Innermost Intercostal 1–11

Thorax · Trunk

Innermost Intercostal 1–11 belongs to the Thorax group of the Trunk. Its origin is Inner surface of the superior rib, and it inserts onto Superior border of the rib below. When it contracts, it act with the internal intercostals to depress the ribs in forced expiration and support the intercostal space. Its nerve supply is the Intercostal nerves (T1–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)

OIANS stands for Origin, Insertion, Action and Nerve, the four facts that uniquely define every skeletal muscle. To learn the Innermost Intercostal 1–11, work through them in order: picture its origin, trace the muscle to its insertion, reason out the action that shortening between those two points must create, then add the nerve that drives it.

Most students remember the Innermost Intercostal 1–11 fastest by linking its action back to its attachments rather than memorising each fact in isolation. Once the origin and insertion make sense, the action usually follows logically.