neuronal activities reflect the conveyance of information about respiration to the cortex, where it may lead to the sensation of dyspnea and perhaps to arousal.

Respiratory-associated rhythmic firing of midbrain neurones in cats: relation to level of respiratory drive

1. We recorded phrenic nerve activities and single unit firing of mesencephalic neurones in unanaesthetized supracollicularly decerebrated, paralysed and ventilated cats, in which vagi and carotid sinus nerves had been ablated. We made these measurements first at low levels of respiratory drive associated with normal PCO2 levels, then with increased respiratory drive and levels of phrenic activity produced by hypercapnia or by carotid sinus nerve stimulation. 2. We found that at least a quarter of the neurones in the central tegmental field of the mesencephalon, which were irregularly tonic or silent at low respiratory drives, developed a rhythmic increase of firing associated with each respiration. There appeared to be a threshold at about 50% of maximum respiratory activity, below which the respiratory-associated rhythm did not occur. Above this level, neuronal firing increased in graded fashion with increasing magnitude of respiratory activity. The latency from onset of phrenic activity to onset of increased neuronal firing was quite long (1.0 s) at drives just above the threshold but shortened to as little as 0.3 s as drive increased towards its maximum. 3. Cutting the spinal cord at C1-C2 had no effect on the ability of increased respiratory activity to generate a respiratory-associated rhythm in mesencephalic neurones. 4. Short-lasting anaesthesia with the agent Saffan caused mesencephalic neurones to lose the respiratory-associated rhythm with little change in phrenic activity and no change in respiratory cycle timing. 5. We also found a mesencephalic response to ventilator-induced chest expansion. The latency of the response from onset of expansion, indexed by fall of airway PCO2, to onset of neurone firing was shorter (0.2 s) than that found with the respiratory-associated rhythm. In seventeen neurones we found both the respiratory-associated rhythm and the independent ventilator-associated rhythm. 6. We interpret our findings to show that the respiratory-associated rhythmic firing of midbrain neurones is not primarily involved in generation or modulation of the motor function of the respiratory oscillator. We believe, instead, that these neurones are part of a sensory pathway conveying information about the magnitude of central neural respiratory drive, as well as spinally transmitted information from receptors in the chest wall, to thalamus and cortex. We suggest that the sensation ultimately generated may be that of 'air hunger' or dyspnoea.

'Air hunger' arising from increased PCO2 in mechanically ventilated quadriplegics

A number of investigators have proposed that the sense of respiratory discomfort accompanying hypercapnia depends on respiratory mechanoreceptors which inform the sensory cortex of reflex increases in breathing. To test this hypothesis, we studied subjects whose respiratory muscles were paralyzed, and who were thus unable to increase breathing in response to hypercapnia. We gradually elevated inspired PCO2 in four tracheostomized quadriplegic subjects supported by constant mechanical ventilation. These subjects reported sensations of 'air hunger' (e.g., "short of breath", "air-starved") when end-tidal PCO2 increased 10 Torr (mean) above their resting levels. In a second experiment we used the forced-choice technique to determine the ability of three of these subjects to detect repeated changes of end-tidal PCO2. Two detected 7 Torr changes, the third detected 11 Torr changes. These data suggest that changes in breathing are not necessary to evoke the sense of 'air hunger'. We conclude that the likely mechanisms are (1) projection of chemoreceptor afferent traffic to the sensory cortex, and (2) projection of corollary discharge from brainstem respiratory centers to the sensory cortex.

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

In anesthetized rabbits, spirogram and diaphragmatic activity were examined during electrical stimulation of the bulbar lateral reticular formation. The activity of bulbar respiratory neurons was recorded contra- or ipsilaterally to the stimulation site. One volley of repetitive stimuli per breath was delivered during either inspiration of expiration. 1. Each volley of about 120 ms duration at 100 pulses per second, delivered early in inspiration, caused an immediate and transient inhibition of the diaphragmatic activity. An inspiratory rebound comprising lengthening of inspiration and increase in tidal volume occurred. a) "Inspiratory" and "expiratory-inspiratory" phase-spanning neurons exhibited inhibition during the volley. The burst discharge was lengthened and the spike density increased after stimulus. The same was true of some "inspiratory-expiratory" phase-spanning units. b) The discharge of most of the "inspiratory-expiratory neurons was not inhibited. "Expiratory" units were excited. In both types of cells activation occurred which outlasted the volley. 2. When applied during expiration, the volley caused a short inspiratory twitch. a) "Inspiratory" and "expiratory-inspiratory" neurons exhibited a short post-stimulus firing and the spike density was increased. In some units of the latter type, however, the burst discharge was shortened. b) Most of the "expiratory" and "inspiratory-expiratory" neurons were not inhibited by the volley. Cells of the former type often produced post-stimulus after-discharge; the burst discharge of units belonging to the latter type was shortened. The effects of expiratory stimuli upon neuronal activity, however, were less consistent than those elicited by inspiratory volleys. 3. During spontaneous irregularities of single inspirations (short interruptions), EI and I neurons exhibited comparable burst pattern changes. The changes in pattern of IE and E units were also comparable and differed distinctly from the behaviour of the EI and I cells. No major differences in behaviour were observed between neurons which were inhibited during lung inflation (alpha units) and those which were activated during inflation (beta cells). The findings are in accord with the effects observed during electrical bulbar stimulation, suggesting that EI and I units are inspiratory-activating cells, whereas IE and E neurons may have an inspiratory-inhibitory function. 4. The conclusion is drawn that the effects of bulbar stimulation on the respiratory movements are the result of manipulation on intrinsic rhythmogenesis.