Muscles of Respiration

Inspiration

The most important muscle of inspiration is the diaphragm. This consists of a rh:r dome-shaped sheet of muscle that is inserted into the lower ribs. It is supplied b the phrenic nerves from cervical segments 3, 4, and 5. When it contracts, the afc dominal contents are forced downward and forward, and the vertical dimensi 1 of the chest cavity is increased. In addition, the rib margins are lifted and mi va out, causing an increase in the transverse diameter of the thorax (Figure 7-1). 1 In normal tidal breathing, the level of the diaphragm moves about 1 cm or >c but on forced inspiration and expirat ion, a total excursion of up to 10 cm may c cur. When the diaphragm is paralyzed, it moves up rather than dou>n with insri ration because the intrathoracic pressure falls. This is known as paradoxical more metit and can be demonstrated at fluoroscopy when the subject sniffs.

The external intercostal muscles connect adjacent ribs and slope downward arn forward (Figure 7-2). When they contract, the ribs are pulled upward and forward causing an increase in both the lateral and anteroposterior diameters of the thq rax. The lateral dimension increases because of the "bucket-handle" movemeru of the ribs. The intercostal muscles are supplied by intercostal nerves that com« off the spinal cord at the same level. Paralysis of the intercostal muscles alon< does not seriously affect breathing because the diaphragm is so effective.

> Passive

Figure 7-1. On inspiration, the dome-shaped diaphragm contracts, the abdominal contents are forced down and forward, and the rib cage is widened. Both increase the volume of the thorax. On forced expiration, the abdominal muscles contra:: and push the diaphragm up.

Figure 7-2. When the external intercostal muscles contract, the ribs are pulled upward and forward, and they rotate on an axis joining the tubercle and head of rib. As a result, both the lateral and anteroposterior diameters of the thorax increase. The internal intercostals have the opposite action.

Figure 7-2. When the external intercostal muscles contract, the ribs are pulled upward and forward, and they rotate on an axis joining the tubercle and head of rib. As a result, both the lateral and anteroposterior diameters of the thorax increase. The internal intercostals have the opposite action.

The accessor} muscles of inspiration include the scalene muscles which elevate the first two ribs, and the sternomastoids which raise the sternum. There is little, if any, activity in these muscles during quiet breathing, but during exercise they may contract vigorously. Other muscles that play a minor role include the alae nasi, which cause flaring of the nostrils, and small muscles in the neck and head.

Expiration

This is passive during quiet breathing. The lung and chest wall are elastic and tend to return to their equilibrium positions after being actively expanded during inspiration. During exercise and voluntary hyperventilation, expiration becomes active. The most important muscles of expiration are those of the abdominal wall, including the rectus abdominis, internal and external oblique muscles, and transversus abdominis. When these muscles contract, intra-abdominal pressure is raised, and the diaphragm is pushed upward. These muscles also contract forcefully during coughing, vomiting, and defecation.

The internal intercostal mwicles assist active expiration by pulling the ribs downward and inward (opposite to the action of the external intercostal muscles), thus decreasing the thoracic volume. In addition, they stiffen the intercostal spaces to prevent them from bulging outward during straining. Experimental studies show that the actions of the respiratory muscles, especially the intercostals, are more complicated than this brief account suggests.

Respiratory Muscles

• Inspiration is active; expiration is passive during rest

• Diaphragm is the most important muscle of inspiration; it is supplied by phrenic nerves which originate high in the cervical region

» Other muscles include the intercostals, abdominal muscles, and accessory muscles

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