The pectoralis major is a thick, fan-shaped muscle, situated at the chest (anterior) of the body. It makes up the bulk of the chest muscles in the male and lies under the breast in the female. Underneath the pectoralis major is the pectoralis minor, a thin, triangular muscle.
The pectoralis major receives dual motor innervation by the medial pectoral nerve and the lateral pectoral nerve, also known as the lateral anterior thoracic nerve. One route of innervation of the pectoralis major originates in the C7, C8 and T1 nerve roots which merge to form the lower trunk of the brachial plexus. The fibers continue in the anterior division of the lower trunk, become the medial cord, and exit the medial cord as the medial pectoral nerve. The medial pectoral nerve then communicates the action potential across the neuromuscular junction by releasing acetylcholine into the neuromuscular junction, inciting a proportional muscle contraction of the sternal head of the pectoralis major. The second source of innervation of the pectoralis major originates from the C5 and C6 nerve roots which merge to form the upper trunk, splits off into the anterior division of the upper trunk which joins with the middle trunk to form the lateral cord. The lateral pectoral nerve branches off of the lateral cord of the brachial plexus and is distributed over the deep surface of the pectoralis major. At the neuromuscular junction, the lateral pectoral nerve provides motor input to the clavicular head of the pectoralis major.
The sensory feedback from the pectoralis major follows the reverse path, returning via first-order neurons to the spinal nerves at C5, C6, C8, and T1 through the anterior rami. After the synapse in the posterior horn of the spinal cord, sensory information concerning movement of the muscle, proprioception, and pressure then travels through a second-order neuron in the posterior corticospinal track to the medulla. There, the fibers decussate to form the medial lemniscus which carries the sensory information the rest of the way to the thalamus, the “gateway to the cortex”. The thalamus diverts some sensory information to the cerebellum and the basal nuclei to complete the motor feedback loop while some sensory information ascends directly to the postcentral gyrus of the parietal lobe of the brain via third-order neurons. Sensory information for the pectoralis major is processed in the superior portion of the sensory homunculus, adjacent to the longitudinal fissure which divides the two hemispheres of the brain.
This tendon consists of two laminae, placed one in front of the other, and usually blended together below.
- The anterior lamina, which is thicker, receives the clavicular and the uppermost sternal fibers. They are inserted in the same order as that in which they arise: the most lateral of the clavicular fibers are inserted at the upper part of the anterior lamina; the uppermost sternal fibers pass down to the lower part of the lamina which extends as low as the tendon of the Deltoid and joins with it.
- The posterior lamina of the tendon receives the attachment of the greater part of the sternal portion and the deep fibers, i. e., those from the costal cartilages.
These deep fibers, and particularly those from the lower costal cartilages, ascend the higher, turning backward successively behind the superficial and upper ones, so that the tendon appears to be twisted.
The posterior lamina reaches higher on the humerus than the anterior one, and from it an expansion is given off which covers the intertubercular groove of the humerus and blends with the capsule of the shoulder-joint.
From the deepest fibers of this lamina at its insertion an expansion is given off which lines the intertubercular groove, while from the lower border of the tendon a third expansion passes downward to the fascia of the arm.
The more frequent variations include greater or less extent of attachment to the ribs and sternum, varying size of the abdominal part or its absence, greater or less extent of separation of sternocostal and clavicular parts, fusion of clavicular part with deltoid, and decussation in front of the sternum.
Deficiency or absence of the sternocostal part is not uncommon.
Absence of the clavicular part is less frequent.
Rarely, the whole muscle is missing. This may accompany absence of the breast in females. (See Poland syndrome).
The pectoralis major has four actions which are primarily responsible for movement of the shoulder joint. The first action is flexion of the humerus, as in throwing a ball side-arm, and in lifting a child. Secondly, it adducts the humerus, as when flapping the arms. Thirdly, it rotates the humerus medially, as occurs when arm-wrestling. Finally it aids in deep inspiration, as in taking a deep breath before jumping in a pool. The pectoralis major is also responsible for keeping the arm attached to the trunk of the body. It has two different parts which are responsible for different actions. The clavicular part is close to the deltoid muscle and contributes to flexion, horizontal adduction, and inward rotation of the humerus. When at an approximately 110 degree angle, it contributes to abduction of the humerus. The sternocostal part is antagonistic to the clavicular part contributing to downward and forward movement of the arm and inward rotation when accompanied by adduction. The sternal fibers can also contribute to extension, but not beyond anatomical position.
A variety of resistance exercises can be used to train the pectoralis major, including bench pressing (using dumbbells, barbells or machines at various angles such as decline, incline and flat where the hips are above, below and level with the head respectively), push ups, flyes (using dumbbells or machines at either flat or inclined angles), cable crossovers or dips. Multi-joint press exercises are better for building muscle mass, while fly and crossovers are more suited for shaping and increasing striations. This muscle is often said to consist of four portions (upper, lower, inner and outer) but the pectoralis actually contracts evenly across all heads during most exercises and as such no portion can be 'targeted'.
The pectoralis can also be trained through a variety of sports as well as by all four Olympic styles of swimming, i.e. the butterfly, backstroke, breaststroke and front crawl. The anaerobic work capacity of the pectoralis is a major determinant of swimming speed, whereas swimming endurance is more influenced by the aerobic capacity of the deltoid muscle (apart from overall cardiopulmonary aerobic capacity).
Injuries and Imaging
Pectoralis major muscle tears are an uncommon sporting injury, most commonly seen in weightlifters, wrestlers and footballers. The injury is characterized by pain in the chest wall, bruising and loss of strength of the muscle. High grade partial or full thickness tears warrant surgery if function is to be preserved, particularly in the athletic population. MRI is the best imaging test to confirm the diagnosis, location and extent of a tear, though in experienced hands, ultrasound can also be used in its place.
Poland’s Syndrome is a congenital anomaly in which there is a malformation of the chest causing the pectoralis major on one side of the body to be absent. Other characteristics of this disease are "unilateral shortening of the index, long, and ring fingers, syndactyly of the affected digits, hypoplasia of the hand, and the absence of the sternocostal portion of the ipsilateral pectoralis major muscle". Although the absence of a pectoralis major is not life threatening, it will have an effect on the person with Poland’s Syndrome. Adduction and medial rotation of the arm will be much harder to accomplish without the pectoralis major. The latissimus dorsi and teres major also aid in adduction and medial rotation of the arm, so they may be able to compensate for the lack of extra muscle. However, some patients with Poland’s Syndrome may also be lacking these muscles, which make these actions nearly impossible.
Researchers from the Department of Rehabilitation Medicine at the Yonsei University College of Medicine in Seoul, Korea reported a case of congenital absence of pectoralis major in 1990. According to Kakulas and Adams, pectoralis major is the most frequently congenitally absent muscle. The case involved a 22 year old marine who had asymmetrical configuration of chest wall who had never experienced difficulties performing daily activities, but who experienced difficulties in the military camp. He had difficulty in some training activities especially those such as throwing a grenade or rope climbing. During a surgery performed to correct the sternal depression, it was found that the right pectoralis major was totally absent. However, previous physical exams did not show deficiencies in muscle strength as the right shoulder was good for flexion, adduction, horizontal adduction and internal rotation. Moreover, his pain and touch sensation were normal. X-rays were also performed and showed normal pictures of the chest's bones. The fact that the absence of pectoralis major did not cause functional loss in ordinary activities in this case of congenital absence showed that other surrounding muscles played a compensation role.
Pectoralis major muscle in rare occasions may develop intramuscular lipomas. Such rare tumors may mimic malignant breast tumors as they look like enlargements of the breasts. They are well-encapsulated radiolucent tumours of fat density. Their location can be accurately identified through computed tomography and magnetic resonance imaging (MRI). The treatment in these cases involves complete surgical excision because of the risk of liposarcoma they post especially large intramuscular liposomas. Partial excision is risky because recurrence may occur.
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