MOVEMENTS OF THORACIC WALL

External Intercostal Muscles Internal Intercostal Muscles Innermost Intercostal Muscles

The intercostal muscles occupy the intercostal spaces.

INTERCOSTAL MUSCLES

External Intercostal Muscles

superficial layer Internal Intercostal Muscles

the inner layerInnermost Intercostal Muscles

internal to the intercostal vessels

Anteriorly, the muscle fibers are replaced bythe external intercostal membranes.These muscles run inferoanteriorly from the ribabove to the rib below. Each muscle attaches superiorly to the inferior border of the rib aboveand inferiorly to the superior border of the ribbelow. The external intercostals are most active during inspiration.

EXTERNAL INTERCOSTAL MUSCLES

(11 pairs) run deep to and at right angles to the external intercostals. Their fibers run inferoposteriorly from the floors of the costal grooves to the superior borders of the ribsinferior to them. The internal intercostals, weaker than the external intercostal muscles,are most active during expiration—especially theirinterosseous (vs. interchondral) portions.

INTERNAL INTERCOSTAL MUSCLES

Similar to the internal intercostals and are essentially their deeper parts. The innermost intercostals are separated from the internal intercostals by intercostal nerves and vessels. These muscles occupy the lateral-most parts of the intercostal spaces. It is likely that their actions are the same as those of the internal intercostal muscles.

INNERMOST INTERCOSTAL MUSCLES

MUSCLE ORIGIN INSERTION INNERVATION FUNCTIONExternal intercostal

Inferior border of ribs

Superior border of ribs below

Intercostal nerve

Elevate ribs during forced inspirationa

Internal intercostal Interosseous part: depresses ribs

Interchondral part: elevates ribs

During active (forced) respirationa

Innermost intercostal

Movements of the thoracic wall and the diaphragm during inspiration produce increases in the intrathoracic volume and diameters of the thorax. Consequent pressure changes result in air being alternately drawn into the lungs (inspiration) through the nose, mouth, larynx, and trachea and expelled from the lungs (expiration) through the same passages. During expiration the diaphragm, intercostal muscles, and other muscles relax, decreasing intrathoracic volume and increasing the intrathoracic pressure. Concurrently, intra-abdominal pressure decreases and abdominal viscera are decompressed. This allows the stretched elastic tissue of the lungs to recoil, expelling most of the air.

MECHANISM OF RESPIRATION

The vertical dimension (height) of the central part of the thoracic cavity increases during inspiration as contraction of the diaphragm causes it to descend, compressing the abdominal viscera.During expiration, the vertical dimension returns to the neutral position as the elastic recoil of the lungs produces sub-atmospheric pressure in the pleural cavities.

The AP dimension of the thorax increases considerably when the intercostal muscles contract. Movement of the ribs (primarily 2nd-6th) at the costovertebral joints around an axis passing through the necks of the ribs causes the anterior ends of the ribs to rise—the pump-handle movement. Because the ribs slope inferiorly, their elevation also results in anterior-posterior movement of the sternum, especially its inferior end.

MOVEMENTS OF THORACIC WALL

PUMP-HANDLE MOVEMENT

When the upper ribs are elevated, the AP dimension of the thorax is

increased (pump-handle movement), with a greater excursion

(increase) occurring inferiorly, at the end of the pump handle

The transverse dimension of the thorax also increases slightly when the intercostal muscles contract, raising the middle (lateral-most parts) of the ribs (especially the lower ones)—the bucket-handle movement.

BUCKET-HANDLE MOVEMENT

The middle parts of the lower ribs move laterally when they are elevated, increasing the transverse dimension

(bucket-handle movement).

The combination of rib movements (arrows) that occurduring forced inspiration increase the AP and transverse dimensions of the thoracic cage.

The combination of all these movements moves the thoracic cage anteriorly, superiorly, and laterally.

D The thorax widens during forced inspiration as the ribs are elevated

E. The thorax narrows during expiration as the ribs are depressed (arrows).