Pectoralis minor muscle
The pectoralis minor muscle (often called just pec minor) is one of the most superficial muscles on the anterior aspect of the chest or thoracic wall, located deep only to the pectoralis major muscle. It is one of the anterior axioappendicular (thoracoappendicular) muscles, together with the pectoralis major, subclavius and serratus anterior.
This article will discuss the anatomy and function of the pectoralis minor muscle.
|Origin||Anterior surface, costal cartilages of ribs 3-5|
|Insertion||Medial border and coracoid process of scapula|
Medial and lateral pectoral nerves (C5-T1)
|Blood supply||Thoracoacromial a. (pectoral and deltoid branches), superior thoracic a., lateral thoracic a.|
|Function||Scapulothoracic joint: draws scapula anteroinferiorly, stabilizes scapula on thoracic wall|
- Origin and insertion
- Blood supply
- Clinical implications of the pectoralis minor dysfunction
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.
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 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.
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.
The vascular supply to the pectoralis minor comes from several sources:
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.
Clinical implications of the 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.
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.