Dominic Stone
The diaphragm is the primary muscle responsible for respiration, and it is assisted by the intercostal muscles. During inhalation, the diaphragm contracts, moving downwards and increasing the volume of the thoracic cavity, which causes a drop in intrathoracic pressure, drawing air into 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 toward the rib above, with the overall effect of raising the rib cage, assisting in inhalation. Accessory muscles that assist in inspiration include the sternocleidomastoid, scalenus anterior, medius, and posterior, pectoralis major and minor, serratus anterior, and latissimus dorsi.
| Characteristics | Values |
|---|---|
| Main muscle of respiration | Diaphragm |
| Primary inspiratory muscles | Diaphragm and external intercostals |
| Accessory inspiratory muscles | Sternocleidomastoid, scalenus anterior, medius, and posterior, pectoralis major and minor, serratus anterior, latissimus dorsi, serratus posterior superior |
| Muscles that elevate the rib cage | External intercostals |
| Muscles that elevate the sternum | Sternocleidomastoid |
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The diaphragm
In addition to its respiratory function, the diaphragm assists in non-respiratory processes. It helps prevent acid reflux by exerting pressure on the oesophagus and aids in expelling vomit, faeces, and urine from the body by increasing intra-abdominal pressure. The diaphragm can be considered as comprising two distinct muscles: the crural and costal diaphragms, which act synchronously during respiration. However, their activities diverge during certain events like swallowing.
To maintain diaphragm health, individuals can perform diaphragmatic breathing exercises, which strengthen the muscle and improve its efficiency. Maintaining a healthy weight, warming up before exercise, and seeking regular check-ups, especially if at higher risk for diaphragm complications, are also recommended. The diaphragm is a vital muscle that not only facilitates respiration but also contributes to overall health and well-being.
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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 responsible for the mechanical aspect of breathing by helping to expand and shrink the chest cavity. There are three layers of intercostal muscles: external, internal, and innermost.
The external intercostal muscles, also known as intercostalis externus, aid in quiet and forced inhalation. They elevate the ribs, bending them more open and thus expanding the transverse dimensions of the thoracic cavity. The muscle fibres are directed downwards, forwards, and medially in the anterior part. The external intercostals are the most superficial intercostal muscles.
The internal intercostal muscles, or intercostalis internus, aid in forced expiration. They originate from the costal groove of one rib, course inferolaterally, and insert into the superior border of the immediate rib below. They are responsible for the depression of the ribs, bending them inward and 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, also referred to as intercostalis intimus, are deep layers of the internal intercostal muscles, separated from them by a neurovascular bundle. The muscle fibres are directed downwards, forwards, and laterally, similar to the intercostalis internus muscle. The innermost intercostals are the deepest intercostal muscles. They originate from the costal groove of one rib and insert into the superior border of the immediate rib below.
All three groups of intercostal muscles support the rib cage and are accessory respiratory muscles that participate in the process of forced breathing. Specifically, the external intercostals facilitate forced inspiration, while the internal and innermost intercostals aid in forced expiration.
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Accessory muscles
Other muscles that have been observed to contribute to respiration include the 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, and subclavius. The levator labii superioris alaeque nasi muscle lifts the sides of the nostrils during inhalation.
The diaphragm is the main muscle of respiration, separating the thoracic and abdominal cavities. During inhalation, the diaphragm contracts, moving downward and expanding the thoracic cavity, while the external and internal intercostal muscles elevate the rib cage. This increase in volume causes a drop in intrathoracic pressure, drawing air into the lungs. The diaphragm also increases intra-abdominal pressure, aiding venous return to the heart.
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Neck muscles
The neck is home to about 30 muscles that allow for a wide range of movements. These muscles are part of a complex musculoskeletal system that connects the base of the skull to the torso. Neck muscles support and stabilise the head, neck, and upper back, and they also help with chewing, swallowing, and breathing.
The sternocleidomastoid and the scalenes (anterior, middle, and posterior) are accessory muscles of respiration that assist in elevating the rib cage during inhalation. The scalenes are consistently physically active during quiet breathing, while the sternocleidomastoids are quiet. With an increase in respiratory volume, the sternocleidomastoids become active. Both muscles are simultaneously activated when one breathes in at the maximal flow rate.
The infrahyoid muscles are four muscles below the hyoid bone that move the larynx (voice box) up and down. The scalene muscles move the first two ribs up and down so that air can be inhaled. They also help to move the head and stabilise the bones in the neck (cervical spine).
Neck accessory muscle breathing (NAMB) has been used in the treatment of patients with ventilator dependency due to spinal cord injury.
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Abdominal muscles
The abdominal muscles are sometimes involved in breathing out (exhalation or expiration). During forceful exhalation, active exhalation can be achieved by contraction of the abdominal wall muscles (rectus abdominis, transverse abdominis, external oblique muscle, and internal oblique muscle). These muscles press the abdominal organs cranially (upward) into the diaphragm, reducing the volume of the thoracic cavity.
The diaphragm is the main inspiratory muscle, and it is supported by the abdominal muscles during exhalation. The diaphragm is a dome-shaped sheet of muscle that separates the chest cavity from the abdomen. 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. This expansion draws air into the lungs.
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 oesophageal hiatus.
The intercostal muscles are another important group of respiratory muscles. The external intercostal muscles are the most important in respiration. These have fibres that are angled obliquely downward and forward from rib to rib. 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 internal intercostal muscles have fibres that are angled obliquely downward and backward from rib to rib. During normal expiration, the external intercostals, together with the diaphragm, relax. The intrathoracic volume decreases, intrapulmonary pressure increases, and air is expelled from the lungs.
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Frequently asked questions
The diaphragm is the main muscle that assists with inspiration. 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). This compresses the abdominal cavity, raises the ribs upward and outward and thus expands the thoracic cavity. This expansion draws air into the lungs.
Accessory muscles of respiration are muscles that assist, but do not play a primary role, in breathing. These include the sternocleidomastoid, the scalenus anterior, medius, and posterior, the pectoralis major and minor, the serratus anterior, and the latissimus dorsi.
Intercostal muscles are those attached between the ribs and are important in manipulating the width of the rib cage. There are three layers of intercostal muscles: external, internal, and innermost. The external intercostal muscles are the most important in respiration. These have fibres that are angled obliquely downward and forward from rib to rib. The contraction of these fibres raises each rib toward the rib above, with the overall effect of raising the rib cage, assisting in inhalation.
During active or forced inspiration, a number of additional 'accessory' muscles may be recruited, including the serratus posterior superior, iliocostalis cervicis, levatores costarum, transversus thoracis, and subclavius.
