When we breathe, what's really going on?

You may think that breathing is a voluntary action, however, your brain disagrees!


Breathing is actually a sensory reaction, which is initiated and controlled by the inspiratory and expiratory centres within the medulla oblongata. (Located in the lower region of the brainstem)


The diaphragm is a key component of the nervous system. It is supplied with spinal nerves from segments c3, c4 and c5. The intercostal musles are supplied by the corresponding thoracic t1 to t12 spinal nerves.
The nerves of the diaphragm lead from the spine, and the brain stem, and run back to collections of neurons in the medulla - whose function is to control unconscious processes such as rate and depth of breathing.


In turn, these are adjoined to a second respiratory centre in the pons (which asists in the rate of movement and breathing). The interaction between these two centres enables automatic breathing, and neurons are both stimulatory and inhibitory.


The areas of the pons are involved in automatic breathing,and if conscious control is excerted, these centres are overpowered by the cerebella cortex. There is a direct link to the respiratory muscles through the relevant motor neurons, coming directly from the motor cortex of the cerebella cortex.

Inbuilt in the pons is a standard rate of breathing, which is variable, allowing connections between the pons and the medulla. The sensors that detect the oxygen and carbon dioxide concentration of blood are located in the aortic arch.


The autonomic control of breathing involves a reflex response...


...There are stretch receptors contained in the walls of the bronchi and bronchioloes which - during inspiration - are stimulated when the lungs inflate.
The receptors are connected to sensory nerves, and they send action potentials along the vagus nerve.

This inhibits the inspiratory centre in the medulla, and makes inspiration stop, and expiration begin. When the lungs deflate, the stretch receptors stop sending action potentials to the inspiratory centre.
At this stage, inhibition stops, and inspiration commences once again.

Also found within the medula are chemoreceptors. These are sensitive to chemical changes in the blood - They are also found in aortic bodies in the aorta, and in the carotid bodies in the carotid arteries carrying blood to the head.
If the chemoreceptors detect a decrease in the PH of the blood, they send a signal to the medulla to send more frequent nerve impulses to the intercostal muscles and diaphragm . This then increases the rate and depth of breathing.

Bibliography:

http://www.nytimes.com/

cgpartnership handout, week 24

www.medical_dictionary.thefreedictionary.com