The lumbar plexus is composed inside the substance of the psoas major by the unification of ventral rami of L1 L3 lumbar nerves and a bigger upper part of the ventral ramus of L4 nerve.
The lower smaller part of the ventral ramus of L4 nerve joins with all the ventral ramus of L5 nerve to create the lumbosacral trunk which takes part in the formation of sacral plexus.
The ventral ramus of the 4th lumbar nerve is sometimescalled nervi furcalis, since it creates the joining linkbetween the lumbar and sacral plexuses.
- The ventral ramus of the L1 nerve supplemented by a twig from T12 (subcostal) nerve splits into a bigger upper branch and smaller lower branch. The bigger upper branch provides rise to iliohypogastric and ilioinguinal nerves.
- The smaller lower branch joins with a twig from the L2 nerve and creates the genitofemoral nerve.
- The L2, L3, L4 nerves break up into dorsal and ventral sections. The dorsal sections of L2, L3, L4 unify to create the femoral nerve. The ventral sections of L2, L3, and L4 join to create the obturator nerve. The accessory obturator nerve if present is originated from the ventral branches of the L3, L4 nerves.
- The main branches of the lumbar plexus are the femoral nerve and the obturator nerve.
Branches of The Lumbar Plexus
Course And Distribution of The Branches
It appears underneath the lateral border of the psoas major muscle, enters downward and laterally in front of the quadratus lumborum. At the lateral border of quadratus lumborum it pierces aponeurotic origin of the transversus abdominis justabove the iliac crest and runs in the anterior abdominal wall. It gives cutaneous innervation to the skin of gluteal region and anterior abdominal wall in the hypogastric region.
It pursues the exact same course as the iliohypogastric nerve, but at a somewhat lower level. It pierces the transversus abdominis near the anterior part of the iliac crest. It gives motor innervation to internal oblique and transversus abdominis muscles and sensory innervation to the skin on the upper medial aspect of the thigh, root of penis, and scrotum in the male, and mons pubis and labium majus in the female.
It enters forwards via the psoas, pierces it and its covering psoas fascia. It runs on the anterior outermost layer of the psoas near its medial border and breaks up above the inguinal ligament into genital and femoral branches. The femoral branch runs along the lateral side of the external iliac artery and enters the femoral sheath, where it is located anterolateral to the femoral artery. It pierces the anterior wall of the sheath to provide the skin over the femoral triangle. The genital branch enters the deep inguinal ring and traverses via the inguinal canal along the spermatic cord in the male and the round ligament of uterus in the female. In the male it provides the cremaster muscle and scrotal skin, and in the female it provides the skin of mons pubis and labium majus.
Lateral Cutaneous Nerve of The Thigh (Lateral Femoral Cutaneous Nerve)
It appears underneath the lateral border of the psoas major above the iliac crest, runs downward and laterally across the iliac fossa in front of the iliacus muscle under cover of the iliac fascia. It enters the thigh by passing below the lateral end of the inguinal ligament. Occasionally it goes through the inguinal ligament. It gives cutaneous innervation to the upper lateral aspect of the thigh.
It appears underneath the lateral border of the psoas major below the iliac crest, runs downward and small laterally in the groove between the psoas major and iliacus under cover of the iliac fascia. It enters the thigh by passing deep to the inguinal ligament where it is located lateral to the psoas sheath. Before going into the thigh it provides the iliacus muscle.
It appears below the medial border of the psoas major in the lumbosacral triangle, crosses the anterolateral angle of the ala of sacrum to run downward and forward along the lateral wall of the true pelvis, and eventually enters the thigh by going through the obturator canal.
Accessory Obturator Nerve
It’s an inconstant nerve. When present, it accompanies the medial border of the psoas major to go into the thigh. It provides the pectineus muscles.