The part of the brain that controls the respiratory system has a remarkable memory and capacity to adapt to various problems that occur during breathing. Whenever there is a lack of oxygen in the blood, the portion of the brain that controls breathing gets activated and stimulates the muscles involved in respiration to act more, so that more air is inhaled. This kind of adjustment by the brain is called respiratory plasticity or long-term facilitation. In the present study, researchers wanted to evaluate the role of the vagus nerve in sending signals to the brain regarding obstruction to airflow, thereby stimulating the muscles involved in breathing in. They also wanted to understand the receptors in the brain involved in this signal transmission.
According to the authors, “It is well known fact that reduced oxygen concentration in blood (hypoxia) induces respiratory plasticity.” But until now, the role of the vagus nerve in producing similar effects was not clearly understood. Researchers in the present study, which involved adult rats, stimulated the vagus nerve, produced obstruction to the airways and measured the activity of the diaphragm and genioglossus, the muscles that are involved in breathing. They also experimented on the part of the brain that controls breathing, the hippocampus, to know the chemicals involved in providing such neural feedback. The results of this study may provide useful clues for devising treatment for patients who suffer from obstruction to air flow while sleeping, a condition which is also called as obstructive sleep apnea.
* This experiment was conducted on 83 anesthetized adult male rats. A tube was inserted in the upper airway of all these rats, which could be occluded whenever desired. In addition, the vagus nerve was dissected in all the rats, so that they could be stimulated. Electrodes were placed in the diaphragm and genioglossus muscles, to assess their activity. Reverse dialysis probes were placed in the hippocampus of the brain, so that various chemicals could be introduced through them.
* Some rats were exposed to obstructive apnea by occluding the tubes inserted in their upper airways. In some rats, the vagus nerve was stimulated by cooling.
* In both the groups, the activity of muscles of respiration was measured by probes placed within them.
* In both the groups, serotonin and adrenalin blocking agents were introduced into the hippocampus, to know which chemical is involved in this signaling.
* Stimulation of the vagus nerve resulted in enhanced activity of the respiratory muscles. In the genioglossus muscle, it was 139 percent above the baseline; in the case of the diaphragm, it was 27 percent above the baseline.
* Recurrent airway occlusions triggered a serious and sustained increase in inspiratory muscle activity that peaked at 60.8 percent above baseline levels by 60 minutes.
Vagus nerve mediated respiratory plasticity is primarily because of increased action of the genioglossus muscle. But it does not have any lasting effect on the diaphragm. Diaphragm is the main muscle involved in inspiration. The biological significance of the action of genioglossus muscle is not clearly understood. Hence, it is obvious that hypoxia mediated plasticity is more important than the plasticity mediated by vagus nerve.
This study highlights how the body adjusts to various adversities it faces during the course of life. Researchers have found out a new pathway through which the brain senses an obstruction to airflow and then increases the action of inspiratory muscles. They could even identify the chemical involved in such involuntary feedback mechanisms. In obstructive sleep apnea, there is a decreased tone of genioglossus, resulting in narrowing of airway. This condition sometimes proves fatal, as the patient may suffer from severe asphyxia. The results of this study may help in the discovery of new drugs for obstructive sleep apnea, which would act at some point of the pathway involving the vagus nerve, hippocampus and genioglossus muscle.
For More Information:
Identification of a Novel Form of Noradrenergic-Dependent Respiratory Motor Plasticity Triggered by Vagal Feedback
The Journal of Neuroscience, December 2010
By Tadjalli Arash; Peever John; University of Toronto, Ontario, Canada
*FYI Living Lab Reports Are Summaries of the Original Research.