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Muscles of Respiration
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Muscles of Respiration

Muscles that expand the ribcage lower the intrathoracic pressure, and therefore assist in inspiration; muscles that contract the ribcage increase intrathoracic pressure and therefore assist in expiration.
Review respiration.
The diaphragm, with the help of the external intercostals, is the primary muscle of inspiration.
In active or forced breathing (as in exercise), several other muscles can assist with inspiration, including the scalenes, sternocleidomastoid, and the muscles of the chest and back (see our notes for information on muscles not included in this tutorial).
Expiration is usually passive; it occurs when the diaphragm and external intercostals relax.
Forced expiration is assisted by several muscles, including the internal and innermost intercostals, transversus thoracis, and the anterolateral abdominal muscles.
Anterior scalene
  • Originates on the transverse processes of C3-C6
  • Inserts on the superior surface of the first rib.
  • Elevates the first rib during forced inhalation, or, when the rib is fixed, flexes the neck (moves the head closer to the chest or shoulder).
Middle scalene
  • Originates on the transverse processes of C2-C7.
  • Inserts with the anterior scalene on the first rib.
  • Elevates the first rib and flexes the neck.
Posterior scalene
  • Originates on the transverse processes of C4-C6.
  • Inserts on the second rib.
  • Elevates rib 2; when this rib is fixed, it flexes the neck.
The scalene muscles are helpful landmarks in the neck: the subclavian artery and the roots of the brachial plexus pass between the anterior and middle scalenes; the subclavian vein passes anteriorly to the anterior scalene.
Three layers of intercostal muscles:
External intercostals
  • Originate on the inferior border of the ribs.
  • Insert into the superior border of the ribs below.
  • Elevate the ribs during inhalation.
Internal and innermost intercostals
  • Originate on the superior border of the ribs.
  • Insert into the inferior border of the ribs above.
  • Depress ribs during forced exhalation (normal exhalation requires no active effort).
The intercostal artery, vein, and nerve traverse along the length of the inferior border of the ribs between the innermost and internal intercostal muscles.
This anatomical relationship is clinically important, as the vessels and nerves must be avoided during thoracotomy or chest tube insertion.
The fibers of the external intercostals and external obliques of the abdominal wall run in the same direction – as if you're putting your hands in your front pants pockets.
Transversus thoracis
  • Originates on the posterior surface of the sternum (body and xiphoid process).
  • Inserts into the internal surfaces of ribs 2-6.
  • Depresses ribs during exhalation.
Diaphragm
  • Originates on the inferior internal surface of the sternum, costal cartilages of ribs 6-12, and the lumbar vertebrae L1-L3.
  • Inserts into the central tendon.
  • Expands the thoracic cavity to decrease intrapulmonary pressure and induce inspiration (inhalation).
  • Innervation of the diaphragm is by the phrenic nerve, which comprises fibers from spinal roots C3-C5 ("C 3,4,5 keep the diaphragm alive!"). Thus, phrenic nerve damage paralyzes the diaphragm and impairs breathing.
The position of the central tendon and top of the "dome" move during respiration.
During expiration, when the diaphragm is relaxed, the top of the diaphragm reaches as high as the fifth rib.
During inspiration, when the diaphragm contracts, it is relatively flat. The right side of the dome lies slightly higher than the left to accommodate the liver in the abdominal cavity.
Features of the diaphragm
Caval opening: where the vena cava passes through the central tendon
Esophageal hiatus: the opening that allows for passage of the esophagus from the thorax to the abdominal cavity (at the level of the tenth thoracic vertebra).
Aortic hiatus: allows for passage of the aorta (level of the 12th thoracic vertebra).
The right and left crura (singular = crus) are on either side of the aortic hiatus. The crura anchor the diaphragm to the first three lumbar vertebrae.
Despite the left-of-midline positioning of the esophageal hiatus, notice that this opening is the product of looping fibers that can be traced to the right crus.
The crura blend with the anterior longitudinal ligament of the vertebral column.

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