Muscles of respiration

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Inspiration and expiration requires changing pressures within the thoracic cage. This is achieved by changing the dimesion(s) of the thoracic cage, which is done by the works of the muscles of respiration

Contents 1 Principal and Accessory Muscles of Inspiration 2 Muscles of Expiration 3 The intercostal muscles 4 References

[edit] Principal and Accessory Muscles of Inspiration

There are two main groups of inspiratory muscles (i.e. contrast with muscles of expiration), Principal and Accessory. Principal muscles are used regardless of the level of breathing required, while Accessory muscles are only used during forced breathing (i.e. during heavy exercise and in some diseases such as Asthma)

The principal muscles are External Intercostal (elevate ribs, thus increasing width of thoracic caity), Intercondral part of internal intercostal (also elevates rib), and diaphragm (descends to increase vertical dimension of the thoracic cavity, also elevates lower ribs)
##Please see the intercostal muscle section below for more explanation##

Accessories muscles of inspiration are: Sternocleidomastoid (elevated sternum) and the scalene muscles (Anterior, middle and posterior scalene). ^[1] After a heavy exercise, people often lay on to a bench with their hands fixed. This has the effect of fixing the insertion of the accessories muscles and thus when the muscles contract, the origin (i.e. the upper ribs and/or the sternum) is pulled to aid respiration

[edit] Muscles of Expiration

During quiet breathing, there is no muscle involved. The process is simply done by the elastic recoil of the lungs and the rib cage.

Duting active breathing, the internal intercostals (except the interchondral part) and Abdominal muscles (Rectus abdominis, external & internal oblique and transversus abdominis) pulls the thoracic cage in and forces the air out. ^[2]
##Please see the intercostal muscle section below for more explanation##

[edit] The intercostal muscles

Along with the diaphragm, these are one of the most important groups of respiratory muscles. These muscles attach from the rib above to the rib below it and are important in manipulating the width of the thoracic cage. There are three layers of intercostal muscles.

The external intercostal have their fibres going forward and downward (antero-inferior -- also called front pocket muscle -- the fibres are in the same direction with a hand going into a front pocket). This have the effect of lifting up the ribs on contraction. Anteriorly, the external intercostal muscles do not directly attach to the sternum, but are replaced by the anterior intercostal membrane.

The internal intercostal have their fibres going backward and upward (postero-superior -- also called the back pocket muscle). The last layer is the innermost intercostals and consists of three separate parts (i.e. not continuous unlike the other two layers)= subcostal (posteriorly), innermost intercostal (laterally - this often cause confusion with the innermost intercostal referring to all three groups) and the transversus thoracis (anteriorly). Innermost intercostal muscles are also "back pocket" muscles. Thus the internal and innermost intercostals have the opposite effect to the external intercostal and aid in forced expiration. The internal intercostal is replaced by posterior intercostal membrane posteriorly.

[edit] References

1. ^ Netter FH. Atlas of Human Anatomy 3rd ed. Icon Learning Systems. Teterboro, New Jersey 2003 - plate 191
2. ^ Netter FH. Atlas of Human Anatomy 3rd ed. Icon Learning Systems. Teterboro, New Jersey 2003 - plate 191