The primary hypothesis of this study was that the cough motor pattern is produced, at least in part, by the medullary respiratory neuronal network in response to inputs from "cough" and pulmonary stretch receptor relay neurons in the nucleus tractus solitarii. Computer simulations of a distributed network model with proposed connections from the nucleus tractus solitarii to ventrolateral medullary respiratory neurons produced coughlike inspiratory and expiratory motor patterns. Predicted responses of various "types" of neurons (I-DRIVER, I-AUG, I-DEC, E-AUG, and E-DEC) derived from the simulations were tested in vivo. Parallel and sequential responses of functionally characterized respiratory-modulated neurons were monitored during fictive cough in decerebrate, paralyzed, ventilated cats. Coughlike patterns in phrenic and lumbar nerves were elicited by mechanical stimulation of the intrathoracic trachea. Altered discharge patterns were measured in most types of respiratory neurons during fictive cough. The results supported many of the specific predictions of our cough generation model and suggested several revisions. The two main conclusions were as follows: 1) The Bötzinger/rostral ventral respiratory group neurons implicated in the generation of the eupneic pattern of breathing also participate in the configuration of the cough motor pattern. 2) This altered activity of Bötzinger/rostral ventral respiratory group neurons is transmitted to phrenic, intercostal, and abdominal motoneurons via the same bulbospinal neurons that provide descending drive during eupnea.

A large body of data suggests that the pontine respiratory group (PRG) is involved in respiratory phase-switching and the reconfiguration of the brain stem respiratory network. However, connectivity between the PRG and ventral respiratory column (VRC) in computational models has been largely ad hoc. We developed a network model with PRG-VRC connectivity inferred from coordinated in vivo experiments. Neurons were modeled in the "integrate-and-fire" style; some neurons had pacemaker properties derived from the model of Breen et al. We recapitulated earlier modeling results, including reproduction of activity profiles of different respiratory neurons and motor outputs, and their changes under different conditions (vagotomy, pontine lesions, etc.). The model also reproduced characteristic changes in neuronal and motor patterns observed in vivo during fictive cough and during hypoxia in non-rapid eye movement sleep. Our simulations suggested possible mechanisms for respiratory pattern reorganization during these behaviors. The model predicted that network- and pacemaker-generated rhythms could be co-expressed during the transition from gasping to eupnea, producing a combined "burst-ramp" pattern of phrenic discharges. To test this prediction, phrenic activity and multiple single neuron spike trains were monitored in vagotomized, decerebrate, immobilized, thoracotomized, and artificially ventilated cats during hypoxia and recovery. In most experiments, phrenic discharge patterns during recovery from hypoxia were similar to those predicted by the model. We conclude that under certain conditions, e.g., during recovery from severe brain hypoxia, components of a distributed network activity present during eupnea can be co-expressed with gasp patterns generated by a distinct, functionally "simplified" mechanism.

This study tested predictions from a network model of ventrolateral medullary respiratory neurone interactions for the generation of the cough motor pattern observed in inspiratory and expiratory pump muscles. Data were from 34 mid‐collicularly decerebrated, paralysed, artificially ventilated cats. Cough‐like patterns (fictive cough) in efferent phrenic and lumbar nerve activities were elicited by mechanical stimulation of the intrathoracic trachea. Neurones in the ventral respiratory group, including the Bötzinger and pre‐Bötzinger complexes, were monitored simultaneously with microelectrode arrays. Spike trains were analysed for evidence of functional connectivity and responses during fictive cough with cycle‐triggered histograms, autocorrelograms, cross‐correlograms, and spike‐triggered averages of phrenic and recurrent laryngeal nerve activities. Significant cross‐correlogram features were detected in 151 of 1988 pairs of respiratory modulated neurones. There were 59 central peaks, 5 central troughs, 11 offset peaks and 2 offset troughs among inspiratory neurone pairs. Among expiratory neurones there were 23 central peaks, 8 offset peaks and 4 offset troughs. Correlations between inspiratory and expiratory neurones included 20 central peaks, 10 central troughs and 9 offset troughs. Spike‐triggered averages of phrenic motoneurone activity had 51 offset peaks and 5 offset troughs. The concurrent responses and multiple short time scale correlations support parallel and serial network interactions proposed in our model for the generation of the cough motor pattern in the respiratory pump muscles. Inferred associations included the following. (a) Excitation of augmenting inspiratory (I‐Aug) neurones and phrenic motoneurones by I‐Aug neurones. (b) Inhibition of augmenting expiratory (E‐Aug) neurones by decrementing inspiratory (I‐Dec) neurones. (c) Inhibition of I‐Aug, I‐Dec and E‐Aug neurones by E‐Dec neurones. (d) Inhibition of I‐Aug and I‐Dec neurones and phrenic motoneurones by E‐Aug neurones. The data also confirm previous results and support hypotheses in current network models for the generation of the eupnoeic pattern.

Current models propose that a neuronal network in the ventrolateral medulla generates the basic respiratory rhythm and that this ventrolateral respiratory column (VRC) is profoundly influenced by the neurons of the pontine respiratory group (PRG). However, functional connectivity among PRG and VRC neurons is poorly understood. This study addressed four model-based hypotheses: 1) the respiratory modulation of PRG neuron populations reflects paucisynaptic actions of multiple VRC populations; 2) functional connections among PRG neurons shape and coordinate their respiratory-modulated activities; 3) the PRG acts on multiple VRC populations, contributing to phase-switching; and 4) neurons with no respiratory modulation located in close proximity to the VRC and PRG have widely distributed actions on respiratory-modulated cells. Two arrays of microelectrodes with individual depth adjustment were used to record sets of spike trains from a total of 145 PRG and 282 VRC neurons in 10 decerebrate, vagotomized, neuromuscularly blocked, ventilated cats. Data were evaluated for respiratory modulation with respect to efferent phrenic motoneuron activity and short-timescale correlations indicative of paucisynaptic functional connectivity using cross-correlation analysis and the "gravity" method. Correlogram features were found for 109 (3%) of the 3,218 pairs composed of a PRG and a VRC neuron, 126 (12%) of the 1,043 PRG-PRG pairs, and 319 (7%) of the 4,340 VRC-VRC neuron pairs evaluated. Correlation linkage maps generated for the data support our four motivating hypotheses and suggest network mechanisms for proposed modulatory functions of the PRG.

Perturbations of arterial blood pressure change medullary raphe neurone activity and the respiratory motor pattern. This study sought evidence for actions of baroresponsive raphe neurones on the medullary respiratory network. Blood pressure was perturbed by intravenous injection of an α1‐adrenergic receptor agonist, unilateral pressure changes in the carotid sinus, or occlusion of the descending aorta in thirty‐six Dial‐urethane‐anaesthetized, vagotomized, paralysed, artificially ventilated cats. Neurones were monitored with microelectrode arrays in two or three of the following domains: nucleus raphe obscurus‐nucleus raphe pallidus, nucleus raphe magnus, and rostral and caudal ventrolateral medulla. Data were analysed with cycle‐triggered histograms, peristimulus time and cumulative sum histograms, cross‐correlograms and spike‐triggered averages of efferent phrenic nerve activity. Prolongation of the expiratory phase and decreased peak integrated phrenic amplitude were most frequently observed. Of 707 neurones studied, 310 had altered firing rates during stimulation; changes in opposite directions were monitored simultaneously in fifty‐six of eighty‐seven data sets with at least two baroresponsive neurones. Short time scale correlations were detected between neurones in 347 of 3388 pairs. Seventeen pairs of baroresponsive raphe neurones exhibited significant offset correlogram features indicative of paucisynaptic interactions. In correlated raphe‐ventrolateral medullary neurone pairs with at least one baroresponsive neurone, six of seven ventrolateral medullary decrementing expiratory (E‐Decr) neurones increased their firing rate during baroreceptor stimulation. Thirteen of fifteen ventrolateral medullary inspiratory neurones correlated with raphe cells decreased their firing rate during baroreceptor stimulation. The results support the hypothesis that raphe neuronal assemblies transform and transmit information from baroreceptors to neurones in the ventral respiratory group. The inferred actions both limit and promote responses to sensory perturbations and match predictions from simulations of the respiratory network.

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1. This study addressed the hypothesis that there is a parallel processing of input from carotid chemoreceptors to brainstem neurones involved in inspiratory phase timing and control of inspiratory motor output amplitude.

The effects of strontium ions, Sr2+, on Ca2+‐dependent feedback mechanisms during excitation‐contraction coupling were examined in voltage‐clamped rat ventricular myocytes in which intracellular [Ca2+] and [Sr2+] were monitored with the fluorescent indicator, indo‐1. Voltage clamp depolarizations and caffeine applications during superfusion in Ca2+‐free, Sr2+‐containing solutions were employed to exchange intracellular Ca2+ with Sr2+. Myocytes were loaded with Sr2+ by applying voltage clamp depolarizations during superfusion in Na+‐free, Sr2+‐containing solutions. Caffeine applications produced large fluorescence transients in Sr2+‐loaded cells. Thus, Sr2+ could be sequestered and released from the sarcoplasmic reticulum. Ca2+ influx, but not Sr2+ influx, via sarcolemmal Ca2+ channels evoked ryanodine‐sensitive fluorescence transients in Sr2+‐loaded cells. These results demonstrated that Ca2+ influx‐induced Sr2+ release (CISR) from the sarcoplasmic reticulum occurred in these experiments, even though Sr2+ influx‐induced Sr2+ release was not observed. The amplitude of the Ca2+ influx‐induced fluorescence transient was 17 ± 1% of the caffeine‐induced transient (n= 5 cells), an indication that fractional utilization of Sr2+ sequestered in the sarcoplasmic reticulum during CISR was low. With increased Sr2+ loading, the amplitude of Ca2+ influx‐ and caffeine‐induced fluorescence transients increased, but fractional utilization of sarcoplasmic reticulum divalent cation stores was independent of the degree of Sr2+ loading. These data suggest that Ca2+ influx directly activated the release of divalent cations from the sarcoplasmic reticulum, but mechanisms promoting positive feedback of Sr2+ release were minimal during CISR. By comparison, in Ca2+‐loaded myocytes, Ca2+ influx‐induced Ca2+ release (CICR) utilized a greater fraction of caffeine‐releasable stores than CISR. Fractional utilization of Ca2+ stores during CICR increased with the degree of Ca2+ loading. Taken together, these results suggest that Ca2+‐dependent feedback mechanisms play a major role in determining the extent of sarcoplasmic reticulum Ca2+ release during cardiac excitation‐contraction coupling under a wide range of conditions.