What are the muscles, nerves, veins, and arteries of the axilla?

Anatomy of the Axilla: Muscles, Nerves, Veins, and Arteries

The axilla is a complex anatomical region containing critical neurovascular structures and muscles that connect the upper limb to the trunk, with detailed knowledge of these structures being essential for surgical procedures, regional anesthesia, and diagnosis of pathological conditions.

Boundaries of the Axilla

The axilla is a pyramidal-shaped space with the following boundaries:

• Anterior: Pectoralis major and minor muscles
• Posterior: Latissimus dorsi, teres major, and subscapularis muscles
• Medial: Serratus anterior muscle and chest wall
• Lateral: Intertubercular groove of the humerus
• Superior: Clavicle, first rib, and scapula
• Inferior: Axillary fascia and skin

Muscles of the Axilla

Anterior Wall

• Pectoralis major: Forms the anterior axillary fold

  • Origin: Clavicle, sternum, and upper ribs
  • Insertion: Lateral lip of intertubercular groove of humerus
  • Function: Adduction, medial rotation, and flexion of the arm
  • Innervation: Medial and lateral pectoral nerves

• Pectoralis minor: Located deep to pectoralis major

  • Origin: 3rd to 5th ribs
  • Insertion: Coracoid process of scapula
  • Function: Depression and protraction of scapula
  • Innervation: Medial pectoral nerve

Posterior Wall

• Latissimus dorsi: Forms the posterior axillary fold

  • Origin: Spinous processes of T7-L5, iliac crest, and thoracolumbar fascia
  • Insertion: Intertubercular groove of humerus
  • Function: Extension, adduction, and medial rotation of the arm
  • Innervation: Thoracodorsal nerve

• Teres major:

  • Origin: Inferior angle of scapula
  • Insertion: Medial lip of intertubercular groove of humerus
  • Function: Adduction and medial rotation of the arm
  • Innervation: Lower subscapular nerve

• Subscapularis:

  • Origin: Subscapular fossa of scapula
  • Insertion: Lesser tubercle of humerus
  • Function: Medial rotation of the arm
  • Innervation: Upper and lower subscapular nerves

Medial Wall

• Serratus anterior:
  • Origin: Upper 8-9 ribs
  • Insertion: Medial border of scapula
  • Function: Protraction and rotation of scapula
  • Innervation: Long thoracic nerve

Nerves of the Axilla

The brachial plexus traverses the axilla and provides the primary innervation to the upper extremity. It originates from C5-T1 nerve roots and forms trunks, divisions, and cords before terminating in peripheral nerves 1.

Major Nerves in the Axilla:

1. Musculocutaneous nerve:

   • From lateral cord
   • Supplies coracobrachialis, biceps brachii, and brachialis
   • Continues as lateral cutaneous nerve of forearm

2. Median nerve:

   • Formed by lateral and medial roots from respective cords
   • Typically forms anterior to the axillary artery
   • Variations in formation occur in approximately 30% of individuals 2

3. Ulnar nerve:

   • From medial cord
   • No branches in axilla
   • Supplies flexor carpi ulnaris, medial half of flexor digitorum profundus, and intrinsic hand muscles

4. Radial nerve:

   • From posterior cord
   • Largest branch of brachial plexus
   • Supplies posterior compartment muscles of arm and forearm
   • Sensory to posterior arm, forearm, and dorsal hand 1

5. Axillary nerve:

   • From posterior cord
   • Supplies deltoid and teres minor
   • Sensory to lateral shoulder

6. Other important nerves:

   • Long thoracic nerve: Supplies serratus anterior
   • Thoracodorsal nerve: Supplies latissimus dorsi
   • Medial and lateral pectoral nerves: Supply pectoral muscles
   • Subscapular nerves: Supply subscapularis and teres major
   • Intercostobrachial nerve: Sensory to medial arm

Arteries of the Axilla

1. Axillary artery: Continuation of the subclavian artery, divided into three parts based on its relationship to pectoralis minor:

   • First part: From outer border of first rib to medial border of pectoralis minor

     • Branch: Superior thoracic artery

   • Second part: Behind pectoralis minor

     • Branches: Thoracoacromial artery and lateral thoracic artery 3

   • Third part: From lateral border of pectoralis minor to teres major

     • Branches: Subscapular artery, anterior circumflex humeral artery, and posterior circumflex humeral artery

2. Thoracoacromial artery: Divides into pectoral, acromial, clavicular, and deltoid branches

3. Lateral thoracic artery: Supplies serratus anterior and pectoral muscles

4. Subscapular artery: Largest branch of axillary artery

   • Divides into circumflex scapular and thoracodorsal arteries

Veins of the Axilla

1. Axillary vein: Continuation of the basilic vein

   • Receives tributaries corresponding to branches of axillary artery
   • Located medial to axillary artery
   • May be duplicated in some individuals (anatomical variation) 3

2. Cephalic vein: Pierces clavipectoral fascia to join axillary vein

3. Tributaries: Include brachial veins, basilic vein, and veins corresponding to branches of axillary artery

Clinical Significance

1. Surgical Considerations:

   • During axillary lymph node dissection for breast cancer, the thoracodorsal, long thoracic, and medial pectoral nerves should be preserved 4
   • Preservation of the intercostobrachial nerve is desirable but may not be possible if preservation compromises adequate excision of grossly positive nodes 4
   • Circumferential stripping of the axillary vein is unnecessary and may increase the incidence of lymphedema 4

2. Anesthetic Applications:

   • Ultrasound-guided axillary brachial plexus block requires knowledge of nerve variations
   • Only 59% of patients show the classic anatomical arrangement of nerves around the axillary artery 5

3. Anatomical Variations:

   • The axillary arch (arch of Langer): A muscular band crossing the axilla, present in approximately 8.5% of individuals, may contribute to thoracic outlet syndrome 6
   • Variations in median nerve formation occur in 30% of individuals, with additional roots from lateral or posterior cords 2
   • Duplicated axillary vein may be present 3

4. Neurovascular Compression:

   • Thoracic outlet syndrome may involve compression of neurovascular structures in the axilla
   • Anatomical variations like the axillary arch may contribute to compression symptoms 6

Imaging of the Axilla

Ultrasound is the preferred initial imaging modality for the axilla, allowing visualization of muscles, vessels, nerves, and lymph nodes in a layer-by-layer approach 7. MRI provides superior soft tissue contrast for evaluating the brachial plexus and is recommended for suspected plexopathy or peripheral nerve injury 1.

Understanding the complex anatomy of the axilla is crucial for clinicians performing procedures in this region, including axillary lymph node dissection, regional anesthesia, and diagnosis of neurovascular compression syndromes.