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The diaphragm is the main inspiratory muscle, which is a thin, dome-shaped muscle that separates the abdominal cavity from the thoracic cavity. During inhalation, the diaphragm contracts, so its centre moves caudally (downward) and its edges move cranially (upward). This compresses the abdominal cavity, raises the ribs upward and outward, and expands the thoracic cavity, drawing air into the lungs. The diaphragm works in conjunction with the intercostal muscles, which are attached between the ribs and manipulate the width of the rib cage. The scalene muscles also play a role in inspiration, consisting of scalenus anterior, scalenus medius, and scalenus posterior.
| Characteristics | Values |
|---|---|
| Main inspiratory muscle | Diaphragm |
| Accessory inspiratory muscles | Sternocleidomastoid, scalenus anterior, medius, and posterior, pectoralis major and minor, serratus anterior, latissimus dorsi, serratus posterior superior |
| Intercostal muscles | External and internal intercostals |
| Function | Drive expansion or contraction of the chest wall during breathing |
| Nerve supply | Phrenic nerve (C3 C4 C5) |
| Sensory supply | Phrenic nerve to central tendon and lower 6 or 7 intercostal nerve to peripheral parts |
| Blood supply | Inferior phrenic arteries, superior phrenic, musculophrenic, and pericardiacophrenic arteries |
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What You'll Learn
The diaphragm is the main inspiratory muscle
During inhalation, the diaphragm contracts, causing its centre to move downward and its edges to move upward. This compresses the abdominal cavity and raises the ribs upward and outward, expanding the thoracic cavity. This expansion draws air into the lungs. When the diaphragm relaxes, the elastic recoil of the lungs causes the thoracic cavity to contract, forcing air out of the lungs, and returning to its dome shape.
The diaphragm is also involved in non-respiratory functions, such as helping to expel vomit, faeces, and urine from the body by increasing intra-abdominal pressure. It also prevents acid reflux by exerting pressure on the oesophagus as it passes through the esophageal hiatus.
The diaphragm works in conjunction with other muscles to facilitate breathing. These include the intercostal muscles, which are attached between the ribs and manipulate the width of the rib cage. The external intercostal muscles are most important in respiration, as their contraction raises the rib cage, assisting in inhalation. The scalene muscles also play a role in inspiration, contributing to the elevation of the ribs.
During exercise, the diaphragm's function as the main inspiratory muscle is supported by the expiratory muscles, which play an active role in breathing. The rib cage muscles contract while the abdominal muscles relax during inspiration, and vice versa during expiration. This mechanism optimises the mechanics of breathing by preventing rib cage distortion, enabling the diaphragm to act as a flow generator, and decreasing the volume of the abdomen.
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Accessory inspiratory muscles
The diaphragm is the main inspiratory muscle. When its muscle fibres are activated in isolation, they shorten, the dome of the diaphragm descends, pleural pressure falls, and abdominal pressure rises. This results in the expansion of the rib cage.
However, there are also accessory inspiratory muscles, which assist in breathing but do not play a primary role. These muscles are typically used during rest in patients with respiratory distress. The accessory inspiratory muscles are:
- Sternocleidomastoid
- Scalenus anterior
- Scalenus medius
- Scalenus posterior
- Pectoralis major
- Pectoralis minor
- Inferior fibres of serratus anterior
- Latissimus dorsi
- Serratus posterior superior
- Iliocostalis cervicis
Technically, any muscle attached to the upper limb and the thoracic cage can act as an accessory inspiratory muscle through reverse muscle action.
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Intercostal muscles
The diaphragm is the main inspiratory muscle. When its muscle fibres are activated, they shorten, the dome of the diaphragm descends, and the rib cage expands. The diaphragm contracts and moves inferiorly, causing a fall in pleural pressure and a rise in abdominal pressure. This results in the expansion of the ventral abdominal wall and the contraction of a large fraction of the rib cage.
The intercostal muscles are a group of intrinsic rib cage muscles that occupy the 11 intercostal spaces between the ribs. They help form and move the chest wall and are mainly involved in the mechanical aspect of breathing by helping to expand and shrink the size of the chest cavity. They comprise three layers of muscles: external, internal, and innermost. These muscles combine to fill the space between the ribs and work together to aid in breathing.
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. These muscles elevate the ribs and bend them open, thereby expanding the transverse dimensions of the thoracic cavity. The muscle fibres are directed downwards, forwards, and medially in the anterior part. The function of the external intercostals is to elevate the ribs during forced inspiration.
The internal intercostal muscles, or intercostalis internus, aid in forced expiration, while quiet expiration is a passive process. They originate on ribs 2–12 and insert on ribs 1–11. These muscles depress the ribs and bend 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 the internal intercostal muscles, separated from them by a neurovascular bundle. They have the same direction of muscle fibre as the internal intercostals. Both the external and internal intercostal muscles are innervated by the intercostal nerves and supplied by the intercostal arteries. The innermost intercostal muscles follow the same vascularisation and innervation patterns as the internal intercostals.
Intercostal muscle strains are typically caused by weakened muscles, overexertion, direct trauma, or repetitive torso twisting. Symptoms include pain, muscle tension and stiffness, and difficulty breathing due to pain, resulting in shallow breathing patterns.
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Sternocleidomastoid and scalenes
Inspiratory muscles are those that drive the expansion or contraction of the chest wall during breathing. The diaphragm is the primary inspiratory muscle, but other muscles also play a role in inspiration.
The sternocleidomastoid and scalenes are accessory inspiratory muscles. The sternocleidomastoid muscle is one of the largest and most superficial cervical muscles. It originates at the manubrium of the sternum and the clavicle and inserts at the mastoid process of the temporal bone of the skull. The primary actions of this muscle are rotating the head to the opposite side and flexing the neck. It also acts as an accessory muscle of respiration. The sternocleidomastoid is innervated by the accessory nerve, which supplies only motor fibres.
The scalenes are a group of muscles in the neck that include the scalenus anterior, medius, and posterior. These muscles are also involved in respiration and are considered inspiratory neck muscles. The scalenes have been shown to undergo greater shortening during inflation compared to the sternocleidomastoid.
Studies have examined the activation of the sternocleidomastoid and scalenes during normoxic and hypoxic incremental inspiratory loading. These studies have found that both muscle groups demonstrate significant increases in activation that are linearly related to respiratory loading during both hypoxic and normoxic conditions. Additionally, sternocleidomastoid activation was greater during hypoxic conditions.
The sternocleidomastoid and scalenes are important accessory inspiratory muscles that contribute to the respiratory process, especially during increased respiratory demands due to hypoxia.
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The mechanics of inspiratory muscles
The diaphragm is the main inspiratory muscle. It is a thin, dome-shaped musculotendinous sheet of internal skeletal muscle located at the inferior-most aspect of the rib cage. It separates the abdominal cavity from the thoracic cavity. During inhalation, the diaphragm contracts, causing its centre to move caudally (downward) and its edges to move cranially (upward). This compresses the abdominal cavity, raises the ribs upward and outward, and expands the thoracic cavity, drawing air into the lungs.
The intercostal muscles are another important group of inspiratory muscles. These muscles are attached between the ribs and are responsible for manipulating the width of the rib cage. There are three layers of intercostal muscles: external, internal, and innermost intercostal muscles. The external intercostal muscles are most important in respiration. The contraction of these fibres raises each rib toward the rib above, with the overall effect of raising the rib cage, assisting in inhalation.
The accessory inspiratory muscles include the sternocleidomastoid, the scalenus anterior, medius, and posterior, the pectoralis major and minor, the inferior fibres of serratus anterior and latissimus dorsi, and the serratus posterior superior. Technically, any muscle attached to the upper limb and the thoracic cage can act as an accessory inspiratory muscle through reverse muscle action.
During quiet breathing, the diaphragm and inspiratory rib cage muscles work together to accomplish breathing. During exercise, the expiratory muscles play an active role in breathing, working in coordination with the inspiratory rib cage muscles. This coordination prevents rib cage distortion, unloads the diaphragm, and optimises the mechanics of breathing.
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Frequently asked questions
Inspiratory muscles are those that contribute to inhalation, or inspiration, when air moves into the lungs.
The diaphragm is the main inspiratory muscle. It is a thin, dome-shaped muscle that separates the abdominal cavity from the thoracic cavity. The intercostal muscles are also key, with the external intercostals being the most important in respiration.
Accessory inspiratory muscles include the sternocleidomastoid and the scalenes (anterior, middle, and posterior). They assist in elevating the rib cage.
