Nerves - Axillary

Question 1

What are the spinal roots of the axillary nerve?

Answer 1

C5 and C6.

Question 2

Sensory functions?

Answer 2

It is delivered by the posterior terminal branch of the posterior terminal branch.

After innervation of the teres minor, the posterior terminal branch continues as the upper lateral cutaneous nerve of the arm. It innervates the skin over the inferior portion of the deltoid (the ‘regimental badge area’).

In a patient with axillary nerve damage, sensation at the regimental badge area may be impaired or absent. The patient may also report paraesthesia in the distribution of the axillary nerve.

Question 3

Motor function?

Answer 3

Innervates two muscles:

1) Teres minor - part of the rotator cuff muscles which act to stabilise the glenohumeral joint. It acts to externally rotate the shoulder joint and is innervated by the posterior terminal branch of the axillary nerve.
2) situated at the superior aspect of the shoulder. It performs abduction of the upper limb at the glenohumeral joint and is innervated by the anterior terminal branch of the axillary nerve.

Question 4

Anatomical course?

Answer 4

Is formed within the axilla areas of the upper limb. It is a direct continuation of the posterior cord from the brachial plexus - therefore contains fibres from the C5 and C6 nerve roots.

in the axilla, the posterior artery is located posterior to the axillary artery and anterior to the subscapularis muscle via the quadrangular space, often accompanied by the posterior circumflex humeral artery and vein.

The axillary nerve then passes medially to the surgical neck of the humerus, where it divides into three branches…

Question 5

What are the three terminal branches of the axillary nerve?

Answer 5

1) Posterior terminal branch - provides motor innervation to the posterior aspect of the deltoid muscle and teres minor. It innervates the skin over the inferior part of the deltoid as the upper lateral cutaneous nerve of the arm.
2) Anterior terminal branch - winds around the surgical neck of the humerus and provides motor innervation to the anterior aspect of the deltoid muscle. It terminates with cutaneous branches to the anterior and anterolateral shoulder.
3) Articular branch - supplies the glenohumeral joint.

Question 6

What is the quadrangular space?

Answer 6

The quadrangular space is a gap in the muscle of the posterior scapular region. It is a pathway for neurovascular structures to move from the axilla anteriorly to the posterior shoulder and arm. It is bounded by:

Superiorly - inferior aspect of teres minor

Inferiorly - superior aspect of teres major

Laterally - surgical neck of the humerus

Medially - long head of triceps brachii

Anteriorly - subscapularis

Question 7

Clinical relevance: injury to the axillary nerve

Answer 7

The axillary nerve is most commonly damaged by direct trauma to the shoulder or proximal humerus. Common mechanisms of injury include fracture of the humeral surgical neck, shoulder dislocation, or iatrogenic injury during shoulder surgery.

Character clinical signs - in long standing cases - the paralysed deltoid muscle atrophies, giving the shoulder a flattened appearance, and the greater tuberosity can be palpated in that area.

Question 8

Clinical relevance: Erb’s palsy

Answer 8

A condition resulting from damage to C5-C6 roots of the brachial plexus. The axillary nerve is affected, and the individual is unusually unable to abduct or externally rotate the shoulder joint.

It commonly occurs where there is an excessive increase in the angle between the neck and shoulder, which stretches (or even tears) the nerve roots.