In the apex of the axilla, the three plexus cords (lateral, medial, and
posterior) form the main terminal nerves of the upper extremity (axillary,
musculocutaneous, median, ulnar, and radial). However, only the last three
nerves accompany the blood vessels through the axilla where the blocks are
performed (Figure 28–1), while the axillary and the
musculocutaneous nerves leave the plexus approximately at the level of the
coracoid process. The axillary nerve departs at a wider angle from the
posterior cord, laterally and dorsally, and the musculocutaneous nerve,
which originates from the lateral cord, runs obliquely laterally into the
coracobrachial muscle and continues downward. The medial antebrachial
and brachial cutaneous nerves run subcutaneously parallel to the
axillary vessels, although the medial antebrachial cutaneous nerve often
follows the median nerve within the neurovascular sheath. In the axilla, the
median and musculocutaneous nerves lie superior to the artery, whereas the
ulnar and radial nerves lie inferior to it. The depths at which the nerves
are found vary. Typically, the median nerve is more superficial than the
musculocutaneous, and the ulnar nerve is more superficial than the radial.
Occasionally, the radial or the musculocutaneous nerves (or both) are found
behind the artery. These two nerves progressively diverge from the
neurovascular sheath, continuing down the upper arm, the musculocutaneous
above (anterior) and the radial below (posterior) to the humerus, where they
can be approached using the midhumeral approach.26