Breathing is a complex series of co-ordinated events. It involves a number of nerves and the contraction and relaxation of corresponding respiratory muscles. Which of the following combination of nerves and respiratory muscles is NOT involved in normal quiet breathing?

Question

BlackTom AI · Accepted Answer

Diaphragmatic contraction via the phrenic nerve (C3–C5) is the principal driver of quiet (eupneic) inspiration, with external intercostal muscles (innervated by thoracic intercostal nerves) contributing modestly to rib elevation. Quiet expiration is not an active muscular event but occurs passively as the lungs and chest wall recoil; therefore expiratory muscles such as the abdominal muscles are not recruited during normal quiet breathing.

1) Relevant knowledge points and concepts
- Inspiration in quiet breathing: active contraction of the diaphragm (phrenic nerve) and modest action of external intercostals (intercostal nerves).  
- Expiration in quiet breathing: passive process due to elastic recoil of lung and chest wall; no active abdominal muscle contraction required.  
- Abdominal muscles (rectus abdominis, internal/external obliques, transversus abdominis) are innervated by lower thoracic and L1 segmental nerves (sometimes broadly termed “abdominal nerves”) and are recruited during forced expiration/coughing, not during quiet breathing.

2) Step-by-step reasoning
- Identify which muscles actively contract during quiet inspiration: diaphragm (phrenic nerve) and external intercostals (intercostal nerves).  
- Determine what drives expiration in quiet breathing: passive elastic recoil of the respiratory system, so no active contraction of expiratory muscles.  
- Conclude that abdominal muscles (innervated by abdominal/lower thoracic nerves) are not required in normal quiet breathing because they produce active expiratory force only when expiration is forced.

3) Calculations
- None required; physiologic reasoning based on muscle recruitment patterns.

4) Why the correct answer is correct
- The abdominal muscles produce active intra-abdominal pressure and upward displacement of the diaphragm only during forced expiration. Because normal quiet breathing uses passive recoil for expiration, the abdominal muscles (and their innervation) are not involved.

5) Why other likely options would be incorrect
- Phrenic nerve → diaphragm: incorrect to say this is not involved, because it is the main inspiratory muscle.  
- Intercostal nerves → external intercostals: incorrect to exclude, since they assist inspiration.  
- Accessory muscles (sternocleidomastoid, scalenes) are not active in quiet breathing but would be listed as “not involved” only if the option named them; typically the abdominal muscles are the classic answer for muscles not used in quiet breathing.

6) Conclusion
Therefore, the combination “The abdominal muscles innervated by the abdominal nerves” is NOT involved in normal quiet breathing, which matches the provided correct answer.

Breathing is a complex series of co-ordinated events. It involves a number of nerves and the contraction and relaxation of corresponding respiratory muscles. Which of the following combination of nerves and respiratory muscles is NOT involved in normal quiet breathing?Single choice

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