Corpus: Diaphragm
1. Definition
The diaphragm is the central respiratory muscle that separates the thoracic cavity from the abdominal cavity.
2. Embryology
The diaphragm develops from a membranous transverse septum in the anterior neck region, originating from the mesoderm. This early development explains its later innervation by parts of the spinal nerves C3 to C5. The transverse septum later fuses with parts of the pleuroperitoneal membranes and the dorsal mesentery. As the embryo grows, the diaphragmatic anlage shifts caudally due to the elongation of the neck and the downward displacement of the heart.
3. Anatomy
3.1. Overview
The diaphragm is composed of a muscle plate originating from the ribs, lumbar spine, and sternum. At the central tendon, the muscle parts converge. It is flanked by the muscular diaphragmatic domes on both sides. The diaphragm is innervated by the phrenic nerve.
3.2. Classification
The diaphragm is divided into three parts based on the origin of its muscle fibers:
3.2.1. Lumbar part
This forms the diaphragmatic legs. The right medial leg originates from the 1st to 4th lumbar vertebrae, while the left medial leg originates from the 1st to 3rd lumbar vertebrae.
The right and left lateral legs arise from two arches: the medial arcuate ligament and the lateral arcuate ligament. The diaphragmatic legs are connected by arcuate fibers known as the median arcuate ligament.[1]
3.2.2. Costal part
The fibers of this section originate on the inner sides of the costal cartilages and adjacent tissue of the lower six ribs on each side, interlocking with fibers of the transversus abdominis muscle.
3.2.3. Sternal part
The fibers here originate from two short, fleshy strips at the back of the xiphoid process of the sternum, radiating into the central tendon.
3.3. Passage points
The diaphragm has several openings for the passage of anatomical structures between the thoracic and abdominal cavities, including:
- Esophageal hiatus: For the esophagus and vagal trunks (vagus nerve)
- Caval opening: For the inferior vena cava and the right phrenicoabdominal branch of phrenic nerve
- Aortic hiatus: For the aorta and thoracic duct
- Larrey's cleft: Often considered the passage for the left superior epigastric artery and vein, according to other opinions free of passing vessels[2]
- Morgagni's hole: Passage for the right superior epigastric artery and vein.
- the medial lumbar fissure for the azygos vein (right), hemiazygos vein (left), the greater splanchnic nerve and the lesser splanchnic nerve
- the lateral lumbar fissure for the sympathetic border cord
A clinically important area is the muscle-free Bochdalek's fissure, a potential site for diaphragmatic hernia.
The left phrenicoabdominal branch of the phrenic nerve typically passes directly through the central tendon or the lumbar part of the diaphragm, independently of the commonly recognized points of passage. In some cases, it may also pass through the esophageal hiatus.
3.4. Projection onto the thoracic wall
At maximum contraction, the diaphragm projects itself
- on the right at the level of the 7th rib (T11 vertebra)
- on the left, one intercostal space lower (T12 vertebra).
At maximum relaxation, it projects
- on the right at the level of the 4th rib (T8 vertebra)
- on the left, half an intercostal space lower (T9 vertebra).
The right dome of the diaphragm lies higher than the left due to the underlying right lobe of the liver.
3.5. Arterial supply
The diaphragm receives blood supply from several arteries:
3.6. Innervation
The diaphragm is innervated by the phrenic nerve, which arises from the cervical plexus (C3 to C5). Additionally, accessory phrenic nerves from C5 to C7 may also contribute to its innervation. A small portion of the diaphragm's musculature is innervated by fibers from the thoracic spinal nerves. The diaphragm's activity is centrally controlled by the respiratory centers in the medulla oblongata and pons, with voluntary control also possible via pathways from the cerebral cortex.
3.7. Function
When the diaphragm contracts, it flattens from a dome shape into a more conical shape, shortening by approximately 30 % in humans. This action increases thoracic volume, driving inspiration. The diaphragm also aids in abdominal pressure regulation during activities such as defecation and childbirth.
4. Histology
The diaphragm's muscle fibers consist of striated muscle, while the central tendon is composed of coarse collagenous tendon fibers. The muscle is highly vascularized with a mix of slow-twitch (type 1) and fast-twitch (type 2) fibers, providing endurance for continuous respiratory function and resistance to fatigue.[3]
5. Physiology
The diaphragm's contraction enlarges the thoracic cavity and changes the shape of the abdominal cavity. This movement increases lung volume, causing negative pressure that draws air into the lungs. Diaphragmatic contraction also aids in increasing abdominal pressure during activities like defecation or childbirth.
6. Clinic
Phrenic nerve palsy can lead to diaphragmatic paralysis. Conditions such as diaphragmatic protrusion or prolapse involve abnormal movement of the diaphragm, and diaphragmatic hernia occurs when abdominal organs pass through diaphragmatic openings into the thorax. Diaphragmitis, an inflammation of the diaphragm, is rare but can occur.
7. Sources
- ↑ Waldeyer et al. Anatomie des Menschen: Lehrbuch und Atlas in einem Band (De Gruyter Studium) (19th totaly rev. ed.), De Gruyter, 2012
- ↑ Kubik, S, Steiner, R: Die Larreysche Spalte, eine anatomische Fehlinterpretation. Swiss Journal of the history of medicine and sciences Band (Jahr): 30 (1973)
- ↑ Polla B et al: Respiratory muscle fibres: specialisation and plasticity. Thorax (2004) Volume 59, Issue 9