In the brain the medulla oblongata contains a respiratory center. This controls breathing. The respiratory center consists of an inspiratory center and an expiratory center. The axons from the nerve cells of these centres lead to the intercostals muscle through the intercostals nerves and the diaphragm via the phrenic nerves. These nerve fibres transmit impulses to the external intercostal muscles and internal intercostal muscles alternately. The walls of the alveoli have sense endings which are stimulated by changes in the tension of alveolar walls.
When the alveoli are stretched at the height of inspiration the receptors send stimuli to the expiratory center of the medulla through the vagus nerve which inhibits further inspiration. This sequence of events is called Herring - Breuer reflex.
In addition the medulla contains a pneumotaxic center which is connected to the breathing centre and helps to ensure rhythmic breathing. During inspiration, the inspiratory part of the respiratory center sends impulses to the pneumotaxic center which responds by sending impulses to the expiratory part of the respiratory center. The expiratory center is then activated and so the inspiratory center is inhibited reflexly, respiratory rhythm is controlled in this manner by these centers in the brain.
Inflammation of the lungs due to infection is called pneumonia. Pneumonia is caused by viruses or bacteria.Viral pneumonia is due to adenovirus, respiratory syncytial virus or a coxsackie virus. The most common bacterial pneumonia is Pneumococcal pneumonia. Pneumonia may also be caused by a mycoplasma (an organism that is intermediate between a bacterium and a virus)