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The human rib cage is a complex structure, and the muscles attached to it play a crucial role in respiration and breathing. The diaphragm is the major muscle of inspiration, but several other muscles aid in this process by elevating the ribs. These include the scalene muscles, the sternocleidomastoid muscle, the serratus anterior muscle, the levatores costarum, and the serratus posterior superior. Additionally, the external and internal intercostal muscles, along with the innermost intercostal muscles, form part of the intercostal muscle group and are responsible for elevating and depressing the ribs during respiration. The quadratus lumborum also plays a role in stabilising the twelfth rib during inspiration.
| Characteristics | Values |
|---|---|
| Muscles that elevate ribs | External intercostal muscles, internal intercostal muscles, innermost intercostal muscles, serratus anterior, scalene muscles, sternocleidomastoid muscle, serratus posterior superior, levatores costarum, diaphragm |
| Number of pairs of external intercostal muscles | 11 |
| Number of pairs of internal intercostal muscles | 11 |
| Origin of external intercostal muscles | Inferior aspect of the rib above |
| Insertion of external intercostal muscles | Superior aspect of the rib below |
| Direction of external intercostal muscle fibres | Oblique, downward, and forward |
| Origin of internal intercostal muscles | Lateral aspect of the costal groove of the rib above |
| Insertion of internal intercostal muscles | Superior aspect of the rib below |
| Direction of internal intercostal muscle fibres | Orthogonal to external intercostal fibres |
| Function of intercostal muscles | Stabilise the position of the ribs during normal respiration, elevate ribs during inspiration, and depress ribs during expiration |
| Function of diaphragm | Major muscle of inspiration, alters thoracic volume during respiration, aids abdominal straining, increases intra-abdominal pressure |
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What You'll Learn
External intercostal muscles
The external intercostal muscles are a set of muscles located between adjacent ribs, attaching along the shafts of the ribs and extending obliquely from the lower border of one rib to the upper border of the adjacent rib below. There are 11 pairs of these muscles on both sides of the body, occupying the 11 intercostal spaces. The external intercostal muscles are the most superficial set of muscles, found superficially to the internal and innermost intercostals.
The external intercostal muscles play a role in respiration, working in unison during inhalation. During forced inspiration, they elevate the ribs and increase the thoracic volume, which facilitates breathing by expanding the lungs and allowing air to enter them. They are functionally classified as accessory respiratory musculature.
The fibres of the external intercostal muscles are directed obliquely downward and laterally on the back of the thorax, and downward, forward, and medially on the front. They are thicker than the internal intercostals, and their fibre direction is the same as that of the external abdominal oblique. The external intercostal muscles receive innervation from intercostal nerves of corresponding intercostal spaces, which are the anterior rami of thoracic spinal nerves. They have an extensive vascular supply, with blood supplied by anterior and posterior intercostal arteries, internal thoracic arteries, musculophrenic arteries, and the costocervical trunk.
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Internal intercostal muscles
The internal intercostal muscles are a part of the muscles of the thoracic wall, located in the intercostal spaces between the ribs. They are one of the three types of intercostal muscles, the others being external and innermost intercostal muscles. There are 11 internal intercostal muscles, filling 11 intercostal spaces by extending between the inferior margin of the superior rib to the superior margin of the first rib below.
The internal intercostal muscles originate from the inferior margin of the costal cartilage and the inner lip of the costal groove. Their fibres course posteroinferiorly and medially to insert into the superior border of the immediate inferior rib. 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.
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. The posterior surface of the superior margin of each internal intercostal encloses the intercostal tunnel with the anterior surface of the corresponding innermost intercostal. The neurovasculature of the intercostal groove; intercostal nerve, artery, and vein traverse this tunnel.
The internal intercostal muscles are primarily muscles of exhalation. During exhalation, the interosseous portions of the internal intercostal muscles depress and retract the ribs, compressing the thoracic cavity and expelling air. They are only used during forceful exhalation, such as coughing or during exercise, and not during relaxed breathing.
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Innermost intercostal muscles
The innermost intercostal muscles (Musculi intercostales intimi) 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 fibres of the innermost intercostals are oriented in an inferior and posterior direction. They originate from the inner surface of the costal groove of the ribs, deep to the origin of the internal intercostal muscles.
The innermost intercostals are often considered deep parts of the internal intercostals due to their similar muscle fibre orientation and common function. However, the two muscle groups are physically separated by the neurovascular bundle of the costal groove (intercostal artery, vein, and nerve) that runs over the superior part of the anterior surfaces of the innermost intercostals.
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. It is suggested that these muscles depress the ribs and aid in pushing air out of the lungs during forced exhalation. This categorises them as accessory respiratory muscles that are only active during forced breathing.
The innermost intercostal muscles are innervated by the intercostal nerves, which are the anterior rami of the thoracic spinal nerves. They receive blood supply from the anterior and posterior intercostal arteries, as well as the internal thoracic and musculophrenic arteries.
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Serratus posterior superior
The serratus posterior superior is an intermediate layer of back muscles. It originates from the C7-T3 vertebrae spinous process and the nuchal ligament. It inserts at the upper border of the 2nd to 5th ribs. The muscle is innervated by the ventral rami of the intercostal nerves of T1-T5.
The serratus posterior superior is part of the group of muscles that form the intermediate group of the extrinsic back muscle. It is located superficial to the thoracic part of the thoracolumbar fascia covering the splenius cervicis. It lies deep to the rhomboids and the trapezius muscle.
The function of the serratus posterior superior is still largely debated. However, it is generally believed that it plays a role in respiration. The muscle elevates the ribs and thus supports inspiration. It is an accessory muscle of inspiration.
The disproportionate use of the serratus posterior superior and other back muscles, such as cradling the phone between the ear and shoulder, can result in a condition known as scapulocostal syndrome. It is characterised by pain and paresthesia along the medial border of the scapula, radiating to the neck, chest, and upper extremities. The pain can be increased by lifting objects with outstretched hands or other activities that cause the scapula to exert pressure on the muscle.
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Scalene muscles
The scalene muscles are a group of three muscles on each side of the neck: the anterior, the middle, and the posterior. They are named from the Ancient Greek "skalēnós", meaning "uneven", as the pairs are all of differing lengths. These muscles are deep and positioned laterally in the neck, forming part of the floor of the posterior triangle of the neck.
The scalene muscles act as accessory muscles of respiration and perform flexion at the neck. They collectively act to elevate the first and second ribs, increasing the intrathoracic volume. This is particularly important for patients with respiratory distress, as the scalene muscles can aid with breathing by increasing the volume of the lungs.
The anterior scalene is the thickest at the level of the cricoid cartilage and thinner at its origin and insertion. It originates from the anterior tubercles of the transverse processes of the third to sixth cervical vertebrae and inserts on the scalene tubercle of the superior surface of the first rib. The middle scalene is the largest and longest of the three scalene muscles. It arises from the posterior tubercles of the transverse processes of the lower six cervical vertebrae and inserts into the upper surface of the first rib. The posterior scalene is the smallest and deepest of the scalene muscles. It arises from the posterior tubercles of the transverse processes of the fourth to sixth cervical vertebrae and is inserted into the outer surface of the second rib.
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Frequently asked questions
The intercostal muscles, serratus anterior, levatores costarum, serratus posterior superior, and diaphragm all elevate the ribs.
The intercostal muscles are the external, internal, and innermost intercostal muscles. There are 11 pairs of external intercostal muscles, which are the most superficial of the intercostal muscles.
The intercostal muscles elevate the ribs during inspiration, increasing the transverse diameter of the thorax.
The diaphragm is the major muscle of inspiration. It attaches to the xiphoid process, the costal margin of the thoracic wall, the lower six ribs, and the lumbar vertebrae. During inspiration, the diaphragm contracts and pulls down, increasing the negative thoracic pressure, which draws air into the thoracic cavity.
The serratus anterior originates on the superolateral surfaces of the first to eighth or ninth ribs and inserts along the superior angle, medial border, and inferior angle of the scapula. It functions to protract the scapula and facilitates scapular rotation.
