Muscles of the thorax that assist in breathing.

External intercostals (in′‐ter‐KOS‐tals; external = closer to surface; inter‐ = between; ‐costa = rib)

Origin: Inferior border of rib above. Insertion: Superior border of rib below. Action: Contraction elevates ribs and increases anteroposterior and lateral dimensions of thoracic cavity, resulting in inhalation; relaxation depresses ribs and decreases anteroposterior and lateral dimensions of thoracic cavity, resulting in exhalation. Thoracic spinal nerves T2-T12.

Internal intercostals (in′‐ter‐KOS‐tals; internal = farther from surface)

Origin: Superior border of rib below. Insertion: Inferior border of rib above. Action: Contraction draws adjacent ribs together to further decrease anteroposterior and lateral dimensions of thoracic cavity during forced exhalation. Thoracic spinal nerves T2-T12.

Diaphragm (DĪ‐a‐fram; dia‐ = across; ‐phragm = wall)

Origin: Xiphoid process of sternum, costal cartilages and adjacent portions of inferior six ribs, lumbar vertebrae and their intervertebral discs. Insertion: Central tendon. Action: Contraction of diaphragm causes it to flatten and increases vertical dimension of thoracic cavity, resulting in inhalation; relaxation of diaphragm causes it to move superiorly and decreases vertical dimension of thoracic cavity, resulting in exhalation. Phrenic nerve, which contains axons from cervical spinal nerves (C3-C5).

Which muscle associated with breathing is innervated by the phrenic nerve?

The diaphragm is innervated by the phrenic nerve.

What is the most important muscle that powers breathing?

The dome‐shaped diaphragm. It also separates the thoracic and abdominal cavities. The diaphragm has a convex superior surface that forms the floor of the thoracic cavity and a concave, inferior surface that forms the roof of the abdominal cavity.