Brachial plexus

Neck

Before it enters the cervicoaxillary canal, the brachial plexus is located within the posterior triangle of the neck. It is a space whose anterior wall is presented with the posterior border of the sternocleidomastoid muscle and the posterior wall is the anterior surface of the trapezius muscle. 

The trunks of the plexus reach the triangle as it passes between the anterior and the posterior scalene muscle together with the subclavian artery. Within this triangle, the plexus is placed superficially while being covered with the platysma. Anteriorly to the plexus, there are the subclavian artery and the subclavian vein. So observed antero-posteriorly, the relations are: vein-artery-nerve (VAN).

The projection of the plexus on the lateral surface of the neck corresponds to the field located under the imaginary line that connects the middle of the clavicle with the posterior edge of the sternocleidomastoid muscle. That means that the trunks of the plexus can be palpated between the sternocleidomastoid muscle and the clavicle, which has a great practical value. Also, a place between the scalene muscles where the subclavian artery and the brachial plexus cross to reach the posterior triangle of the neck, is the place where a compression of the brachial plexus can occur, which is clinically called the anterior scalene muscle syndrome.
 
The brachial plexus leaves the posterior triangle of the neck as it passes under the clavicle and enters the cervicoaxillary canal.

Axilla

The cervicoaxillary canal is a structure that is anteriorly bordered by the clavicle, posteriorly by the scapula, and medially by the first rib. This canal connects the neck with the arm and it is used by the brachial plexus, among other structures, to reach the arm.
 
When the brachial plexus reaches the axilla, it is at first located laterally and posteriorly to the axillary artery. At this point, the neuronal fibers are organized in the form of the divisions. Not long after the axilla is reached, the divisions merge to form the cords, which take their own positions related to the axillary artery: laterally to the artery is the lateral cord, medially to it is the medial cord, and posteriorly to the axillary artery is the posterior cord. It is clear that the cords are named just according to their relations to the artery, so that will make them easier to remember.

Anterior Branches

Lateral Pectoral Nerves

These nerves usually arise from the lateral cord of the brachial plexus, and they carry the fibers that originate from the C5, C6 and the C7. They cross anteriorly the axillary vessels (axillary artery, axillary vein) and then penetrate the pectoral fascia. In this way, they reach the posterior surface of the major pectoral muscle which they innervate.

Through the anastomoses with the medial pectoral nerves, the lateral nerves participate in the innervation of the minor pectoral muscle.

Medial Pectoral Nerves

The medial pectoral nerves emerge from the medial cord of the brachial plexus. They are directed downwards and in that way they cross between the axillary artery and the axillary vein, aiming towards the thoracic muscles. Only a few of them penetrate the lesser pectoral muscle and terminate within the inferior portion of the greater pectoral muscle.

Subclavian Nerve

This nerve emerges from the superior trunk of the brachial plexus, and it carries the fibers originating from the C5. It extends inferiorly, along with the edge of the anterior scalene muscle. Medially to the subclavian nerve is the phrenic nerve.

The subclavian nerve then crosses posteriorly to the clavicle and anteriorly to the subclavian artery and vein, to finally end in the subclavian muscle, which it innervates.

Posterior Lateral Branches

Dorsal Scapular Nerve

The dorsal scapular nerve arises from the superior trunk of the brachial plexus and it carries the C5 fibers. This nerve either penetrates the anterior scalene muscle, either it crosses across its surface. In any case, it is directed laterally and backward as it crosses over the first rib and the superior portions of the serratus anterior muscle.
By reaching the posterior surface of the levator scapulae muscle, it elongates several fibers that innervate it, and after it continues downwards rhomboid muscle in which it terminates.

Suprascapular Nerve

This nerve emerges from the superior trunk and it contains the fibers of the anterior branches of the fifth and the sixth spinal nerves. It is directed laterally, posteriorly and downwards and it crosses over the supraclavicular fossa by following the inferior belly of the omohyoid muscles.

After this, the nerve passes through the scapular incisure then enters the fossa of the supraspinatus muscle and sends the branches for the supraspinatus muscle. After that, the nerve goes around the lateral edge of the scapular spine and reaches the fossa of the infraspinatus muscle which it innervates.

The scapular incisure is closed with the strong fibrous stripe. Excessive movement of the shoulder joint can cause this stripe to compress the nerve which presents under the condition called the suprascapular nerve syndrome.

Subscapular Nerves

There are mostly two of these nerves: superior and the inferior. They rise from the posterior cord carrying the fibers of the C5. They descend laterally across the posterior wall of the axilla. The superior subscapular nerve innervates the superior portion of the subscapularis muscle, whereas the inferior subscapular nerve innervates the rest of the subscapularis muscle and as well as the teres major muscle.

Thoracodorsal Nerve

This nerve arises from the posterior cord of the brachial plexus as it carries the fibers from the C7 and C8. It courses laterally and downwards over the anterior surface of the subscapularis muscle. At first, it follows the subscapular artery, and then it follows the artery terminal branch, the thoracodorsal artery, and finally reaches the latissimus dorsi muscle which it innervates.

Long Thoracic Nerve

This nerve arises from the three neuronal rootlets that originate from the C5, C6 and C7. These rootlets merge above the first rib to form the nerve. The long thoracic nerve then descends over the anterior surface of the first rib, placed posteriorly to the axillary artery and reaches the superior edge of the anterior serratus muscle. It then descends across the lateral wall of the thorax, or actually the anterior serratus muscle which it innervates.

Musculocutaneous Nerve

This is a mixed terminal branch of the lateral cord of the brachial plexus. This nerve arises from the lateral cord at the level of the inferior edge of the greater pectoral muscle, and then it descends laterally and downwards. Medially to it is the axillary artery and the median nerve, whereas laterally to it is the coracobrachialis muscle. When it reaches the upper arm, the musculocutaneous nerve penetrates the coracobrachialis muscle and enters the space between the biceps brachii muscle and the brachialis muscle and sends the motor fibers that innervate both of these muscles. In the lower one-third of the upper arm, the nerve penetrates the deep fascia and takes the superficial position just under the skin, and it continues as its own terminal branch, the lateral cutaneous nerve of the forearm, which innervates the skin of the same named region.

The motor fibers of the musculocutaneous nerve extend in the upper arm, and they innervate:

• Biceps brachii muscle
• Coracobrachialis muscle
• Brachialis muscle

With its sensory terminal branch, it innervates the lateral surface of the forearm.

Median Nerve

The median nerve is created from two of its roots:

• The lateral root which is the terminal branch of the lateral cord of the brachial plexus
• The medial root, that is the branch of the medial cord

These roots merge anteriorly to the axillary artery and form the median nerve.

Course and Relations

The median nerve descends downwards, going anteriorly to the axillary artery. It soon leaves the axilla at the inferior margin of the major muscle. After entering the arm, the nerve enters the bone canal called medial bicipital sulcus. At first, the median nerve is located laterally to the brachial artery, but approximately at the middle of the arm, the nerve passes over the arteries anterior surface and positions itself to the medial side of the artery. The brachial artery and the median nerve share common fascia, along which they both descend towards the elbow, located superficially during that road.
 
The nerve enters the cubital fossa where it is positioned superficially. Laterally to the nerve is the tendon of the biceps muscle while medially is the pronator teres muscle. Posteriorly to the nerve is the brachialis muscle, on which the nerve lies directly. Finally, anteriorly to the nerve is the cubital vein, above which is the skin. 

The nerve leaves the cubital fossa by passing between the two heads of the pronator teres muscle and enters the anterior region of the forearm. In this area, the nerve is not located superficially, but instead it takes a deeper position by diving between the superficial flexor digitorum muscle which is superficial to the nerve, and the deep flexor digitorum muscle that is inferior to the nerve. The nerve holds this deep position all the way through the forearm until it reaches the wrist.

At the wrist, where the superficial and the deep flexor digitorum muscle extend their tendons, the nerve passes between the tendons and again, becomes superficial lying between the tendons of the flexor carpi radialis muscle and the palmaris longus muscle. The nerve enters the hand through the canal called the carpal canal. In this canal, laterally to the nerve is the tendon of the common flexor of the fingers whereas the medial position takes the long flexor of the thumb. Superficially to the median nerve is the flexor retinaculum. Passing through this canal, the median nerve enters the palm. When it enters the palm, the nerve is located superficially in the ventral side of the palm, and the only thing that separates it from the skin is the palmar aponeurosis.

After entering the palm, the nerve extends to its terminal branches:

• The recurrent branch that innervates the:
  • Three thenar muscles
  • Opponens pollicis muscle
• The palmar digital nerves that provide sensory innervation for: 
  • The skin of the lateral one-half of the palm
  • The ventral skin of the lateral three and a half fingers (starting from the thumb)
  • The dorsal skin of the lateral two and a half fingers on the level of the middle and distal phalanges
  • Two lateral lumbrical muscles

Major motor branch of the median nerve arises at the point where the muscle enters the forearm. It is called the anterior interosseous nerve, and it innervates all the muscles of the deep layer of the arm except for the:

• Adductor of the thumb muscle
• Medial one-half of the deep flexor digitorum muscle

This field of motor innervation of the median nerve suggests why the injury of the median nerve causes a clinical state called the preacher's hand in which the patient has its thumb, the index and the middle finger extended and is unable to flex them, with the thumb adducted.

Ulnar Nerve

The ulnar nerve is a mixed nerve, and also the terminal branch of the medial cord of the brachial plexus. It extends through the axillary fossa by descending posteriorly to the axillary artery, and soon after that, it enters the anterior region of the arm.

The ulnar nerve then enters the medial bicipital sulcus in which it follows the medial side of the brachial artery. The nerve then penetrates the medial intermuscular septum of the upper arm and reaches the posterior region of the arm, where it is located anteriorly to the medial head of the triceps brachii muscle. 

By extending downwards, the nerve passes through the ulnar nerve sulcus located on the posterior surface of the medial epicondyle of the humerus and enters the anterior compartment of the forearm.

The ulnar nerve extends through the medial part of this region by going between the ulnar carpal flexor muscle and the deep flexor digitorum muscle. At the distal portion of the forearm, the ulnar artery joins the ulnar nerve at its pathway, and it is placed laterally to the nerve. Then they both enter the hand by passing superficially to the flexor retinaculum. Within the hand, the ulnar nerve splits into two of its terminal branches:

• The superficial branch, that is mostly sensory
• The deep branch, which is mainly motor

Field of Innervation

The motor branches that arise in the forearm innervate the following muscles:

• The flexor carpi ulnaris muscle
• The medial one-half of the flexor digitorum profundus muscle
• Lateral two lumbrical muscles
• The flexor pollicis brevis muscle

The sensory branches arise from the superficial terminal branch of the ulnar nerve, and they innervate the skin of the medial one half of both of the dorsal and the ventral surface of the palm and the medial one and the one half of the fingers.

The deep branch of the nerve that arises in the hand innervates the:

• Hypothenar muscles
• The long flexors of the digits
• The two medial lumbrical muscles
• The adductor pollicis muscle

Radial Nerve

The radial nerve is the terminal branch of the posterior cord of the brachial plexus. Within the axillary fossa, the nerve is located posteriorly to the axillary artery, and then it passes to the posterior compartment of the arm by crossing the inferior margin of the pronator teres muscle.

Within the arm, the nerve is located posteriorly to the brachial artery, and then it passes through the triangular interval and enters the posterior compartment of the arm. Here,  the nerve descends directed slightly laterally through the sulcus of the radial nerve. It then penetrates the lateral intermuscular septum and enters the anterior compartment of the arm. After that, the radial nerve extends through the lateral bicipital sulcus and enters the lateral portion of the cubital fossa by passing anteriorly to the lateral epicondyle of the humerus.

Within the arm, the nerve elongates its sensory and motor fibers. The sensory fibers are:

• The inferior lateral cutaneous nerve of the arm 
• The posterior cutaneous nerve of the forearm 

Both of these branches penetrate the triceps muscle and become subcutaneous.

The motor fibers innervate the:

• Triceps brachii muscle
• Brachioradialis muscle
• Extensor carpi radialis longus muscle

Within the cubital fossa, the nerve extends its branches:

• The superficial branch, which is sensory and extends through the forearm by going deep to the brachioradialis muscle. When it reaches the wrist, it turns backward and enters the dorsum of the hand. 
• The deep branch is mostly motor. It descends through the posterior compartment of the arm and ends within the dorsum of the hand, too.

The motor (deep) branches of the radial muscle innervate:

• The muscles of the posterior compartment of the arm
• The muscles of the posterior compartment of the forearm
• The muscles of the superficial compartment of the forearm

The sensory (superficial) branches supply:

• The skin of the posterior surface of the arm
• The distal skin of the lateral surface of the forearm
• The skin of the lateral surface of the dorsum of the hand
• The dorsal skin of the lateral two and the half of the fingers