Breathing can be a conscious or unconscious act. We can hold our breath or breathe deep even while the rest of our body is asleep. Without the respiratory system our body could not get the vital oxygen it needs to function. Lungs themselves are made of a soft, spongy material that contain no muscle at all. The lungs have to rely on external muscles and bones to allow for respiration.
The external muscles are numerous in nature but only share one name. The intercostal muscles are tiny muscles located in between each rib. There are 12 sets of ribs and these muscles are located between each rib and on each side. In between each rib there is two bands of muscles, one called the internal intercostal muscles and the other called the external intercostal muscles. In between each of these muscles is the nerve and blood supply. The "European Respiration Journal" reported a study in 1993 called "Respiratory Function of the Rib Cage Muscles". In this study both dogs and humans were observed breathing awake and anesthetized. In humans it was found that the most important muscles for breathing were the internal intercostal muscles closest to the sternum. During inhalation, these external intercostal muscles nearest the sternum contract and lift the rib cage up and out to make more room for the lungs. As we exhale the internal intercostal muscles contract and allow the weight of the ribs to move back down. This article went on to explain that in addition to the intercostal muscles, there are the scalene muscles, deep neck muscles attached to the upper ribs to move the upper ribs up during inspiration.
The most important muscle to breathing is the diaphragm. The diaphragm has its own nerve supply and can operate as a voluntary muscle or involuntary muscle, thus allowing us to hold our breath or slow our breathing if we wish to. When the diaphragm contracts, it moves down towards the stomach. This creates a vacuum in the cavity containing the lungs. This vacuum causes the lungs to expand and pull air down and in. When we breath out and the diaphragm relaxes and moves up again, no longer causing a vacuum. This in combination with the rib bones relaxing to their normal position help to push air back out of the lungs. The involuntary act of breathing is driven by carbon dioxide sensors in the body. These carbon dioxide sensors will send a message directly to the brain to force the body to breath again, i.e., make the diaphragm and intercostal muscles contract.
Accessory Muscles of Breathing
During times of stress, injury or just difficulty breathing, other muscles will kick in to help out. These muscles are called accessory muscles and are not used during normal breathing. These muscles are the front of the neck (sternocleidomastoid), the chest pectorals muscles and the abdomen. When these muscles are working there may be other more prominent issues going on, like a panic attack, but it is important to know that these muscles can assist with breathing.