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The sternum, or breastbone, is a flat, T-shaped bone located in the centre of the chest. It is divided into three segments: the manubrium, body, and xiphoid process. The sternum is a crucial part of the skeletal system, providing support and protection to the torso and its organs, such as the heart, lungs, and blood vessels. It also serves as an attachment point for various muscles, including the pectoralis major, transversus thoracis, and the diaphragm. Several muscles of the neck, thorax, and anterior abdominal wall also connect to the sternum. This article will explore the muscles that attach to the sternum and their functions.
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What You'll Learn
Transversus thoracis muscle
The transversus thoracis muscle, also known as triangularis sterni, is a muscle found on the inner surface of the anterior chest wall. It is one of the intrinsic muscles of the chest wall, along with the intercostals, subcostal, levatores costarum, and serratus posterior muscles.
The transversus thoracis originates from three points: the inferior third of the posterior surface of the body of the sternum, the posterior surface of the xiphoid process, and the sternal ends of the costal cartilages of ribs 4 to 7. The xiphoid process is the lowest part of the sternum, a flat, T-shaped bone located in the centre and front of the chest. The muscle fibres diverge and course superolaterally, forming 4 to 5 slips on each side of the sternum. These slips insert into the inner surfaces of the costal cartilages of ribs 2 to 6. The lowest fibres of the transversus thoracis are horizontal and continuous with the transversus abdominis muscle. The intermediate fibres run obliquely in a superolateral direction, while the superiormost fibres lie nearly vertically.
The transversus thoracis is an accessory respiratory muscle that aids in forced expiration. It pulls ribs 2 to 6 towards the sternum during forced exhalation, resulting in rib depression. The muscle is innervated by the intercostal nerves, specifically the anterior rami of the thoracic spinal nerves (T2-T6). The blood supply to the muscle comes from the internal thoracic artery.
The transversus thoracis is typically a thin plane of muscular and tendinous fibres. However, in athletic individuals, it can be a thick and well-developed muscle. It separates the pleura from the intercostal nerves and is located deep to the central part of the thoracic cage, forming the anterior wall of the anterior mediastinum.
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Sternocleidomastoid muscle
The sternocleidomastoid (SCM) muscle is a powerful neck muscle that allows you to bend your neck and turn or tilt your head. It also helps to maintain your posture and stabilize your neck. The SCM is the largest muscle in the front of your neck and can be felt on both sides of the neck. It is thick and serves as a primary landmark of the neck, dividing it into anterior and posterior cervical triangles. This helps to define the location of structures such as the lymph nodes for the head and neck.
The SCM extends from the mastoid process at the base of the skull to the collarbones (clavicles) and breastbone (sternum). The SCM ends at the mastoid process, which is a section of bone located behind the ears at the base of the skull. The SCM helps to lift the breastbone and collarbone when you inhale, creating space for the lungs to take in air.
The SCM is associated with various conditions and injuries. Sternocleidomastoid syndrome, for example, is a condition involving neck stiffness, pain, and other symptoms. It occurs when the SCM develops tightened, sensitive areas, or trigger points. Torticollis, or wryneck, is another condition where shortened SCM muscles cause the head to naturally turn or tilt to one side. SCM muscle injuries can lead to pain and stiffness, and treatments may include stretching, physical therapy, and osteopathic manipulation.
The SCM has a variable innervation arrangement, with the "classical anastomotic pattern" observed in about half of cases. It is supplied by the accessory nerve, which conveys signals to motor endplates on the muscle fibres located at the clavicle. The SCM is also associated with the internal carotid artery and the brachial plexus.
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Sternohyoid muscle
The sternohyoid muscle is a flat, strap-like muscle located on both sides of the neck. It is one of the four infrahyoid muscles, which also include the omohyoid, sternothyroid, and thyrohyoid muscles. The sternohyoid muscle is contained within the muscular triangle of the neck, a subdivision of the anterior triangle.
The sternohyoid muscle originates from the upper posterior aspect of the manubrium of the sternum, the posterior surface of the medial end of the clavicle, the sternoclavicular ligament, and the superoposterior portion of the manubrium. It then ascends the neck and inserts into the inferior border of the body of the hyoid bone, where it adjoins the insertion of the contralateral sternohyoid muscle.
The primary function of the sternohyoid muscle is to depress the hyoid bone after it has been elevated by the suprahyoid muscles. This action occurs during swallowing, closing the airways to prevent food from being inhaled. Once the food passes from the pharynx to the oesophagus, the sternohyoid muscle aids in reopening the laryngeal inlet by depressing the hyoid bone and larynx, thus reinstating the breathing process. The muscle also plays a role in speech, primarily modulating speech volume rather than intonation.
The sternohyoid muscle receives its arterial blood supply from the superior thyroid artery, a branch of the external carotid artery. The lymphatic drainage of the sternohyoid muscles differs between the right and left sides. The right sternohyoid muscle drains lymph fluid into the right supraclavicular or cervical lymph nodes, which then drain into the right lymphatic duct. On the left side, the lymph drains into the left supraclavicular or cervical lymph nodes and eventually into the thoracic duct.
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Sternothyroid muscle
The sternothyroid muscle is a paired strap muscle located in the muscular triangle of the neck. It is part of the infrahyoid muscle group, which consists of four muscles grouped into superficial and deep layers. The sternothyroid is comparatively wider and shorter than the other infrahyoid muscles.
The sternothyroid originates from the posterior edge of the costal cartilage of the first rib, as well as the posterior surface of the manubrium of the sternum. At its origin, the muscle is in contact with the contralateral sternothyroid. The vertically oriented fibres then diverge as they travel superiorly to insert on the oblique line of the thyroid cartilage. Here, the muscle outlines the upper limit of the thyroid gland.
The sternothyroid muscle plays a crucial role in depressing the hyoid bone and larynx, which are collectively referred to as the voice box. By depressing the larynx, the sternothyroid contributes to speaking, swallowing, and chewing. Additionally, it helps protect other important structures in the neck.
The sternothyroid muscle has several important anatomical relationships with neurovascular and glandular structures in the neck. It lies anterior to the brachiocephalic trunk on the right and the common carotid artery and brachiocephalic vein on the left. The muscle is innervated by the anterior rami of the first three cervical spinal nerves (C1-3), which provide motor innervation through the ansa cervicalis. The blood supply to the sternothyroid is provided by the branches of the lingual and superior thyroid arteries.
In certain surgical procedures, such as a thyroidectomy, the sternothyroid muscle may need to be divided to allow access to the superior pedicle of the thyroid gland. However, there is currently no evidence suggesting that dividing this muscle leads to adverse functional outcomes postoperatively.
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Pectoralis major muscle
The pectoralis major muscle is a thick, fan-shaped or triangular convergent muscle that makes up the bulk of the chest muscles. It is located on the anterior surface of the thoracic cage, beneath the breast. In males, it is covered by the deep layer of fascia, subcutaneous tissue, and the adjacent skin. In females, it is covered by the breast. The pectoralis major muscle is innervated by the lateral and medial pectoral nerves, which stem from the brachial plexus. It is vascularized by the pectoral branches of the thoracoacromial artery and the perforating branches of the internal thoracic artery.
The pectoralis major muscle has three parts that originate from three different sites: the clavicular part, the sternocostal part, and the abdominal part. The clavicular part originates from the anterior surface of the medial half of the clavicle. The sternocostal part originates from the anterior surface of the sternum and the anterior aspects of the costal cartilages of ribs 1-6. The abdominal part, the smallest of the three, originates from the anterior layer of the rectus sheath.
The main function of the pectoralis major muscle is the adduction and internal rotation of the arm in the shoulder joint. It acts as a strong adductor and internal rotator of the humerus at the shoulder joint. The clavicular portion of the muscle flexes the humerus up to 90 degrees in a horizontal plane. The sternocostal portion produces the antagonistic movement, extending the humerus back to the anatomical position. When acting together with the latissimus dorsi muscle, the pectoralis major pulls the trunk forwards or upwards when its humeral attachment is fixed. This action is important in activities such as climbing.
The pectoralis major muscle can be targeted through various exercises, such as the barbell bench press, dumbbell bench press, and machine bench press, which induce high activation in the sternocostal region. Tears in the pectoralis major muscle are rare but can occur in high-impact contact sports such as powerlifting. Poland syndrome is a rare congenital condition where the pectoralis major muscle is completely missing, usually on one side of the body.
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Frequently asked questions
The sternum is a flat, T-shaped bone located in the middle and front of the chest. It is also known as the breastbone.
The sternum protects the organs in the torso, such as the heart, lungs, and chest blood vessels. It also provides support and acts as a connection point for other bones and muscles.
The sternum is divided into three segments: the manubrium, the body, and the xiphoid process. The manubrium is the top part of the sternum, while the body is the longest and middle portion. The xiphoid process is the lowest part of the sternum and is usually pointed.
Several muscles attach to the sternum, including the transversus thoracis, pectoralis major, sternocleidomastoid, sternohyoid, sternothyroid, and the diaphragm.
Some common issues related to the sternum include costochondritis, sternal fracture, sternoclavicular joint injury, muscle strain, and pectus carinatum, which can cause pain and discomfort in the chest area.
