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The diaphragm is the main muscle of respiration, and it is involved in the process of ventilation. During quiet breathing, the diaphragm contracts, lowering alveolar pressure and drawing air into the lungs. The diaphragm is more resistant to fatigue than other limb muscles. During quiet expiration, the diaphragm relaxes, and the elastic lung and chest wall return to their resting volume. The breathing cycle is controlled by the respiratory centre located inside the medulla oblongata and the pons of the brain stem. The dorsal respiratory group within the dorsal portion of the medulla is responsible for the largest part of the breathing cycle.
| Characteristics | Values |
|---|---|
| Main muscle of respiration | Diaphragm |
| Diaphragm function | Lengthens and shortens the chest cavity |
| Diaphragm contraction | Lowers alveolar pressure, drawing air in |
| Diaphragm relaxation | Allows lung tissue to recoil, expelling air |
| Diaphragm fatigue | More resistant than other limb muscles |
| Inspiratory muscles | External intercostals, rib cage inspiratory muscles, abdominal muscles |
| Expiratory muscles | Internal intercostals, abdominal muscles |
| Accessory muscles | Scalene, Sternomastoid, Iliocostalis, Longissimus, Serratus posterior inferior, Quadratus lumborum, Obliques |
| Accessory muscle function | Assist with inspiration and forced expiration |
| Breathing control | Respiratory centre in the brain stem |
| Brain centres involved in pulmonary ventilation | Medulla oblongata, Pontine respiratory group |
| Brain function | Maintains breathing rhythm, controls rate and depth of breathing |
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What You'll Learn
The diaphragm is the main muscle of respiration
During inspiration, the diaphragm contracts, and its domes descend, expanding the thoracic cavity and lung volume. This expansion creates a lower pressure within the lungs than the atmosphere, causing air to be drawn into the lungs. During quiet breathing, inspiration is usually active, while expiration is passive. The diaphragm is responsible for this mechanism, as it lengthens and shortens the chest cavity.
The diaphragm is also involved in forced respiration, where it acts as an accessory muscle, assisting with inspiration. It causes the elevation and outward movement of the rib cage. During forced inspiration, the diaphragm contracts, pushing the abdominal organs upward against the diaphragm, helping to push the diaphragm further into the thorax and increase lung volume.
The breathing cycle is controlled by the respiratory centre located inside the medulla oblongata and the pons of the brain stem. The dorsal respiratory group within the medulla oblongata is responsible for the largest part of the breathing cycle, sending impulses through the phrenic nerve to the diaphragm to initiate breathing. The ventral respiratory group in the ventrolateral part of the medulla plays a role in forced expiration.
Other muscles involved in respiration include the intercostals, which are divided into internal, external, and parasternal intercostals. During inspiration, the external intercostals contract, raising the lower ribs up and out, increasing the dimensions of the thorax. During expiration, the internal intercostals contract, pulling the ribs downwards and inwards, reducing the size of the thoracic cavity.
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The role of the intercostal muscles
The intercostal muscles are an essential component of the respiratory system, working in tandem with the diaphragm to facilitate the process of breathing. These muscles, located between the ribs, play a crucial role in both inspiration (inhalation) and expiration (exhalation).
During inspiration, the external intercostal muscles contract, raising the lower ribs up and out. This action increases the lateral and anteroposterior dimensions of the thorax, expanding the thoracic cavity and lung volume. As a result, the pressure within the lungs decreases, becoming lower than the atmospheric pressure, which draws air into the lungs.
In contrast, during expiration, the internal intercostal muscles, along with the abdominal muscles, aid in forcing air out of the lungs. These muscles help compress the rib cage, reducing the volume of the thoracic cavity. This compression further increases the pressure within the lungs, causing air to be expelled.
The intercostal muscles are divided into three types: internal intercostal, external intercostal, and parasternal intercostal. While the external intercostals are primarily active during inspiration, the internal intercostals come into play during expiration. However, during quiet breathing, expiration is typically passive, with the respiratory muscles relaxing and the lungs and chest wall recoiling to their resting volume.
The intercostal muscles, along with the diaphragm, are considered the most important muscles for breathing. Their rhythmic and continuous contraction ensures the constant exchange of air, maintaining the necessary oxygen and carbon dioxide balance for our survival.
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The scalenes and how they contribute to respiration
The scalenes are a group of three muscles found on each side of the neck: scalenus anterior (anterior scalene), scalenus medius (middle scalene), and scalenus posterior (posterior scalene). They are accessory muscles of respiration, meaning they are only active during forced inspiration.
During quiet breathing, the diaphragm is the primary muscle responsible for inspiration, while expiration is passive. However, the scalenes are consistently active during the inspiratory phase of the breathing cycle, even when the increase in lung volume is small. They contribute to chest wall expansion in healthy individuals by elevating the ribs during forced inspiration.
The scalenus anterior, when fixed to the vertebral column, can lift the first rib superiorly. Combined with the contraction of the external intercostal muscles, this action expands the anteroposterior diameter of the thorax. The scalenus medius is the largest and longest of the scalenes. It originates from the transverse processes of the cervical vertebrae and inserts into the superior border of the first rib. The contraction of the scalenus medius produces strong ipsilateral flexion of the neck.
The scalenus posterior, when fixed to the costal attachment, results in ipsilateral lateral flexion of the neck. When fixed to the vertebral attachment, it helps stabilize or elevate the second rib during respiration.
In summary, the scalenes are accessory muscles of respiration that contribute to quiet breathing by actively participating in the inspiratory phase and expanding the chest wall. They work in conjunction with the diaphragm and other respiratory muscles to facilitate inhalation and exhalation during the respiratory cycle.
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How the sternomastoid muscle works
The sternomastoid muscle, also known as the sternocleidomastoid muscle (SCM), is a two-headed neck muscle that plays a crucial role in respiration and neck movement. It gets its name from its attachments: "sterno-" from the sternum, "-cleido-" from the clavicle, and "-mastoid" from the mastoid process.
The sternomastoid muscle originates from two distinct sites: the upper edge of the sternal manubrium (sternal head) and the medial portion of the clavicle (clavicular head). These two heads merge to form a single muscle belly, which inserts into the mastoid process of the temporal bone and the lateral half of the superior nuchal line of the occipital bone. This fusion occurs below the middle of the neck, resulting in a thick, rounded muscle with a strong tendon attachment.
The primary function of the sternomastoid muscle is to facilitate neck movement. A unilateral contraction of the muscle causes lateral flexion of the neck to the same side and rotation of the head to the opposite side. Conversely, bilateral contraction results in elevation of the head and dorsal extension of the upper cervical joints, while also flexing the lower cervical column, leading to an overall bending of the neck towards the chest. Additionally, it assists in expanding the thoracic cavity during inspiratory breathing.
The sternomastoid muscle is classified as an accessory muscle of respiration, along with the scalene muscles of the neck. During quiet breathing in healthy individuals, the sternomastoid muscle is typically electrically silent, with the diaphragm and intercostals being the primary muscles responsible for respiration. However, during rigorous respiratory activity, the sternomastoid muscle becomes active and contributes to the movement of the upper rib cage, particularly in patients with COPD.
The sternomastoid muscle is innervated by the accessory nerve (cranial nerve XI) and direct branches of the cervical plexus (C2-C3). It receives its blood supply from the superior thyroid artery, a branch of the external carotid artery. The muscle is closely associated with several neurovascular structures passing through the neck and is easily visible and palpable.
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Accessory muscles and their role in respiration
The muscles of respiration are also known as the 'breathing pump muscles'. They form a complex arrangement in the form of semi-rigid bellows around the lungs. All muscles attached to the human rib cage have the potential to cause a breathing action. Muscles that expand the thoracic cavity are called inspiratory muscles, and those that compress it are expiratory muscles.
The diaphragm is the primary muscle of respiration and is the only muscle used in normal breathing. It contracts during inspiration, lengthening and shortening the chest cavity, and is more resistant to fatigue than other limb muscles. However, accessory muscles are often recruited during quiet breathing in patients with obstructive lung disease or respiratory muscle fatigue.
Accessory muscles are those that provide assistance to the main breathing muscles, primarily when additional power is needed, such as during exercise or in those with airway pathologies like COPD. They are also recruited during dysfunction in the respiratory system. Accessory inspiratory muscles include the sternocleidomastoid, scalenus anterior, medius, and posterior, pectoralis major and minor, serratus anterior, and latissimus dorsi. The abdominal muscles are accessory expiratory muscles, including the rectus abdominis, external oblique, internal oblique, and transversus abdominis.
In patients with obstructed airways, the abdominal muscles help to expel air across the obstruction. In patients with respiratory muscle fatigue, these muscles contract at the end of expiration to compress the lungs so that the subsequent inspiration can occur passively. Accessory muscle use is also observed in over 90% of patients hospitalised with acute exacerbations of chronic obstructive lung disease, although this decreases over time.
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Frequently asked questions
The diaphragm is the main muscle of respiration and is responsible for quiet inspiration.
During quiet inspiration, the diaphragm contracts, causing the rib cage to expand and move outward, increasing the volume of the thoracic cavity and the lungs. This creates a lower pressure within the lungs, causing air to be drawn into the lungs.
The external intercostal muscles are responsible for quiet inspiration, while the internal intercostals are responsible for expiration. During inspiration, the external intercostals contract, raising the lower ribs up and out, increasing the dimensions of the thorax. During expiration, the internal intercostals help to compress the rib cage, reducing the volume of the thoracic cavity.
