External And Internal Intercostals

David Muir

2 min read

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06-12-2024

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The intercostal muscles, a group of 11 pairs located between the ribs, play a crucial role in breathing and maintaining the integrity of the thoracic cage. Understanding their distinct functions is key to comprehending respiratory mechanics and related pathologies. This article will explore the external and internal intercostals, highlighting their anatomical differences and contrasting actions.

External Intercostal Muscles: Inspiration's Helpers

The external intercostal muscles are situated superficially, running inferoanteriorly (downwards and forwards) from the rib above to the rib below. Their fibers are oriented obliquely, a characteristic that distinguishes them from their internal counterparts.

Function:

Their primary function is inspiration (inhalation). During this process, their contraction elevates the ribs, increasing the volume of the thoracic cavity. This expansion creates a negative pressure gradient, drawing air into the lungs. While other muscles contribute significantly to inspiration, the external intercostals provide a foundational role in rib cage expansion.

Innervation:

The external intercostals receive innervation from the intercostal nerves, specifically the anterior rami of the thoracic spinal nerves (T1-T11).

Internal Intercostal Muscles: Expiration's Contributors

Located deep to the external intercostals, the internal intercostal muscles run inferoposteriorly (downwards and backwards) from the rib above to the rib below. This opposite fiber orientation is a key anatomical difference influencing their action.

Function:

Primarily, the internal intercostals assist in forced expiration (exhalation). Their contraction depresses the ribs, reducing the volume of the thoracic cavity. This compression increases the pressure within the lungs, forcing air out. It's important to note that quiet expiration is largely passive, relying on the elastic recoil of the lungs and thoracic cage. The internal intercostals become more active during forceful exhalation, such as during strenuous exercise or coughing.

Innervation:

Similar to the external intercostals, the internal intercostals are innervated by the intercostal nerves (anterior rami of T1-T11).

Key Differences Summarized:

Clinical Significance

Dysfunction of the intercostal muscles can significantly impact respiratory function. Conditions affecting these muscles can lead to reduced respiratory capacity and increased breathing difficulty. Injuries, inflammation, or neuromuscular diseases can all compromise their ability to contribute to normal breathing. Understanding their roles is crucial for diagnosing and managing such respiratory conditions.