The Axilla and Brachial Plexus

Question 1

What is the axilla?

Answer 1

The axilla is the name given to an area that lies underneath the glenohumeral joint, at the junction of the upper limb and the thorax.

This region is a passage by which structures such as vessels and nerves can enter and leave the upper limb.

Question 2

Describe the shape of the axilla.

Answer 2

The overall 3D shape of the axilla looks slightly like a pyramid. The borders consist of four sides and a base with an opening at the apex.

The size and shape of the axilla region varies with arm abduction. It decreases in size most markedly when the arm is fully abducted – at this point, the contents of the axilla are at most risk of injury

Question 3

Describe the borders of the axilla and the structures that form them.

Answer 3

• Apex – Also known as the axillary inlet, this is formed by lateral border of the first rib, superior border of scapula, and the posterior border of the clavicle.
• Lateral wall – Formed by intertubecular groove of the humerus.
• Medial wall – Consists of the serratus anterior and the thoracic wall (ribs and intercostal muscles).
• Anterior wall – Contains the pectoralis major and the underlying pectoralis minor and the subclavius muscles.
• Posterior wall – Formed by the subscapularis, teres major and latissimus dorsi.

Question 4

What are the 3 main routes of exit in the axilla?

Answer 4

The main route of exit is immediately inferiorly and laterally, into the upper limb. The majority of contents of the axilla region leave by this method.

Another pathway is via the quadrangular space.This is a gap in the posterior wall of the axilla, allowing access to the posterior arm and shoulder area. Structures passing through include the axillary nerve and posterior circumflex humeral artery (a branch of the axillary artery.

The last passageway is the clavipectoral triangle, which is an opening in the anterior wall of the axilla. It is bounded by the pectoralis major, deltoid, and clavicle. The cephalic vein enters the axilla via this triangle, while the medial and lateral pectoral nerves leave.

Question 5

Name the main clinically important structures of the axilla (muscles, nerves, vasculature and lymphatics).

Answer 5

• Axillary artery
• Axillary vein
• Brachial plexus
• Biceps brachii and corocobrachialis – These muscle tendons move through the axilla, where they attach to the coracoid process of the scapula.
• Axillary Lymph nodes

Question 6

What is the axillary artery?

Answer 6

The main artery supplying the upper limb. It is commonly refered as having three parts, one medial to the pectoralis minor, one posterior to pectoralis minor, and one lateral to pectoralis minor. The medial and posterior parts travel in the axilla.

Question 7

What is the axillary vein?

Answer 7

The main vein draining the upper limb, its two largest tributaries are the cephalic and basilic veins.

Question 8

What is the brachial plexus?

Answer 8

A collection of spinal nerves that form the peripheral nerves of the upper limb.

Question 9

What are the axillary lymph nodes?

Answer 9

The axillary lymph nodes filter lymph that has drained from the upper limb and pectoral region. In women, axillary lymph node enlargement is an non-specific indicator of breast cancer.

Question 10

What is thoracic outlet syndrome?

Answer 10

The apex of the axilla region is an opening between the clavicle, first rib and the scapula. In this apex, the vessels and nerves may become compressed between the bones – this is called thoracic outlet syndrome. Common causes of TOS are trauma (e.g fractured clavicle) and repetitive (seen commonly in occupations that require lifting of the arms) It often presents with pain in the affected limb, (where the pain is depends on what nerves are affected), tingling, muscle weakness and discolouration.

Question 11

Why would you take a lymph node biopsy from the axilla?

Answer 11

Approximately 75% of lymph from the breast drains into the axilla lymph nodes, so can be biopsied if breast cancer is suspected. If breast cancer is confirmed, the axillary nodes may need to be removed to prevent the cancer spreading. This is known as axillary clearance. During this procedure, the long thoracic nerve may become damaged, resulting in winged scapula.

Question 12

What is the brachial plexus?

Answer 12

The brachial plexus is a network of nerve fibres that supplies the skin and musculature of the upper limb. It begins in the root of the neck, passes through the axilla, and enters the upper arm.

Question 13

What is the brachial plexus formed from?

Answer 13

The plexus is formed by the anterior rami (divisions) of the cervical spinal nerves C5, C6, C7 and C8, and the first thoracic spinal nerve, T1.

Question 14

What are the 5 parts of the brachial plexus?

Answer 14

Roots, trunks, divisions, cords, branches.
(Really Thirsty, Drink Cold Beer)

There are no functional differences between these divisions – they are simply used to aid explanation of the brachial plexus.

Question 15

What are the roots of the brachial plexus?

Answer 15

The ‘roots’ refer the beginning of the brachial plexus. They are formed by the spinal nerves C5, C6, C7, C8 and T1.

At each vertebral level, paired spinal nerves arise. They leave the spinal cord via the intervertebal foramina of the vertebral column.

Each nerve then divides into anterior and posterior nerve fibres. The roots of the brachial plexus are formed by the anterior divisions (ventral rami) of spinal nerves C5-T1 (the posterior divisions (dorsal rami) go on to innervate the skin and musculature of the trunk).

After their formation, these nerves pass between the anterior and medial scalene muscles to enter the base of the neck.

Question 16

What are the trunks of the brachial plexus?

Answer 16

At the base of the neck, the roots of the brachial plexus converge, forming three trunks. These structures are named by their anatomical position:

Superior trunk: A combination of C5 and C6 roots.
Middle trunk: A continuation of C7.
Inferior trunk: A combination of C8 and T1 roots.
The trunks begin to move laterally, crossing the posterior triangle of the neck.

Question 17

What are the divisions of the brachial plexus?

Answer 17

Within the posterior triangle of the neck, each trunk divides into two branches. One division travels anteriorly (toward the front of the body) and the other posteriorly (towards the back of the body). Thus, they are known as the anterior and posterior divisions.

We now have three anterior and three posterior nerve fibres. These divisions leave the posterior triangle and pass into the axilla region. They recombine in the next part of the brachial plexus.

Question 18

What are the cords of the brachial plexus?

Answer 18

• Once the anterior and posterior divisions have entered the axilla, they combine together to form three nerves. These nerves are named by their position relative to the axillary artery.
• There are 3 cords: lateral, posterior and medial
• The cords give rise to the major branches of the brachial plexus..

Question 19

What forms the lateral cord of the brachial plexus?

Answer 19

The lateral cord is formed by:
The anterior division of the superior trunk
The anterior division of the middle trunk

Question 20

What forms the posterior cord of the brachial plexus?

Answer 20

The posterior cord is formed by:
The posterior division of the superior trunk
The posterior division of the middle trunk
The posterior division of the inferior trunk

Question 21

What forms the medial cord of the brachial plexus?

Answer 21

The medial cord is formed by:

The anterior division of the inferior trunk.

Question 22

Where do the cords in the brachial plexus give rise to the branches?

Answer 22

In the axilla and the proximal aspect of the upper limb, the three cords give rise to five major branches. These nerves continue into the upper limb to provide innervation to the muscles and skin present.

Question 23

Which branch is formed from the lateral cord of the brachial plexus?

Answer 23

Musculocutaneous nerve

Roots: C5, C6, C7.

Motor Functions: Innervates the brachialis, biceps brachii and corocobrachialis muscles.

Sensory Functions: Gives off the lateral cutaneous branch of the forearm, which innervates the lateral half of the anterior forearm, and a small lateral portion of the posterior forearm.

Question 24

Which 2 branches are formed from the posterior cord of the brachial plexus?

Answer 24

The axillary nerve

The radial nerve

Question 25

What are the roots, motor functions and sensory functions of the axillary nerve?

Answer 25

Roots: C5 and C6.

Motor Functions: Innervates the deltoid, teres minor and the long head of the triceps brachii.

Sensory Functions: Gives off the superior lateral cutaneous nerve of arm, which innervates the inferior region of the deltoid (“regimental badge area”).

Question 26

What are the roots, motor functions and sensory functions of the radial nerve?

Answer 26

Roots: C5-C8 and T1.

Motor Functions: innervates the triceps brachii, and the extensor muscles in the posterior compartment of the forearm.

Sensory Functions: Innervates the posterior aspect of the arm and forearm, and the posterior, lateral aspect of the hand.

Question 27

Which branch arises from the lateral and medial cords of the brachial plexus?

Answer 27

Median nerve

Roots: C6 – C8 and T1.

Motor Functions: Innervates most of the flexor muscles in the forearm, the thenar muscles, and the two lateral lumbrical muscles that move the index and middle fingers.

Sensory Functions: Gives off the palmar cutaneous branch, which innervates the lateral part of the palm, and the digital cutaneous branch, which innervates the lateral three and a half fingers on the anterior (palmar) surface of the hand.

Question 28

Which branch arises from the medial cord of the brachial plexus?

Answer 28

Ulnar nerve

Roots: C8 and T1.

Motor Functions: Innervates the muscles of the hand (apart from the thenar muscles and two lateral lumbricals), flexor carpi ulnaris and medial half of flexor digitorum profundus.

Sensory Functions: Innervates the anterior and posterior surfaces of the medial one and half fingers, and associated palm area.

Question 29

Which structure is important when trying to identify the structures of the brachial plexus?

Answer 29

The important structure to look for is an ‘M’ shape. This is formed by the musculocutaneous, median and ulnar nerves, arranged around the brachial artery.

Question 30

What are the minor branches of the roots of the brachial plexus?

Answer 30

Dorsal scapular nerve

Long thoracic nerve (supplies serratus anterior)

Question 31

What are the minor branches of the trunks of the brachial plexus?

Answer 31

Suprascapular nerve

Nerve to subclavius

Question 32

What are the minor branches of the lateral cord of the brachial plexus?

Answer 32

Lateral percoral nerve (supplies pectoralis major and minor)

Question 33

What are the minor branches of the medial cord of the brachial plexus?

Answer 33

Medial pectoral nerve (supplies pectoralis major and minor)
Medial cutaneous nerve of the arm
Medial cutaneous nerve of the forearm

Question 34

What are the minor branches of the posterior cord of the brachial plexus?

Answer 34

Superior subscapular nerve
Thoracodorsal nerve
Inferior subscapular nerve

Question 35

What happens when the brachial plexus is injured?

Answer 35

An intact brachial plexus is vital for the normal function of the upper limb. There are two major types of injuries that can affect the brachial plexus. An upper brachial plexus injury affects the superior roots, and a lower brachial plexus injury affects the inferior roots.

Question 36

What is Erb’s Palsy?

Answer 36

-An injury to the upper brachial plexus.
Erb’s palsy commonly occurs where there is excessive increase in the angle between the neck and shoulder – this stretches (or can even tear) the nerve roots, causing damage. It can occur as a result of result of a difficult birth or shoulder trauma.
-Nerves affected: Nerves derived from solely C5 or C6 roots; musculocutaneous, axillary, suprascapular and nerve to subclavius.
-Muscles paralysed: Supraspinatus, infraspinatus, subcalvius, biceps brachii, brachialis, coracobrachialis, deltoid and teres minor.
-Motor functions: The following movements are lost or greatly weakened – abduction at shoulder, lateral rotation of arm, supination of forearm, and flexion at shoulder.
-Sensory functions: Loss of sensation down lateral side of arm, which covers the sensory innervation of the axillary and musculcutaneous nerves.
-The affected limb hangs limply, medially rotated by the unapposed action of pectoralis major. The forearm is pronated due to the loss of biceps brachii. This is position is known as ‘waiter’s tip’, and is characteristic of Erb’s palsy.

Question 37

What is Klumpke Palsy?

Answer 37

A lower brachial plexus injury resulting from excessive abduction of the arm (e.g person catching a branch as they fall from a tree). It has a much lower incidence than Erb’s palsy.

Nerves affected: Nerves derived from the T1 root – ulna and median nerves.

Muscles paralysed: All the small muscles of the hand (the flexors muscles in the forearm are supplied by the ulna and median nerves, but are innervated by different roots).

Sensory functions: Loss of sensation along medial side of arm.

The metacarpophalangeal joints are hyperextended, and the interphalangeal joints are flexed. This gives the hand a clawed appearance.

Question 38

Which muscle does the axillary nerve innervate?

Answer 38

Deltoid.