There are differing theories concerning how the central nervous system controls external respiration. Only one of those theories is presented here.
Respiratory control centers are located in the pons and the medulla. The medullary dorsal respiratory group (DRG), also called the 'inspiratory center', sends stimulatory signals to the diaphragm by way of the phrenic nerve (solid line labeled P). These signals last approximately 2 seconds and, during that time, increase in frequency until suddenly stopping. This is called a 'ramp potential.'
The strength of the ramp potential is increased if the acidity (H+) of the cerebrospinal fluid increases. This direct relationship is represented by the solid arrow pointing from H+ to DRG. Hydrogen ions can not cross the blood-brain barrier; these hydrogen ions result from the reaction between carbon dioxide (CO2) and water in the CSF as indicated by the solid arrow. The level of carbon dioxide is the same as in the blood.
The strength of the ramp potential is also increased by increased acidity (H+) of the systemic arterial blood. Chemoreceptors in the carotid and aortic bodies (CAB hexagon) send signals to the dorsal respiratory group (DRG) via the glossopharyngeal nerve here represented as a solid line labeled GP. Carbon dioxide (CO2) can also directly stimulate these receptors as indicated by the solid arrow.
These receptors are also sensitive to decreasing levels of oxygen (O2) but only when the level is very low. This inverse relationship is shown by the dashed arrow. The reverse is also true, that is, high oxygen levels can inhibit respiration.
This medullary group consists of two parts; one stimulates expiratory muscles and the other stimulates inspiratory muscles other than the diaphrage. If one part is active is inhibits the other as shown by the pair of dashed lines. Some theories hold that the ventral respiratory group (VRG) is activated by 'spill over' from the DRG when it's activity is increased. This is indicated by the solid line representing stimulatory nerve fibers.
These sensory receptors are located throughout the connective tissue in the lungs. When the alveolar pressure (Pa) decreases the stretch receptors (SR hexagon) are stimulated. This inverse relationship is represented by the dashed arrow.
Inhibitory signals originating in the stretch receptors travel in the vagus nerve to the DRG. This inverse relationship between stimulated stretch receptors inhibiting the 'inspiratory center' is represented by a dashed line. Stimulatory signals are simultaneously sent to the pontine pneumotaxic center (PC circle) represented as a solid line branching from the vagus nerve. When the pneumotaxic center is stimulated it sends inhibitory signals (dashed line) to the 'inspiratory center.' These neural pathways stop inspiration and this action is called the 'Hering-Breuer reflex."
Last Updated: 7/18/2005