3.6 The respiratory center: 

The respiratory center is composed of several groups of neurons located bilaterally in the medulla oblongata and pons of the brain stem. It is divided into three major collections of neurons: a dorsal respiratory group, located in the dorsal portion of the medulla, which mainly causes inspiration; a ventral respiratory group, located in the ventro lateral part of the medulla, which mainly causes expiration; and the pneumotaxic center, located dorsally in the superior portion of the pons, which mainly controls rate and depth of breathing. The dorsal respiratory group of neurons plays the most fundamental role in the control of respiration.

The respiratory centre are affected by

• Oxygen (O2) lack: Lack of oxygen stimulates the respiratory centre reflex via the chemo- receptors, however oxygen lack directly depresses the respiratory centre. The overall action of oxygen lack on the respiratory centre is stimulation. Respiration is increased and the oxygen lack is minimized.
• Carbon dioxide (CO2) excess or lack: An excess of CO2 stimulates the respiratory centre directly. The respiratory centre is most sensitive to a rise in CO2. The respiratory centre is stimulated not only directly but also through the chemoreceptors. A fall in CO2 can depress the respiratory centre. This is sometimes seen following a general anaesthetic in which the patient has been over ventilated. The respiratory centre is depressed and breathing is slow to return.
• Changes in the acid base balance of the blood: If the acid content of the blood is increased, then the respiratory centre is stimulated.
• Reflex control: e.g., The chemoreceptors (the carotid bodies found at the bifurcation of the common carotid artery and the aortic bodies near the aortic arch) are a special group of cells in the neck. Oxygen lack, carboondioxide excess and increase acidity of blood stimulate the chemoreceptors and cause impulses to travel up these nerves to the respiratory centre.