Question 21

College Answer

The diaphragm is the principal muscle of respiration. Important and unique features of its 
anatomy include a central tendon that blends with the pericardium above and the fibrous 
capsule of the liver below, arcuate ligaments and crura that are important points of muscle 
insertion. There are also three major and three minor openings that allow passage of structures 

between the thoracic and abdominal cavities. Candidates who had studied anatomy of the 
diaphragm were clearly distinguishable from those who had not.
Candidates who followed a traditional template for anatomy answers scored better, providing 
answers that covered the breath of the topic.

Discussion

The following data comes from, and is structured like, Last's textbook (9^th ed, p. 248-251). You can't get more traditional than Last's.

• Basic structural anatomy: 
  • Thin sheet of skeletal muscle, oval in shape, composed of a central noncontractile tendon and two discrete muscular portions, the costal and crural diaphragm.
  • From the circumference, fibres arch upwards into a pair of domes and then descend to a central tendon which has no bony attachment.
  • The right dome is higher than the left
  • The central tendon is at the level of the xiphisternum
• Relations: 
  • Superiorly: pericardium and basal lung segments
    (the central tendon is continuous with the pericardium)
  • Inferiorly: 
    • Right: liver, adrenal gland, kidney (the central tendon is also blended with the fibrous capsule of the liver)
    • Left: stomach, adrenal gland, kidney and spleen 
  • Posteriorly: crura (right and left crus), plus 3 arcuate ligaments: median (joins the two crura), medial (a thickening over the psoas), and the lateral (a thickening over the quadratus lumborum)
    • Also aorta, azygos veins, oesophagus, vagus nerve, pleura
  • Anteriorly: tendinous origin is from the  lower six costal cartilages and posterior aspect of the xiphoid process
• Openings in the diaphragm:
  • Aortic opening (at the level of T12)
  • Oesophageal opening (at the level of T10)
  • Vena cava foramen (at the level of T8)
  • Smaller openings for the hemiazygos vein, splanchnic nerves, superior epigastric vessels, lymphatics
• Blood supply: ​​​​​
  • Costal margin supplied by the lower five intercostal and subcostal arteries.
  • Main central mass supplied on their abdominal surface by right and left
    inferior phrenic arteries from the abdominal aorta
  • The phrenic nerve is supplied by the pericardiacophrenic artery 
• Innervation: 
  • Motor: right and left phrenic nerves (C3, 4 and 5, but mainly C4)
  • The lower intercostal nerves send some proprioceptive fibres to
    the periphery of the diaphragm

Anatomical features which are important to its function, if one could isolate those, would probably have to include the following:

• Bilateral nerve supply (i.e. the loss of one phrenic nerve does not adversely affect the function of the entire diaphragm)
• Motor innervation is solely from C3,4 and 5 via the phrenic nerves, which renders it vulnerable to high spinal cord damage
• Slow twitch fibres (favour sustained activity)
• Circumferential attachment to the costal margin and dome-like shape allows the diaphragm to increase intrathoracic volume by its contraction.
• Nerve supply extends radially from the centre, which has implications for diaphragmatic injuries and lacerations 

Gauthier, ALAIN P., et al. "Three-dimensional reconstruction of the in vivo human diaphragm shape at different lung volumes." Journal of applied physiology 76.2 (1994): 495-506.

De Troyer, André, Peter A. Kirkwood, and Theodore A. Wilson. "Respiratory action of the intercostal muscles." Physiological Reviews 85.2 (2005): 717-756.

Poole, DAVID C., et al. "Diaphragm structure and function in health and disease." Medicine and science in sports and exercise 29.6 (1997): 738-754.

Hart, Nicholas, et al. "Effect of severe isolated unilateral and bilateral diaphragm weakness on exercise performance." American journal of respiratory and critical care medicine165.9 (2002): 1265-1270.

Luce, John M., and Bruce H. Culver. "Respiratory muscle function in health and disease." Chest 81.1 (1982): 82-90.