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Respiratory neuron activity in the urethane anaethetized ground squirrel : responses to changes in respiratory drive Franks, Sarah Kate


These experiments examined the effects of arousal state and respiratory drive on the activity of central respiratory neurons in urethane anaesthetized ground squirrels. EEG recordings were used to score arousal state. Three arousal states were identified, state I (wake-like), state II (drowsy) and state III (slow wave sleeplike). Extracellular recordings were made from 21 neurons with respiratory related discharge in the ventrolateral medulla. Six different cell types were identified, these included inspiratory continuous cells, inspiratory augementing tonic cells, inspiratory augmenting cells, late inspiratory cells, expiratory inspiratory phase spanning cells and expiratory decrementing cells. Increases in neuron activity (specifically peak neuron activity) were observed during state I compared to state III. This increased activity was observed in all cell types during state I. These results are analogous to those observed in unanaesthetized animals (Orem, 1977) in wake compared to slow wave sleep, suggesting that urethane does not interfere with the changes in the respiratory related activity of these cells associated with changes in cortical activation state. Increases in peak neuron activity (spikes per second) were observed in respiratory related neurons in the present study upon exposure to hypercapnia. These results are also consistent with those of other studies (Batsel, 1966, Nesland et al., 1966, St. John, 1981, Foldgering and Smolders, 1979, St. John, 1977, St. John and Bianchi, 1985) examining the effects of hypercapnia on respiratory related activity of neurons in the medulla of unanaesthetized animals. This suggests that urethane also has little effect on the response of respiratory neurons to hypercapnia associated with the increased respiratory drive under hypercapnic conditions, in the golden-mantled ground squirrel. The present study demonstrated an increase in peak neuron activity in response to hypoxia. However, there was a large range of variation between cells with some exhibiting increases whilst others exhibited decreases in activity. Similarly, previous studies in unanaesthetized animals have also produced inconsistent results in response to hypoxic stimuli despite the fact hypoxia too increased respiratory drive in these animals. The eta squared (η²) value is a statistic that quantifies the strength and consistency of the discharge of respiratory related cells compared to the oscillations of the respiratory cycle. In the present study the η² varied from one cell to the next regardless of the cell type examined. Arousal state had variable influences on the η² value of each cell, some cells exhibited increases in this statistic in a particular state whereas others decreased their η² value in that same state. These results suggest that each of the cells recorded from had levels of afferent input that varied from one state to the next. Upon exposure to hypercapnia or hypoxia there was a general trend towards a decrease in the η² value of the respiratory neurons. These results were contrary to predictions of a relatively immutable η² value. This suggests that η² is an endogenous property of each cell and that any variation in input, including respiratory tonic input, introduces variation in the cells output and alters the coupling of the discharge to the respiratory cycle.

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