Axilla and Brachial Plexus

The axilla is a pyramid-shaped space located between the upper thorax and the arm Arm The arm, or "upper arm" in common usage, is the region of the upper limb that extends from the shoulder to the elbow joint and connects inferiorly to the forearm through the cubital fossa. It is divided into 2 fascial compartments (anterior and posterior). 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 Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Structure and Function of the 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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Editorial responsibility: Stanley Oiseth, Lindsay Jones, Evelin Maza

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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 Arm The arm, or "upper arm" in common usage, is the region of the upper limb that extends from the shoulder to the elbow joint and connects inferiorly to the forearm through the cubital fossa. It is divided into 2 fascial compartments (anterior and posterior). Arm.

  • Base: hair and sweat gland–bearing axillary skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Structure and Function of the 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

Boundaries of the axilla

Image by BioDigital, edited by Lecturio
The axillary inlet

Axillary inlet

Image by Lecturio.

Contents of the Axilla

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

Axillary lymph

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 Arm The arm, or "upper arm" in common usage, is the region of the upper limb that extends from the shoulder to the elbow joint and connects inferiorly to the forearm through the cubital fossa. It is divided into 2 fascial compartments (anterior and posterior). Arm, formed by the cephalic and basilic veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. 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 Arm The arm, or "upper arm" in common usage, is the region of the upper limb that extends from the shoulder to the elbow joint and connects inferiorly to the forearm through the cubital fossa. It is divided into 2 fascial compartments (anterior and posterior). Arm and pectoral region, including the breast (important in the diagnosis and management of breast cancer Breast cancer Breast cancer is a disease characterized by malignant transformation of the epithelial cells of the breast. Breast cancer is the most common form of cancer and 2nd most common cause of cancer-related death among women. Breast Cancer).

Brachial plexus

The brachial plexus is formed from the anterior rami of spinal cord Spinal cord The spinal cord is the major conduction pathway connecting the brain to the body; it is part of the CNS. In cross section, the spinal cord is divided into an H-shaped area of gray matter (consisting of synapsing neuronal cell bodies) and a surrounding area of white matter (consisting of ascending and descending tracts of myelinated axons). Spinal Cord segments C5T1, which are the roots of the brachial plexus and the network of nerves that supply the upper arm Arm The arm, or "upper arm" in common usage, is the region of the upper limb that extends from the shoulder to the elbow joint and connects inferiorly to the forearm through the cubital fossa. It is divided into 2 fascial compartments (anterior and posterior). 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
Segment Branch Function
Roots Dorsal 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 trunk Suprascapular nerve (C5–C6) Supplies the supraspinatus and infraspinatus muscles
Nerve to subclavius (C5–C6) Supplies the subclavius muscle and the sternoclavicular joint
Lateral cord Lateral 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 Forearm The forearm is the region of the upper limb between the elbow and the wrist. The term "forearm" is used in anatomy to distinguish this area from the arm, a term that is commonly used to describe the entire upper limb. The forearm consists of 2 long bones (the radius and the ulna), the interosseous membrane, and multiple arteries, nerves, and muscles. 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 cord Medial pectoral nerve (C8, T1) Supplies the pectoralis major and minor muscles
Medial cutaneous nerve of the arm Arm The arm, or "upper arm" in common usage, is the region of the upper limb that extends from the shoulder to the elbow joint and connects inferiorly to the forearm through the cubital fossa. It is divided into 2 fascial compartments (anterior and posterior). Arm (medial brachial cutaneous nerve) (C8, T1) Supplies the skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Structure and Function of the Skin on the medial side of the arm Arm The arm, or "upper arm" in common usage, is the region of the upper limb that extends from the shoulder to the elbow joint and connects inferiorly to the forearm through the cubital fossa. It is divided into 2 fascial compartments (anterior and posterior). Arm
Medial cutaneous nerve of the forearm Forearm The forearm is the region of the upper limb between the elbow and the wrist. The term "forearm" is used in anatomy to distinguish this area from the arm, a term that is commonly used to describe the entire upper limb. The forearm consists of 2 long bones (the radius and the ulna), the interosseous membrane, and multiple arteries, nerves, and muscles. Forearm (medial antebrachial cutaneous nerve) (C8, T1) Supplies the skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Structure and Function of the Skin on the medial side of the forearm Forearm The forearm is the region of the upper limb between the elbow and the wrist. The term "forearm" is used in anatomy to distinguish this area from the arm, a term that is commonly used to describe the entire upper limb. The forearm consists of 2 long bones (the radius and the ulna), the interosseous membrane, and multiple arteries, nerves, and muscles. 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 cord Upper 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 diagram

Brachial plexus

Image by Lecturio.

Axillary Spaces

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

Axillary spaces (posterior view)

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
Space Borders Content
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
Mnemonic of axillary spaces

Spaces of arm Arm The arm, or "upper arm" in common usage, is the region of the upper limb that extends from the shoulder to the elbow joint and connects inferiorly to the forearm through the cubital fossa. It is divided into 2 fascial compartments (anterior and posterior). Arm using a finger arrangement

Image by Lecturio.

Clinical Relevance

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

  • Axillary lymphadenopathy Lymphadenopathy Lymphadenopathy is lymph node enlargement (> 1 cm) and is benign and self-limited in most patients. Etiologies include malignancy, infection, and autoimmune disorders, as well as iatrogenic causes such as the use of certain medications. Generalized lymphadenopathy often indicates underlying systemic disease. Lymphadenopathy: inflammation Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body's defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. Inflammation or disease of the lymph nodes. Common causes include infection and breast cancer Breast cancer Breast cancer is a disease characterized by malignant transformation of the epithelial cells of the breast. Breast cancer is the most common form of cancer and 2nd most common cause of cancer-related death among women. Breast Cancer; lymph node dissection is often part of the staging Staging Cancer is the 2nd leading cause of death in the US after cardiovascular disease. Many malignancies are treatable or curable, but some may recur. Thus, all malignancies must be assigned a grade and stage in order to guide management and determine prognosis. Grading, Staging, and Metastasis of breast cancer Breast cancer Breast cancer is a disease characterized by malignant transformation of the epithelial cells of the breast. Breast cancer is the most common form of cancer and 2nd most common cause of cancer-related death among women. Breast Cancer disease.
  • Axillary nerve injury: The axillary nerve innervates the deltoid and teres minor muscles and the skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Structure and Function of the Skin of the upper lateral and dorsal areas of the upper arm Arm The arm, or "upper arm" in common usage, is the region of the upper limb that extends from the shoulder to the elbow joint and connects inferiorly to the forearm through the cubital fossa. It is divided into 2 fascial compartments (anterior and posterior). Arm through the superior lateral brachial cutaneous nerve. Common causes of injury include shoulder dislocation, fracture Fracture A fracture is a disruption of the cortex of any bone and periosteum and is commonly due to mechanical stress after an injury or accident. Open fractures due to trauma can be a medical emergency. Fractures are frequently associated with automobile accidents, workplace injuries, and trauma. Overview of Bone Fractures of the surgical neck of the humerus, and scapular fracture Fracture A fracture is a disruption of the cortex of any bone and periosteum and is commonly due to mechanical stress after an injury or accident. Open fractures due to trauma can be a medical emergency. Fractures are frequently associated with automobile accidents, workplace injuries, and trauma. Overview of Bone Fractures
  • Radial nerve injury: commonly injured in the upper arm Arm The arm, or "upper arm" in common usage, is the region of the upper limb that extends from the shoulder to the elbow joint and connects inferiorly to the forearm through the cubital fossa. It is divided into 2 fascial compartments (anterior and posterior). Arm at the spiral groove of the humerus after fracture Fracture A fracture is a disruption of the cortex of any bone and periosteum and is commonly due to mechanical stress after an injury or accident. Open fractures due to trauma can be a medical emergency. Fractures are frequently associated with automobile accidents, workplace injuries, and trauma. Overview of Bone Fractures, or from pressure on the axilla from a chair (also known as “Saturday night palsy”) or from crutches. 
  • Brachial plexus injuries Brachial plexus injuries The brachial plexus is a network of nerves that originate from the lower cervical and upper thoracic nerve roots. The causes of brachial plexopathies include traumatic injuries, birth-related injuries, iatrogenic procedures, neoplastic processes, and previous treatment with radiation. 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 Labor Labor is the normal physiologic process defined as uterine contractions resulting in dilatation and effacement of the cervix, which culminates in expulsion of the fetus and the products of conception. Normal and Abnormal 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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