Dominic Stone
The human rib cage is a complex structure, and the muscles attached to it play a crucial role in breathing. During inhalation, the muscles attached to the rib cage expand the thoracic cavity, while during exhalation, they compress it. The diaphragm is the major muscle of inspiration, but several other muscles aid in rib elevation during breathing. These include the scalene muscles, sternocleidomastoid muscle, serratus anterior muscle, levatores costarum, serratus posterior superior, and the external and internal intercostal muscles.
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What You'll Learn
External intercostal muscles
The external intercostal muscles are the most superficial set of muscles that occupy the 11 intercostal spaces. They are also known as the intercostales externi. There are eleven on both sides, extending from the tubercles of the ribs behind to the cartilages of the ribs in front. They end in thin membranes, the external intercostal membranes, which continue forward to the sternum.
The external intercostal muscles work in unison when inhalation occurs. They contract and raise the lateral part of the ribs, increasing the transverse diameter of the thorax in a bucket-handle motion. This increases the transverse and anteroposterior diameter of the lungs, which in turn decreases the intrapleural pressure. This process expands the lungs and facilitates the entry of air into them.
The external intercostal muscles are thicker than the internal intercostals, and their fibres are directed obliquely downward and laterally on the back of the thorax, and downward, forward, and medially on the front. They originate from the sharp inferior costal border of one rib and insert along the outer lip of the superior border of the immediate rib below.
The external intercostal muscles are separated from the internal intercostal muscles by a thin fascia. Superficial to the external intercostal muscles lie the pectoralis major and minor, serratus anterior, and the upper part of the rectus abdominis muscle.
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Internal intercostal muscles
The internal intercostal muscles are a group of 11 muscles that lie between the ribs in the intercostal spaces. They form the intermediate layer of the three layers of intercostal muscles, which also include the external and innermost intercostal muscles. The internal intercostal muscles originate from the lateral aspect of the costal groove of the rib above and insert into the superior aspect of the rib below. This arrangement allows them to depress the ribs and reduce the volume of the thoracic cavity during forced expiration or exhalation.
The internal intercostal muscles are located deep to the external intercostals, from which they are separated by a thin fascia. They lie superficial to the innermost intercostal muscles, which some authors consider a part of the internal intercostals. The posterior surface of the superior margin of each internal intercostal encloses the intercostal tunnel, along with the anterior surface of the corresponding innermost intercostal. The neurovasculature of the intercostal groove, including the intercostal nerve, artery, and vein, traverses this tunnel.
The internal intercostal muscles are innervated by the intercostal nerves of the corresponding intercostal spaces. They receive their blood supply from the muscular branches of the anterior and posterior intercostal arteries, as well as the intercostal branches of the internal thoracic, musculophrenic arteries, and costocervical trunk. The venous blood is conveyed by the anterior and posterior intercostal veins to either the brachiocephalic or azygos venous system, which both empty into the superior vena cava.
Along with the external and innermost intercostal muscles, the internal intercostal muscles assist in breathing and support the integrity of the thoracic cage. They are primarily muscles of exhalation, as they depress and retract the ribs during forceful exhalation, such as coughing or during exercise. However, the parasternal part of the internal intercostal muscles, which lies between the cartilage portion of the superior and inferior ribs, is involved in inspiration by aiding in elevating the ribs and expanding the thoracic cavity.
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Innermost intercostal muscles
The innermost intercostal muscles are the deepest layer of intercostal muscles. They are located deep within the internal and external intercostals, filling the 11 intercostal spaces between the ribs. The intercostal muscles are a part of the intrinsic muscles of the thoracic wall, which also includes the serratus posterior, levatores costarum, subcostal, and
The innermost intercostal muscles originate from the inner surface of the costal groove of the ribs, deep to the origin of the internal intercostal muscles. They are not equally developed in every intercostal space, with the least developed muscles in the superior intercostal spaces and the most developed in the inferior intercostal spaces. Even the most developed innermost intercostal muscles do not fill the entire length of the intercostal spaces, but rather fill the middle two-quarters.
The precise function of the innermost intercostal muscles is not yet known, but it is likely to be similar to that of the internal intercostal muscles. It is suggested that these muscles depress the ribs and aid in forced expiration by pushing air out of the lungs. This makes them accessory respiratory muscles that are only active during forced breathing. Additionally, they are constantly tonically contracted, increasing the rigidity of the chest wall and making it suitable for manipulation by the diaphragm.
The innermost intercostal muscles are innervated by the intercostal nerves, which are the anterior rami of the thoracic spinal nerves. They receive their blood supply from the vessels of the thoracic wall, including the muscular branches of the anterior and posterior intercostal arteries, as well as the intercostal branches of the internal thoracic and musculophrenic arteries.
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Scalene muscles
The scalene muscles are a group of three muscles on each side of the neck: the anterior, the middle, and the posterior. They are named after the Greek word 'skalēnós', meaning 'uneven', as the pairs are all of differing lengths. These muscles are deep and positioned laterally in the neck, with the anterior scalene being the thickest at the level of the cricoid cartilage. The scalene muscles are attached at one end to bony protrusions on vertebrae C2 to C7 and at the other end to the first and second ribs.
The scalene muscles act as accessory muscles of respiration, elevating the first and second ribs during forced inspiration and increasing the intrathoracic volume. They also contribute to neck movement, including flexion, lateral inclination, and the initial degrees of head rotation. The scalene muscles are involved in upper limb movement and have significant surgical importance.
The anterior and middle scalene muscles lift the first rib and bend the neck to the side they are on. The posterior scalene lifts the second rib and tilts the neck to the same side. The middle scalene is the largest and longest of the three scalene muscles, with several long, thin muscle bellies arising from the cervical spine. The posterior scalene is the smallest and most deeply seated of the scalene muscles.
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Sternocleidomastoid muscle
The sternocleidomastoid muscle (SCM) is a large and superficial cervical muscle in the neck. It is one of over 20 pairs of muscles in the neck region and is a prominent anatomical landmark, dividing the neck into anterior and posterior cervical triangles.
The SCM has dual innervation and multiple functions. It originates at the manubrium of the sternum and the clavicle, with an insertion at the mastoid process of the temporal bone of the skull. The muscle is thick and narrow at its centre, broadening and thinning at each end. The SCM acts as a lateral neck flexor, rotating the head to the opposite side and flexing the neck. It also aids in respiration, assisting in raising the sternum and the clavicles.
The SCM plays an important role in neck posture and is involved in complex physiological movements, such as allowing correct temporomandibular joint (TMJ) function. It has a unique organisational pattern, with five distinct topographical parts: the superficial sternomastoid, profound sternomastoid, sterno-occipital, cleidomastoid, and cleido-occipital. These parts are arranged in superficial and deep layers, with muscle fibres lying within a common fascial sheath and traversing in the same direction.
The SCM is innervated by the accessory nerve, which supplies motor fibres. The signalling process to contract or relax the SCM begins in Cranial Nerve XI, with fibres from the accessory nerve nucleus travelling upward to enter the cranium via the foramen magnum. Acetylcholine (ACH) is released, initiating an action potential that spreads along the muscle fibre, resulting in SCM contraction.
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Frequently asked questions
The muscles that elevate the ribs are called inspiratory muscles as they help in inhalation. These include the scalene muscles, sternocleidomastoid muscle, serratus anterior muscle, and the external and internal intercostal muscles.
Intercostal muscles are the muscles of the thoracic wall that occupy the intercostal spaces. There are 11 pairs of external intercostal muscles and 11 pairs of internal intercostal muscles. They are located between adjacent ribs and play a role in respiration.
The external intercostal muscles are the most superficial layer of the intercostal muscles. They originate from the inferior aspect of the rib above and attach to the superior aspect of the rib below. During inspiration, these muscles contract and pull the ribs upward and forward, increasing the transverse diameter of the thorax.
The internal intercostal muscles form the intermediate layer of the intercostal muscles. They originate from the lateral aspect of the costal groove of the rib above and attach to the superior aspect of the rib below. The fibres of the internal intercostal muscles are arranged orthogonally to those of the external intercostals.
