Victor Forge
The muscles that control inspiration, or inhalation, are known as the muscles of respiration. These muscles aid in the expansion and contraction of the thoracic cavity, which changes the volume of the lungs and, subsequently, the pressure within them. The diaphragm is the major muscle responsible for breathing, and it is assisted by the intercostal muscles. During inspiration, the diaphragm contracts, moving downward and compressing the abdominal cavity, while raising the ribs upward and outward, thus expanding the thoracic cavity. This expansion draws air into the lungs. The intercostal muscles are attached between the ribs and manipulate the width of the rib cage. The contraction of these fibres raises the rib cage, assisting in inhalation.
| Characteristics | Values |
|---|---|
| Main muscles controlling inspiration | Diaphragm, intercostal muscles, abdominal muscles, scalenus anterior, scalenus medius, scalenus posterior |
| Diaphragm function | Contracts to expand the rib cage and lungs, allowing air to enter the lungs |
| Intercostal muscles function | Contract to elevate the ribs and sternum, expanding the rib cage and lungs |
| Abdominal muscles function | Relax during inspiration, allowing the diaphragm to contract and expand the lungs |
| Scalenus anterior function | Extends from C3-C6 vertebrae to the first rib, contributing to its elevation |
| Scalenus medius function | Extends from the axis and C3-C7 vertebrae to the first rib, raising it |
| Scalenus posterior function | Extends from C4-C6 vertebrae to the second rib, helping to elevate it |
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What You'll Learn
The diaphragm
Upon exhalation, the diaphragm relaxes and returns to its dome-like shape, allowing air to be forced out of the lungs. This relaxation of the diaphragm reduces the volume of the abdomen, which aids in the breathing process.
Various conditions, injuries, and diseases can affect the diaphragm's function, leading to symptoms such as difficulty breathing and chest pain. These issues can arise from autoimmune disorders, lung problems, heart disease, neuromuscular disorders, and specific viruses. Diaphragm function can be improved through special breathing exercises that strengthen the muscle.
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Intercostal muscles
The external intercostal muscles are the outermost layer and are directly under the skin. They originate from the lower border of the rib and run obliquely to insert into the upper border of the rib below. During inhalation, the contraction of these fibres raises each rib toward the rib above, with the overall effect of raising the rib cage and assisting in inhalation. They are the most important intercostal muscles in respiration.
The internal intercostal muscles are the intermediate layer. They originate from the costal groove near the inferior border of the rib above and insert into the upper border of the rib below. They help collapse the lung during expiration.
The innermost intercostal muscles are the deepest layer. They originate from the costal groove of one rib and insert into the superior border of the immediate rib below. They assist the internal and external intercostals in their function.
Intercostal muscle strain can occur due to overexertion, direct trauma, or repetitive torso twisting. Symptoms include pain in the upper back or rib cage, muscle tension and stiffness, and shallow breathing to avoid pain, which may lead to reduced blood oxygenation.
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Abdominal muscles
The abdominal muscles play a crucial role in the respiratory process, working in conjunction with other muscle groups to facilitate inhalation and exhalation. While the diaphragm is the primary muscle of inspiration, the abdominal muscles are essential for expiration.
During inspiration, the diaphragm contracts and moves downward, increasing the volume of the thoracic cavity and expanding the lungs, which draws air into them. Simultaneously, the external intercostal muscles, a type of rib cage muscle, contract and elevate the rib cage, further contributing to the expansion of the thoracic cavity.
In contrast, the abdominal muscles are primarily active during expiration. The rectus abdominis, one of the abdominal muscles, pulls the ribs down during active expiration. Additionally, during forced expiration, the internal intercostal muscles are recruited, along with the rectus abdominis. This combination of muscles increases intra-abdominal pressure, pushing the diaphragm upwards, and pulling the ribs downwards, reducing the size of the thoracic cavity and facilitating exhalation.
The abdominal muscles, along with the diaphragm and rib cage muscles, form the three main groups of respiratory muscles. The coordination of these muscle groups is vital for optimal breathing mechanics. During inspiration, the rib cage muscles contract, while the abdominal muscles relax, preventing rib cage distortion and allowing the diaphragm to function effectively as a "flow generator." Conversely, during expiration, the abdominal muscles contract, generating the pressure required to move the abdomen and the lower part of the rib cage.
The external oblique is the largest of the three anterolateral abdominal muscles. Its fibres run inferomedially, originating from the external surface of ribs 5-12 and inserting on the linea alba, pubic tubercle, and iliac crest. The rectus abdominis, another important abdominal muscle, is divided into four sections by tendinous intersections. It originates at the pubic symphysis and pubic crest and attaches to the xiphoid process and the 5th to 7th costal cartilages.
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Scalenus muscles
The scalenus muscles are a group of three muscles on each side of the neck: the scalenus anterior, scalenus medius, and scalenus posterior. They are also referred to as the scalene muscles or scalenes. A fourth muscle, the scalenus minimus, is present in approximately 30% to 71% of individuals.
The scalene muscles develop from myoblasts originating in the hypaxial portion of the cervical myotomes, along with the prevertebral and geniohyoid muscles, during the 7th week of embryonic development. The scalene muscles receive their blood supply from the first portion of the subclavian artery, referred to as the "prescalene area."
The scalenus anterior muscle (Latin: scalenus anterior) lies deeply at the side of the neck, behind the sternocleidomastoid muscle. It arises from the anterior tubercles of the transverse processes of the third to sixth cervical vertebrae and is inserted by a narrow, flat tendon into the scalene tubercle on the inner border of the first rib. The middle scalene, or scalenus medius (Latin), 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, or scalenus posterior (Latin), is the smallest and most deeply seated of the scalene muscles. It arises from the posterior tubercles of the transverse processes of the fourth to sixth cervical vertebrae and is inserted by a thin tendon into the outer surface of the second rib.
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. In patients with respiratory distress, the scalene muscles may be used as accessory muscles of respiration to aid with breathing. However, they are not required in the respiration of a healthy individual, and their use is an important clinical sign of respiratory distress.
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Sternocleidomastoid muscles
The sternocleidomastoid (SCM) muscle is a powerful neck muscle that allows you to bend your neck and turn or tilt your head. It is one of over 20 pairs of muscles acting on the neck. The SCM is an important landmark in the neck which divides it into an anterior and a posterior triangle.
The SCM muscle ends at the mastoid process, a section of bone located at the base of the skull behind the ears. The SCM extends from the base of the skull and down both sides of the neck until it reaches the sternum (sternal head) or clavicle. It is a superficially palpable muscle that acts as an anatomical landmark within the neck region.
The SCM has multiple functions, including aiding in various complex physiological movements beyond its principal function as a lateral neck flexor. It helps to stabilize the neck and maintain posture, even when one is not moving. The SCM also plays a role in inspiration by lifting the sternum and clavicles, creating space for the lungs to take in air.
Injuries, tension, sprains, strains, atrophy, and tumors can all damage the SCM muscle. Conditions such as sternocleidomastoid syndrome and torticollis can result from SCM issues, leading to neck stiffness, pain, and other symptoms. Treatment options include massage, osteopathic manipulation, physical therapy, and surgery in severe cases. Maintaining good posture, managing stress, and regular exercise can help care for the SCM muscle.
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Frequently asked questions
The diaphragm is the major muscle responsible for breathing and is the primary muscle of inspiration. It is a thin, dome-shaped muscle that separates the abdominal cavity from the thoracic cavity. During inhalation, the diaphragm contracts, moving caudally (downward) and cranially (upward) to compress the abdominal cavity and expand the thoracic cavity.
The intercostal muscles are another important group of respiratory muscles. They are attached between the ribs and help manipulate the width of the rib cage. The external intercostal muscles are most important for respiration, raising the rib cage to assist in inhalation. The scalene muscles also play a role in inspiration, with the scalenus medius being the most significant for breathing in this group.
During inspiration, the diaphragm contracts and flattens, moving downward to expand the lower part of the rib cage. The rib cage muscles, including the intercostals, contract to raise the rib cage, acting on the upper part of the rib cage. While the rib cage muscles contract, the abdominal muscles relax. This mechanism prevents rib cage distortion and allows the diaphragm to act as a flow generator, optimising the mechanics of breathing.
