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PERIPHERAL NERVES


PERIPHERAL NERVES
The cutaneous nerves of the upper limb are for the most part derived from the brachial plexus, although the uppermost nerves to the shoulder are derived from the cervical plexus. The supraclavicular nerves (C3, C4) become superficial at the posterior border of the sternocleidomastoid muscle within the posterior triangle of the neck. They pierce the superficial layer of the cervical fascia and the platysma muscle, radiating in three lines: (1) over the clavicle medial supraclavicular nerves, (2) toward the acromion intermediate supraclavicular nerves, and (3) over the scapula lateral, or posterior, supraclavicular nerves.

The superior lateral cutaneous nerve of the arm (C5, C6) is the termination of the lower branch of the axillary nerve of the brachial plexus. Leaving the axillary nerve, it turns superficially around the posterior border of the lower third of the deltoid muscle to pierce the brachial fascia. Its cutaneous distribution is the lower half of the deltoid muscle and the long head of the triceps brachii.


The inferior lateral cutaneous nerve of the arm (C5, C6) is derived from the posterior antebrachial cutaneous nerve shortly after this nerve branches from the radial nerve. The inferior lateral brachial cutaneous nerve becomes superficial in line with the lateral intermuscular septum a little below the insertion of the deltoid muscle. It accompanies the lower part of the cephalic vein and distributes in the lower lateral and the anterior surface of the arm.
The posterior cutaneous nerve of the arm (C5-C8) arises within the axilla as a branch of the radial nerve.
Traversing the medial side of the long head of the triceps brachii muscle, the nerve penetrates the brachial fascia to distribute in the middle third of the back of the arm above and behind the distribution of the medial brachial cutaneous nerve and the intercostobrachial nerve.
The medial cutaneous nerve of the arm (C8, T1) arises from the medial cord of the brachial plexus in the lower axilla. It descends along the medial side of the brachial artery to the middle of the arm, where it pierces the brachial fascia and supplies the skin of the posterior surface of the lower third of the arm as far as the olecranon.


The intercostobrachial nerve (T2) is the larger part of the lateral cutaneous branch of the second thoracic nerve. In the second intercostal space at the axillary line, it pierces the serratus anterior muscle to enter the axilla. Here, it usually anastomoses with the medial brachial cutaneous nerve and then pierces the brachial fascia just beyond the posterior axillary fold. Its cutaneous distribution is along the medial and posterior sur- faces of the arm from the axilla to the elbow.
A complete neurologic examination of the shoulder tests the just-mentioned dermatomes as well as the coordinated contraction of the shoulder girdle musculature (T11). One commonly encountered neuropathy is long thoracic nerve dysfunction, which can result from axillary lymph node dissection. Physical examination reveals medial winging of the scapula when the arm is placed anterior to the plane of the body, which is exaggerated by pushing against a wall.