Dominic Stone
The intercostal muscles are a group of intrinsic rib cage muscles that occupy the 11 intercostal spaces. They are divided into three groups: external, internal, and innermost intercostal muscles. The external intercostal muscles are the most superficial set of muscles, while the internal intercostal muscles lie deep to the external intercostals, and the innermost intercostal muscles are the deepest layer of the thorax wall. The intercostal muscles are innervated by the intercostal nerves of corresponding intercostal spaces, which are the anterior rami of thoracic spinal nerves. These nerves play a crucial role in the functioning of the intercostal muscles, which are primarily involved in the mechanical aspect of breathing by helping to expand and shrink the size of the chest cavity during respiration.
| Characteristics | Values |
|---|---|
| Number of intercostal muscles | 11 |
| Groups | External, internal, innermost |
| Function | Accessory respiratory muscles that participate in the process of forced breathing |
| External intercostals | Facilitate forced inspiration |
| Internal and innermost intercostals | Aid forced expiration |
| Innervation | Intercostal nerves (T1-T11) |
| Blood supply | Anterior and posterior intercostal arteries, costocervical trunk, internal thoracic and musculophrenic arteries |
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What You'll Learn
External intercostal muscles
The external intercostal muscles, also known as the intercostales externi, are the most superficial set of muscles that occupy the 11 intercostal spaces. They are located superficially to the internal and innermost intercostals. The external intercostal muscles consist of 11 pairs of muscles, with each muscle originating from the sharp inferior costal border of one rib. The fibres of each muscle course inferomedially and insert along the outer lip of the superior border of the rib below.
The external intercostal muscles are accessory respiratory muscles that aid in quiet and forced inhalation. They work in unison during inhalation, contracting and causing the expansion of the chest cavity and an influx of air into the lungs. The external intercostal muscles elevate the ribs during forced inhalation, increasing the transverse and anteroposterior diameter of the lungs, which in turn decreases intrapleural pressure. This process expands the lungs and facilitates the entry of air into them.
The external intercostal muscles receive innervation from the intercostal nerves of corresponding intercostal spaces, which are the anterior rami of thoracic spinal nerves. The first two posterior intercostal arteries arise from the supreme intercostal artery, a branch of the costocervical trunk. The remaining nine arise from the posterior aspect of the thoracic aorta.
The most posterior fibres of each muscle fuse with the superior costotransverse ligaments, while the most anterior fibres blend with the external intercostal membrane, which fills the remainder of the corresponding intercostal space from the costochondral junction to the sternum.
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Internal intercostal muscles
The intercostal muscles are the muscles that occupy the 11 intercostal spaces between the ribs. They are divided into three groups, from superficial to deep: external, internal, and innermost intercostal muscles. The internal intercostal muscles are the middle layer of the intercostal muscle group. They originate from the inferior margin of the costal cartilage and inner lip of the costal groove. Their fibres course posteroinferiorly and medially to insert into the superior border of the immediate inferior rib.
The internal intercostal muscles are located deep to the external intercostals, from which they are separated by a thin fascia. They lie superficially to the innermost intercostal muscles, which some authors consider a part of the internal intercostals. The internal intercostal muscles form the intermediate layer of the thoracic wall, which is made up of five muscles in total. The other muscles are the external intercostal muscles, innermost intercostal muscles, subcostalis, and transversus thoracis.
The internal intercostal muscles are innervated by the intercostal nerves of corresponding intercostal spaces. Intercostal nerves are the anterior rami of thoracic spinal nerves. The muscle fibres of each internal intercostal muscle fill the intercostal space from the lateral border of the sternum to the posterior angle of the ribs onto which they attach. At the level of the posterior angle of the rib, the fibres of the internal intercostal muscle blend with the internal intercostal membrane, which fills the intercostal space from the angle of the rib to the spine.
The internal intercostal muscles are accessory respiratory muscles. They aid in forced expiration by depressing the ribs and bending them inward, thus decreasing the transverse dimensions of the thoracic cavity. The external intercostal muscles, in contrast, aid in forced inhalation by elevating the ribs and bending them more open, thereby expanding the thoracic cavity.
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Innermost intercostal muscles
The innermost intercostal muscles, also known as the intercostalis intimus, are the deepest layer of intercostal muscles. They are located deep to the internal and external intercostals, filling the 11 intercostal spaces between the ribs. The precise function of the innermost intercostal muscles is not yet known, but it is highly likely that they share the same function as the internal intercostal muscles. They are believed to depress the ribs and aid in pushing air out of the lungs during forced expiration. This places them in the category of accessory respiratory muscles, which are only active during forced breathing.
The innermost intercostal muscles originate from the inner surface of the costal groove of the ribs, deep to the origin of the internal intercostal muscles. They are separated from the internal intercostal muscles by a neurovascular bundle of the costal groove (intercostal artery, vein, and nerve) that runs over the superior part of their anterior surfaces. The posterior surfaces of these muscles are in relation to the endothoracic fascia and parietal pleura. The muscle fibres are directed downwards, forwards, and laterally, similar to the intercostalis internus muscle.
The innermost intercostal muscles are innervated by the intercostal nerves, which are the anterior rami of the thoracic spinal nerves. They receive their blood supply from the muscular branches of the anterior and posterior intercostal arteries, as well as the intercostal branches of the internal thoracic, musculophrenic arteries, and costocervical trunk. The venous drainage occurs via the anterior and posterior intercostal veins, which empty into either the azygos or brachiocephalic veins.
The innermost intercostal muscles are not equally developed in every intercostal space. The least developed muscles are found in the superior intercostal spaces, while the most developed ones are located in the inferior intercostal spaces. Even the most developed muscles do not fill the entire length of the intercostal spaces but rather occupy their middle two quarters.
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Intercostal nerves
The intercostal muscles are a group of intrinsic rib cage muscles that occupy the 11 intercostal spaces. They are divided into three groups, from superficial to deep: external intercostals, internal intercostals, and innermost intercostals. All three groups of muscles support the rib cage and are accessory respiratory muscles that participate in the process of forced breathing.
The external intercostals are the most superficial intercostal muscles. They are responsible for forced inhalation and the elevation of the ribs, thus expanding the thoracic cavity. The internal intercostals lie deep to the external intercostals and extend from the rib above to the one below, but in an opposite direction. They are responsible for depressing the ribs and decreasing the thoracic cavity during forced expiration. The innermost intercostals are the deepest intercostal muscles and are similar in structure to the internal intercostals. They assist in the depression of ribs during forced expiration.
The intercostal muscles are innervated by the intercostal nerves of corresponding intercostal spaces. Intercostal nerves are the anterior rami of thoracic spinal nerves. The external intercostal muscles receive innervation from intercostal nerves T2-T6, while the internal intercostal muscles are innervated by intercostal nerves T1-T11.
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Intercostal arteries and veins
The intercostal muscles are a group of intrinsic rib cage muscles that occupy the 11 intercostal spaces. They are divided into three groups, from superficial to deep: external, internal, and innermost intercostal muscles. All three groups of muscles support the rib cage and are accessory respiratory muscles, participating in the process of forced breathing.
The external intercostals facilitate forced inspiration by elevating the ribs and bending them more open, thus expanding the transverse dimensions of the thoracic cavity. The internal and innermost intercostals aid in forced expiration by depressing the ribs and bending them inward, thereby decreasing the transverse dimensions of the thoracic cavity.
The intercostal muscles are innervated by the intercostal nerves, which are the anterior rami of thoracic spinal nerves. The vascularization of the internal intercostals is through the blood supply of the thoracic wall. The internal intercostals are supplied by the muscular branches of the anterior and posterior intercostal arteries, as well as the intercostal branches of the internal thoracic, musculophrenic arteries, and costocervical trunk.
The venous blood from the internal intercostals is conveyed by the anterior and posterior intercostal veins to either the brachiocephalic or azygos venous system, which both drain into the superior vena cava. The anterior intercostal veins follow the same course as the arteries and drain into the internal thoracic and musculophrenic veins. The posterior intercostal veins drain into the azygos vein on the right side and the hemiazygos and accessory hemiazygos veins on the left side.
The intercostal arteries are a group of arteries that pass within the intercostal spaces, with 9 anterior and 11 posterior intercostal arteries on each side of the body. The anterior intercostal arteries are branches of the internal thoracic artery, while the posterior intercostal arteries are branches of the supreme intercostal artery and thoracic aorta. The subcostal arteries are direct branches of the thoracic aorta and are analogous to the posterior intercostal artery.
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Frequently asked questions
The intercostal muscles are the 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.
There are three types of intercostal muscles: external, internal, and innermost intercostal muscles.
The external intercostal muscles are the most superficial set of muscles. They facilitate forced inspiration and are responsible for the elevation of the ribs, thus expanding the thoracic cavity.
The internal intercostal muscles are located deep to the external intercostals. They aid in forced expiration and are responsible for depressing the ribs, thus reducing the thoracic cavity.
The intercostal muscles are innervated by the intercostal nerves (T1-T11) of corresponding intercostal spaces. The intercostal nerves are the anterior rami of thoracic spinal nerves.
