Nerve impulses are passed from the brain down the brain stem and along the spinal nerves to the diaphragm and intercostal muscles. This causes the contraction and relaxation of the diaphragm and intercostal muscles during breathing.
The area of the brain that these spinal nerves carry impulses from is called the 'Medulla oblongata' also known as the Respiratory centers. The Respiratory is connected to a second Respiratory center in the 'Pons'.
The two respiratory centers allow for automatic breathing.
The respiratory centers can be over ridden by a part of the brain called the 'cerebella cortex'. This allows for us to control our breathing rate and hold our breath, all thoe this control can only be exerted for a short time at which point the medulla oblongata and the pons would override the cerebella cortex and automatic breathing would begin again.
The pons control the standard rate of breathing but it is also connected to the medulla and sensors that detect oxygen and carbon dioxide in the blood to allow for the breathing rate to change.
sorry that ref should have been http.//www.classes.design.ucla.edu/Winter06/16/B/projects/ed/nervous.gif
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