Explanation of pre- and postaxial nerves of the brachial plexus:

In order to understand this concept, you must first understand that for there are pre- and postaxial structures, they must be oriented around an axis.

Axis - a center line to which parts of a structure or body may be referred. (The American Heritage® Stedman's Medical Dictionary, 2002, Houghton Mifflin Company.)

Now, notice that the brachial plexus consists of five nerve roots (C5-T1). Logically, the C7 root is the center (middle or ‘midline’) of this group. This means that C7 is the axis of this nerve grouping.

As the lecture notes indicate, pre-axial nerves are found cranial to this axis, and postaxial nerves are found caudal to the axis (although for practical purposes the axis can be included in either subgroup due to nerve fiber contribution). Therefore, the musculocutaneous (C5-C7) and axillary (C5-C6) nn. are pre-axial, since they are formed by contributions from C7 and above. The ulnar (C7-T1) n. is postaxial, since it is formed by contributions from C7 and below.

Note that the radial and median nn. are not considered to be either pre- or postaxial. This is because they receive contributions from all of the roots of the brachial plexus, and therefore cannot be grouped with those branches formed only from roots above or below the axis…