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The diaphragm is the main muscle responsible for inspiration, or breathing in. It is a thin, dome-shaped muscle that separates the abdominal cavity from the thoracic cavity. During inhalation, the diaphragm contracts, moving down and increasing the volume of the thoracic cavity, which results in air being drawn into the lungs. The diaphragm is aided by the intercostal muscles, which are attached between the ribs and manipulate the width of the rib cage. The scalene muscles also play a role in inspiration, consisting of scalenus anterior, scalenus medius, and scalenus posterior, all of which are involved in breathing.
| Characteristics | Values |
|---|---|
| Main inspiratory muscle | Diaphragm |
| Diaphragm shape | Double-domed sheet of muscle |
| Diaphragm origin | Xiphoid process (posterior surface), lower six ribs and their costal cartilage (inner surface) and upper three lumbar vertebrae as right crus and upper two lumbar vertebrae as left crus |
| Diaphragm nerve supply | Motor nerve supply by Phrenic nerve (C3 C4 C5) and sensory supply by phrenic nerve to central tendon and lower 6 or 7 intercostal nerve to peripheral parts |
| Diaphragm blood supply | Inferior phrenic arteries, superior phrenic arteries, musculophrenic arteries, and pericardiacophrenic arteries |
| Diaphragm action | Contracts and moves in an inferior direction, increasing the vertical diameter of the thoracic cavity and producing lung expansion |
| External intercostal muscles | Most important in respiration |
| External intercostal muscle fibres | Angled obliquely downward and forward from rib to rib |
| External intercostal muscle action | Contraction raises each rib toward the rib above, raising the rib cage and assisting in inhalation |
| Internal intercostal muscles | Recruited during expiration |
| Accessory inspiratory muscles | Sternocleidomastoid, scalenus anterior, medius, and posterior, pectoralis major and minor, inferior fibres of serratus anterior and latissimus dorsi, serratus posterior superior, iliocostalis cervicis |
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What You'll Learn
The diaphragm is the main muscle of inspiration
The diaphragm is the primary muscle responsible for inspiration, or breathing in. It is a large, dome-shaped muscle located below the lungs, at the inferior-most aspect of the rib cage. It separates the thoracic cavity from the abdominal cavity and acts as the floor of the former and the roof of the latter.
During inspiration, the diaphragm contracts and flattens, moving inferiorly and increasing the vertical diameter of the thoracic cavity. This, in turn, expands the lungs and creates a vacuum, drawing air into them. The diaphragm consists of a right and left dome, which rise to the level of the fourth intercostal space. The right dome is usually slightly higher than the left, due to the presence of the liver.
The diaphragm is innervated by the phrenic nerve, with the right half innervated by the right phrenic nerve and the left half by the left phrenic nerve. The majority of its arterial supply is delivered via the inferior phrenic arteries, with the remaining supply coming from the superior phrenic, musculophrenic, and pericardiacophrenic arteries.
During expiration, or breathing out, the diaphragm relaxes and returns to its original dome shape. This reduces the volume of the thoracic cavity, and air is forced out of the lungs. While inspiration is typically an active process, requiring muscle contraction, expiration is usually passive unless forced.
In addition to the diaphragm, other muscles also play a role in inspiration, including the scalene muscles (scalenus anterior, scalenus medius, and scalenus posterior) and the external intercostal muscles, which elevate the ribs and sternum. However, the diaphragm remains the main muscle responsible for inspiration, as it is the only muscle that directly lengthens and shortens the thoracic cavity, enabling inhalation and exhalation.
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External intercostal muscles aid inspiration
The diaphragm is the main muscle that controls inspiration or breathing in. It is a dome-shaped muscle that separates the thoracic and abdominal cavities. During inspiration, the diaphragm contracts and moves in an inferior direction, increasing the vertical diameter of the thoracic cavity and causing the lungs to expand. This, in turn, causes air to be drawn into the lungs.
The external intercostal muscles aid inspiration by raising the ribs and sternum, thereby extending the anterior/posterior dimension of the thoracic cavity. These muscles are located between the ribs, starting at the first rib and extending down to the 11th rib. They are relatively small but powerful, and their contraction allows for timely access to the lungs. The external intercostals receive nerve signals from the intercostal nerves, which stem from the spinal cord, and they receive blood from three major arteries.
The accessory inspiratory muscles include the sternocleidomastoid, scalenus anterior, scalenus medius, scalenus posterior, pectoralis major, pectoralis minor, serratus anterior, latissimus dorsi, and serratus posterior superior. These muscles help to raise the ribs and sternum, and some also assist with raising the shoulder blade to add extra space in the rib cage.
During exercise, the diaphragm and external intercostal muscles are the main muscles involved in inspiration. The costal diaphragm blood flow exceeds the blood flow to all other inspiratory or expiratory muscles, and the same is true of the external intercostal muscles, which see a large increase in blood flow during exercise. This indicates that these muscles are active and probably stiffening the rib cage to resist the deformation caused by the change in pleural pressure.
The external intercostal muscles can be strained as a result of too much stress or pressure, and this is common during a thoracostomy, a procedure used to remove fluid from the lungs. Symptoms of a strained external intercostal include sharp pain in the upper back and ribs, which worsens with coughing, sneezing, and deep breathing.
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Scalenus muscles are involved in inspiration
The diaphragm is the main muscle involved in inspiration. During inspiration, the diaphragm contracts and moves in an inferior direction, increasing the vertical diameter of the thoracic cavity and causing the lungs to expand. This results in air being drawn into the lungs.
In addition to the diaphragm, the scalenus muscles also play a role in inspiration. There are three scalenus muscles: the scalenus anterior, scalenus medius, and scalenus posterior. These muscles are located in the neck and are involved in breathing by elevating the ribs during forced inspiration.
The scalenus anterior muscle contributes to the elevation of the first rib. It arises from the anterior tubercles of the transverse processes of the C3 to C6 vertebrae and inserts into the first rib. The scalenus medius muscle also raises the first rib and runs from the transverse processes of the axis and the transverse process of C3 to C7 to the first rib. The scalenus posterior muscle helps elevate the second rib and passes from the posterior tubercles of the transverse process of C4-6 to the second rib.
The scalenus muscles are accessory muscles of respiration and are important for breathing. They facilitate accessory breathing and contribute to neck and head movement, as well as postural stability between the neck and head. Dysfunction of these muscles can lead to pathological conditions such as thoracic outlet syndrome (TOS), reduced ventilatory capacity, and cervical pain.
Therefore, the scalenus muscles, including the scalenus anterior, scalenus medius, and scalenus posterior, are involved in inspiration by elevating the ribs and assisting the diaphragm in expanding the thoracic cavity during breathing.
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Serratus anterior muscles assist in breathing
The diaphragm is the main muscle responsible for controlling inspiration, or breathing in. It is dome-shaped and separates the thoracic and abdominal cavities. During inspiration, the diaphragm contracts and moves in an inferior direction, increasing the vertical diameter of the thoracic cavity and expanding the lungs, which draws air in.
The diaphragm is not the only muscle that controls inspiration, however. The serratus anterior is a fan-shaped muscle that originates on the superolateral surfaces of the first to eighth or ninth ribs at the lateral wall of the thorax. Its main function is to assist in scapular protraction and upward rotation, keeping the shoulder blades against the ribcage at rest and during movement.
The serratus anterior is also an accessory inspiratory muscle, meaning it assists in breathing when respiratory distress occurs. It lifts the ribcage and supports breathing. The accessory inspiratory muscles are controlled by both voluntary and involuntary mechanisms.
The serratus anterior is commonly referred to as the "boxer's muscle" because it is largely responsible for the protraction of the scapula, which occurs when throwing a punch. It is also important in respiration, and when it becomes dysfunctional, symptoms such as back and neck pain can arise.
The scalene muscles also play a role in inspiration. They consist of scalenus anterior, scalenus medius, and scalenus posterior, all of which are involved in breathing.
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Sternocleidomastoid muscles are accessory inspiratory muscles
The diaphragm is the main muscle responsible for controlling inspiration, or breathing in. During inspiration, the diaphragm contracts and moves in an inferior direction, increasing the vertical diameter of the thoracic cavity and expanding the lungs, which draws air in.
However, the sternocleidomastoid muscles are also accessory inspiratory muscles. They originate from the manubrium of the sternum and sternal end of each clavicle, and insert on the mastoid process of the temporal bone and superior nuchal line of the occipital bone. This allows them to elevate the sternum and clavicle, subsequently lifting the ribs during inhalation.
The sternocleidomastoid muscle is a two-headed neck muscle, which has attachments to the manubrium of the sternum, the clavicle, and the mastoid process of the temporal bone. It is a long, bilateral muscle of the neck, which functions to flex the neck both laterally and anteriorly, as well as rotate the head contralaterally to the side of contraction.
A unilateral contraction of the sternocleidomastoid muscle flexes the cervical vertebral column to the same side and rotates the head to the opposite side. A bilateral contraction elevates the head by dorsally extending the upper cervical joints, while also flexing the lower cervical column, causing an overall bending of the neck towards the chest. If the head is fixed, it elevates the sternum and clavicle and, thus, expands the thoracic cavity.
Other accessory inspiratory muscles include the scalenus anterior, medius, and posterior, the pectoralis major and minor, the serratus anterior, and the latissimus dorsi.
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Frequently asked questions
The diaphragm is the main inspiratory muscle. It is a dome-shaped muscle that separates the abdominal cavity from the thoracic cavity. During inspiration, it contracts and moves down, increasing the volume of the thoracic cavity and producing lung expansion, which draws air into the lungs.
Intercostal muscles are one of the most important groups of respiratory muscles. There are three types: external, internal, and innermost intercostal muscles. External intercostal muscles are most important in respiration. They contribute to quiet inspiration by raising the rib cage.
Accessory muscles assist in breathing but do not play a primary role. They are typically recruited during forced inspiration. The sternocleidomastoid and scalenes (anterior, middle, and posterior) are included in the accessory muscles.
Inspiration is the process of breathing in, while expiration is the process of breathing out. Inspiration occurs through the active contraction of muscles, while expiration is usually passive, relying on the elastic recoil of the lungs.
