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Human respiratory muscles: sensations, reflexes and fatiguability
S C Gandevia1, G M Allen, J E Butler
1Prince of Wales Medical Research Institute, Sydney, New South Wales, Australia. S.Gandevia@unsw.edu.au
Clinical and Experimental Pharmacology & Physiology
|October 24, 1998
Summary
Respiratory muscles possess specialized sensory and motor functions. Increased carbon dioxide (CO2) causes discomfort, and inspiratory muscles may not fatigue as previously thought.
Area of Science:
- Physiology
- Respiratory System
- Neuromuscular Function
Background:
- The ventilatory 'pump' muscles are crucial for breathing and may have unique sensory and motor adaptations.
- Traditionally, blood gas changes were not considered direct sensory influences, but new evidence suggests otherwise.
Purpose of the Study:
- To review unexpected properties of the human respiratory muscle system.
- To explore sensory feedback and fatigue mechanisms in respiratory muscles.
Main Methods:
- Review of existing studies on respiratory muscle function in human subjects.
- Analysis of data from paralyzed subjects regarding CO2 sensitivity.
- Examination of inspiratory muscle reflexes using airway occlusion.
- Assessment of diaphragm fatigue using phrenic nerve stimulation.
Main Results:
- Elevated arterial CO2 levels induce significant sensations of respiratory discomfort in paralyzed individuals.
- Inspiratory muscles exhibit initial inhibition during brief inspiratory occlusions, independent of pulmonary/upper airway receptors.
- Diaphragm fatigue during resistive loading may not occur before hypoventilation or voluntary cessation of loading.
Conclusions:
- The respiratory muscle system exhibits specialized sensory and motor properties.
- Carbon dioxide retention is a potent stimulus for respiratory discomfort.
- Current understanding of inspiratory muscle fatigue may need revision, with CO2 retention being a more immediate limiting factor than muscle exhaustion.