Pectoralis Minor Muscle Anatomy

Location & Overview

The pectoralis minor muscle is a small, somewhat triangular-shaped muscle that is sometimes referred to as the “pec minor”. It is located in the superior and anterior part of the chest and lies beneath the larger pectoralis major muscle, hence its designation as the ‘minor’ component of the pectorals, or ‘pecs’. Given its position deep to the pectoralis major and deltoid, it is not visible externally. The pectoralis minor is nestled close to the anterior chest wall, situating it deeper than many of the other muscles in this region.^[1,2,3]

The pectoralis minor forms part of the anterior wall of the axilla. The axilla is also known as the armpit. This anatomical position makes it an important muscle for the movement and stability of the shoulder joint.^[1,2,3]

Origin & Insertion

The pectoralis minor muscle originates from the anterior surface of the third, fourth, and fifth ribs, specifically from the margins adjacent to their costochondral junctions. These are the points where the bony part of the rib meets the cartilage that connects the ribs to the breastbone, or sternum. Additionally, the deep fascia overlying these intercostal spaces – the spaces between the ribs – also contributes to the origin of the pectoralis minor. This muscular origin region resides just lateral to the sternum, the long flat bone located in the center of the chest.^[2,3,4]

From this origin, the fibers of the pectoralis minor ascend superolaterally, angling upward and laterally toward the shoulder. The fibers eventually converge to form a flat tendon that inserts onto the coracoid process of the scapula. The coracoid process is a small hook-like structure that projects anteriorly from the superior aspect of the scapula. The pectoralis minor attaches to the medial border and superior surface of the coracoid process.^[2,3,4]

Action & Function

The pectoralis minor primarily contributes to shoulder girdle movements and also aids in the process of respiration as a secondary action. One of its primary shoulder functions is to depress the scapula, an action that is seen when one actively lowers their scapula/shoulder, moving it downward. In this motion, the pectoralis minor pulls the coracoid process of the scapula downward, causing the shoulder to move downward.^[2,3]

Another primary action of the pectoralis minor is to contribute to the protraction of the scapula. During protraction, the muscle acts to draw the scapula laterally and forward along the chest wall, toward the front of the body. This motion can be visualized by imagining reaching forward to push an object, such as when pushing open a door.^[2,3]

In addition to its role in shoulder movements, the pectoralis minor also serves as an accessory muscle of respiration.^[5] This occurs when other muscles hold the scapula in place, allowing the pectoralis minor to act on the ribs. During the process of inspiration, especially deep or forced inspiration, the pectoralis minor can lift the third, fourth, and fifth ribs. This action assists other muscles in expanding the chest cavity, which then allows for a greater volume of air to enter the lungs.^[4,6]

Beyond these specific actions of scapula depression, scapula protraction, and elevation of ribs, the pectoralis minor also plays a role in stabilizing the scapula.^[3]

Innervation

The pectoralis minor muscle is innervated by the medial pectoral nerve and the lateral pectoral nerve.^[2,3]

The medial pectoral nerve originates from the brachial plexus and is an offshoot of the medial cord. It carries impulses from the anterior rami of spinal nerves C8 and T1.^[4]

The lateral pectoral nerve also contributes to the innervation of the pectoralis minor, although to a lesser extent. This nerve branches from the lateral cord of the brachial plexus, carrying impulses from the anterior rami of spinal nerves C5, C6, and C7.^[4]

Blood Supply

The pectoralis minor is supplied by multiple branches of the axillary arterial system. Cadaveric injection data indicate variable contributions from the thoracoacromial, lateral thoracic, superior thoracic, abdominal-part, and coracoid/coracobrachial branches, with the major supplying arteries described as the abdominal-part artery, lateral thoracic artery, and thoracoacromial artery.^[7] The thoracoacromial artery, also called the thoracoacromial trunk or acromiothoracic artery, typically arises from the second part of the axillary artery near the superior or superomedial border of pectoralis minor, pierces the clavipectoral fascia or costocoracoid membrane, and classically gives rise to pectoral, deltoid, acromial, and clavicular branches.^[8,9]