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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 is the major muscle responsible for breathing, but other muscles, including the intercostal muscles, also play a role in ventilation. These muscles work to change the diameter of the chest cavity, allowing air to enter and leave the lungs. During inhalation, the diaphragm contracts, compressing the abdominal cavity and raising the ribs, which expands the thoracic cavity and draws air into the lungs. When the diaphragm relaxes, the thoracic cavity contracts, forcing air out of the lungs. The intercostal muscles are located in the rib cage and are involved in manipulating the width of the rib cage. There are three layers of intercostal muscles: external, internal, and innermost. Accessory muscles of respiration, such as the sternocleidomastoid and scalenes, can also assist in inspiration during situations of increased ventilatory demand, such as during exercise or when other inspiratory muscles are impaired.
| Characteristics | Values |
|---|---|
| Primary muscles | Diaphragm, external intercostals |
| Accessory muscles | Sternocleidomastoid, scalenus anterior, medius, and posterior, pectoralis major and minor, serratus anterior, latissimus dorsi, iliocostalis cervicis, serratus posterior superior |
| Expiratory muscles | Internal intercostals, intercostalis intimi, subcostals, abdominal muscles |
| Abdominal muscles | Rectus abdominis, external oblique, internal oblique, transversus abdominis |
| Function | Expansion and contraction of the lungs |
| Controlled by | Voluntary and involuntary mechanisms |
| Structure | Same as other skeletal muscles |
| Composition | Fatigue-resistant muscle fibres |
| Location | Diaphragm is under the lungs, separating the thoracic and abdominal cavities |
| Innervation | Phrenic nerve, intercostal nerves |
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What You'll Learn
The diaphragm
During inhalation, the diaphragm contracts and flattens, enlarging the chest cavity and creating a vacuum that pulls air into the lungs. This movement also expands the abdomen and the lower part of the rib cage. Conversely, upon exhalation, the diaphragm relaxes and returns to its dome-like shape, forcing air out of the lungs. This rhythmic and continual contraction and relaxation of the diaphragm alter the volume of the thoracic cavity and the lungs, producing inspiration and expiration.
A healthy diaphragm can be maintained and strengthened through diaphragmatic breathing or abdominal breathing exercises. These involve inhaling deeply and slowly through the nose, filling the lungs with air as the belly expands. Such exercises not only strengthen the diaphragm but also offer additional benefits such as reduced stress and lower blood pressure.
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Intercostal muscles
The intercostal muscles are a group of intrinsic rib cage muscles that occupy the 11 intercostal spaces between the ribs. They are composed of fatigue-resistant muscle fibres and are controlled by both voluntary and involuntary mechanisms. They receive neuronal inputs from intercostal nerves that arise from the thoracic nerves of the spinal cord.
There are three types of intercostal muscles: external, internal, and innermost. The external intercostal muscles are the outermost layer and lie directly under the skin. They originate from the lower border of the rib above and run obliquely, inserting into the upper border of the rib below. They are responsible for the elevation of the ribs and bending them more open, thus expanding the chest wall and the transverse dimensions of the thoracic cavity during inhalation. The internal intercostal muscles are the intermediate layer, originating from the costal groove near the inferior border of the rib above and inserting into the upper border of the rib below. They are responsible for depressing the ribs and bending them inward, decreasing the transverse dimensions of the thoracic cavity during expiration. The innermost intercostal muscles are deep layers of the internal intercostal muscles, separated from them by a neurovascular bundle. They originate from the costal groove of the rib, posteriorly to the origin of the internal intercostals, and insert into the superior border of the immediate rib below.
Strains of the intercostal muscles can occur due to overexertion, direct trauma, or repetitive torso twisting. Symptoms of a strain include pain, muscle tension and stiffness, and difficulty breathing due to pain.
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Abdominal muscles
The abdominal muscles are one of the three groups of respiratory muscles, the other two being the diaphragm and rib cage muscles. They play a role in breathing, especially at higher levels of chemical drive or at increased end-expiratory lung volume. The rectus abdominis pulls the ribs down during active expiration. Its point of origin is the pubic symphysis and pubic crest, and it attaches to the xiphoid process and the 5th to 7th costal cartilages. The rectus abdominis also increases intra-abdominal pressure, pushing the diaphragm upwards.
The abdominal muscles act on the abdomen and abdominal rib cage and are expiratory. They are accessory expiratory muscles, along with the external oblique, internal oblique, and transversus abdominis. During constant work rate exercise, EMG activities increased to 40-50% and 5-10% of the peak in rectus and external oblique muscles, respectively. This suggests that abdominal muscles play a role in regulating the ventilatory response to progressive intensity bicycle exercise.
The abdominal muscles also play a supporting role in determining the ventilatory response to heavy, constant work rate cycling exercise in healthy human subjects. Their significance is illustrated by the respiratory difficulties encountered by patients with spinal cord injuries, degenerative diseases, or after upper abdominal surgery. Patients with airway obstruction or chronic obstructive pulmonary disease typically have active abdominal muscles. Physiotherapy of the abdominal muscles improves exercise capacity and maximal expiratory pressure generation in patients with chronic obstructive pulmonary disease.
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Sternocleidomastoid muscles
The sternocleidomastoid muscle, or SCM, is a two-headed neck muscle that attaches to the manubrium of the sternum, the clavicle, and the mastoid process of the temporal bone. It is a long, bilateral muscle that is easily visible and palpable, and it plays a crucial role in neck posture and movement. The SCM acts as an accessory muscle of inspiration, aiding in forceful expiration.
The SCM has multiple functions, including flexing the neck both laterally and anteriorly, as well as rotating the head to the opposite side of the contracting muscle. When acting together, the muscles on both sides flex the neck and extend the head. Unilaterally, the SCM flexes the cervical vertebral column to the same side and rotates the head to the opposite side. Bilaterally, the SCM elevates the head by dorsally extending the upper cervical joints.
The SCM is an important landmark in the neck, dividing it into anterior and posterior triangles. It also protects vital structures in the neck, such as the vertical neurovascular bundle, branches of the cervical plexus, deep cervical lymph nodes, and soft tissues. The muscle is innervated by the accessory nerve (cranial nerve XI) and direct branches of the cervical plexus (C2-C3). Its blood supply comes from the superior thyroid artery, a branch of the external carotid artery.
The SCM is a unique muscle with variable innervation arrangements, and it has been shown to act in concert with the entire muscular group of the cervicofacial region, aiding in various complex physiological movements beyond its principal function as a lateral neck flexor. Knowledge of the SCM's anatomy is crucial for surgical planning, especially in reconstructive procedures and muscle flap harvesting.
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Accessory muscles of respiration
The accessory inspiratory muscles include the sternocleidomastoid, the scalenus anterior, medius, and posterior, the pectoralis major and minor, the inferior fibres of serratus anterior and latissimus dorsi, and the serratus posterior superior may help in inspiration. The iliocostalis cervicis is also considered an accessory inspiratory muscle, and any muscle attached to the upper limb and the thoracic cage can act as one 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 are accessory expiratory muscles. The internal intercostals, intercostalis intimi, subcostals, and abdominal muscles help in forceful expiration.
The muscles of inspiration elevate the ribs and sternum, and the muscles of expiration depress them. The rib cage muscles, including the intercostals, the parasternals, the scalene, and the neck muscles, mostly act on the upper part of the rib cage (pulmonary rib cage) and are both inspiratory and expiratory. The abdominal muscles act on the abdomen and the abdominal rib cage and are expiratory.
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Frequently asked questions
The diaphragm and external intercostals are the primary muscles that aid in ventilation.
Intercostal muscles are attached between the ribs and are important in manipulating the width of the rib cage. There are three layers of intercostal muscles: external, internal, and innermost intercostal muscles.
Accessory muscles of respiration include sternocleidomastoid, scalenes, trapezii, latissimus dorsi, platysma, and pectoralis major and minor muscles. They can expand the rib cage and assist inspiration during situations of increased ventilatory demand.
The muscles of expiration depress the ribs and sternum. They include the internal intercostals, intercostalis intimi, subcostals, and the abdominal muscles.
