There are chemoreceptors in the carotid and aortic bodies that affect respiration. For instance, an increase in P CO2 will result in an increase in the number of impulses traveling over C.N.s IX and X to the caudal nucleus solitarius. Neurons in nucleus solitarius project to the phrenic nucleus , which consists of a group of neurons in the ventral horn of the spinal cord from C3-C5.
Axons arising from the phrenic nucleus comprise the phrenic nerve that innervates the diaphragm.
Cells in nucleus solitarius also project to neurons in the spinal cord that innervate the intercostal muscles. Therefore, an increase in P CO2 will result in an increase in the depth and rate of breathing , while a decrease in P CO2 will have the opposite effect. However, it is extremely important for you to remember that the brain stem, especially the medulla, is an important region for the control of respiration and cardiovascular functions.