The lumbosacral plexus is less complex than the brachial plexus (Figure 230.1). The formation of the lumbosacral plexus can be summarized as follows: each ventral ramus splits into 2 divisions, 1 anterior and 1 posterior, except the ventral ramus of L4 and S3. The ventral ramus of L4 splits into 4 divisions, 2 anterior and 2 posterior. The ventral ramus of S3 does not divide.

Figure 230.1.— Schematic representation of the divisions of the ventral rami. Each ventral ramus splits into 2 divisions (1 anterior [A] and 1 posterior [P]) except for the ventral ramus of L4 that divides into 4 (2 anterior and 2 posterior divisions), and the ventral ramus of S3 which does not divide.

The anterior divisions of the L2 and L3, and 1 of the anterior divisions of the L4, join to form the obturator nerve (Figure 230.2 [A]). The posterior division of the L2 and L3, and 1 of the posterior divisions of L4 join to form the nerve to the iliopsoas muscle and the femoral nerve (Figure 230.2 [B]).

A	B

Figure 230.2.— Schematic representation of the formation of the obturator nerve (ON), iliopsoas nerve (IPN), and femoral nerve (FN).