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.

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.

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.

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

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.

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.