External versus internal intercostal muscles
External intercostal muscles (the muscles between the ribs) lift the ribs in inspiration, and internal intercostals pull them down when breathing out hard.
Remember
External intercostals lift the ribs to breathe in, internal intercostals pull them downwards when breathing out hard, and the two pull in different directions.
Memory aid
hands in pockets for the externals, and the internals cross the other way
The external fibres run downwards and forwards like hands going into pockets, and the internal fibres cross them at right angles.
Between the ribs lie three layers of muscle, the intercostal muscles. All three are supplied by the intercostal nerves. From the surface inwards they are the external intercostals, the internal intercostals and the innermost intercostals.
The external layer is the most superficial. Its fibres (the parts of the muscle whose direction sets the direction of pull) run downwards and forwards, that is inferiorly and anteriorly. When these muscles contract, the ribs lift and swing upwards and outwards like the handle of a bucket, and the width of the chest increases. These muscles lift the ribs in inspiration.
The internal layer lies deeper, and its fibres run the other way, downwards and backwards (inferiorly and posteriorly). These muscles pull the ribs down in forced expiration, meaning breathing out hard. The innermost layer, the deepest of the three, is used alongside the internal layer during forced expiration.
Quiet breathing out needs none of these muscles, because it is passive. The external intercostals do help with quiet breathing in, producing around 30% of chest expansion, but the diaphragm is the main muscle of inspiration.
So the direction of the fibres tells you the job: downwards and forwards means the ribs are lifted for breathing in, and downwards and backwards means the ribs are pulled down for breathing out hard.
Key facts
- Layers
- Three layers of muscle lie between the ribs: from the surface inwards, the external, internal and innermost intercostals.
- External fibres
- They run downwards and forwards, like the hands in a pocket.
- External action
- They lift the ribs in inspiration and produce around 30% of chest expansion in quiet breathing.
- Internal fibres
- They run downwards and backwards, at right angles to the externals.
- Internal action
- They pull the ribs down in forced expiration, which is breathing out hard.
- Innermost layer
- It is the deepest layer and assists the internal intercostals in forced expiration.
Exam traps
- Intercostal muscles are not supplied by the phrenic nerve (the nerve of the diaphragm). The external intercostals lift the ribs and are supplied by the intercostal nerves.
- Giving the internal intercostals fibres that run downwards and forwards is wrong, because that direction belongs to the external intercostals and the internal fibres run downwards and backwards.
- The diaphragm is the main muscle of quiet inspiration, but it is a separate sheet of muscle and not one of the intercostal muscle groups.
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