The diaphragm is supplied with spinal nerves from segments c3 c4 and c5 the intercostals muscles are supplied by the thoracic spinal nerves t1 to t12 the nerves of the diaphragm lead from the brain stem and down the spine they run back to neurones in the medulla oblongata which are known as the respiratory centres. These in turn are connected to a second respiratory centre in the pons which enables automatic breathing, there are two recognised centres in the pons the pneumotaxic and the apneustic centres, these two areas are also involved in automatic breathing, if we consciously hold our breath then these centres would override it by the cerebella cortex, the cerebella cortex has a direct line to the respiratory muscles through the relevant neurones.
Built into the pons is a standard rate of breathing this is variable, there are connections between the pons and medulla, in the aortic arch there are sensors which detect the oxygen and carbon dioxide concentration of blood.
references
cgp respiratory system handout
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