The pectoralis major muscle is the large chest muscle. It participates in motor control of the arm (internal rotation, adduction, anteversion) and as an accessory muscle in respiration. In Poland syndrome, a rare dysplasia, the pectoralis major muscle may be completely absent or underdeveloped.
What is the pectoralis major muscle?
The pectoralis major muscle is the large chest muscle. It belongs to the skeletal musculature and participates in the movement of the arm as well as breathing. The skeletal musculature tenses (contraction) or relaxes (relaxation) its fibers in response to voluntary effort on the part of its owner. Control and coordination are primarily the responsibility of the motor center of the brain; it transmits its signals via efferent nerve pathways to the muscle, where the motor end plate converts the electrical signal into a biochemical one, to which the muscle filaments respond accordingly. Conversely, the brain also receives information from the muscle, for example about the degree of tension or stretch. This transmission occurs via the afferent nerve pathways. In a healthy person, the large pectoral muscle is strongly developed and stands out visibly under the skin.
Anatomy and structure
The pectoralis major muscle lies beneath the pectoral fascia, which is part of the trunk fascia and delineates the muscle above. Under the pectoralis major muscle is the pectoralis minor muscle, which is the small pectoral muscle that completely disappears under its large counterpart and is also involved in certain arm movements as well as breathing. The pectoralis major muscle attaches to the bone of the upper arm and originates from the sternum, clavicle and cartilage of the six uppermost ribs. The connection between the arm and chest areas is particularly well seen at the armpit, where the pectoralis major muscle forms an arch. Its structure is composed of three areas, whose names indicate their location in the pectoralis major muscle: The clavicular portion (pars clavicularis) forms the uppermost section, followed by the sternocostal portion (pars sternocostalis) and abdominal portion (pars abdominalis). Like all muscles of the skeletal musculature, the large pectoral muscle belongs to the striated type. The striated muscle owes its name to the fact that in cross-section under the microscope it shows a distinct linear structure, which distinguishes it from the smooth muscle.
Function and tasks
The pectoralis major muscle has two main areas of function: it provides a respiratory support muscle and it is responsible for certain arm movements. When a person rotates the arm inward, he or she does so by tensing the pectoralis major muscle so that it pulls the limb inward. This process is also known in medicine as internal rotation. Anteversion, on the other hand, is a forward movement with which the pectoralis major muscle rotates the arm at the shoulder joint. Through a third type of tension, the muscle can also shorten so that the arm is pulled toward the body (adduction). The pectoralis major is also one of the respiratory muscles. Physicians classify it as part of the inspiratory respiratory support muscles because it can primarily participate in inhalation when the person supports his or her arms. Unlike the respiratory muscles themselves, the respiratory accessory muscles cannot independently control the inflow and outflow of air into the lungs, but can only play a supporting role. This classification is controversial, however, because it is almost impossible to make a clear distinction and, with the exception of the intercostal muscles as well as the diaphragm, all other muscles involved in breathing are considered respiratory accessory muscles.
Diseases
In Poland syndrome, the pectoralis major muscle is completely absent on one side or parts of the muscle are not developed. The sterno-rib portion and the abdominal portion are particularly commonly affected. Poland syndrome is a congenital malformation, the causes of which medical science has not yet been able to fully explain. The dysplasia also affects the mammary gland and the external appearance of the breast. The side of the body on which Poland syndrome manifests itself has a smaller nipple with a particularly dark areola and less fatty tissue compared to the healthy breast, while the other breast may have too much fatty tissue.This additional mass not only depends on the body weight and the total fat percentage, but is also asymmetrically displaced in Poland syndrome. The thorax and ribs may show deviations in shape, and the pectoralis minor muscle may also be missing. Other malformations may also be present: Fingers may be missing or fused together; the arm may be less strong or developed shorter. In rare cases, dysplasia of the lower extremities occurs. Of the internal organs, kidneys and/or heart may be affected, but the latter is rare. The majority of affected individuals are male and represent 0.01-0.001% of the population. Treatment of Poland syndrome is not always necessary, often limited to surgical approximation of the breasts. Significantly more common than Poland syndrome are adhesions of the large pectoral muscle to the large dorsal muscle (latissimus dorsi muscle) or deltoid muscle (deltoid muscle).
