Stimulation of Breathing
There are two pathways of motor neuron stimulation of the respiratory muscles. The first is the control of voluntary breathing by the cerebral cortex. The second is involuntary breathing controlled by the medulla oblongata. There are chemoreceptors in the aorta, the carotid arteries, and in the medulla oblongata of the brainstem that are sensitive to pH. As carbon dioxide levels increase there is a buildup of carbonic acid, which releases hydrogen ions and lowers pH. Thus, the chemoreceptors do not respond to changes in oxygen levels (which actually change much more slowly), but to pH, which is an indirect measure of carbon dioxide levels. In other words, CO2 is the driving force for breathing. The receptors in the aorta and the carotid arteries stimulate an immediate increase in breathing rate and the receptors in the medulla stimulate a sustained increase in breathing until blood pH returns to normal. This response can be experienced by running a 100 meter dash. During this exertion (or any other sustained exercise) your muscle cells must metabolize ATP at a much faster rate than usual, and thus will produce much higher quantities of CO2. The blood pH drops as CO2 levels increase, and you will involuntarily increase breathing rate very soon after beginning the sprint. You will continue to breathe heavily after the race, thus expelling more carbon dioxide, until pH has returned to normal.