Lumbar plexus

Question 1

Outline the lumbar plexus.

Answer 1

• After the anterior rami of the upper 4 lumbar nerves have supplied. psoas and quadratus lumborum segmentally, they form the plexus in the substance of psoas major.
• The plexus innervates part of the lower abdominal wall but is chiefly concerned in supplying skin and muscle in the lower limb.
• It reinforces the sacral plexus, which is the true plexus of the lower limb.

Question 2

What are the branches of the lumbar plexus?

Answer 2

• L1 - iliohypogastric and ilioinguinal
• L1, 2 - genitofemoral
• L2, 3 (posterior divisions) - lateral femoral cutaneous
• L2, 3, 4 (posterior divisions) - femoral
• L2, 3, 4 (anterior divisions) - obturator

Question 3

Outline the iliohypogastric and ilioinguinal nerves (L1).

Answer 3

• These are only by convention included in the lumbar plexus.
• They are really just the first lumbar segmental body-wall nerve and its collateral branch, in series with the thoracic nerves.
• Apart from supplying skin over the inguinal region and the front of the scrotum, they provide the important motor supply for the fibres of internal oblique and transversus that form the roof of the inguinal canal and reach the conjoint tendon.

Question 4

Outline the genitofemoral nerve (L1, 2).

Answer 4

• The femoral part supplies an area of skin below the middle of the inguinal ligament.
• The genital part supplies that part of the abdominal wall herniated into the scrotum for the descent of the testis (i.e. the spermatic cord).
• It is sensory to tunica vaginalis and the spermatic fasciae, and motor to cremaster muscle. It supplies a small area of anterior scrotal and labial skin.

Question 5

Outline the lateral femoral cutaneous nerve (L2, 3 - posterior divisions).

Answer 5

• The nerve is wholly sensory, to the iliac fascia and peritoneum of the iliac fossa, and to the lateral side of the thigh down to the knee.
• It emerges from the lateral border of psoas and enters the thigh by passing through or under the lateral part of the inguinal ligament.

Question 6

Outline the femoral nerve (L2, 3, 4 - posterior divisions).

Answer 6

• This nerve issues from the lateral border of psoas and crosses the iliac fossa in the gutter between psoas and iliacus, deep to the iliac fascia.
• It supplies iliacus in the abdomen and passes beneath the inguinal ligament lateral to the femoral sheath.
• As it enters the femoral triangle it supplies pectineus and breaks up at once into several branches.
• The nerve to pectineus runs behind the femoral sheath to reach the muscle.
• Of two nerves to sartorius one often pierces the muscle and continues on as an intermediate femoral cutaneous nerve which supplies skin and fascia lata over the front of the thigh down to the knee.
• The medial femoral cutaneous nerve supplies the medial side of the thigh, and an anterior branch reaches the front of the knee.
• The nerve to rectus femoris is usually double, and the upper branch also supplies the hip joint.
• The nerve to vastus lateralis runs down with the descending branch of the lateral femoral circumflex artery between rectus femoris and vastus intermedius.
• The nerve to vastus intermedius sinks into the anterior surface of that muscle.
• The nerve to vastus medialis enters the upper part of the subsartorial canal and sinks into the muscle.
• The nerves to all three vasti also supply the knee joint.
• The saphenous nerve crosses in front of the femoral artery in the subsartorial canal, and emerges from behind the posterior border of sartorius. Its infrapatellar branch pierces sartorius to run into the patellar plexus.
• The saphenous nerve pierces the fascia lata between the tendons of sartorius and gracilis and descends to supply skin and periosteum over the subcutaneous surface of the tibia.
• It runs with the great saphenous vein in front of the medial malleolus and ends on the medial side of the foot just short of the big toe.

Question 7

Outline the obturator nerve (L2, 3, 4 - anterior divisions).

Answer 7

• Emerging from the medial side of psoas the nerve lies on the ala of the sacrum lateral to the lumbosacral trunk.
• It slants down to the side wall of the pelvis between the origin of the internal iliac artery and the ilium.
• From the angle between external and internal iliac vessels it runs straight to the obturator foramen, supplying the parietal peritoneum of the side wall of the pelvis (in the female the ovary lies here).
• As it emerges from the obturator foramen it splits into anterior and posterior divisions.
• The posterior division supplies obturator externus, then pierces the upper part of that muscle and runs into the thigh deep to adductor brevis. It runs down on adductor magnus, whose pubic part it supplies. A slender branch accompanies the femoral artery into the popliteal fossa to supply the knee joint.
• The anterior division passes over obturator externus and, emerging into the thigh, it supplies the hip joint. It runs down over adductor brevis, deep to pectineus and adductor longus. It supplies these two adductors and may supply pectineus. It also supplies gracilis. It supplies the medial side of the thigh by a cutaneous branch which runs through the subsartorial plexus.
• The accessory obturator nerve, when occasionally present, passes over the superior pubic ramus to supply pectineus.