Diaphragm Anatomy

The Diaphragm

• The diaphragm attaches to the boundaries of the thoracic outlet and rises up to the 4th rib on the right and 5th intercostal space on the left.
• During inspiration, the diaphragm descends, and in forced breathing, the ribs ascend.
• The 12th rib is held down to prevent the diaphragm from ascending with the ribcage, achieved by contraction of the quadratus lumborum muscle.

Quadratus Lumborum

• The quadratus lumborum muscle is attached to the iliac crest of the hip bones below and the 12th rib above.
• It is also attached to the transverse processes of the lower lumbar vertebrae and iliolumbar ligament.
• The key attachment is to the iliac crest.
• It is innervated by the anterior rami of T12-L3.

Posture and Breathing

• In “quiet” breathing, the rib cage does not need to move much, and expansion of the thoracic cavity is achieved through descent of the diaphragm.
• In the upright position, gravity assists the diaphragm by causing descent of the abdominal organs.
• The abdominal wall undergoes reflex relaxation to assist this process, allowing the abdominal organs to be displaced.
• To expire, the opposite occurs – the abdominal wall undergoes reflex contraction to force the abdominal organs and diaphragm upwards.

Breathing in the Supine Position

• When lying down, the effects of gravity have a negative effect on breathing, since the liver moves upwards towards the chest.
• This is due to the presence of a curve in the vertebral column.

Muscles Attached to the Inlet

• The sternocleidomastoid muscle attaches to the sternum, clavicle, and mastoid process of the skull.
• It can turn the head to the side, or acting with its partner on the opposite side, pull the head forward and tilt it backwards.
• It elevates the sternum when the head is stable.
• It is supplied by the spinal accessory nerve (11th cranial nerve).

Scalene Muscles

• The scalene muscles attach to the cervical vertebrae proximally and 1st and 2nd ribs distally.
• They elevate the ribs when the neck is stable.
• They are innervated by nerves of the cervical plexus.

Intercostal Muscles

• The external intercostal muscles are inspiratory and internal ones expiratory, but this reverses beyond 120 degrees as the muscles approach the sternum.
• The external intercostal muscle is largely inspiratory, and the interchondral portion of the internal intercostal muscle is also inspiratory.

Accessory Muscles of Respiration

• The extrinsic muscles of the chest aid movement of the ribcage.
• The anterior abdominal wall muscles stabilize the 1st and 2nd ribs.
• The rectus abdominis and external oblique abdominis muscles pull the ribs downwards.
• The transversus abdominis muscle causes compression of the abdominal cavity, which assists the elevation of the relaxed diaphragm towards the thoracic cavity.

Serratus Posterior Muscles

• The superior serratus posterior muscle arises from the spinous processes and supraspinous ligaments of C7-T2 and inserts onto ribs 2-5.
• The inferior serratus posterior muscle arises from the spinous processes and supraspinous ligaments of T11-L2 and inserts onto ribs 9-12.
• These muscles are thought to be involved in measuring stresses at the top and bottom of the thoracic spine, having a proprioceptive role for the vertebral column.

Levator Costarum Muscles

• The levator costarum muscles arise from the transverse processes of C7 to T11.
• They elevate the ribs, but are small and weak, and do not contribute any significant action during respiration.

Muscles of Inspiration and Expiration

• Forced inspiration involves the action of the scalene group of muscles, sternocleidomastoid muscle, external intercostal and interchondral part of the internal intercostal muscles, and the diaphragm.
• Quiet expiration requires relaxation of the inspiratory muscles, and the ribcage is lowered, and the diaphragm elevates.
• In forced breathing, the intercostal part of the internal intercostal muscle possibly depresses the ribs, and the abdominal muscles contract to depress the ribcage and compress the abdominal cavity.