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CHAPTER 8
Summary and Conclusion
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The vagus nerves are the primary sensory afferents arising from the tracheobronchial tree and lungs. Reflexes evoked from pulmonary vagal afferent activation are responsible for regulation of breathing pattern and protection of the respiratory tract. Activities of the UAW motor nerves play a role in regulating the respiratory airflow by changing geometry and compliance of the UAW. Present dissertation demonstrated that modulatory effects of pulmonary vagal afferents on the UAW motor outputs. Chapter 2 ~ 4 revealed capsaicin-induced activation of PCF receptors produced a significant inhibition on phasic activity and discharge onset of the hypoglossal branches innervating the tongue protrudors and retractors due to reduction of discharge rate and onset of EI and I hypoglossal motoneurons.
Furthermore, moderate and high doses of capsaicin (1.25 and 5 μg/kg) evoked obvious tonic activity of the medial hypoglossal branch by recruiting silent hypoglossal motoneurons. Chapter 5 provides evidences showing decrease in the protrusive and retractive tongue force development following PCF receptor activation.
It is established that activity of the hypoglossal nerve and tongue muscles is essential to maintain the UAW patency. Therefore, present results revealed that the UAW patency might be reduced during conditions that could evoke excitation of PCF receptors, such as airway inflammation, pulmonary edema and embolism.
Chapter 6 reported that rhythmic activity of the UAW motor nerves could be uncoupled from the phrenic discharge during elevation of PEEP, which has been demonstrated to alter activity of SARs and RARs. Moreover, this study provided detailed information regarding responses of the UAW motoneurons to increase of PEEP. Present results showed that EI and Pre-I but not I motoneurons persist rhythmic discharge during PEEP-induced phrenic quiescent. The linear regression analysis indicated differential influence of PEEP on discharge rate of EI motoneurons during pre-inspiration vs inspiration. Accordingly, I proposed the UAW motor nerves might
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receive different modulatory inputs during pre-inspiration and inspiration. This notion is also supported by results from Chapter 5 revealing capsaicin induced a more inhibition on discharge rate of EI motoneuron during expiration than inspiration.
Chapter 7 further evaluated neurotransmitter responsible for preceding onset (Pre-I activity) of the hypoglossal nerve. The results demonstrated that blockade of glycine receptor could attenuate preceding onset of the hypoglossal nerve during control, increase of PEEP and lung inflation, indicating changes of lung volume may regulate Pre-I UAW activity by altering glycine transmission.
