John Orem

Respiratory neurons of the pneumotaxic center during sleep and wakefulness.

Extracellular recordings were made from respiratory related units (RRUs) in the pneumotaxic center (PNC) of unanesthetized cats across states of consciousness: wakefulness, NREM and REM sleep. Contrary to expectations from results in acute preparations, well-modulated respiratory activity was detected in chronic animals with vagi intact. Pontine RRU activity decreased during sleep in 75% of the cells studied. The greatest reduction occurred during REM sleep.

Behavioral control of breathing in the cat.

Respiration depends upon brainstem neuronal circuits that produce the respiratory rhythm and relay it, via the ventrolateral columns, to motor neurons in the spinal cord. This brainstem system produces respiration automatically, i.e. without conscious effort, and is responsive to chemical and mechanical stimuli that signal imbalances in respiratory homeostasis. In addition to this automatic/metabolic respiratory system, there is a voluntary/behavioral system that controls the respiratory muscles during speaking, breath holding, and other voluntary respiratory acts. It has been proposed that this behavioral system involves corticofugal fibers that bypass the automatic system, course in the dorsolateral columns, and end at the level of the respiratory motor neurons. According to this scheme, the integration of behavioral control with automatic/metabolic control occurs at the level of the motor neurons and not within the automatic system. This proposed scheme has not been investigated experimentally. In the present study, we trained cats to control their respiration and recorded the activity of cells within the automatic system in the medulla during this behavioral control. We trained the animals to terminate inspiration and prolong expiration when a tone sounded. Microelectrode recordings from 40 medullary respiratory neurons showed that most cells, inspiratory and expiratory, became inactive during the behavioral apneic response. The exceptions were some expiratory cells that were activated during the task. These results suggest that the integration of behavioral influences occurs within the automatic system.

Endogenous excitatory drive to the respiratory system in rapid eye movement sleep in cats

1 A putative endogenous excitatory drive to the respiratory system in rapid eye movement (REM) sleep may explain many characteristics of breathing in that state, e.g. its irregularity and variable ventilatory responses to chemical stimuli. This drive is hypothetical, and determinations of its existence and character are complicated by control of the respiratory system by the oscillator and its feedback mechanisms. In the present study, endogenous drive was studied during apnoea caused by mechanical hyperventilation. We reasoned that if there was a REM‐dependent drive to the respiratory system, then respiratory activity should emerge out of the background apnoea as a manifestation of the drive. 2 Diaphragmatic muscle or medullary respiratory neuronal activity was studied in five intact, unanaesthetized adult cats who were either mechanically hyperventilated or breathed spontaneously in more than 100 REM sleep periods. 3 Diaphragmatic activity emerged out of a background apnoea caused by mechanical hyperventilation an average of 34 s after the onset of REM sleep. Emergent activity occurred in 60 % of 10 s epochs in REM sleep and the amount of activity per unit time averaged approximately 40 % of eupnoeic activity. The activity occurred in episodes and was poorly related to pontogeniculo‐occipital waves. At low CO2 levels, this activity was non‐rhythmic. At higher CO2 levels (less than 0.5 % below eupnoeic end‐tidal percentage CO2 levels in non‐REM (NREM) sleep), activity became rhythmic. 4 Medullary respiratory neurons were recorded in one of the five animals. Nineteen of twenty‐seven medullary respiratory neurons were excited in REM sleep during apnoea. Excited neurons included inspiratory, expiratory and phase‐spanning neurons. Excitation began about 43 s after the onset of REM sleep. Activity increased from an average of 6 impulses s−1 in NREM sleep to 15.5 impulses s−1 in REM sleep. Neuronal activity was non‐rhythmic at low CO2 levels and became rhythmic when levels were less than 0.5 % below eupnoeic end‐tidal levels in NREM sleep. The level of CO2 at which rhythmic neuronal activity developed corresponded to eupnoeic end‐tidal CO2 levels in REM sleep. 5 These results demonstrate an endogenous excitatory drive to the respiratory system in REM sleep and account for rapid and irregular breathing and the lower set‐point to CO2 in that state.

Erroneous classification of neuronal activity by the respiratory modulation index

Failure to record respiratory activity in the mesencephalon of the chronic cat led us to analyze the formula (the respiratory modulation index, RMI) used by Hugelin and his colleagues to discriminate respiratory neurons. Using computer simulations, we compared RMI with the analysis of variance (F) and the non-parametric Friedmans test (chi 2). Samples were drawn repeatedly from simulated distributions of neuronal activity and were allocated to successive bins representing the respiratory cycle. Allocations of bins were made randomly so that only a chance relationship existed between the simulated activity and respiratory cycle. These simulations revealed that the RMI erroneously yields values indicative of a respiratory relationship and does so as a function of sample size and the variability and shape of the distribution of non-respiratory activity. Although some of the simulated conditions violated assumptions of the F test and, to a lesser degree, the chi 2, these statistics erred at rates close to the chosen 5% level. When respiratory activity was stimulated, chi 2 and F were more sensitive than RMI in detecting the relationship. We conclude that the high incidence of respiratory activity reported by the Hugelin group is based upon a faulty statistic and is highly questionable.

The activity of retrofacial expiratory cells during behavioral respiratory responses and active expiration

The activity of retrofacial expiratory cells was recorded from cats trained to inhibit inspiration in response to a tone. Because retrofacial expiratory cells inhibit inspiratory cells, we thought they might mediate this response. We found, however, that these cells were inactive during the response and thus could not be the mediators thereof. Moreover, retrofacial expiratory cells were inactive also during sneezing and thus were not acting as expiratory upper motoneurons during these active expirations. We propose that they act to promote and synchronize inspiratory activity.

Experimental control of the diaphragm and laryngeal abductor muscles by brain stem arousal systems

The effect of mesencephalic central tegmental field (FTC) stimulation in barbiturate anesthetized cats on the activity of the diaphragm and the laryngeal abductors was studied. With brief stimulus trains, two effects were observed: (1) a short latency, stimulus-specific activation of these muscles and (2) phase-switching of the respiratory cycle. The characteristics of short latency driving were as follows: (1) the latency and threshold for activation of the laryngeal abductors was less than for activation of the diaphragm; (2) driving continued for the duration of the stimulus only; and (3) during expiration, the threshold for short latency driving was lowest in the early part of the phase and progressively increased throughout it. Phase-switching had these characteristics: (1) expiration-to-inspiration (E-to-I) phase-switching was obtained in all cases and, in 17% of the cases, stimulation of the same FTC site also produced I-to-E phase-switching; (2) phase-switching was a function of stimulus intensity and the time of stimulation; and (3) during expiration, phase-switching showed a threshold profile opposite to that for short latency driving. These effects could be obtained after bilateral dorsolateral pontine lesions, bilateral vagotomy, and transection at the C8 level. It was concluded that the FTC could influence breathing by two systems. One is relatively direct to respiratory motoneurons and the other engages the oscillator.

The activity of late inspiratory cells during the behavioral inhibition of inspiration

Cats can be trained to stop inspiration behaviorally--a response mediated by inactivation of brainstem inspiratory neurons. Neurons that discharge late in the inspiratory phase of the respiratory cycle may terminate that phase; therefore, such cells may be activated behaviorally to inhibit inspiration. To test this hypothesis, we studied the activity of late-onset inspiratory neurons located in the dorsal and ventral medullary respiratory groups in cats trained to stop inspiration behaviorally. Twenty-eight of 112 respiratory neurons were classified as late-onset inspiratory neurons. They had an average eta 2 value of 0.58 (+/- 0.13, S.D.) and an average maximal discharge rate of 42 Hz (+/- 18, S.D.). For most cells, the period of activity varied under different conditions: some extended their activity into early inspiration; others, into early expiration. Eighteen of these late-onset inspiratory neurons were completely silent when the animals stopped inspiration behaviorally, and 10 discharged only a few action potentials. The latter response was weak and inconsistent, and we conclude that late inspiratory cells do not inhibit other brainstem inspiratory cells when animals stop inspiration behaviorally.

Respiratory pattern generator model using Ca++-induced Ca++ release in neurons shows both pacemaker and reciprocal network properties

Abstract.There are two contradictory explanations for central respiratory rhythmogenesis. One suggests that respiratory rhythm emerges from interaction between inspiratory and expiratory neural semicenters that inhibit each other and thereby provide reciprocal rhythmic activity (Brown 1914). The other uses bursting pacemaker activity of individual neurons to produce the rhythm (Feldman and Cleland 1982). Hybrid models have been developed to reconcile these two seemingly conflicting mechanisms (Smith et al. 2000; Rybak et al. 2001). Here we report computer simulations that demonstrate a unified mechanism of the two types of oscillator. In the model, we use the interaction of Ca++-dependent K+ channels (Mifflin et al. 1985) with Ca++-induced Ca++ release from intracellular stores (McPherson and Campbell 1993), which was recently revealed in neurons (Hernandez-Cruz et al. 1997; Mitra and Slaughter 2002a,b; Scornik et al. 2001). Our computations demonstrate that uncoupled neurons with these intracellular mechanisms show conditional pacemaker properties (Butera et al. 1999) when exposed to steady excitatory inputs. Adding weak inhibitory synapses (based on increased K+ conductivity) between two model neural pools surprisingly synchronizes the activity of both neural pools. As inhibitory synaptic connections between the two pools increase from zero to higher values, the model produces first dissociated pacemaker activity of individual neurons, then periodic synchronous bursts of all neurons (inspiratory and expiratory), and finally reciprocal rhythmic activity of the neural pools.

Intercostal muscle activity during sleep in the cat: an augmentation of expiratory activity.

Intercostal-muscle activity (I-EMG) was studied in sleep and wakefulness in ten adult cats. The animals were implanted with electrodes for recording the electroencephalogram (EEG), electrooculogram (EOG), and pontogeniculooccipital (PGO) waves as well as intercostal- and neck-muscle activity. A small-animal pneumotachograph connected to a tracheal cannula was used to monitor respiration. The data were obtained in 87 recording sessions, each lasting 5-6 h. Total EEG power (fast Fourier analysis) and integrated I-EMG activity were computed over consecutive 5.12-sec periods of wakefulness and nonrapid eye movement (NREM) sleep. Cycle-triggered histograms based on 50 breaths were computed also in wakefulness and NREM sleep. I-EMG was classified as expiratory or inspiratory according to when the activity was maximal during the respiratory cycle. Expiratory I-EMG augmented progressively during NREM sleep with the maximal activity occurring just before rapid eye movement (REM) sleep. The mean correlation between total EEG power and expiratory I-EMG for all expiratory muscles was 0.441 (+/- 0.16 SD; n=24). In contrast to the consistent augmentation of expiratory I-EMG, inspiratory I-EMG changes from wakefulness to deep NREM sleep varied within and between muscle groups: in 21% of the cases, activity decreased from wakefulness to NREM sleep; in 16% of the cases, activity increased; in 56% of the cases, activity did not change; and in 7% of the cases, activity increased in only the expiratory phase. We conclude the consistent augmentation of expiratory intercostal muscle indicates an active, NREM sleep-specific respiratory process that may have important implications for lung mechanics in that state.

Inspiratory neurons that are activated when inspiration is inhibited behaviorally

Respiration can be automatic or controlled behaviorally. Behavioral control in the cat occurs, at least in part, through control of the brainstem respiratory neurons that constitute the automatic system. Thus, when inspiration is inhibited behaviorally, inspiratory neurons in the medulla are inactivated. Reported herein are data on inspiratory cells, located in both the dorsal and ventral respiratory groups, that were activated when other inspiratory cells there were inhibited behaviorally. During spontaneous breathing, their activity showed much variability unattributable to the respiratory cycle--indicating that they receive a considerable non-respiratory input. These cells might act as the interface through which behavioral inhibition of inspiration occurs.