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The diaphragm is the main muscle responsible for inhalation, but it is not alone in this task. The intercostal muscles and neck muscles also help move the rib cage and assist in breathing. The diaphragm is a dome-shaped muscle that separates the chest cavity from the abdomen. During inhalation, it contracts and moves downward, increasing the length and diameter of the chest cavity, which expands the lungs. The intercostal muscles are attached between the ribs and manipulate the width of the rib cage. The external intercostal muscles are the most important in respiration, as their contraction raises each rib, lifting the rib cage and aiding inhalation. Accessory muscles, such as the sternocleidomastoid and scalenes, also assist in elevating the rib cage and play a role in inspiration.
| Characteristics | Values |
|---|---|
| Main inspiratory muscle | Diaphragm |
| Diaphragm's function | Contracts and moves in an inferior direction, increasing the vertical diameter of the thoracic cavity and producing lung expansion |
| Diaphragm's origin | Costal groove (lower part of the inner surface of the rib near the inferior border) |
| Diaphragm's blood supply | Anterior and posterior intercostal arteries, internal thoracic and musculophrenic arteries, and costocervical trunk |
| Accessory inspiratory muscles | Sternocleidomastoid, scalenus anterior, medius, and posterior, pectoralis major and minor, inferior fibres of serratus anterior and latissimus dorsi, serratus posterior superior, and iliocostalis cervicis |
| Intercostal muscles | External, internal, and innermost |
| Intercostal muscles' function | Assist in the function of external and internal intercostal muscles |
| Intercostal muscles' origin | Costal groove of the rib above |
| Intercostal muscles' blood supply | Anterior and posterior intercostal arteries, internal thoracic and musculophrenic arteries, and costocervical trunk |
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The diaphragm
During inhalation, the diaphragm contracts and moves in an inferior direction, increasing the vertical diameter of the thoracic cavity and producing lung expansion. This, in turn, draws air into the lungs. The diaphragm pulls the lower surfaces of the lungs downwards, and simultaneously, the muscles of inspiration, mainly the external intercostals, elevate the rib cage.
During inspiration, while the rib cage muscles contract, the abdominal muscles relax. This mechanism prevents rib cage distortion, and also means the diaphragm is unloaded and can act as a flow generator. During exercise, the diaphragm functions primarily as a flow generator, while the rib cage and abdominal muscles are pressure generators.
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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 divided into three groups, from superficial to deep: external intercostals, internal intercostals, and innermost intercostals.
The external intercostal muscles, or intercostalis externus, are the outermost layer of intercostal muscles. They originate on ribs 1–11 and insert on ribs 2–12. During inhalation, they elevate the ribs, bending them open and expanding the transverse dimensions of the thoracic cavity. This expansion increases the intrathoracic volume, allowing the lungs to expand. The muscle fibres are directed downwards, forwards, and medially in the anterior part.
The internal intercostal muscles, or intercostalis internus, are the intermediate layer. They originate from the costal groove near the inferior border of the rib above and insert into the upper border of the rib below. These muscles aid in forced expiration by depressing the ribs and bending them inward, thus decreasing the transverse dimensions of the thoracic cavity. The muscle fibres are directed downwards, forwards, and laterally, forming a right angle with the external intercostal muscle.
The innermost intercostal muscles, or intercostalis intimus, are the deepest layer of intercostal muscles. They cross more than one intercostal space and assist the internal and external intercostals in their functions. The muscle fibres are directed downwards, forwards, and laterally, similar to the internal intercostal muscles.
All three groups of intercostal muscles support the rib cage and participate in the process of forced breathing. The external intercostals facilitate forced inspiration, while the internal and innermost intercostals aid in forced expiration.
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Sternocleidomastoid
The sternocleidomastoid (SCM) muscle is a powerful neck muscle that allows you to bend your neck and turn or tilt your head. It is the largest neck muscle in the front of your neck. The SCM muscle extends from the mastoid process at the base of your skull to your collarbones (clavicles) and breastbone (sternum).
The SCM muscle is considered an accessory muscle for respiration. It supports inhalation when lung volume nears 100% vital capacity or when inhalation is very rapid. During inspiration, the SCM muscle elevates the sternum and clavicle, subsequently lifting the ribs.
Injuries, tension, sprains, strains, atrophy, and tumours can all damage the SCM muscle. Sternocleidomastoid syndrome is a condition involving neck stiffness, pain, and other symptoms. It occurs when the SCM muscle develops tightened, sensitive areas, or trigger points. Treatment options include stretching, physical therapy, osteopathic manipulation, surgery, and managing stress and anxiety.
To care for your SCM muscle, it is important to maintain good posture. This includes positioning digital devices properly to avoid craning your neck and supporting your neck while sleeping.
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Scalenus
The scalenus muscles are a group of neck muscles that aid in inhalation. They consist of three types: scalenus anterior, scalenus medius, and scalenus posterior. All three types of scalenus muscles are involved in breathing and play a role in inspiration.
The scalenus anterior muscles extend from the anterior tubercles of the transverse processes of C3 to C6 vertebrae to the first rib, contributing to its elevation. This subtle action of lifting the first rib during inhalation classifies the scalenus anterior as an accessory breathing muscle.
The scalenus medius, also known as the medial scalene, resides between the anterior and posterior branches of the scalene muscle. It originates from the posterior tubercles of the transverse processes of the lower cervical vertebrae (C2 to C7) and runs to the first rib, raising it. The scalenus medius can contract unilaterally or bilaterally. When only one side contracts, it lifts the first rib, flexes, and laterally bends the neck to the same side. When both medial scalenes contract, they flex the neck. These movements are enabled by the proximity of the attachment sites to those of the scalenus anterior.
The scalenus posterior is also considered an accessory breathing muscle as it lifts the second rib during inhalation. The scalenus anterior, scalenus medius, and the first rib together form an anatomical area called the scalene triangle or scalene fissure. This formation is important as the brachial plexus nerve complex branches pass through it.
The scalenus muscles are prone to tension and tightness, which can lead to neck problems such as torticollis, or wry neck, where neck muscles remain contracted in chronically shortened positions.
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Abdominal muscles
The abdominal muscles play a key role in the breathing process, alongside the diaphragm and rib cage muscles. While the diaphragm is the primary muscle responsible for inhalation, the abdominal muscles are crucial for exhalation or expiration. During inhalation, the diaphragm contracts and moves downward, increasing the vertical diameter of the chest cavity and expanding the lungs. Simultaneously, the muscles of inspiration, primarily the external intercostals, elevate the rib cage.
The abdominal muscles come into play during exhalation, when they contract and relax in a coordinated manner with the inspiratory muscles. This mechanism prevents rib cage distortion and allows the diaphragm to act as a "flow generator," optimising the mechanics of breathing. The abdominal muscles also contribute to protective reflexes such as coughing, sneezing, and vomiting, as well as generating the pressure required for defecation and parturition.
The abdominal muscles can be categorised into internal and external muscles. The internal abdominal muscles, namely the internal oblique and transverse abdominis, are more sensitive to increases in chemical or volume-related drive. These muscles respond to changes in lung volume and respiratory drive, such as hypercapnia and hypoxia. The external abdominal muscles, including the rectus abdominis and external oblique, play a role in expiration by increasing intra-abdominal pressure and pushing the diaphragm upwards.
The rectus abdominis, in particular, pulls the ribs down during active expiration, reducing the size of the thoracic cavity. This muscle originates from the pubic symphysis and pubic crest and attaches to the xiphoid process and the 5th to 7th costal cartilages. The external oblique, internal oblique, and transversus abdominis are also accessory expiratory muscles that contribute to the breathing process.
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Frequently asked questions
The diaphragm is the main muscle used for inhalation. It is a dome-shaped muscle that separates the chest cavity from the abdomen. During inhalation, the diaphragm contracts and moves downward, increasing the length and diameter of the chest cavity and expanding the lungs.
Accessory muscles that aid in inhalation include the sternocleidomastoid, scalenus anterior, scalenus medius, and scalenus posterior. These muscles help elevate the rib cage and sternum, assisting in inhalation.
In addition to the diaphragm and accessory muscles, the intercostal muscles, neck muscles, and abdominal muscles also play a role in respiration. The intercostal muscles are attached between the ribs and help manipulate the width of the rib cage. The abdominal muscles are primarily involved in exhalation but can also be recruited during inhalation in certain situations.
