Internal Intercostals

Thorax · Trunk

Internal Intercostals belongs to the Thorax group of the Trunk. Its origin is Inferior border of each rib (inner aspect), and it inserts onto Superior border of the rib below. It is responsible for several movements: Interosseous part depresses ribs in forced expiration; interchondral part elevates ribs. Its nerve supply is the Intercostal nerves (T1–T11).

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)

OIANS stands for Origin, Insertion, Action and Nerve, the four facts that uniquely define every skeletal muscle. To learn the Internal Intercostals, 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 Internal Intercostals 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.