Victor Forge
The process of inspiration, or breathing in, is vital for providing oxygen to tissues and removing carbon dioxide from the body. The diaphragm is the major muscle responsible for driving respiration during quiet breathing. It is a thin, dome-shaped muscle that separates the abdominal cavity from the thoracic cavity. During inhalation, the diaphragm contracts, so that its centre moves caudally (downward) and its edges move cranially (upward), expanding the thoracic cavity and drawing air into the lungs. In addition to the diaphragm, the intercostal muscles, serratus anterior, scalene muscles, and sternocleidomastoid are also involved in the process of inspiration.
| Characteristics | Values |
|---|---|
| Main muscle of inspiration | Diaphragm |
| Other names for the diaphragm | Thoracic cage, respiratory muscle, muscle of respiration, chief muscle of inspiration, main muscle of respiration |
| Diaphragm's function | Lengthens and shortens the chest cavity, expands the rib cage, moves up and down, expands and contracts the lungs |
| Diaphragm's structure | Musculotendinous domed structure, consisting of a right and left dome |
| Diaphragm's contribution to increase in thoracic volume | 75% |
| Secondary muscles of inspiration | External intercostals, sternocleidomastoids, scalenes, serratus anterior, pectoralis major and pectoralis minor, trapezius, latissimus dorsi, erector spinae, iliocostalis, quadratus lumborum, serratus posterior superior, serratus posterior inferior, levatores costarum, transversus thoracis |
| External intercostals' contribution to increase in thoracic volume | 25% |
| Function of the scalene muscles | Elevate the rib cage |
| Function of the levator labii superioris alaeque nasi muscle | Lifts the sides of the nostrils |
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The diaphragm
During inspiration, or inhalation, the diaphragm contracts and moves down, allowing the lungs to fill with air. This downward movement of the diaphragm compresses the abdominal cavity, raising the ribs outward and upward, which expands the thoracic cavity. This expansion creates a pressure gradient, causing air to be pulled into the lungs. The diaphragm is the primary muscle responsible for this action, and it has a high duty cycle, contracting 10 to 20 times per minute during quiet breathing.
During expiration, or exhalation, the diaphragm relaxes and moves back up, reducing the space inside the chest and pushing air out of the lungs. This movement is facilitated by the elastic recoil of the lungs, causing the thoracic cavity to contract and return to its dome shape. The diaphragm's function during exhalation is supported by the contraction of abdominal wall muscles, which further reduce the volume of the thoracic cavity.
In addition to its respiratory function, the diaphragm also has non-respiratory roles. It helps to increase intra-abdominal pressure, aiding in the expulsion of vomit, faeces, and urine from the body. Additionally, the diaphragm prevents acid reflux by exerting pressure on the oesophagus as it passes through the diaphragm. This dual functionality has led to differing perspectives on the classification of the diaphragm, with respiratory physiologists focusing on its respiratory function and gastrointestinal physiologists emphasising its role in gastric processes.
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Intercostal muscles
The intercostal muscles are a group of intrinsic rib cage muscles that occupy the 11 intercostal spaces between the ribs. They are accessory respiratory muscles that participate in the process of forced breathing. They are divided into three groups, from superficial to deep: external intercostals, internal intercostals, and innermost intercostals.
The external intercostal muscles, or intercostalis externus, aid in quiet and forced inhalation. They originate on ribs 1–11 and insert on ribs 2–12. During inhalation, the contraction of these muscle fibres raises each rib toward the rib above, elevating the ribs and bending them more open. This expands the transverse dimensions of the thoracic cavity, allowing air to enter the lungs. The external intercostals are the most superficial intercostal muscles and are most important in respiration.
The internal intercostal muscles, or intercostalis internus, aid in forced exhalation or expiration. They originate on ribs 2–12 and insert on ribs 1–11. Their fibres are directed downwards, forwards, and laterally, forming a right angle with the external intercostal muscle. The internal intercostals depress the ribs and bend them inward, decreasing the transverse dimensions of the thoracic cavity, which helps to expel air from the lungs.
The innermost intercostal muscles, or intercostalis intimus, are deep layers of the internal intercostal muscles, separated from them by a neurovascular bundle. Their fibres are directed downwards, forwards, and laterally, similar to the intercostalis internus muscle. Deficient innermost intercostal muscles may not provide adequate tension to the intercostal spaces, leading to internal and external movements of the chest wall during thoracic pressure changes and resulting in respiratory insufficiency.
Overall, the intercostal muscles play a crucial role in respiration by helping to expand and shrink the chest cavity during inhalation and exhalation, respectively. They work in conjunction with other respiratory muscles, such as the diaphragm, to facilitate the mechanical aspect of breathing.
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Accessory muscles
There is no definitive list of accessory muscles, but the sternocleidomastoid and the scalenes (anterior, middle, and posterior) are typically included, as they assist in elevating the rib cage. The scalenes are consistently physically active during quiet breathing, while the sternocleidomastoids are quiet. With an increase in respiratory volume, the sternocleidomastoids also become active. Both muscles are simultaneously activated when one breathes in at the maximal flow rate.
The serratus anterior, pectoralis major and pectoralis minor, trapezius, latissimus dorsi, erector spinae, iliocostalis, quadratus lumborum, serratus posterior superior, serratus posterior inferior, levatores costarum, and transversus thoracis have also been observed contributing to respiration.
The subclavius, which usually stabilizes the rib cage, is another example of an accessory muscle. It lies between the collarbone and the top rib.
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Neck muscles
In addition to the diaphragm, several neck muscles contribute to inspiration. These include the scalene muscles, consisting of the scalenus anterior, scalenus medius, and scalenus posterior. The scalenus anterior and scalenus medius muscles contribute to elevating the first rib, while the scalenus posterior elevates the second rib. The elevation of the ribs helps expand the thoracic cavity, aiding in inhalation.
Another important neck muscle during inspiration is the sternocleidomastoid. This muscle originates from the sternum and clavicle and inserts on the mastoid process of the temporal bone and occipital bone. Bilateral contraction of the sternocleidomastoid flexes the neck, bringing the head towards the chest. When the head and neck are fixed, this muscle assists in elevating the sternum and clavicle, thereby expanding the thoracic cavity and facilitating inspiration.
The sternocleidomastoid and scalene muscles work together during quiet breathing, with the sternocleidomastoid becoming more active during increased respiratory volume. They are considered accessory muscles of respiration, providing additional support to the diaphragm during inspiration.
Furthermore, the sternothyroid muscle, located in the neck, plays a role in opening the laryngeal inlet, which aids in forced inspiration. By depressing the hyoid bone and larynx, the sternothyroid helps to reestablish breathing after swallowing.
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Abdominal muscles
The abdominal muscles, along with the diaphragm and external intercostals, are the primary muscles involved in inspiration. During inspiration, the diaphragm contracts and moves inferiorly, increasing the vertical diameter of the thoracic cavity and facilitating lung expansion, which draws air into the lungs. The abdominal muscles assist in this process by helping to raise the rib cage.
The abdominal muscles play a significant role in breathing, particularly during periods of increased chemical drive or end-expiratory lung volume. They contribute to pulmonary ventilation, which is the movement of air between the atmosphere and the lung alveoli. During inspiration, the abdominal muscles aid in expanding the thoracic cavity, allowing air to enter the lungs.
While the diaphragm is the primary muscle responsible for inspiration, the abdominal muscles provide crucial support. They contract during inspiration, assisting in the elevation of the rib cage and creating the necessary space for the lungs to expand. This coordinated action between the diaphragm and the abdominal muscles ensures effective inhalation.
Additionally, the abdominal muscles exhibit remarkable adaptability in respiratory functions. In individuals with absent or impaired abdominal muscles, such as those with prune belly syndrome, compensatory mechanisms involving other respiratory muscles, including the pectorals, maintain satisfactory ventilation and exercise capacity. This highlights the body's ability to adjust and optimise respiratory processes even in the presence of muscular deficiencies.
The abdominal muscles also play a role in forceful exhalation or expiration. During this phase of the breathing cycle, they contract and press the abdominal organs cranially, reducing the volume of the thoracic cavity and facilitating air expulsion from the lungs. This dual function of the abdominal muscles in both inspiration and expiration underscores their importance in maintaining optimal respiratory function.
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Frequently asked questions
The diaphragm is the main muscle of inspiration. It is a thin, dome-shaped muscle that separates the abdominal cavity from the thoracic cavity. During inspiration, the diaphragm contracts, moving down and increasing the volume of the thoracic cavity, which results in air being drawn into the lungs.
Intercostal muscles are attached between the ribs and are important in manipulating the width of the rib cage. The external intercostal muscles are most important in respiration, contributing to quiet inspiration. They cause the rib cage to move up and outward, increasing the thoracic volume.
Accessory muscles are muscles that assist, but do not play a primary role in breathing. During forced inspiration, accessory muscles may be recruited, including the sternocleidomastoids and scalenes. The scalenes are a trio of prevertebral muscles capable of flexing the neck and elevating the upper two ribs.
The abdominal muscles and rib cage muscles also function during inspiration. The abdominal muscles act on the abdomen and abdominal rib cage and are expiratory, relaxing during inspiration. The rib cage muscles, including the intercostals, parasternals, scalene and neck muscles, mostly act on the upper part of the rib cage and are both inspiratory and expiratory.
