| Summary: | 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. === Science, Faculty of === Zoology, Department of === Graduate
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