Muscles of Respiration - Intercostal Muscles

Intercostal Muscles

Along with the diaphragm, the intercostal muscles are one of the most important groups of respiratory muscles. The external intercostals are responsible for ~25% of the air that enters the lungs during quiet breathing. These muscles are attached between each rib and are important in manipulating the width of the thoracic cage. There are three layers of intercostal muscles:

• The external intercostal have fibres that go forward and downward (obliquely downward and forward from rib to rib). These fibres are called the antero-inferior, and are sometimes referred to as the front pocket muscle since the fibres mimic the direction a hand would enter a front pocket. The external intercostals assist in deep inspiration by increasing the anterioposterior diameter of the chest.
• The internal intercostal also have fibres, which go backward and downward (obliquely downward and posteriorly from rib to rib). These are called the postero-superior, or the back pocket muscles. The internal intercostals assist in expiration by pulling the ribcage down.
• The last layer is the innermost intercostals and consists of three separate parts; the subcostal (posteriorly), the innermost intercostal (laterally, which often causes confusion with the innermost intercostal referring to all twenty groups) and the transversus thoracis (anteriorly).

The antero-inferior fibres have the effect of lifting up the ribs during contraction. Anteriorly, the external intercostal muscles do not directly attach to the sternum, but are replaced by the anterior intercostal membrane.

The innermost intercostal muscles are also "back pocket" muscles. The internal and innermost intercostals have the opposite effect of the external intercostals and aid in forced expiration. The internal intercostal is replaced by the posterior intercostal membrane posteriorly.