Logan Steele
Accessory muscles are a crucial but often overlooked component of the respiratory system. They are not primarily responsible for breathing but are recruited during physical exertion or when the primary muscles of respiration are insufficient, such as in medical conditions like asthma or COPD. Accessory muscles help to expand and contract the thoracic cavity, facilitating adequate gas exchange. They are also implicated in neurovascular compression and can be useful in reconstructive surgery. Examples include the sternocleidomastoid, scalene muscles, abdominal muscles, and pectoralis minor.
| Characteristics | Values |
|---|---|
| Definition | Accessory muscles refer to muscles that provide assistance to the main breathing muscles, mainly when additional power is needed, for example during exercise or those with airway pathologies. |
| Activation | Accessory muscles are not active during regular breathing for healthy people. They may be used when taking a deliberately deep breath, such as when swimming underwater or blowing out birthday candles. |
| Conditions | Accessory muscles may be automatically activated during typical breathing for people with conditions that make breathing more difficult, such as COPD, hypoxemia, hypercapnia, or metabolic acidosis. |
| Life Stages | Accessory muscles are commonly used by infants and young children whose muscles are not yet fully developed. |
| Examples | Accessory muscles of respiration include the sternocleidomastoid, scalene, subclavius, abdominal oblique, and pectoralis major muscles. |
| Clinical Use | Accessory muscles can be used in reconstructive surgery and may be encountered in axillary lymph node removal. |
| Symptoms | Accessory muscles are usually asymptomatic but may produce clinical symptoms such as palpable swelling or mass effect on neurovascular structures. |
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What You'll Learn
Accessory muscles of respiration
The diaphragm and intercostal muscles are the primary muscles for respiration. The diaphragm is the major muscle responsible for breathing. It is a thin, dome-shaped muscle that separates the abdominal cavity from the thoracic cavity. During inhalation, the diaphragm contracts, and its centre moves caudally (downward) while its edges move cranially (upward). This compresses the abdominal cavity, raises the ribs upward and outward, and expands the thoracic cavity, drawing air into the lungs.
Accessory inspiratory muscles include the sternocleidomastoid, scalenus anterior, medius, and posterior, pectoralis major and minor, serratus anterior, and latissimus dorsi. Technically, any muscle attached to the upper limb and the thoracic cage can act as an accessory muscle of inspiration through reverse muscle action. Accessory expiratory muscles include the abdominal muscles: rectus abdominis, external oblique, internal oblique, and transversus abdominis.
The use of accessory muscles during rest is often interpreted as a sign of respiratory distress. Conditions that make breathing more difficult, such as COPD, may force the body to activate accessory muscles for respiration.
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Accessory muscles in infants
Accessory muscles refer to muscles that provide assistance to the main breathing muscles, typically during exercise or for those with respiratory conditions. In infants, accessory muscles are used to help them breathe when their diaphragm fatigues. The diaphragm of a newborn is not as strong and is more prone to fatigue compared to older children and adults. When this happens, the newborn's body recruits accessory muscles to help them keep breathing.
Accessory muscles of respiration include the sternocleidomastoid and the scalene muscles (anterior, middle and posterior scalene). The sternocleidomastoid muscle is located in the neck and is responsible for neck rotation and flexion. The scalene muscles are located in the neck and assist in breathing by helping to elevate the rib cage.
Other accessory muscles that may be used during respiration include the subclavius, which usually stabilises the rib cage, and the intercostal muscles, which are located between the ribs. However, the intercostal muscles in infants are not yet properly developed and are therefore not as effective in their role as breathing accessory muscles.
In addition to respiration, accessory muscles can also be found in other parts of the body. For example, accessory muscles of the anterior thoracic wall include the sternalis muscle and variations of pectoralis major such as the pectoralis minimus, pectoralis quartus, and pectoralis intermedius.
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Accessory muscles in radiology
Accessory muscles refer to muscles that assist the primary muscles responsible for movement. They are relatively rare anatomical variations where duplication of a muscle may appear anywhere in the muscular system. In most cases, accessory muscles are asymptomatic and are found incidentally during imaging or surgery. However, in some cases, they may produce clinical symptoms such as palpable swelling or mass effect on neurovascular structures.
Accessory muscles of respiration, for example, the sternocleidomastoid and scalene muscles, aid the diaphragm during inspiration. During quiet breathing, the diaphragm is the primary muscle responsible for inspiration, with contributions from the abdominal and external intercostal muscles. Accessory muscles of respiration are typically recruited during exercise or in individuals with airway pathologies, such as COPD, when additional breathing power is required.
In radiology, knowledge of accessory muscles is essential for accurate diagnosis and treatment. Magnetic resonance imaging (MRI) is often used to identify accessory muscles and differentiate them from soft tissue tumors. Accessory muscles around the ankle, such as the flexor digitorum accessorius longus, peroneocalcaneus internus, accessory soleus, and accessory peroneal muscles, can cause pain, compressive neuropathy, compartment syndrome, or rigid hindfoot deformities. Similarly, accessory muscles in the forearm, wrist, and hand have been implicated in various clinical conditions and may mimic pathological conditions during imaging.
Additionally, accessory muscles in the pectoral region, such as the pectoralis major or minor, may be absent or doubled. These variations are important to consider during axillary lymph node removal to avoid complications. Overall, the presence of accessory muscles can have significant implications for patient evaluation, diagnosis, and treatment, especially when they contribute to or influence clinical symptoms.
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Accessory muscles in the pectoral region
Accessory muscles refer to muscles that assist the main muscles, usually during exertion. An accessory muscle can also refer to a rare anatomical variation where a muscle is duplicated anywhere in the muscular system. The pectoral or chest region contains four muscles: the pectoralis major, pectoralis minor, serratus anterior, and subclavius.
The pectoralis major is the most superficial muscle in the pectoral region. It is large and fan-shaped, composed of a sternal head and a clavicular head. The sternal head originates from the anterior surface of the sternum, the superior six costal cartilages, and the aponeurosis of the external oblique muscle. The clavicular head originates from the anterior surface of the medial clavicle.
The pectoralis minor lies underneath the pectoralis major, forming part of the anterior wall of the axilla region. It originates from the 3rd to 5th ribs and inserts into the coracoid process of the scapula. The pectoralis minor stabilises the scapula by drawing it anteroinferiorly against the thoracic wall.
Variations of the pectoralis minor include the pectoralis minimus, which passes from the first rib to the coracoid process. The pectoralis minor is also known to exhibit variation in its number of ribs and level. For example, its costal attachments can be from the 2nd to 5th ribs, 3rd to 5th ribs, or 2nd to 4th ribs.
Accessory pectoral muscles have been observed in the literature. In one case, an accessory muscle was found between the pectoralis major and pectoralis minor, fusing with the fibres of the pectoralis minor. This accessory muscle was 14 cm long and 4 cm broad at its origin. Another case described an accessory muscle deep to the left pectoralis minor, attached to ribs 4-6 near the midclavicular line. This muscle did not fuse with other muscle fibres but ran parallel to the pectoralis minor.
The presence of accessory pectoral muscles can have clinical implications. They may be mistaken for masses or tumours during CT or MRI scans, and they can cause difficulties during pectoral flap surgeries. On the other hand, the pectoralis major is useful in head and neck reconstructions, and the increased size of the pectoralis minor through accessory muscles may enhance shoulder function.
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Accessory muscles in the hand
The hand and wrist are a complex network of bones, muscles, nerves, connective tissue, and blood vessels. There are 34 muscles in each hand, which work together to help you move your hand and fingers. These muscles are categorised into groups, including:
Thenar Muscles
These muscles control the thumb and can be felt as a bulge at the base of the thumb in the palm of the hand. There are three thenar muscles, which are named abductor pollicis brevis, flexor pollicis brevis, and opponens pollicis. A fourth muscle, the adductor pollicis, is located in this region but is not part of the thenar muscle group. The thenar muscles are responsible for various thumb movements, including abduction, flexion, and opposition.
Hypothenar Muscles
These muscles line the outer edges of the palm on the outside of the little finger. The abductor digiti minimi is the most superficial of the hypothenar muscle group. It originates from the pisiform and the tendon of the flexor carpi ulnaris and attaches to the base of the proximal phalanx of the little finger. The flexor digiti minimi brevis lies laterally to the abductor digiti minimi in the hand.
Interossei Muscles
These muscles are located between the metacarpal bones in the palm. They help the fingers move side-to-side.
Lumbrical Muscles
These muscles are located at the base of the four non-thumb fingers and help flex the fingers. There are four lumbricals in the hand, each associated with a finger. They are crucial to finger movement, linking the extensor tendons to the flexor tendons.
Accessory muscles of the hand and wrist are usually incidental findings. They are asymptomatic anatomical variants that might be mistaken for pathological conditions such as tumours or mass lesions. Diagnosis depends on familiarity with normal muscular anatomy and awareness of places in the wrist where muscles should not exist.
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Frequently asked questions
Accessory muscles are muscles that are not primarily responsible for respiration but can assist in the act of breathing when there is an increased demand for oxygen.
Accessory muscles include the sternocleidomastoid, scalene muscles, and various abdominal muscles.
Accessory muscles are used when breathing demands increase, such as during physical exertion or respiratory distress due to medical conditions like asthma or COPD.
Accessory muscles help to expand and contract the thoracic cavity to facilitate adequate gas exchange. They also help to push up against the diaphragm during exhalation, allowing air to be expelled more forcefully.
