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The muscles of ventilation are the muscles that enable breathing by aiding in the expansion and contraction of the thoracic cavity. The diaphragm is the primary muscle responsible for breathing, and it separates the abdominal cavity from the thoracic cavity. The intercostal muscles are another important group of respiratory muscles that are attached between the ribs and manipulate the width of the rib cage. The contraction of these muscles raises the rib cage, assisting in inhalation. Accessory muscles of respiration include the sternocleidomastoid and the scalenes, which also assist in elevating the rib cage.
| Characteristics | Values |
|---|---|
| Definition | Muscles that contribute to inhalation and exhalation, by aiding in the expansion and contraction of the thoracic cavity |
| Location | Located in the rib cage |
| Function | Expand the thoracic cavity to drive air into the lungs |
| Types | Diaphragm, External Intercostals, Internal Intercostals, Innermost Intercostals, Scalenus Anterior, Scalenus Medius, Scalenus Posterior, Abdominal Muscles, Serratus Anterior, Sternocleidomastoid, Trapezius, Pectoralis Major, Pectoralis Minor, Latissimus Dorsi, Erector Spinae, Iliocostalis, Quadratus Lumborum, Serratus Posterior Superior, Serratus Posterior Inferior, Levatores Costarum, Transversus Thoracis, Subclavius, Levator Labii Superioris Alaeque Nasi |
| Nerve Supply | Phrenic nerve, Intercostal nerves |
| Blood Supply | Anterior and Posterior Intercostal arteries, Internal Thoracic and Musculophrenic arteries, Costocervical trunk |
| Activation | Controlled by both voluntary and involuntary mechanisms |
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What You'll Learn
The diaphragm is the main inspiratory muscle
The diaphragm is a dome-shaped muscle that sits underneath the lungs and separates the thoracic cavity from the abdominal cavity. It is the main inspiratory muscle, responsible for driving respiration during quiet breathing. During inspiration, the diaphragm contracts and moves in an inferior direction, increasing the vertical diameter of the thoracic cavity and producing lung expansion, which draws air into the lungs. This contraction also compresses the abdominal cavity, raising the ribs upward and outward, further expanding the thoracic cavity.
The diaphragm is innervated by the phrenic nerve, which originates in the medullary respiratory centre in the medulla of the brain. It receives its motor nerve supply from the phrenic nerve (C3-C5) and sensory supply from the phrenic nerve to the central tendon. The phrenic nerve also supplies the sensory nerves to the diaphragm's peripheral parts and the lower six or seven intercostal nerves.
The intercostal muscles are another important group of respiratory muscles, located in the rib cage between the ribs. They assist in the function of the diaphragm and external intercostal muscles, and there are three types: external, internal, and innermost intercostal muscles. The external intercostal muscles are the most important in respiration, as they raise each rib toward the rib above, elevating the rib cage and assisting in inhalation. The internal intercostal muscles have fibres that angle obliquely downward and backward from rib to rib. The innermost intercostal muscles are the deepest layer and assist in forced exhalation.
The accessory inspiratory muscles include the sternocleidomastoid, scalenus anterior, medius, and posterior, pectoralis major and minor, and the serratus anterior, among others. These muscles assist in elevating the rib cage and lifting the ribs and clavicle during inspiration. They are recruited during exercise due to the increased metabolic need and ventilatory demands, and during dysfunction in the respiratory system.
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External intercostal muscles assist in inhalation
The ventilation muscles are the muscles that contribute to inhalation and exhalation, aiding in the expansion and contraction of the thoracic cavity. The diaphragm is the main inspiratory muscle. However, the intercostal muscles are also one of the most important groups of respiratory muscles.
The intercostal muscles are located in the rib cage and are attached between the ribs. They 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 the most important in respiration. They are the most superficial layer of the intercostal muscle group, located in the intercostal spaces between the ribs.
The external intercostal muscles assist in inhalation by raising the rib cage. They have fibres that are angled obliquely downward and forward from rib to rib. When these fibres contract, they raise each rib toward the rib above, with the overall effect of elevating the rib cage. This action increases the volume of the thoracic cavity, which in turn draws air into the lungs.
The contraction of the external intercostal muscles is a voluntary and involuntary process. During inhalation, these muscles contract, raising the rib cage and expanding the thoracic cavity. During exhalation, they relax, allowing the thoracic cavity to contract and air to be forced out of the lungs.
The external intercostal muscles work in conjunction with the diaphragm to facilitate breathing. The diaphragm is a dome-shaped muscle that sits underneath the lungs, separating the thoracic cavity from the abdominal cavity. During inhalation, the diaphragm contracts, moving downward and increasing the vertical diameter of the thoracic cavity, which produces lung expansion and draws air into the lungs. During exhalation, the diaphragm relaxes, allowing the elastic recoil of the lungs to cause the thoracic cavity to contract and air to be expelled.
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Muscles involved in expiration
Ventilation muscles are those that contribute to inhalation and exhalation, aiding in the expansion and contraction of the thoracic cavity. The diaphragm and, to a lesser extent, the intercostal muscles drive respiration during quiet breathing. The diaphragm is the major muscle responsible for breathing. It 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. This expansion draws air into the lungs. The external intercostal muscles are 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 quiet breathing, there is little to no muscle contraction involved in exhalation. This process is driven by the elastic recoil of the lungs. When forceful exhalation is required, or when lung elasticity is reduced, active exhalation can be achieved by contraction of the abdominal wall muscles. These include the rectus abdominis, transverse abdominis, external oblique muscle, and internal oblique muscle. These abdominal muscles press the abdominal organs upward into the diaphragm, reducing the volume of the thoracic cavity.
The internal intercostal muscles have fibres that are angled obliquely downward and backward from rib to rib. These muscles help in forceful expiration, along with the intercostalis intimi, subcostals, and the abdominal muscles.
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Accessory muscles of respiration
The muscles of respiration, also known as the 'breathing pump muscles', form a complex arrangement in the form of a semi-rigid bellows around the lungs. All muscles attached to the human rib cage have the potential to cause a breathing action. The diaphragm is the major muscle of ventilation and is the only muscle used in normal breathing. It is a thin, dome-shaped muscle that separates the abdominal cavity from the thoracic cavity. During inhalation, the diaphragm contracts, moving its centre downwards and its edges upwards, compressing the abdominal cavity and raising the ribs outward, which expands the thoracic cavity and draws air into the lungs.
The accessory inspiratory muscles include the sternocleidomastoid, the scalenus anterior, medius, and posterior, the pectoralis major and minor, the serratus anterior and latissimus dorsi, and the iliocostalis cervicis. 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. The contraction of the sternocleidomastoid and scalene muscles lifts the clavicles and first ribs, helping to expand the thorax of distressed patients, especially those with chronic obstructive lung disease.
The accessory expiratory muscles are the abdominal muscles: rectus abdominis, external oblique, internal oblique, and transversus abdominis. In the thoracolumbar region, the lowest fibres of iliocostalis and longissimus, the serratus posterior inferior and quadratus lumborum are also accessory expiratory muscles.
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The role of intercostal muscles
The intercostal muscles are located in the rib cage and play a crucial role in respiration, alongside the diaphragm. They receive neuronal inputs from intercostal nerves that arise from the thoracic nerves of the spinal cord. There are three types of intercostal muscles: external, internal, and innermost intercostal muscles. The external intercostal muscles are the most superficial and important for respiration. They have fibres that run obliquely downward and forward from rib to rib. When these fibres contract, they raise each rib towards the rib above, elevating the rib cage and assisting in inhalation. Conversely, the internal intercostal muscles have fibres that run obliquely downward and backward from rib to rib. These muscles, along with the intercostalis intimi, subcostals, and abdominal muscles, aid in forceful exhalation.
The mechanical advantages of the external and internal intercostals depend on their orientation, position within the interspaces, and the number of interspaces. The external intercostals in the dorsal portion of the rostral interspaces have a significant inspiratory mechanical advantage, which decreases as you move ventrally and caudally. On the other hand, the internal interosseous intercostals in the caudal interspaces have a large expiratory mechanical advantage, which diminishes cranially and ventrally in the upper interspaces.
The intercostal muscles are composed of fatigue-resistant muscle fibres and are controlled by both voluntary and involuntary mechanisms. They are continuously in use, especially during periods of low breathing, such as rest, when only the inspiratory muscles are active. During high breathing activities like exercise, the expiratory muscles also become active. The intercostal muscles, along with the diaphragm, contribute to inhalation and exhalation by aiding in the expansion and contraction of the thoracic cavity, which is crucial for respiratory health and optimal function.
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Frequently asked questions
Ventilation muscles, also known as respiratory muscles, are the muscles that contribute to inhalation and exhalation by aiding in the expansion and contraction of the thoracic cavity.
The diaphragm is the main inspiratory muscle. The diaphragm contracts and moves in an inferior direction, increasing the vertical diameter of the thoracic cavity and producing lung expansion, in turn, air is drawn in. The external intercostals are also primary muscles of ventilation.
The secondary muscles of ventilation are the scalenes, sternocleidomastoid, trapezius, and serratus anterior. These muscles lift the ribs and clavicle. The abdominal muscles, internal intercostals, and relaxation of the diaphragm facilitate expiration.
Inspiration is when air moves into the lungs, and expiration is when air leaves the lungs. Inspiration is driven by the contraction of the diaphragm and external intercostal muscles, while expiration occurs when these muscles relax.
