Internal Intercostals
Thorax · Trunk
The internal intercostals form the middle of the three intercostal muscle layers, lying deep to the external intercostals. Each arises from the inner aspect of the inferior border of a rib and passes obliquely downward and backward to the superior border of the rib below, so its fibres run roughly at right angles to those of the external layer. This arrangement gives the muscle two functional regions: the interosseous part between the bony ribs depresses the ribs in forced expiration, while the interchondral part between the costal cartilages elevates the ribs and assists inspiration. They are supplied by the intercostal nerves (T1 to T11). Posteriorly they are replaced by the internal intercostal membrane.
Origin, Insertion, Action & Nerve
Origin
Inferior border of each rib (inner aspect)
Insertion
Superior border of the rib below
Action
Interosseous part depresses ribs in forced expiration; interchondral part elevates ribs
Nerve
Intercostal nerves (T1–T11)
Attachments explained
The Internal Intercostals is defined first by where it attaches. Its origin is the more fixed anchor, usually the proximal or more stable end that stays put during contraction, while its insertion is the more mobile point that is pulled toward the origin when the muscle shortens.
Origin
Inferior border of each rib (inner aspect)
Insertion
Superior border of the rib below
Action & function
When the Internal Intercostals contracts, it produces the following movements: Interosseous part depresses ribs in forced expiration; interchondral part elevates ribs.
As part of the Thorax group of the Trunk, it works alongside neighbouring muscles to generate smooth, coordinated movement and to stabilise the structures it acts on.
Nerve supply & clinical relevance
The Internal Intercostals receives its nerve supply from the Intercostal nerves (T1–T11). This nerve carries fibres from spinal segment(s) T1, T11.
Because a muscle can only contract when its nerve is intact, injury to the Intercostal nerves or to its spinal roots (T1, T11) can weaken or paralyse the Internal Intercostals, impairing the movements it normally produces (interosseous part depresses ribs in forced expiration). This is why knowing the innervation is central to localising nerve lesions in clinical practice.
How to study the Internal Intercostals (the OIANS method)
Fix the internal intercostals by contrast with the external layer: their fibres run downward and backward, roughly perpendicular to the outer muscle, and this reversed direction hints at their main role in pulling the ribs down during forced expiration, such as coughing or blowing out hard.
Do not overlook the exception built into this muscle, the interchondral part near the cartilages actually helps elevate the ribs for inspiration, so the same muscle can aid breathing both in and out depending on the region. Sharing the intercostal nerves (T1 to T11) with the other layers, it belongs to the same segmental neurovascular pattern coursing along each rib.
Frequently asked questions
Where is the Internal Intercostals located?
The Internal Intercostals is a muscle of the Thorax group, located in the Trunk.
What is the origin of the Internal Intercostals?
Inferior border of each rib (inner aspect)
What is the insertion of the Internal Intercostals?
Superior border of the rib below
What movements does the Internal Intercostals produce?
Interosseous part depresses ribs in forced expiration; interchondral part elevates ribs
What nerve supplies the Internal Intercostals?
Intercostal nerves (T1–T11)
Is the Internal Intercostals free to study in OIANS?
The Internal Intercostals is always free to browse. Its full origin, insertion, action and nerve details are open to everyone in the Muscle Directory. Quiz and Flashcard practice for the Trunk is part of the one-time Lifetime upgrade, though; only the Upper Limb decks are free to test yourself on.
Related muscles
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