The lumbar anterior primary rami contribute to the formation of both lumbar and sacral nerves.
This is done through the formation of a plexus, as for the upper limb nerves. Again the nerves fall
into either anterior or posterior groups. The lumbar plexus forms within the psoas muscle, and
the nerves emerge either at the medial or lateral borders of the psoas, or through the muscle to
emerge on its anterior surface. The nerves emerging from the lateral border of psoas are the
iliohypogastric, ilioinguinal, lateral cutaneous nerve of the thigh and femoral. Emerging from the
medial border of the psoas muscle is the obturator nerve and the lumbosacral trunk.
The iliohypogastric nerve
The iliohypogastric nerve is formed by fibres from L1, with some contribution from T12. The
nerve runs obliquely across the quadratus lumborum muscle behind the kidney. Close to the iliac
crest the nerve pierces the transversus abdomins muscle. Its lateral branch pierces the muscle of
the lateral abdominal wall to supply the skin over the lateral gluteal region. The anterior branch
runs forwards and down wards between transversus abdominis and internal oblique to supply the
skin above the pubis.
The ilioinguinal nerve
The ilioinguinal nerve is formed in common with the iliohypogastric nerve. The nerve lies on the
quadratus lumborum muscle and the iliacus until it perforates the body wall near the anterior iliac
crest. It lies between the internal and external oblique to pass through the superficial inguinal
ring. It distributes sensory fibres to superomedial thigh, root of the penis and upper part of the
scrotum in the male, or mons pubis and labium majus in the female.
The genitofemoral nerve
The genitofemoral nerve is formed from L1,2 and passes through the psoas to emerge on its
anterior surface. It runs down wards on the psoas and divides into genital and femoral branches.
The genital branch enters the inguinal canal through the deep inguinal ring to supply the cremaster
muscle and a small area of overlying skin. The femoral branch passes behind the inguinal ligament
to enter the femoral sheath and supply the skin over the femoral triangle.
Lateral cutaneous nerve of the thigh
The lateral cutaneous nerve of the thigh emerges at the lateral body of the psoas muscle. It forms
from the posterior branches of the L2,3 anterior primary rami. It crosses the iliacus muscle to
emerge close to the anterior superior iliac spine. Its anterior and posterior branches supply the
lateral thigh.
The femoral nerve
The femoral nerve arises from posterior branches of the L2,3,4 anterior primary rami. The nerve
lies between psoas and iliacus and enters the thigh behind the inguinal ligament, lateral to the
femoral artery. The nerve will be considered further in the section on lower limb nerves.
The obturator nerve
The nerve arises from anterior branches of the L2,3,4 anterior primary rami. It emerges medial to
the psoas muscle and curves around the lateral wall of the pelvis on the obturator internus muscle.
The nerve leaves the pelvis through the obturator foramen as anterior and posterior branches. An
accessory obturator nerve is sometimes present arising from L3,4. The nerve passes over the
superior ramus of the pubis to enter the pectineus.
The lumbosacral trunk
Part of the anterior branch of the L4 primary ramus and the L5 primary ramus form the
lumbosacral trunk. The trunk lies anterior to the ala of the sacrum to join the S1 anterior primary
ramus.
Muscle nerves
The T12 and lumbar primary rami send short nerves into neighboring muscles, the quadratus
lumborum, psoas and iliacus.
