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Pectoralis major
Pectoralis major.png
Pectoralis major
Latin Musculus pectoralis major
Gray's subject #122 436
Origin Clavicular head: anterior surface of the medial half of the clavicle.
Sternocostal head: anterior surface of the sternum, the superior six costal cartilages, and the aponeurosis of the external oblique muscle
Insertion    intertubercular groove of the humerus
Artery pectoral branch of the thoracoacromial trunk
Nerve lateral pectoral nerve and medial pectoral nerve
Clavicular head: C5 and C6
Sternocostal head: C7, C8 and T1
Actions Clavicular head: flexes the humerus
Sternocostal head: extends the humerus
As a whole, adducts and medially rotates the humerus. It also draws the scapula anteriorly and inferiorly.
MeSH Pectoralis+Muscles

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.

Contents

Origin and insertion

It arises from the anterior surface of the sternal half of the clavicle; from breadth of the half of the anterior surface of the sternum, as low down as the attachment of the cartilage of the sixth or seventh rib; from the cartilages of all the true ribs, with the exception, frequently, of the first or seventh and from the aponeurosis of the abdominal external oblique muscle. From this extensive origin the fibers converge toward their insertion; those arising from the clavicle pass obliquely downward and outwards (laterally), and are usually separated from the rest by a slight interval; those from the lower part of the sternum, and the cartilages of the lower true ribs, run upward and laterally, while the middle fibers pass horizontally. They all end in a flat tendon, about 5 cm in breadth, which is inserted into the lateral lip of the bicipital groove of the humerus. Electromyography suggests that it consists of at least six groups of muscle fibres that can be independently coordinated by the central nervous system.[1]

Innervation

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 costal 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 sternoclavicular 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.

Laminae

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.

Variations

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).

Actions

The pectoralis major has four actions which are primarily responsible for movement of the shoulder joint[2]. 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[3], 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[4]. [5]It has two different parts which are responsible for different actions. The clavicular part is close to the deltoid muscle and contributes to flexation, 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.

Training

The pectoralis muscle can be trained for athletic or bodybuilding purposes[6]. The following compound exercises are often used to increase the strength and mass of the muscle:

Isolation exercises include:

Poland's Syndrome

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" [7]. 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.

References

  1. ^ Brown JM, Wickham JB, McAndrew DJ, Huang XF. (2007). Muscles within muscles: Coordination of 19 muscle segments within three shoulder muscles during isometric motor tasks. J Electromyogr Kinesiol. 17(1):57-73. PMID 16458022 doi:10.1016/j.jelekin.2005.10.007
  2. ^ Saladin, KS (2010). Anatomy & Physiology: The Unit of Form and Function. 5th ed. New York: McGraw-Hill. Changes made by Kari Thomas.
  3. ^ Saladin, KS (2010). Anatomy & Physiology: The Unit of Form and Function. 5th ed. New York: McGraw-Hill. Changes made by Kari Thomas.
  4. ^ Saladin, KS (2010). Anatomy & Physiology: The Unit of Form and Function. 5th ed. New York: McGraw-Hill. Changes made by Kari Thomas.
  5. ^ Hamilton, N, Luttgens, K, Weimar, W (2008). Kinesiology. 11th ed. Boston: Mcgraw Hill. Changes made by Kari Thomas
  6. ^ Chest Exercise Menu
  7. ^ [www.polands-syndrome.com>

External links

This article was originally based on an entry from a public domain edition of Gray's Anatomy. As such, some of the information contained within it may be outdated.

Notes








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