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These terms and definitions cover muscles of inspiration and expiration in respiration.

This is the partition of muscle and aponeurosis that separates the thoracic and abdominal cavities.


Extending from rib above to rib below, these muscles function as a single sheet of muscle to pull the lower ribs upward toward the first rib.

External intercostals

Extending from the shoulder to upper ribs 2-5, this muscle helps raise the rib cage.

Pectoralis minor

Only found in the upper thorax, these muscles originate at the spinous process of C7 through T3 and insert into ribs 2 through 5; on contraction, they help to elevate the ribs.

Serratus posterior superior

Extending from the skull to the sternum and the clavicle, this muscle pulls the chest upward and helps elevate the rib cage.


Fanning out from the upper arms to the sternum and the clavicle, this muscle draws the sternum upward and helps elevate the rib cage.

Pectoralis major

Extending from C2-C7 to the first and/or second rib, they pull the chest upward and help elevate the rib cage.


This muscle is primarily responsible for increasing thoracic volume in the vertical dimension.


These 12 pairs of small muscles extend from the transverse process of vertebrae to the rib below; they help pull the rib cage upward.

Levator costarum brevis

These four muscles of the upper and lower back support inspiration by stabilizing the rib cage.

Trapezius Levator, scapulae, Rhomboidus major, Rhomboidus minor

The major accessory muscles of inspiration, these are primarily responsible for increasing the thoracic cavity in the transverse dimension.

External intercostals

This is the primary muscle of inspiration.


When it contracts, the diaphragm pulls (up, down) on the central tendon.


Extending from ribs 9 through 12—originating on the transverse process, skipping a rib and inserting into the next rib below—these muscles help elevate the ribs.

Levator costarum longis

the sternal head of this neck muscle raises the rib cage.


These three sets of posterior thoracic muscles aid in inspiration by elevating the rib cage.

Levator costarum brevis, Levator costarum longis, Serratus posterior superior

Primarily involved in expiration, this muscle also aids in forced inspiration.

Interchondral portion of the internal intercostals

These muscles originate on the lower edge of each rib (except 12) and course medially and inferiorly to insert in the rib below; they increase the vertical dimension of the thorax.

External intercostals

This is the portion of the diaphragm into which the muscle fibers insert.

Central tendon

Extending from pubis to sternum, and contained in the fibrous connective tissue of the abdominal aponeurosis, this muscle pulls the rib cage down.

Rectus abdominus

Extending upward and inward from the iliac crest and inguinal ligament to the lowest three ribs and abdominal aponeurosis, this muscle pulls the lower ribs down and compresses the abdominal viscera.

Internal oblique abdominis

Extending from hipbone to lowest rib, this muscle anchors this floating rib against the pull of diaphragmatic muscles.

Quadratus lumborum

The most superficial of the abdominal muscles, this muscle assists in lowering the rib cage.

External oblique abdominis

Extending horizontally from the lower ribs, lumbar vertebrae, and hipbone to the abdominal aponeurosis, this muscle compresses the abdominal viscera.

Transversus abdominis

Extending upward and outward from vertebrae to the lowest four ribs, this muscle pulls those ribs downward.

Serratus posterior inferior

On the inside of the rib cage, these muscles extend upward and outward from the lowest to higher ribs to pull them downward and inward.


Sweeping upward and outward inside the chest from sternum to upper ribs, this muscle pulls the chest downward.

Transversus thoracis

Extending from rib below to rib above, these muscles are thought to pull rib cage down and stiffen the lowest rib.

Interosseous portion of the internal intercostals

This anterior abdominal muscle fans medially from the inguinal ligament and iliac crest to the cartilaginous portion of the lower ribs and the abdominal aponeurosis near the rectus abdominis; it helps compress the abdominal viscera.

Internal oblique abdominis

The deepest of the anterior abdominal muscles, this muscle runs laterally and its contraction significantly reduces the volume of the abdomen.

Transversus abdominis

This runs from the xiphoid process of the sternum to the pubic symphysis and forms the midline structure for muscle attachment.

Linea alba

Extending upward and outward from the pelvis to the lower eight ribs, this muscle pulls the rib cage down.

External oblique abdominis

These are the prominent midline muscles of the abdomen; contraction compresses abdominal contents and allows us to do sit-ups.

Rectus abdominis

This cranial nerve provides parasympathetic (rest and digest) innervations to the lungs.

Vagus (X)

This nerve provides innervation to the diaphragm. It arises from cervical spinal nerves (C3, C4, & C5).

Phrenic nerve

Nerve bundles in these two spinal regions provide sympathetic (fight or flight) innervations to the lungs.

Cervical, Thoracic

This is an example of a parasympathetic lung/respiratory response.

Slowed respirations

This is an example of a sympathetic lung/respiratory response.

Increased respirations

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