Pectoralis minor muscle

Origin and insertion

The pectoralis minor muscle arises as 3 separate heads from the anterior surface of the 3rd, 4th, and 5th ribs near the corresponding costal cartilages, as well as the fascia overlying the adjacent intercostal muscles found in the intercostal spaces.

The muscle extends superolaterally to form a flat tendon, which inserts into the medial border and coracoid process of scapula.

Some anatomic variation is seen in the orgin of the muscle. Some fibres may also arise from the 2nd or 5th ribs, or more rarely both. The muscle may be altogether absent when the pectoralis major is absent, such as in Poland's syndrome.

Relations

The main anterior relation is the pectoralis major muscle, found superficial to the pectoralis minor and almost completely covering it. Found between the two muscles are the lateral pectoral nerve and the pectoral branches of the thoracoacromial artery. Located deep to the pectoralis major and its covering fascia is another connective tissue layer known as clavicopectoral fascia. It covers the pectoralis minor, superior to it forming a fascial layer known as the costocoracoid membrane and inferior to it forming the suspensory ligament of the axilla, continuous with the axillary fascia. Together the pectoralis minor and minor, as well as their associated fascia, form the anterior wall of the axilla.

Posteriorly the pectoralis minor is related to the serratus anterior and intercostal muscles, the ribs and several neurovascular and lymphatic structures, described below.

The pectoralis minor is important clinically and as a surgical landmark, due to the structures that lie below or deep to the muscle and its 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

The pectoralis minor is used as the reference point for 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 posterior to the muscle is the second part, and lateral to the lateral border of the muscle, is the third part of the axillary artery.

Innervation

The primary nerve supply to the pectoralis minor muscle comes 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. That results in the pectoralis minor receiving innervation from the spinal roots of C5-T1.

Blood supply

The vascular supply to the pectoralis minor comes from several sources:

• Thoracoacromial artery (branch of the second part of axillary artery) gives two supplying branches - pectoral and deltoid.
• Superior thoracic artery (branch of the first part of axillary artery).
• Lateral thoracic artery (branch of the axillary artery).

Function

Pectoralis muscle has several functions, mostly related to the movement of the scapula.

• Together with the serratus posterior it acts in protraction of the scapula, i.e. moving it laterally and anteriorly against the ribcage. This movement is important in reaching the arm forward.
• Medial or downward rotation (the inferior angle of the scapula moves medially) of the scapula against resistance is achieved by the pectoralis minor exerting force on the corocoid process, which pulls the lateral aspect of the scapula inferiorly, while the levator scapulae and the rhomboids pull upwards on the medial side of the rotation axis.
• Depression of the scapula can normally be carried out by gravity alone, however, when additional force is required, the action is aided by the pectoralis minor and serratus anterior muscles.
• When the scapula is fixed the pectoralis minor can be considered an accessory muscle of respiration when inspiration is deep and forced, as it will help raise  ribs 3-5 during inspiration and aid in expanding the thoracic cavity.