Axilla and Brachial Plexus

The axilla is a pyramid-shaped space located between the upper thorax and the arm. The axilla has a base, an apex, and 4 walls (anterior, medial, lateral, posterior). The base of the pyramid is made up of the axillary skin. The apex is the axillary inlet, located between the 1st rib, superior border of the scapula, and clavicle. The apex houses various vessels and nerves, including the axillary artery and its branches, the axillary vein and its tributaries, the branches of the brachial plexus, and the axillary lymph nodes.

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Boundaries of the Axilla

The axilla is a pyramid-shaped space below the glenohumeral joint that is the passageway for nerves and vessels to pass into the upper arm.

  • Base: hair and sweat gland–bearing axillary skin
  • Apex (axillary inlet): between the 1st rib, scapula, and clavicle
  • 4 converging walls: 
    • Anterior wall: pectoralis major and minor
    • Medial wall: serratus anterior, thoracic wall
    • Lateral wall: intertubercular sulcus of the humerus
    • Posterior wall: subscapularis, latissimus dorsi, teres major muscles

Boundaries of the axilla

Image by BioDigital, edited by Lecturio

Axillary inlet

Image by Lecturio.

Contents of the Axilla

The contents of the axilla are structures enclosed in the axillary sheath.

Embedded in the axillary fat, enclosed within the axillary sheath, the contents of the axilla include: the axillary artery and its branches, the axillary vein and its tributaries, the branches of the brachial plexus, and the axillary lymph nodes.

Image by BioDigital, edited by Lecturio

Axillary artery

The axillary artery is a direct continuation of the subclavian artery and is composed of 3 parts:

  • 1st part: 
    • Superior to the pectoralis minor muscle
    • Branches: superior thoracic artery
  • 2nd part: 
    • Deep to the pectoralis minor muscle
    • Branches: thoracoacromial and lateral thoracic artery
  • 3rd part: 
    • Inferior to the pectoralis minor muscle and extending to the lower border of the axilla
    • Branches: subscapular artery, anterior, and posterior circumflex humeral artery

Axillary vein

The axillary vein is the primary venous drainage of the upper arm, formed by the cephalic and basilic veins.

Axillary lymph nodes

There are 5 groups of axillary lymph nodes draining to the apical lymph nodes:

  1. The anterior or pectoral group drains the anterolateral abdominal wall and lateral quadrants of the breast.
  2. The posterior or subscapular group drains the superficial layers of the back.
  3. The lateral group drains the upper limb.
  4. The central group drains the anterior, posterior, and lateral groups, which drain into the apical or subclavicular group at the apex of the axilla (apical → subclavian lymph trunk → thoracic duct on the left and right lymphatic trunk on the right).
  5. The infraclavicular or deltoid group (not found within the axilla) drains the superficial layers of the upper limb.

These lymph nodes filter lymph fluid from the arm and pectoral region, including the breast (important in the diagnosis and management of breast cancer).

Brachial plexus

The brachial plexus is formed from the anterior rami of spinal cord segments C5T1, which are the roots of the brachial plexus and the network of nerves that supply the upper arm.

The Brachial Plexus

The brachial plexus is a nerve plexus formed by intercommunication of the ventral rami of C5T1 nerves. The brachial plexus serves as the origin of all the peripheral nerves that innervate the upper limb and shoulder.

Table: Branches of the brachial plexus
SegmentBranchFunction
RootsDorsal scapular nerve (C5)Supplies the rhomboid major and minor muscles and the levator scapulae muscles
Long thoracic nerve (C5–C7)Supplies the serratus anterior muscles
Upper trunkSuprascapular nerve (C5–C6)Supplies the supraspinatus and infraspinatus muscles
Nerve to subclavius (C5–C6)Supplies the subclavius muscle and the sternoclavicular joint
Lateral cordLateral pectoral nerve (C5–C7)Supplies the pectoralis major muscle
Musculocutaneous nerve (C5–C7)Supplies the coracobrachialis muscle, biceps brachii, and brachialis muscles, terminates as the lateral cutaneous nerve of forearm
Lateral root of the median nerve (C5–C7)Joins a similar branch from the medial cord to form the median nerve (opposition of the thumb and flexion of the first 3 fingers)
Medial cordMedial pectoral nerve (C8, T1)Supplies the pectoralis major and minor muscles
Medial cutaneous nerve of the arm (medial brachial cutaneous nerve) (C8, T1)Supplies the skin on the medial side of the arm
Medial cutaneous nerve of the forearm (medial antebrachial cutaneous nerve) (C8, T1)Supplies the skin on the medial side of the forearm
Medial root of the median nerve (C8, T1)Joins with the lateral root to form the median nerve
Ulnar nerve (C7–T1)Flexion of the 4th and 5th fingers
Posterior cordUpper subscapular nerve (C5–C6)Supplies the upper part of the subscapularis muscle
Thoracodorsal nerve (C6–C8)Supplies the latissimus dorsi muscle
Lower subscapular nerve (C5–C6)Supplies the lower part of the subscapularis and teres major muscles
Axillary nerve (C5–C6)Supplies both the deltoid and teres minor muscles
Radial nerve (C5–T1)Extension of the wrist and fingers

Brachial plexus

Image by Lecturio.

Axillary Spaces

The axillary spaces are anatomic spaces in the axilla where important nerves and vessels exit the axilla.

Posterior view of the axilla, featuring the scapulohumeral muscles and the axillary spaces: quadrangular and triangular spaces

Image by BioDigital, edited by Lecturio
Table: Axillary spaces
SpaceBordersContent
Quadrangular space
  • Medial: long head of triceps
  • Lateral: humeral shaft
  • Superior: teres minor
  • Inferior: teres major
  • Axillary nerve
  • Posterior humeral circumflex vessels
Triangular space
  • Inferior: teres major
  • Lateral: long head of triceps
  • Superior: lower border of teres minor
Scapular circumflex artery and vein
Triangular interval
  • Superior: teres major
  • Lateral: lateral head of the triceps or the humerus
  • Medial: long head of the triceps
  • Profunda brachii artery
  • Radial nerve

Spaces of arm using a finger arrangement

Image by Lecturio.

Clinical Relevance

The following are common conditions and pathologies associated with the axilla:

  • Axillary lymphadenopathy: inflammation or disease of the lymph nodes. Common causes include infection and breast cancer; lymph node dissection is often part of the staging of breast cancer disease.
  • Axillary nerve injury: The axillary nerve innervates the deltoid and teres minor muscles and the skin of the upper lateral and dorsal areas of the upper arm through the superior lateral brachial cutaneous nerve. Common causes of injury include shoulder dislocation, fracture of the surgical neck of the humerus, and scapular fracture. 
  • Radial nerve injury: commonly injured in the upper arm at the spiral groove of the humerus after fracture, or from pressure on the axilla from a chair (also known as “Saturday night palsy”) or from crutches. 
  • Brachial plexus injuries:
    • Erb’s palsy: a type of obstetrical palsy; obstetrical palsy is an injury due to trauma during childbirth. Erb’s palsy is a stretch injury to the brachial plexus, specifically C5C6, during labor and delivery. 
    • Klumpke’s palsy: another type of obstetrical palsy that results in injury to the lower trunk of the brachial plexus.

References

  1. Drake, R.L., Vogl, A.W., & Mitchell, A.W.M. (2014). Gray’s Anatomy for Students (3rd ed.). Philadelphia, PA: Churchill Livingstone.

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