The pattern of motor stimuli during breathing can be divided into inspiratory and expiratory phases. Inspiration shows a sudden, ramped increase in motor discharge to the inspiratory muscles (including pharyngeal dilator muscles). Before the end of inspiration, there is a decline in motor discharge. Exhalation is usually silent, except at high minute ventilation rates.
The mechanism of generation of the ventilatory pattern is not completely understood, but involves the integration of neural signals by respiratory control centers in the medulla and pons. The nuclei known to be involved are divided into regions known as the following:
Levels of CO2 rise in the blood when the metabolic use of O2 is increased beyond the capacity of the lungs to expel CO2. CO2 is stored largely in the blood as bicarbonate (HCO3-) ions, by conversion first to carbonic acid (H2CO3), by the enzyme carbonic anhydrase, and then by disassociation of this acid to H+ and HCO3-. Build-up of CO2 therefore causes an equivalent build-up of the disassociated hydrogen ion, which, by definition, decreases the pH of the blood.
During moderate exercise, ventilation increases in proportion to metabolic production of carbon dioxide. During strenuous exercise, ventilation increases more than needed to compensate for carbon dioxide production. Increased glycolysis facilitates release of protons from ATP and metabolites lower pH and thus increase breathing.
Mechanical stimulation of the lungs can trigger certain reflexes as discovered in animal studies. In humans, these seem to be more important in neonates and ventilated patients, but of little relevance in health. The tone of respiratory muscle is believed to be modulated by muscle spindles via a reflex arc involving the spinal cord.
The upper airway receptors are responsible for reflex responses such as, sneezing, coughing, closure of glottis, and hiccups.
The spinal cord reflex responses include the activation of additional respiratory muscles as compensation, gasping response, hypoventilation, and an increase in breathing frequency and volume.
The nasopulmonary and nasothoracic reflexes regulate the mechanism of breathing through deepening the inhale. Triggered by the flow of the air, the pressure of the air in the nose, and the quality of the air, impulses from the nasal mucosa are transmitted by the trigeminal nerve to the breathing centres in the brainstem, and the generated response is transmitted to the bronchi, the intercostal muscles and the diaphragm.
Voluntary control of respiration
In addition to involuntary control of respiration by respiratory neuronal networks in the brainstem, respiration can be affected by higher brain conditions such as emotional state, via input from the limbic system, or temperature, via the hypothalamus, or free will. Voluntary or conscious control of respiration is provided via the cerebral cortex, although chemoreceptor reflex is capable of overriding it.
While breathing can obviously be controlled both consciously and unconsciously, all other basic functions provided by the brainstem can not be controlled voluntarily. Only conscious control of respiratory neuronal networks in the reticular formation can effect other basic functions regulated by the brainstem, because of the inter-meshed character of the reticular formation, e.g. the heart rate in yoga and meditation ("to take a deep breath").
^Coates EL, Li A, Nattie EE. Widespread sites of brain stem ventilatory chemoreceptors. J Appl Physiol. 75(1):5-14, 1984.
^Cordovez JM, Clausen C, Moore LC, Solomon, IC. A mathematical model of pH(i) regulation in central CO2 chemoreception. Adv Exp Med Biol. 605:306-11, 2008.