Pectoralis minor muscle - want to learn more about it?

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Pectoralis minor muscle

So often when discussing muscles, the nuances of their clinical impact can be lost if they are discussed in isolation from surrounding anatomical structures. This is very much the case with the pectoralis (pec) minor muscle.

Key Facts
Origins	Anterior surface of the 3rd, 4th, and 5th ribs and the fascia overlying the intercostal spaces
Insertions	Medial border and superior surface of the coracoid process of the scapula
Innervation	Medial pectoral nerve
Function	Protraction of the scapula, pulls the coracoid process anteriorly and inferiorly, accessory muscle in respiratory

To understand this muscle from a clinical perspective, this article will first look at the anatomy of the muscle, including bony attachments (the origin and insertion), its actions, and its nerve supply. Next, the location of the pectoralis minor muscle will be discussed in relation to the important surrounding anatomical structures. Finally, the article will discuss the anatomy of the pectoralis minor muscle and its connection to the surrounding structures, with clinical implications associated with dysfunction of the muscle.

Contents

Pectoralis Minor Anatomy
Surrounding Anatomical Structures
Clinical Implications of Pectoralis Minor Dysfunction
1. Impingement Syndrome
2. Thoracic outlet syndrome (TOS)
Related diagrams and images

Pectoralis Minor Anatomy

The pectoralis minor muscle is one of the most superficial muscles on the anterior aspect of the chest or thoracic wall. The most superficial is the pectoralis major, under which is located the pectoralis minor.

Pectoralis minor muscle - ventral view

Although it does not move the arm directly, as does the pectoralis major, the pectoralis minor can have an impact on arm and shoulder movement due to its bony attachments, specifically to the scapula.

Attachments

Bony attachments
Origins	Anterior surface of the 3rd, 4th, and 5th ribs and the fascia overlying the intercostal spaces
Insertion	Coracoid process of the scapula

Actions

It is important to note that when considering muscle actions, one should be aware that the action of a muscle will depend on which bony attachment is ‘fixed’, or in other words, unmoving or stable. When described as origin and insertion, the origin is thought to be the bony attachment that is not moving and the insertion will move towards the origin. However, referring to bony attachments as distal and proximal indicates that technically, either bony attachment could be moving towards the other. Different textbooks will use either origin/insertion, or proximal/distal attachment. Neither will be 100% accurate, as sometimes it can be quite difficult to tell which is the proximal or distal attachment, and the insertion does not always move towards the origin.

Action when the ribs are fixed: Contraction of the pectoralis minor will pull the coracoid process of the scapula anteriorly and pull or rotate the coracoid process (and the scapula and glenoid cavity) inferiorly. It will also cause protraction of the scapula (the medial border of the scapula moves away from the spine) and pulls it against the posterior thoracic wall.
Action when the scapula is fixed: The pectoralis minor can be considered an accessory muscle in respiration when inspiration is deep and forced, as it will help raise the ribs during inspiration and aid in expanding the thoracic cavity.

Nerve Supply

The primary nerve supply to the pectoralis minor muscle comes primarily via the medial pectoral nerve (C8,T1), one of the minor branches of the brachial plexus that arises from the cervical portion of the spinal cord. Innervation to the pectoralis minor is also received from the lateral pectoral nerve, via a communicating branch known as the 'ansa pectoralis', which is usually found anterior to the first part of the axillary artery.

Medial pectoral nerve - ventral view

Surrounding Anatomical Structures

The pectoralis minor is important clinically and as a surgical landmark, due to the structures that lie below or deep to the muscle and tendon. Running deep to the pectoralis minor muscle are the nerves and blood supply to the upper limb:

the posterior, lateral, and medial cords of the brachial plexus
the axillary artery and vein

As the pectoralis minor crosses the axilla from the coracoid process, it is used to define the three divisions of the axillary artery. Medial to the medial border of the pectoralis minor is the first part of the axillary artery, directly behind the muscle is the second part, and lateral to the lateral border of the muscle, is the third part of the axillary artery.

Clinical Implications of Pectoralis Minor Dysfunction

The overabundance of sedentary activities that negatively affect good upper body posture can be detrimental to the proper functioning of the pectoralis minor muscle. Recall that this muscle attaches to, and acts on, the scapula via the coracoid process, pulling it forward and inferiorly. It also rotates the glenoid cavity inferiorly and causes protraction of the scapula. Postures in which there are extended periods of scapular protraction will cause the pectoralis minor muscle to shorten, keeping the scapula in that undesirable position. This can lead to the implication of pectoralis minor in a number of clinical conditions.

Impingement Syndrome

Full abduction of the arm (180 degrees) is a result of movement occurring at two ‘joints’. One hundred and twenty degrees of abduction occurs at the glenohumeral joint. The remaining 60 degrees is a result of the scapula rotating on the posterior thoracic wall (or scapulothoracic joint). If this 60 degrees of rotation is limited or lost completely, moving the shoulder into full abduction can cause discomfort or pain.

The pain is a result of soft tissue structures surrounding the glenohumeral joint (supraspinatus tendon, subacromial bursa, joint capsule) being pinched between two bony surfaces: the greater tubercle of the humerus and the acromion process of the scapula. Because the pectoralis minor muscle inferiorly rotates the glenoid cavity when shortened, this could limit the ability of the scapula to fully rotate in the opposite direction, i.e. superiorly, and allow for a full range of abduction. This limitation of abduction could lead to a soft tissue injury due to impingement.

Thoracic outlet syndrome (TOS)

This syndrome is the result of compression or irritation of the neurovascular structures that serve the upper limb. These include the brachial plexus and the blood vessels running to and from the arm (the subclavian and axillary arteries and veins). Historically, TOS has been associated with compression of these structures in three regions:

between the anterior and middle scalene muscles, and the 1st rib;
behind the clavicle;
in the subcoracoid space between the pectoralis minor tendon and where it attaches to the coracoid process.

In spite of this third site of compression, pectoralis minor has only recently been identified as being implicated in cases of TOS. The close proximity of pectoralis minor to these structures, and the tendency of this muscle to become shortened with poor posture should mean that treatment of neurovascular symptoms in the arm and hand include pectoralis minor, once other more serious causes have been eliminated.

Pectoralis minor muscle - want to learn more about it?

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Show references

References:

R. L. Drake, A. W. Vogle, A. W. M. Mitchell: Gray’s Clinical Anatomy for Students, 3rd edition, Churchill Livingstone (2015), p. 723-725.
G. Fitzgerald: Thoracic outlet syndrome of pectoralis minor etiology mimicking cardiac symptoms on activity: a case report. The Journal of the Canadian Chiropractic Association (2012), volume 56, issue 54, p.311-315.
K. L. Moore, A. D. Dalley II, A. M. R. Agur: Clinically Oriented ANATOMY, 7th edition, Lippincott Williams & Wilkins (2014), p. 414-431, 465-470.
M. Provencher, P. Golijanin, D. Gross et al.: Surgical Release of the Pectoralis Minor Tendon for Scapular Dyskinesia and Shoulder Pain. The Journal of Arthroscopic and Related Surgery (2016), volume 32, issue 6, p.e23.
R. S. Snell: Clinical Anatomy by Regions, 9th edition, Lippincott Williams & Wilkins (2012), p. 350.
Berish Strauch et al: Grabb's Encyclopedia of Flaps, volume 1,

Lippincott Williams & Wilkins (2009), p. 444