Dominic Stone
The diaphragm is the primary 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 supported by the intercostal muscles, which are attached between the ribs and help to elevate the rib cage. During forced inspiration, accessory muscles such as the scalenes and sternocleidomastoid are recruited to assist with breathing. These muscles act as an accessory by elevating the rib cage and aiding in the expansion of the thoracic cavity.
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What You'll Learn
The diaphragm is the main inspiratory muscle
The diaphragm is a large, dome-shaped muscle located below the lungs, separating the thoracic and abdominal cavities. It is the primary muscle responsible for inspiration or inhalation. During inspiration, the diaphragm contracts and moves inferiorly, increasing the vertical diameter of the thoracic cavity and producing lung expansion, which draws air into the lungs. This movement also results in a decrease in pleural pressure and a rise in abdominal pressure, contributing to the expansion of the ventral abdominal wall.
The diaphragm's contraction leads to a lengthening and shortening of the chest cavity, which is one of the two primary mechanisms of breathing. The other mechanism involves changing the anteroposterior diameter of the chest cavity by elevating and depressing the ribs. While the diaphragm is the main inspiratory muscle, other muscles also play a role in inspiration. These include the external intercostals, which contribute to quiet inspiration by moving the rib cage up and outward, and the internal intercostals, which assist in forceful expiration.
Accessory inspiratory muscles provide additional support during active or forced inspiration. These include the sternocleidomastoid, scalenus anterior, scalenus medius, scalenus posterior, pectoralis major and minor, serratus anterior, and latissimus dorsi. The scalene muscles, consisting of the scalenus anterior, scalenus medius, and scalenus posterior, are particularly important for breathing as they contribute to elevating the ribs.
The diaphragm's function as the main inspiratory muscle is vital as it generates pressure and creates a vacuum effect during inhalation, facilitating the expansion of the lungs and thoracic cavity. This negative pressure within the pleural cavity ensures the lungs expand and air rushes in during inspiration. During exhalation or expiration, the diaphragm relaxes, allowing the lungs to recoil and air to be expelled.
In summary, the diaphragm is the primary muscle of inspiration, and its contraction and descent lead to lung expansion and air inhalation. Its movement alters the volume and pressure within the thoracic cavity, making it crucial for the inhalation phase of breathing.
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Intercostal muscles assist in inhalation
The process of breathing involves the coordination of the chest wall, the lungs, and the muscles that move them. The diaphragm, located directly below the lungs, and the intercostal muscles, which lie between the ribs, are two muscle groups with important roles in breathing.
The intercostal muscles assist in inhalation by working in tandem with the diaphragm. When the diaphragm contracts, it moves downward, increasing the volume of the thoracic cavity and creating more room for the lungs to expand. Similarly, when the intercostal muscles contract, the ribs move upward and the rib cage expands, further increasing the volume of the thoracic cavity. This expansion of the thoracic cavity by the intercostal muscles during contraction is facilitated by the fact that they are an incomplete muscle layer that crosses more than one intercostal space.
The diaphragm is considered the main inspiratory muscle, but the intercostal muscles also play a significant role in inhalation. The external intercostal muscles are the most superficial muscles of the intercostal muscle group, and they contribute to quiet inspiration by causing the rib cage to move up and outward. This movement of the rib cage by the external intercostal muscles contributes to around 25% of the increase in thoracic volume during inhalation.
During active or forced inspiration, additional accessory muscles are recruited to assist in inhalation, including the sternocleidomastoids and scalenes. The scalenes are a trio of prevertebral muscles that can flex the neck and elevate the upper two ribs during forced inspiration. The internal intercostal muscles, on the other hand, are considered more active during expiration.
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The scalenus anterior, medius and posterior muscles are involved in breathing
The scalenus anterior, medius, and posterior muscles are involved in breathing. These muscles are deep and positioned laterally in the neck. They are also known as scalene muscles. The scalene muscles are the three muscles found on each side of the neck, spanning between the transverse processes of the cervical vertebrae and the upper two ribs. The scalenus anterior muscle extends from the anterior tubercles of the transverse processes of C3 to C6 vertebrae to the first rib, contributing to its elevation. The scalenus medius is the largest and longest of the scalenes. It originates from the transverse processes of the axis and the transverse process of C3 to C7 until the first rib, also raising it. The scalenus posterior passes from the posterior tubercles of the transverse process of C4-6 to the second rib, helping to elevate it.
The scalenus muscles are accessory muscles of respiration, elevating the ribs during forced inspiration. They are also involved in the flexion, lateral flexion, and rotation of the neck. The scalenus medius is the most significant for breathing in this group. The scalenus muscles facilitate accessory breathing and contribute to neck and head movement, as well as postural stability between the neck and head.
The diaphragm and the external intercostals are the muscles primarily responsible for inspiration. The diaphragm is the main inspiratory muscle. It contracts and moves in an inferior direction, increasing the vertical diameter of the thoracic cavity and producing lung expansion, which results in air being drawn in. The external intercostals contribute to quiet inspiration, causing the rib cage to move up and outward, contributing to an increase in thoracic volume.
During forced inspiration, the scalenus muscles act as accessory muscles, capable of elevating the upper two ribs. They are also involved in elevating the first rib during forced respiration.
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Sternocleidomastoid acts as an accessory muscle during forced inspiration
The diaphragm is the main inspiratory muscle, and during inspiration, it contracts and moves in an inferior direction, increasing the volume of the thoracic cavity and producing lung expansion. The external intercostal muscles are the other muscles primarily responsible for inspiration.
During forced inspiration, a number of additional 'accessory' muscles may be recruited, including the sternocleidomastoid and scalenes. The sternocleidomastoid muscle is a two-headed neck muscle that bears 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.
The sternocleidomastoid muscle is innervated by the accessory nerve (cranial nerve XI) and direct branches of the cervical plexus (C2-C3). A unilateral contraction of the sternocleidomastoid muscle flexes the cervical vertebral column to the same side (lateral flexion) and rotates the head to the opposite side. A bilateral contraction elevates the head by dorsally extending the upper cervical joints. At the same time, it flexes 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 (inspiratory breathing muscle).
The scalenes are a trio of prevertebral muscles capable of flexing the neck. During forced inspiration, they act as an accessory muscle and are capable of elevating the upper two ribs.
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The muscles of inspiration elevate the ribs and sternum
The external intercostals are the muscles that are primarily responsible for inspiration, along with the diaphragm. They cause the rib cage to move up and outward, contributing to around 25% of the increase in thoracic volume. The external intercostals are the most superficial layer of the intercostal muscle group, which is located in the intercostal spaces between the ribs. There are 11 pairs of external intercostals, extending between the tubercles of the ribs and the costochondral joints. They run in an infero-anterior direction between the borders of two adjacent ribs.
The scalene muscles also play a role in inspiration. They consist of scalenus anterior, scalenus medius, and scalenus posterior. All three are involved in breathing. Scalenus anterior muscles extend from the anterior tubercles of transverse processes of C3 to C6 vertebrae to the first rib, contributing to its elevation. Scalenus medius runs from transverse processes of the axis and the transverse process of C3 to C7 until the first rib, also raising it. The scalenus posterior passes from the posterior tubercles of the transverse process of C4-6 to the second rib. Therefore, it helps elevate the second rib.
The sternocleidomastoid muscles originate from the manubrium of the sternum and sternal end of each clavicle. They insert into the mastoid process of the temporal bone and the superior nuchal line of the occipital bone. This allows them to elevate the sternum and clavicle, subsequently lifting the ribs during inhalation. Therefore, they are used as accessory muscles in pulmonary ventilation.
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Frequently asked questions
The diaphragm and external intercostals are the primary muscles for inspiration.
The diaphragm is the main inspiratory muscle. It is a thin, 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 causing air to be drawn in.
Intercostal muscles are attached between the ribs and help manipulate the width of the rib cage. There are three types: external, internal, and innermost intercostal muscles.
The contraction of the external intercostal muscles raises each rib toward the rib above, with the overall effect of raising the rib cage and assisting in inspiration.
The accessory muscles for inspiration include the sternocleidomastoid and the scalenes (anterior, middle, and posterior). They assist in elevating the rib cage.
