🫁 Muscular System Lesson

EIEL: intercostal muscles for breathing

Two muscle layers between the ribs, running in opposite fiber directions, producing opposite effects on breathing.

Ext

Inspir.

Int

Expir.

📖 Full Breakdown

Opposite fiber directions, opposite breathing functions

The fiber direction itself predicts the function — a useful shortcut for remembering which layer does what.

External intercostals

Fibers run downward and forward

Elevate the ribs during inspiration — remember: External = Elevates = Inspiration.

Internal intercostals

Fibers run downward and backward

Depress the ribs during FORCED expiration — notably, these muscles are not needed for quiet, passive expiration, only forced expiration.

The diaphragm

The primary muscle of quiet breathing

Contracts and flattens during inspiration, doing most of the work during normal, relaxed breathing — the intercostals mainly assist with deeper breathing.

Accessory muscles

Used during respiratory distress

The sternocleidomastoid (SCM) and scalenes assist during heavy breathing or respiratory distress, beyond what the diaphragm and intercostals alone can provide.

🩺 Clinical / Exam Application

A patient with severe COPD is observed using visible neck muscle effort (accessory muscles) just to breathe, even at rest. This indicates their diaphragm and intercostals alone are no longer sufficient to meet their breathing demands, forcing recruitment of the SCM and scalenes — muscles that healthy individuals only use during heavy exertion, not resting breathing. Recognizing accessory muscle use at rest is a visible, immediate sign of significant respiratory compromise.

⚠️ Exam Alert

A frequently tested clarification: the diaphragm, not the intercostals, is the primary muscle of QUIET breathing — the internal intercostals specifically are only needed for FORCED expiration, not normal passive exhalation, which happens largely through elastic recoil of the lungs.

🚧 Common Trap

Don't assume both intercostal layers are equally active during normal, quiet breathing. External intercostals assist with inspiration during normal breathing, but internal intercostals are essentially inactive until FORCED expiration is needed (like during exercise or coughing) — quiet exhalation is largely passive.

✅ Quick Check

Using fiber direction, explain why external intercostals elevate the ribs while internal intercostals depress them.

📝 Exam Prep

Common Exam Questions

❓ What is the function of the external vs internal intercostal muscles?

✅ External intercostals elevate the ribs during inspiration. Internal intercostals depress the ribs during forced expiration. The diaphragm does most of the work during quiet breathing, with intercostals assisting during deeper breathing.

❓ When are accessory muscles like the SCM and scalenes used in breathing?

✅ Accessory muscles are recruited during heavy breathing or respiratory distress, when the diaphragm and intercostal muscles alone are insufficient to meet breathing demands — their use at rest is a sign of significant respiratory compromise.