Innermost Intercostal 1–11
Thorax · Trunk
The innermost intercostals form the deepest of the three muscular layers of the intercostal spaces, lying internal to the internal intercostals and separated from them by the intercostal neurovascular bundle. Each muscle runs from the inner surface of a rib to the superior border of the rib below, its fibres oriented in the same direction as the internal intercostals with which it is functionally continuous. Working together with the internal intercostals, they depress the ribs during forced expiration and support the intercostal space, helping maintain its integrity during pressure changes in breathing. They are supplied by the intercostal nerves running in the plane just superficial to them, at levels T1 to T11.
Origin, Insertion, Action & Nerve
Origin
Inner surface of the superior rib
Insertion
Superior border of the rib below
Action
Act with the internal intercostals to depress the ribs in forced expiration and support the intercostal space
Nerve
Intercostal nerves (T1–T11)
Attachments explained
The Innermost Intercostal 1–11 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
Inner surface of the superior rib
Insertion
Superior border of the rib below
Action & function
When the Innermost Intercostal 1–11 contracts, it produces the following movement: Act with the internal intercostals to depress the ribs in forced expiration and support the intercostal space.
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 Innermost Intercostal 1–11 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 Innermost Intercostal 1–11, impairing the movements it normally produces (act with the internal intercostals to depress the ribs in forced expiration and support the intercostal space). This is why knowing the innervation is central to localising nerve lesions in clinical practice.
How to study the Innermost Intercostal 1–11 (the OIANS method)
Fix the three-layer arrangement of the intercostal space from outside in: external, internal, then innermost. The clinically important point is that the neurovascular bundle lies between the internal and innermost layers, which is why the innermost muscle is the deep boundary you visualise when placing a chest drain above a rib to avoid the vessels and nerve.
Because the innermost intercostals share fibre direction and function with the internal intercostals, group them together as expiratory and space-supporting muscles rather than learning a separate action. Their supply by the intercostal nerves at T1 to T11 mirrors that of the other intercostal layers, so a single segmental nerve rule covers the whole space.
Frequently asked questions
Where is the Innermost Intercostal 1–11 located?
The Innermost Intercostal 1–11 is a muscle of the Thorax group, located in the Trunk.
What is the origin of the Innermost Intercostal 1–11?
Inner surface of the superior rib
What is the insertion of the Innermost Intercostal 1–11?
Superior border of the rib below
What movements does the Innermost Intercostal 1–11 produce?
Act with the internal intercostals to depress the ribs in forced expiration and support the intercostal space
What nerve supplies the Innermost Intercostal 1–11?
Intercostal nerves (T1–T11)
Is the Innermost Intercostal 1–11 free to study in OIANS?
The Innermost Intercostal 1–11 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
Study the Innermost Intercostal 1–11
Test yourself on Trunk muscles with interactive quizzes and flashcards in the free OIANS app.
Quiz yourself → Flashcards Open the OIANS app
Browsing is always free. A one-time Lifetime unlock adds every region's quizzes, flashcards, progress tracking and explore tools.