The plexus gives rise to direct muscular branches which supply the quadratus lumborum (T12; L1 to L4), psoas major (L2 to L4), psoas minor (L1), and iliacus (L2, L3) muscles as well as six major terminal nerves, as described below (Figs. 11.4 and 11.5).

A branch of the L1 nerve root (anterior ramus), the iliohypogastric nerve (Fig. 11.6), emerges from behind the upper part of the psoas major and courses laterally along the posterior abdominal wall underneath the subcostal nerve (T12). It crosses posterior to the inferior pole of the kidney obliquely, sandwiched between it and the anterior surface of the quadratus lumborum muscle. The nerve then pierces the posterior aspect of the transversus abdominis to run anteriorly along the iliac crest in the plane between the transversus abdominis and the internal oblique, dividing into its lateral and anterior cutaneous branches. The lateral branch pierces the internal and external oblique muscles above the iliac crest to be distributed to the posterolateral skin of the gluteal region. The anterior branch continues between the transversus abdominis and the internal oblique muscles and supplies them, coursing above and parallel to the inguinal ligament. It then pierces the internal oblique just medial to the anterior superior iliac spine and exits through the aponeurosis of the external oblique above the superficial inguinal ring to become cutaneous and supply the skin of the suprapubic region. The iliohypogastric nerve often has communicating branches with the subcostal (T12) and ilioinguinal (L1) nerves.

Another branch of the L1 nerve root, the ilioinguinal nerve (Fig. 11.6), is smaller than the iliohypogastric nerve. The nerve emerges from beneath the lateral border of the upper part of the psoas major with or just below the iliohypogastric nerve (the two may arise as a common trunk and split more distally at a variable distance) and crosses the inferior pole of the kidney obliquely just below the iliohypogastric nerve anterior to the quadratus lumborum. It then pierces the transversus abdominis near the anterior aspect of the iliac crest and enters the internal oblique, supplying it, after which it travels through the inguinal canal below the spermatic cord. The nerve emerges from the canal, lying inferior to the cord, through the superficial inguinal ring to be distributed to the skin on the proximal medial aspect of the thigh and the external genitalia. The nerve occasionally joins the iliohypogastric nerve as it enters the transversus abdominis, or it may be absent altogether, in which case the iliohypogastric nerve takes over supply to the associated areas.

The genitofemoral nerve, with contributions from the L1 and L2 nerve roots, originates within the substance of the psoas major muscle. The nerve pierces through the muscle approximately at the level of L3/L4 and travels retroperitoneally along the muscle’s anterior surface before dividing into its genital and femoral branches at the level of L5 or S1. It can also divide within the psoas major, emerging as two distinct nerves.