Control of Respiration
Breathing is controlled automatically and modified by neural and chemical reflexes designed to maintain oxygen delivery to the tissues and remove waste carbon dioxide. Within the brainstem, the pons and medulla, there are groups of neurons known as the pneumotaxic center and the respiratory center. Automatic and rhythmic discharge of neurons in the medullary respiratory center produce inspiration. The basic rhythm is determined by cells within the inspiratory center that spontaneously generate nerve impulses periodically to initiate inspiration. Impulses travel via the phrenic nerve to the diaphragm.
Other neurons in the brainstem, known as expriratory neurons, are active when ventilation is increased over resting rates. When breathing is rapid, signals from the inspiratory area spill over into the expiratory area so that expiration is stimulated. Inspiratory and expiratory neurons also have inhibitory connections to one another. In the pons, the pneumotaxic center acts as an integrator of inspiratory and expiratory inputs to coordinate and make the process of breathing smooth.
Stretch receptors in the lungs also act to modify respiration. The Hering-Breuer reflex is stimulated when the lungs are stretched. Stretch receptors in the bronchi and bronchioles transmit inhibitory signals to the inspiratory center. This is a protective measure to prevent the lungs from being overinflated. Stretch receptors also send information about lung deflation to the brainstem. Respiration is affected by emotional cues and input from the position sensors (proprioceptors) in joints and muscle.
Chemoreceptors in the carotid and aortic bodies provide additional information to the integrating centers in the brainstem regarding peripheral levels of oxygen and carbon dioxide. Such regulation maintains alveolar PCO 2 at normal levels, holds H+ in check and elevates PO 2 as necessary. Alveolar PO 2 is higher than needed to saturate hemoglobin. Therefore, changes in alveolar PCO 2 have a greater effect on respiration. The carotid bodies are located at the branch point of the carotid arteries and the aortic bodies are found within the aortic arch. Carotid chemosensors sense a decrease in arterial PO 2 , especially below 50 mmHg, and increased PCO 2 and H+. The aortic bodies sense increased PCO 2 and H+. Chemoreceptors in the brain monitor the concentration of H+ in cerebrospinal fluid which closely parallels the PCO 2 in blood.