ARTICLE ON MUSCLES OF THE UPPER LIMB (PECTORALIS MAJOR AND MINOR)
INTRODUCTION
The pectoralis major (from Latin: pectus, breast) is a thick, fan-shaped muscle, situated at the chest (anterior) of the human body. It makes up the bulk of the chest muscles and lies under the breast. Below (deep to) the pectoralis major is the pectoralis minor, a thin, triangular muscle. In sports as well as bodybuilding, the pectoral muscles may colloquially be referred to as "pecs", "pectoral muscle" or "chest muscle" due to its being the larger and most superficial muscle in the chest area.
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.
STRUCTURE
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 (intertubercular sulcus) of the humerus.
NERVE SUPPLY
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 out 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 sternalis
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 posterior 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 dorsal column medial lemniscus tract 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.
Electromyography suggests that it consists of at least six groups of
muscle fibres that can be independently coordinated by the central
nervous system.
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.
VARIATION
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 and more frequent than absence of the clavicular part. Poland syndrome
is a rare congenital condition in which the whole muscle is missing,
most commonly on one side of the body. This may accompany absence of the
breast in females. The sternalis muscle may be a variant form of the pectoralis major or the rectus abdominis.
FUNCTIONS
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
underhand, 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. Fourthly 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 adduction 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.
Hypertrophy of the pectoralis major increases functionality. Maximal
activation of the pectoralis major occurs in the transverse plane
through pressing motions. Both multi-joint and single-joint exercises
induce pectoralis major hypertrophy. A combination of both single-joint
and multi-joint exercises will result in a maximum hypertrophic
response. The pectorals can be targeted from numerous training angles
along the sternum and clavicle.
Exercises that include horizontal adduction and elbow extensions such
as the barbell bench press, dumbbell bench press, and machine bench
press induce high activation of the pectoralis major in the sternocostal
region. Heavy loads are strongly correlated with pectoralis major
activation.
INJURIES
Tears of the pectoralis major are rare and typically affect otherwise
healthy individuals. This type of injury (while uncommon) is known to
affect athletic population, namely in high-impact contact sports such as
powerlifting, and may result in pain, weakness, and disability. Most
lesions are located at the musculotendinous junction and result from
violent, eccentric contraction of the muscle, such as during bench
press.
A less frequent rupture site is the muscle belly, usually as a result
of a direct blow. In developed countries, most lesions occur in male
athletes, especially those practicing contact sports and weight-lifting
(particularly during a bench press maneuver). Women are less susceptible
to these tears because of larger tendon-to-muscle diameter, greater
muscular elasticity, and less energetic injuries.
The injury is characterized by sudden and acute pain in the chest wall
and shoulder area, bruising and loss of strength of the muscle. High
grade partial or full thickness tears warrant surgical repair as the
preferred treatment if function is to be preserved, particularly in the
athletic population. Acting fast, obtaining the correct diagnoses, and
getting the surgical repair as soon as possible is a key to successful
recovery. Waiting can cause the acute injury to become chronic and
chances of success is greatly diminished as a result. After surgery, the
impacted arm is then immobilized with a sling for about six to eight
weeks to minimize and avoid movement of the arm and potentially
re-rupturing the surgery site. About two months after the surgery,
physical therapy is typically introduced for about six months, after
which point strengthening of the muscle is needed to achieve good
results. Most patients are able to return to activity after six months
to a year following surgery with high patient satisfaction and slightly
reduced strength compared to pre-injury. Both US and MRI
are useful to confirm the diagnosis, location and extent of a tear,
though the first may be more cost-effective in experienced hands.
POLAND SYNDROME
Poland 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.
OTHER DISEASES
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.
Post a Comment