External And Internal Intercostals

Ed Bradley

2 min read

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31-12-2024

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The intercostal muscles, located between the ribs, play a crucial role in respiration. Understanding their distinct functions is key to comprehending how we breathe. This article delves into the anatomy and physiology of the external and internal intercostals, highlighting their differences and collaborative roles.

External Intercostal Muscles: The Inspirators

The external intercostal muscles, eleven in each intercostal space, are the more superficial of the two groups. Their fibers run inferoanteriorly – downwards and forwards – from the inferior border of one rib to the superior border of the rib below. This orientation is crucial to their function.

Function in Respiration:

During inspiration (inhalation), the external intercostals contract. This contraction elevates the ribs, increasing the anteroposterior and lateral dimensions of the thoracic cavity. This increase in volume reduces the intrapleural pressure, drawing air into the lungs. Essentially, they actively help expand the chest for breathing in.

Innervation:

The external intercostals are innervated by the intercostal nerves, which are branches of the thoracic spinal nerves.

Internal Intercostal Muscles: The Expirators (and more)

The internal intercostal muscles, also eleven in each space, lie deep to the external intercostals. Their fibers run superoanteriorly – upwards and forwards – almost perpendicular to the external intercostals.

Function in Respiration:

While primarily associated with forced expiration (exhalation), the role of the internal intercostals is more nuanced than simply pushing air out. During quiet breathing, their role is less prominent. However, during forceful exhalation, their contraction depresses the ribs, decreasing the thoracic cavity volume, and increasing the intrapleural pressure, thus expelling air from the lungs.

Other Functions:

Beyond respiration, the internal intercostals contribute to stability of the thoracic cage and protection of the underlying structures.

Innervation:

Similar to the external intercostals, the internal intercostals receive their innervation from the intercostal nerves.

Key Differences Summarized:

Conclusion: A Coordinated Effort

The external and internal intercostal muscles work in a coordinated manner to facilitate respiration. While the external intercostals are primarily responsible for inspiration, the internal intercostals play a significant role in forceful exhalation and overall thoracic cage stability. Understanding their distinct anatomical features and functions is essential for a comprehensive understanding of the respiratory mechanics.