• Is formed by the ventral primary rami of the lower four cervical nerves and the first thoracic nerve
• Has roots that pass between the scalenus anterior and medius muscles.
• Is enclosed with the axillary artery and vein in the axillary sheath , which is formed by a prolongation of the prevertebral fascia.
• Has the following subdivisions:

Dorsal scapular nerve (C5)

• Pierces the scalenus medius muscle to reach the posterior cervical triangle and descends deep to the levator scapulae and the rhomboid minor and major muscles.
• Innervates the rhomboids and frequently the levator scapulae muscles.

Long thoracic nerve

• Descends behind the brachial plexus and runs on the external surface of the serratus anterior muscle, which it supplies.

Suprascapular nerve

• Runs laterally across the posterior cervical triangle.
• Passes through the scapular notch under the superior transverse scapular ligament, whereas the suprascapular artery passes over the ligament.
  
• Supplies the supraspinatus muscle and the shoulder joint and then descends through the notch of the scapular neck to innervate the infraspinatus muscle.

Nerve to subclavius (C5)

• Descends in front of the brachial plexus and the subclavian artery and behind the clavicle to reach the subclavius muscle.
• Also innervates the sternoclavicular joint.
• Usually branches to the accessory phrenic nerve (C5) , which enters the thorax to join the phrenic nerve.
  

Lateral pectoral nerve

• Innervates the pectoralis major muscle primarily and also supplies the pectoralis minor muscle by way of a nerve loop.
• Sends a branch over the first part of the axillary artery to the medial pectoral nerve and forms a nerve loop through which the lateral pectoral nerve conveys motor fibers to the pectoralis minor muscle.
• Pierces the costocoracoid membrane of the clavipectoral fascia.
• Is accompanied by the pectoral branch of the thoracoacromial artery.

Musculocutaneous nerve

• Pierces the coracobrachialis muscle, descends between the biceps brachii and brachialis muscles, and innervates these three muscles.

Medial pectoral nerve

• Passes forward between the axillary artery and vein and forms a loop in front of the axillary artery with the lateral pectoral nerve.
• Enters and supplies the pectoralis minor muscle and reaches the overlying pectoralis major muscle.

Medial brachial cutaneous nerve

• Runs along the medial side of the axillary vein.
• Innervates the skin on the medial side of the arm.
• May communicate with the intercostobrachial nerve , which arises as a lateral branch of the second intercostal nerve.

Medial antebrachial cutaneous nerve

• Runs between the axillary artery and vein and then runs medial to the brachial artery.
• Innervates the skin on the medial side of the forearm.

Ulnar nerve

• Runs down the medial aspect of the arm but does not branch in the brachium.

• Is formed by heads from both the medial and lateral cords.
• Runs down the anteromedial aspect of the arm but does not branch in the brachium.

Upper subscapular nerve

• Innervates the upper portion of the subscapularis muscle.

Thoracodorsal nerve

• Runs behind the axillary artery and accompanies the thoracodorsal artery to enter the latissimus dorsi muscle.

Lower subscapular nerve

• Innervates the lower part of the subscapularis and teres major muscles.
• Runs downward behind the subscapular vessels to the teres major muscle.

Axillary nerve

• Innervates the deltoid and teres minor muscles and gives rise to the lateral brachial cutaneous nerve.
• Passes posteriorly through the quadrangular space accompanied by the posterior circumflex humeral artery and winds around the surgical neck of the humerus (may be injured when this part of the bone is fractured).

Radial nerve

• Is the largest branch of the brachial plexus and occupies the musculospiral groove on the back of the humerus with the profunda brachii artery.