Logan Steele
The breathing muscles, also known as respiratory muscles, are the mechanical effectors of the breathing system. They are divided into three main groups: the inspiratory muscles, the expiratory muscles, and the accessory muscles of respiration. The diaphragm, the primary muscle of inspiration, is the most powerful inspiratory muscle, and accounts for approximately 70% of the inhaled tidal volume. The intercostal muscles are another important group of respiratory muscles, and they assist in the elevation of the rib cage during inspiration. The scalene and sternomastoid are accessory muscles of respiration, which assist in elevating the rib cage. During exercise, the diaphragm is primarily a flow generator, while the rib cage and abdominal muscles are pressure generators.
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What You'll Learn
The diaphragm is the primary muscle of inspiration
The diaphragm is a thin, double-domed sheet of skeletal muscle located at the inferior-most aspect of the rib cage. It separates the thoracic cavity from the abdominal cavity. The diaphragm contracts and relaxes, altering the volume of the thoracic cavity and the lungs, producing inspiration and expiration.
During inhalation, the diaphragm contracts and flattens, and the chest cavity enlarges. This contraction creates a vacuum, which pulls air into the lungs. The diaphragm is the primary muscle responsible for breathing, accounting for approximately 70% of inhaled tidal volume in a normal individual. Its contraction results in a downward piston motion of the muscle and outward and upward movement of the ribs through the zone of apposition.
The intercostal muscles are another important group of respiratory muscles. These muscles are attached between the ribs and are important in manipulating the width of the rib cage. There are three layers of intercostal muscles, with the external intercostal muscles being the most important in respiration. 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 accessory muscles of respiration, such as the sternocleidomastoid and scalenes, can expand the rib cage and assist in inspiration during situations of increased ventilatory demand, such as exercise or when other inspiratory muscles are impaired. These muscles also function during minimal exertion and even during quiet tidal breathing.
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Intercostal muscles assist in breathing
The intercostal muscles are one of the most important groups of respiratory muscles. They are attached between the ribs and play a crucial role in breathing by manipulating the width of the rib cage. The intercostal muscles contract during inspiration, assisting in the elevation of the ribs and sternum, which creates an expansion of the thoracic cavity. This expansion increases the volume of the thoracic cavity and the lungs, enabling the lungs to expand and perform inspiration.
There are three types of intercostal muscles: external, internal, and innermost. The external intercostal muscles are the most important in respiration. 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 raising the rib cage and assisting in inhalation. The external intercostals are also involved in promoting inhalation during singing, remaining active until the lungs are fully filled.
The internal intercostal muscles are another important group of respiratory muscles. They are the most important for normal speech and singing, as they are responsible for propelling air out through the mouth and nose. The internal intercostals pull down on the rib cage, pushing air out of the lungs. This action also contributes to the expiratory phase of the breathing cycle.
The innermost intercostal muscles are also involved in breathing, although their specific function is not clearly mentioned in the sources. However, it is known that all three types of intercostal muscles receive blood supply from anterior and posterior intercostal arteries, as well as internal thoracic and musculophrenic arteries.
The intercostal muscles work in conjunction with other respiratory muscles, such as the diaphragm and accessory muscles like the scalenes, to facilitate breathing. The diaphragm is the major muscle of inspiration, accounting for approximately 70% of the inhaled tidal volume. It contracts to increase the space in the thoracic cavity, allowing the lungs to expand. During expiration, the diaphragm relaxes, and the lungs recoil to their original volume, decreasing the size of the thoracic cavity and facilitating exhalation.
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Accessory muscles of respiration
The accessory muscles of respiration are those that assist the main breathing muscles, particularly when additional power is required, such as during exercise or when other respiratory muscles are impaired. They are also used during quiet breathing in people with respiratory conditions such as COPD.
Accessory muscles are additional muscles that the body activates to help inhale and exhale air into the lungs. They include the scalene, the pectoralis major, the trapezius, and the external intercostals. The levator labii superioris alaeque nasi muscle, for example, lifts the sides of the nostrils.
The accessory inspiratory muscles are the sternocleidomastoid, the scalenus anterior, medius, and posterior, the pectoralis major and minor, the serratus anterior, the latissimus dorsi, the serratus posterior superior, the serratus posterior inferior, the levatores costarum, the transversus thoracis, and the subclavius. The sternocleidomastoid and the scalenes assist in elevating the rib cage. 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 accessory expiratory muscles are the abdominal muscles: the 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 the quadratus lumborum are also accessory expiratory muscles.
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Abdominal muscles are expiratory
The respiratory muscles are divided into three major groups: the inspiratory muscles, the expiratory muscles, and the accessory muscles of respiration. The diaphragm is the major muscle of inspiration and accounts for approximately 70% of the inhaled tidal volume in a normal individual.
During inspiration, the diaphragm contracts, causing a downward piston motion of the muscle and an outward and upward movement of the ribs through the zone of apposition. This increases the space in the thoracic cavity, decreases pressure, and enables the lungs to expand and perform inspiration.
During expiration, the diaphragm relaxes, the lungs recoil back to their original volume, and the size of the thoracic cavity decreases, forcing air out of the lungs. Expiration is a passive process driven by the elastic recoil of the lungs. However, during forced expiration, the abdominal muscles, specifically the rectus abdominis, are recruited along with the internal intercostals.
The abdominal muscles act on the abdomen and the abdominal rib cage and are expiratory. During inspiration, the abdominal muscles gradually relax, and during expiration, they contract, decreasing the volume of the abdomen. This mechanism prevents rib cage distortion, unloads the diaphragm, and optimises the mechanics of breathing.
In summary, the abdominal muscles are expiratory, and their contraction during expiration decreases the volume of the abdomen, playing a crucial role in the respiratory cycle.
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Neck muscles and upper limb muscles
While breathing is typically an involuntary process, it can be influenced by several factors, including physical activity, the condition of the air, and even your emotional state. The act of breathing involves a complex interplay of various muscles, including those in the neck and upper limbs.
Neck Muscles
The neck muscles play a crucial role in assisting the primary inspiratory and expiratory muscles. The sternocleidomastoid and scalene muscles on the neck are accessory muscles that aid in pulmonary ventilation. The sternocleidomastoid muscles elevate the sternum and clavicle, lifting the ribs during inhalation. The scalenes, consisting of scalenus anterior, scalenus medius, and scalenus posterior, also contribute to the elevation of the ribs during inspiration.
Upper Limb Muscles
The muscles of the upper limbs, particularly those in the shoulder and arm, can influence breathing patterns. The serratus anterior, which originates from the first to eighth pairs of ribs and inserts onto the medial border of the scapula, is important in laboured breathing. By fixing the scapula in position, this muscle aids in stabilisation during challenging breathing situations, such as grasping a support or assuming a tripod position. Additionally, the shoulder girdle and arm fixators may be recruited during times of increased respiratory demand or metabolic needs.
It is worth noting that while not directly related to the neck and upper limb muscles, the diaphragm, as the primary muscle of inspiration, plays a vital role in breathing. Located below the lungs, it contracts to increase the space in the thoracic cavity, enabling the lungs to expand and facilitating inhalation. During exhalation, the diaphragm relaxes, allowing the lungs to recoil and the air to flow out.
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Frequently asked questions
The breathing muscles are the mechanical effectors of the breathing system. They are divided into three major groups: the inspiratory muscles, the expiratory muscles, and the accessory muscles of respiration.
The diaphragm is the major inspiratory muscle. It is a thin, dome-shaped muscle that separates the abdominal cavity from the thoracic cavity. Other inspiratory muscles include the rib cage muscles, such as the intercostals, the parasternals, and the neck muscles.
The abdominal muscles are the primary expiratory muscles. They act on the abdomen and the abdominal rib cage to facilitate exhalation.
The accessory muscles of respiration assist in breathing but do not play a primary role. They include the sternocleidomastoid, scalenes, trapezii, latissimus dorsi, platysma, and pectoralis major and minor muscles.
The breathing muscles work in a highly coordinated manner during the respiratory cycle. During inspiration, the diaphragm contracts, causing it to move downward while the ribs move upward and outward, expanding the thoracic cavity and drawing air into the lungs. During expiration, the diaphragm relaxes, the ribs move downward and inward, and air flows out of the lungs.
