The 4 respiratory centers of the brain, various respiratory organs and neural stimulation play a role in regulating breathing.
1. Respiratory Center: There are four respiratory centers in the brain. One pair is aneustic and pneumotaxic located in the pons and the other pair is the inspiratory center and expiratory center located in the medulla oblongata.
Anaesthetic center is stimulated in the presence of CO2 in the blood. This stimulus reaches the respiratory center (Dorsal Respiratory Group-DRG). Then the stimulus from the respiratory center comes simultaneously to the diaphragm, intercostal muscles and the pneumotactic center. As the stimulus reaches the diaphragm and intercostal muscles, the diaphragm and peritoneum contract and respiration begins. This increases the volume of the thoracic cavity, allows air to enter the lungs, and inflates the lungs.
Then at the same time nerve impulses come directly from the pneumotactic center and from the inflated lungs via the vagus nerve to the aneustic center. As a result, the anesthetic center becomes depressed and respiration stops. On the other hand, at the same time the nerve impulse from the pneumotactic center reaches the respiratory center (Ventral Respiratory Group-VRG) and respiration begins. As a result, air escapes from the lungs.
Nerve impulses from the pneumotactic center reach the inspiratory center and the expiratory center simultaneously, resulting in simultaneous cessation of inspiration and initiation of inspiration. During inspiration, the contractile stimulus of the lung does not reach the aneustic center. So the anesthetic center is re-stimulated and the stimulus is transmitted to the respiratory center and respiration starts. In this way breathing is regulated by repetition of events in phases.
2. Reflex action: Various organs related to the respiratory system control the breathing process through reflex action. For example-
(i) Tension receptor cells in the lung wall are stimulated when the lungs are filled with air during inhalation. This stimulation reaches the anesthetic center via the vagus nerve and dampens the activity of this center. As a result, breathing stops. Wall tension receptor cells are not stimulated as the lungs contract during exhalation. It activates the anesthetic center and causes respiration. The reflex action caused by the contraction and expansion of the lungs is called the Herring-Breyer reflex.
(ii) Stimulation of the mucous membrane of the nasal cavity through the olfactory nerve produces a reflex action and changes breathing.
(iii) When a foreign substance enters the trachea, the mucous membrane is stimulated, triggering the coughing reflex through the vagus nerve and altering breathing.
(iv) Stimulation of pharyngeal wall during swallowing of food causes Gag reflex action through glossopharyngeal nerve and stop respiration.
(v) Stimulations from the skin, muscles, viscera, joints etc. of the body regulate respiration by inducing reflex action.
3. Nerve Stimulation: Cerebral cortex, midbrain, hypothalamus etc. of the brain produces nerve stimulation which affects respiration. Areas of the cerebral cortex associated with speech, smell, chewing, and swallowing modulate breathing. For example, rapid breathing occurs after a long breath while speaking. Any painful stimulus to the body increases the rate of respiration. Again, more painful stimuli temporarily stop breathing.