The Arm

Cutaneous innervation of the arm

The skin of the lateral side of the arm, over the upper half of  deltoid, is innervated by the supraclavicular nerve. While, the skin over the lower half of deltoid is innervated by the upper lateral cutaneous nerve of the arm, which is a branch from the axillary nerve. The skin of the lateral side of the arm, below deltoid, is innervated by the lower lateral cutaneous nerve of the arm, a branch from the radial nerve.  The skin of the medial side is innervated by intercostobrachial nerve, and the medial cutaneous nerve of the arm which is a branch from the medial cord. The skin of the back of the arm is innervated by the posterior cutaneous nerve of the arm, a branch which originates from the radial nerve.

 

                              Figure 1& 2: Medial & lateral intermuscular septums

Compartments of the arm

The arm is divided by the humerus and two intermuscular septa into two compartments, the anterior and posterior compartment.  The medial intermuscular septum is a fascial septum which extends from the deep fascia to the medial supracondylar ridge of the humerus and is pierced by the ulnar nerve. The lateral intermuscular septum extends from the deep fascia to the lateral supracondylar ridge and is pierced by the radial nerve.

                                        Figure3: Biceps, coracobrachialis ( anterior compartment of the arm)

 

Muscles of the Anterior Compartment

The three muscles of the anterior compartment are coracobrachialis, biceps brachii, and brachialis.

Coracobrachialis
Origin
From the tip of the coracoid process.

Insertion
In the middle of the medial border of the humerus.

Nerve supply
Musculocutaneous nerve.

Actions
Flexes and adducts the arm.

                                                   Figure4: Coracobrachialis.

Biceps brachii (or biceps)
Origin
The two head of biceps brachii (biceps) originates from two sources.
1) The long head:
Originates from the supraglenoid tubercle of the scapula.
2) The short head:
Originates from the tip of the coracoid process.

Insertion
Inserted in the radial tuberosity.

Nerve supply
Musculocutaneous nerve.

Actions
1) Strong supinator of the forearm.
2) Strong flexor of the forearm.
3) Weak flexor of the arm.

                                                        Figure5: Biceps.

                                             Figure6: Brachialis.

Brachialis
Origin
Lower half of the anterior surface of the humerus with the related intermuscular septa.

Insertion
In the coronoid process of the ulna.

Nerve supply
1) Musculocutaneous nerve.
2) Radial nerve supplies the lateral part of it.

Action
Flexes the elbow.

                                                                      Figure7: Brachialis

Vessels of the Anterior Compartment

            Figure8: Anterior compartment of the arm

Brachial artery

The brachial artery is the continuation of the axillary artery. It begins at the lower border of teres major. It runs downwards and laterally in the medial side of the arm then passes to the front of the elbow. It terminates in the cubital fossa at the level of the neck of the radius by dividing into the radial and ulnar arteries.

Relations
Anterior
1) The medial margin of biceps and coracobrachialis.
2) The median nerve crosses it in the middle.
3) The bicipital aponeurosis crosses the artery in the cubital fossa.

Posterior
1) Triceps muscle.
2) Coracobrachialis muscle.
3) Brachialis.

Medial
1) In the upper part: the ulnar nerve and the basilic vein.
2) In the lower part: the median nerve.

Lateral
1) In the upper part: the median nerve, biceps and coracobrachialis.
2) In the lower part: the biceps tendon.

 
Branches
1) Profunda brachii:
The profunda brachii is the largest branch and accompanies the radial nerve in the spiral groove.
2) Nutrient artery of the humerus
3) Superior ulnar collateral:
It arises in the approximate centre of the arm. It pierces the medial intermuscular septum and accompanies the ulnar nerve in the posterior compartment.
4) Inferior ulnar collateral:
Arises just above the elbow and forms part of the arterial anastomosis around the elbow.
5) Muscular branches:
Supplies the surrounding muscles.
6) Terminal branches:
The radial and ulnar arteries.

                              Figure9: Arteries & nerves of the anterior compartment.

                     Figure10: Ulnar nerve and superior ulnar collateral artery

Nerves of the Anterior Compartment

1)Musculocutaneous nerve (C5, 6, 7)

The musculocutaneous nerve is a branch of the lateral cord of the brachial plexus. First, it descends in the axilla then leaves it and enters the front of the arm by piercing coracobrachialis. In the arm it runs downwards and laterally between biceps and brachialis to reach the lateral side of the tendon of the biceps. It ends by piercing the deep fascia and forms the lateral cutaneous nerve of the forearm.


Branches
1) Muscular branches
a) Coracobrachialis.
b) Biceps brachii.
c) Brachialis.

2) Articular branches
Supplies the elbow joint.

3) Cutaneous branches
The lateral cutaneous nerve of the forearm that supplies the skin on the lateral side of the forearm.

2) Median Nerve (C5, 6, 7, 8, T1)

It originates as two supplies, or roots,  the lateral root from the lateral cord and the medial root from the medial cord.  Its course is closely related to the brachial artery in the upper part of the arm where it lies lateral to the artery; in the lower part of the arm it lies medial to the artery. It has no branches in the arm.

3) Ulnar nerve (C7, 8, T1)

Originating from the medial cord of the brachial plexus. It descends in the upper part of the arm then pierces the medial intermuscular septum with the superior ulnar collateral artery and goes to the posterior compartment of the arm. It passes behind the back of the medial epicondyle. It has no branches in the arm.

                                     Figure11: Ulnar nerve

The Back of the Arm

This region contains the:
1) Triceps brachii,
2) Radial nerve, and
3) Profunda brachii artery.

                                     Figure12: Triceps brachii.

Triceps brachii
Triceps is the only muscle that occupies the posterior compartment of the arm. It is formed of 3 heads as follows: the long head, the lateral head and the medial head.

Origin
1) The long head originates from the infraglenoid tubercle of the scapula.
2) The lateral head originates from the oblique ridge on the back of the humerus above the radial groove.
3) The medial head originates from the posterior surface of the humerus below the radial groove.

Insertion
Inserted into the upper part of the olecranon process of the ulna by a common tendon.

Nerve supply
Radial nerve.

Action
Extensor of the forearm.

                                                         Figure13: Triceps brachii.

Radial nerve (C5, 6, 7, 8, T1)

The radial nerve is the largest branch of the posterior cord of the brachial plexus. It descends behind the third part of the axillary artery and the proximal part of the brachial artery. After which it leaves the frontal compartment of the arm with the profunda brachii vessels through the triangular interval to the posterior compartment where it lies in the radial groove. In the radial groove it runs with the profunda vessels between the lateral and medial heads of triceps. At the lower end of the radial groove, it pierces the lateral intermuscular septum and passes to the anterior compartment. In the lower part of the arm it passes between brachialis and brachioradialis.

Branches
1) Muscular branches:
a) Triceps.
b) Anconeus.
c) Brachialis.
d) Brachioradialis.
e) Extensor carpi radialis longus.

2) Cutaneous branches:
a) Inferior lateral cutaneous nerve of the arm that supplies the skin over the lateral side of the arm below the deltoid.
b) Posterior cutaneous nerve of the forearm that supplies the skin of the middle of the back of the forearm.

3) Articular branches:
It supplies the elbow joint.

                                    Figure14: Posterior compartment of the arm.

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