Axillary nerve

Template:Infobox nerve The axillary nerve is a nerve of the human body, that comes from the brachial plexus (upper trunk, posterior division, posterior cord) at the level of the axilla (armpit) and has nerve fibers from the C5 and C6 nerve roots. The axillary nerve goes through the quadrangular space with the posterior circumflex humeral artery and vein. It is sometimes called the circumflex nerve.

Structure

The nerve starts behind the axillary artery, and in front of the subscapularis muscle, and goes downw to the lower border of that muscle.

It then goes backward, together with the posterior humeral circumflex artery, through a quadrilateral (four-sided) space surrounded above by the subscapularis, below by the teres major, medially by the long head of the triceps brachii, and laterally by the surgical neck of the humerus. Then it divides into an anterior (front), a posterior (back), and a collateral (side) branch which innervates (sends signals to) the long head of the triceps brachii muscle.

• The anterior branch (upper branch) winds around the surgical neck of the humerus, beneath the deltoid muscle, with the posterior humeral circumflex vessels. It goes as far as the anterior border of the deltoid to send signals making the muscle move. The anterior branch also gives off a few small cutaneous (skin) branches, which go through the muscle receive sense from the skin.

• The posterior branch (lower branch) supplies the teres minor and the posterior part of the deltoid. The posterior branch goes through the deep fascia and becomes the superior (or upper) lateral cutaneous nerve of arm. This goes around the posterior border of the deltoid and carries sense from the skin over the lower two-thirds of the posterior part of this muscle, as well as that covering the long head of the triceps brachii.

• The nerve that sends motor (movement) signals to the long head of the triceps brachii comes about 6 mm (range 2–12 mm) from the last part of the posterior cord.

• The axillary nerve gives off an articular filament which enters the shoulder joint below the subscapularis muscle.

Function

The axillary nerve sends and receives signals from three muscles in the arm: deltoid (a muscle of the shoulder), teres minor (one of the rotator cuff muscles) and the long head of the triceps brachii. Scientists used to think that only the radial nerve sent and received signals from the triceps. However, a study in 2004 showed that, in 20 cadaveric (dead) specimens and 15 live participants, the long head of the triceps always got signals from the axillary nerve.

The axillary nerve sends signals to two muscles in the forearm, the flexor carpi ulnaris and the medial (inside) half of the flexor digitorum profundus. In the hand, it sends signals the two medial lumbricals, the muscles in the hypothenar compartment, the interossei, and parts of the adductor pollucis.

The axillary nerve also carries sensory information from the shoulder joint, as well as the skin covering the lower part of the deltoid muscle.

The posterior (back) cord of the brachial plexus splits below to the shoulder joint. The two parts are the axillary nerve, which goes around the surgical neck of the humerus, and the radial nerve which goes around in front of the humerus and along its lateral (outside) border.

Injuries and problems

The axillary nerve can get injured from shoulder dislocation, squashing the armpit using a crutch or a broken arm. An example of injury to the axillary nerve includes axillary nerve palsy. Injury to the nerve results in:

• Paralysis of the teres minor muscle and deltoid muscle, resulting in trouble moving the arm. Paralysis of deltoid and teres minor muscles results in flat shoulder deformity.
• Loss of sensation in the skin over a small part of the lateral upper arm.

Additional images

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  Brachial plexus with courses of spinal nerves shown
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  Suprascapular and axillary nerves of right side, seen from behind.
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  Cutaneous nerves of right upper extremity.
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  Diagram of segmental distribution of the cutaneous nerves of the right upper extremity.
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  Axillary nerve
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  Axillary nerve
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  Axillary nerve
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  Axillary nerve

External links

• Template:DukeOrtho

Template:Brachial plexus