Dominic Stone
The muscles that control breathing are divided into three groups: inspiratory, expiratory, and accessory muscles. The diaphragm is the major muscle of inspiration, while the abdominal muscles are the most important for exhalation. The accessory muscles, such as the scalenes and sternocleidomastoid, assist in elevating the rib cage during inhalation. During exercise, the expiratory muscles become active, working in coordination with the inspiratory muscles to optimise breathing. The breathing cycle is controlled by the respiratory centre located in the brain stem, which sends impulses to the diaphragm and intercostal muscles to initiate breathing.
| Characteristics | Values |
|---|---|
| Primary inspiratory muscles | Diaphragm, external intercostals |
| Accessory inspiratory muscles | Sternocleidomastoid, scalenus anterior, medius, and posterior, pectoralis major and minor, serratus anterior and latissimus dorsi, serratus posterior superior, iliocostalis cervicis |
| Accessory expiratory muscles | Rectus abdominis, external oblique, internal oblique, transversus abdominis, iliocostalis, longissimus, serratus posterior inferior, quadratus lumborum |
| Muscles that maintain patency of the upper airway | Abductors of the vocal cords, palatal elevators, retractors of the tongue, dilators of the nares |
| Muscles that control nostril shape | Levator labii superioris alaeque nasi |
| Neural control | Phrenic nerve, intercostal nerves, thoracoabdominal nerves |
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What You'll Learn
The diaphragm is the primary muscle for inhalation
The diaphragm is the primary muscle responsible for inhalation. It is a dome-shaped sheet of muscle that separates the chest cavity from the abdomen. The diaphragm is attached to the base of the sternum, the lower parts of the rib cage, and the spine. During inhalation, the diaphragm contracts, causing its centre to move downwards and its edges to move upwards and outwards. This increases the length and diameter of the chest cavity, expanding the lungs and drawing air into them.
The diaphragm is the major muscle of inspiration and accounts for approximately 70% of the inhaled tidal volume in a normal individual. The contraction of the diaphragm results in a downward piston motion of the muscle, as well as outward and upward movement of the ribs. This compresses the abdominal cavity, raising the ribs and expanding the thoracic cavity.
The intercostal muscles, including the external intercostals, also play a role in inhalation. The external intercostals are the most superficial layer of the intercostal muscle group, located in the intercostal spaces between the ribs. They are the most important muscles for raising the rib cage. During inspiration, the rib cage muscles contract, while the abdominal muscles relax. This mechanism prevents rib cage distortion and allows the diaphragm to act as a flow generator.
During quiet breathing, the diaphragm and inspiratory rib cage muscles work together in a highly coordinated manner. This coordination ensures that undesirable effects, such as inward or outward motion during inspiration, do not occur. The scalenes, a type of accessory muscle, are consistently physically active during quiet breathing, assisting in elevating the rib cage.
During exercise, the expiratory muscles become active and work in coordination with the inspiratory muscles to meet the increased ventilatory demands. The abdominal muscles, the most important expiratory muscles, contract and push against a relaxed diaphragm, causing air to be exhaled.
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Intercostal muscles aid inhalation and exhalation
The intercostal muscles are the muscles between the ribs. They are divided into three types: external intercostals, internal intercostals, and innermost intercostals. The external intercostal muscles connect the ribs in such a way that their contraction lifts the ribs and rib cage, expanding the rib cage and, in turn, the chest. This action promotes inhalation, allowing the lungs to fill with air. The external intercostals remain active until stretch receptors within them indicate that the lungs are full, after which they shut off, signalling the internal intercostals to contract.
The internal intercostals produce the opposite effect: they lower the ribs and reduce the rib cage's dimensions, thereby pushing air out of the lungs and aiding exhalation. The internal intercostals are the most important respiratory muscles for normal speech and singing, as they are responsible for propelling air out through the mouth and nose. The greater the pressure of the escaping air, the louder one's voice.
During vocalization, there is a short period of inhalation followed by a longer period of exhalation. This is in contrast to breathing when silent, where inhalation and exhalation occur at a more-or-less even rate. Singers, particularly professional singers, are skilled at regulating the amount of air they inhale and the speed with which they do so, depending on the length and loudness of the phrase they are about to sing.
The intercostal muscles are also involved in forceful exhalation, along with other muscles such as the abdominal muscles, which include the rectus abdominis, external oblique, internal oblique, and transversus abdominis. These muscles are particularly important during vigorous exercise, when a number of muscles participate in exhalation.
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Abdominal muscles are key to exhalation
The abdominal muscles are key to exhalation. While the diaphragm is the major muscle of inhalation, the abdominal muscles are the most important muscles used for exhalation. The diaphragm separates the chest cavity from the abdomen and is attached to the base of the sternum, the lower parts of the rib cage, and the spine. As the diaphragm contracts, it moves down and increases the length and diameter of the chest cavity, expanding the lungs.
During exhalation, the diaphragm relaxes, and the abdominal muscles contract, raising abdominal pressure and pushing the diaphragm against the lungs, causing air to be pushed out. The abdominal muscles that act on the abdomen and the abdominal rib cage are expiratory. These include the rectus abdominis, external oblique, internal oblique, and transversus abdominis.
During quiet breathing, there is little to no muscle contraction involved in exhalation. This process is driven by the elastic recoil of the lungs and surface tension. However, during vigorous exercise, a number of muscles participate in exhalation, with the abdominal muscles being the most important. The abdominal muscles contract, increasing intra-abdominal pressure, and pushing the diaphragm upwards. This reduces the volume of the thoracic cavity, forcing air out of the lungs.
The breathing cycle is controlled by the respiratory centre located inside the medulla oblongata and the pons of the brain stem. To initiate breathing, the dorsal respiratory group sends impulses through the phrenic nerve towards the diaphragm, causing it to contract. For exhalation to occur, the dorsal respiratory group stops firing impulses, allowing the diaphragm to relax. When forceful exhalation is required, impulses from the respiratory group reach the ventral group, activating it. The ventral respiratory group in the ventrolateral part of the medulla plays a role in forced exhalation.
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Accessory muscles can aid breathing
The muscles of respiration are those that contribute to inhalation and exhalation by 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 and is attached to the base of the sternum, the lower parts of the rib cage, and the spine. As the diaphragm contracts, it moves down and increases the length and diameter of the chest cavity, thus expanding the lungs.
The accessory muscles of respiration are muscles that assist, but do not play a primary role, in breathing. The use of these muscles while at rest is often interpreted as a sign of respiratory distress. There is no definitive list of accessory muscles, but the sternocleidomastoid and the scalenes (anterior, middle, and posterior) are typically included, as they assist in elevating the rib cage. The involvement of these muscles seems to depend on the degree of respiratory effort. During quiet breathing, the scalenes are consistently physically active, while the sternocleidomastoids are quiet. With an increase in respiratory volume, the sternocleidomastoids also become active. Both muscles are simultaneously activated when one breathes in at the maximal flow rate.
Apart from the above neck muscles, the serratus anterior, pectoralis major and pectoralis minor, trapezius, latissimus dorsi, erector spinae, iliocostalis, and quadratus lumborum have also been observed contributing to respiration. The accessory inspiratory muscles are the sternocleidomastoid, the scalenus anterior, medius, and posterior, the pectoralis major and minor, the inferior fibres of serratus anterior and latissimus dorsi, the serratus posterior superior may help in inspiration also the iliocostalis cervicis. Technically, any muscle attached to the upper limb and the thoracic cage can act as an accessory muscle of inspiration through reverse muscle action.
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. The accessory muscles are recruited during times of exercising because of the increased metabolic need and also during dysfunction in the respiratory system.
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Neural control is required for muscle contraction
The diaphragm, a dome-shaped muscle that separates the chest cavity from the abdomen, is the primary muscle used for inhalation. The intercostal and abdominal muscles also play a role in inhalation, while the abdominal muscles are the most important for exhalation.
During inspiration, the diaphragm contracts and moves down, increasing the length and diameter of the chest cavity and expanding the lungs. This contraction is controlled by the neural activation of diaphragm muscle fibres (motor units). The basic functional unit underlying force generation and contraction in skeletal muscles is the cross-bridge, formed by the binding of the myosin heavy chain (MyHC) head to the actin filament. The final common output of neural control in skeletal muscles is the motor unit, consisting of a motor neuron and the group of muscle fibres it innervates. When the motor neuron is activated, the resulting action potential propagates along axonal branches to activate all muscle fibres of the motor unit.
The neural networks direct muscles that form the walls of the thorax and abdomen, producing pressure gradients that move air into and out of the lungs. Ventilation is the movement of air in and out of the lungs, which facilitates gas exchange. The respiratory rhythm and the length of each phase of respiration are set by reciprocal stimulatory and inhibitory interconnections of brain-stem neurons. The respiratory system can adjust breathing patterns to changes in the internal milieu and external environment. For example, ventilation increases and decreases in proportion to swings in carbon dioxide production and oxygen consumption caused by changes in metabolic rate.
Voluntary respiration is under conscious control and is managed via the motor cortex in the cerebrum, which receives inputs from the limbic system and hypothalamus. Signals are sent to the spinal cord from the motor cortex, which are then passed on to the respiratory muscles.
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Frequently asked questions
The diaphragm is the major muscle responsible for breathing, contributing to inhalation and exhalation by aiding in the expansion and contraction of the thoracic cavity.
The intercostal muscles and neck muscles help move the rib cage and assist in breathing. The abdominal muscles are also involved in breathing out.
Accessory muscles of respiration assist but do not play a primary role in breathing. These include the sternocleidomastoid, scalenes, trapezii, and pectoralis major and minor muscles.
Accessory muscles are recruited during times of increased ventilatory demand, such as during exercise, or when other inspiratory muscles are impaired.
