Intercostal Muscles

The intercostal muscles are a group of muscles located between the ribs, running in between each rib. They play a crucial role in facilitating breathing and stabilising the chest wall during respiration, assisting in both inhalation and exhalation, and maintaining the structure and stability of the chest wall^[1].

There are three layers of intercostal muscles: the external intercostals, internal intercostals, and innermost intercostals^[2].

1. External Intercostal Muscles: These muscles are positioned on the outermost layer and are primarily involved in inspiration (breathing in). When they contract, they raise the ribcage, expanding the chest cavity, and aiding in inhalation. This muscle action helps draw air into the lungs.

2. Internal Intercostal Muscles: These muscles lie beneath the external intercostals, in the middle layer.. The internal intercostal muscles have two subtypes: the interosseous portion and the interchondral portion. Both portions play a role in expiration (breathing out) and forced exhalation. When they contract, they depress the ribcage, decreasing the space in the chest cavity, and assisting in exhalation by pushing air out of the lungs^[4].

3. Innermost Intercostal Muscles: These muscles are situated on the innermost layer, deep to the internal intercostal muscles^[2]. They are more limited in extent and have a similar role to the internal intercostal muscles, aiding in exhalation^[5].

Together, the intercostal muscles help regulate and control the movement of the ribcage during breathing. When we inhale, the external intercostal muscles contract, lifting the ribcage upward and outward, expanding the chest cavity. This expansion creates a negative pressure inside the lungs, drawing air in. During exhalation, the internal intercostal muscles contract, pulling the ribcage downward and inward, reducing the size of the chest cavity and pushing air out.

The intercostal muscles also contribute to the stability and protection of the thoracic region, helping to maintain the integrity of the ribcage and supporting the vital organs within it.