Lumbar Plexus

Question 1

What is the lumbar plexus?

Answer 1

The lumbar plexus is a network of nerve fibres that supplies the skin and musculature of the lower limb. It is located in the lumbar region, within the substance of the psoas major muscle and anterior to the transverse processes of the lumbar vertebrae.

Question 2

Which spinal nerves form the lumbar plexus?

Answer 2

The spinal nerves L1 – L4 form the basis of the lumbar plexus. At each vertebral level, paired spinal nerves leave the spinal cord via the intervertebal foramina of the vertebral column. Each nerve then divides into anterior and posterior nerve fibres.

The lumbar plexus begins as the anterior fibres of the spinal nerves L1, L2, L3, and L4.

Question 3

Describe the Iliohypogastric Nerve

Answer 3

• first major branch of the lumbar plexus.
• runs to the iliac crest, across the quadratus lumborum muscle of the posterior abdominal wall. It then perforates the transversus abdominis, and divides into its terminal branches.

Roots: L1 (with contributions from T12).

Motor Functions: Innervates the internal oblique and transversus abdominis.

Sensory Functions: Innervates the posterolateral gluteal skin in the pubic region.

Question 4

Describe the Ilioinguinal Nerve

Answer 4

• follows the same anatomical course as the larger iliohypogastric nerve.
• after innervating the muscles of the anterior abdominal wall, it passes through the superficial inguinal ring to innervate the skin of the genitalia and middle thigh.

Roots: L1.

Motor Functions: Innervates the internal oblique and transversus abdominis.

Sensory Functions: Innervates the skin on the upper middle thigh. In males, it also supplies the skin over the root of the penis and anterior scrotum. In females, it supplies the skin over mons pubis and labium majus.

Question 5

Describe the Genitofemoral Nerve

Answer 5

After leaving the psoas major muscle, the genitofemoral nerve quickly divides into a genital branch, and a femoral branch.

Roots: L1, L2.

Motor Functions: The Genital branch innervates the cremasteric muscle.

Sensory Functions: The Genital branch innervates the skin of the anterior scrotum (in males) or the skin over mons pubis and labium majus (in females). The Femoral branch innervates the skin on the upper anterior thigh.

Question 6

Describe the Lateral Cutaneous Nerve of the Thigh

Answer 6

This nerve has a purely sensory function. It enters the thigh at the lateral aspect of the inguinal ligament, where it provides cutaneous innervation to the skin there.

Roots: L2, L3

Motor Functions: None.

Sensory Functions: Innervates the anterior and lateral thigh down to the level of the knee.

Question 7

Describe the obturator nerve

Answer 7

Roots: L2, L3, L4.

Motor Functions: Innervates the muscles: obturator externus, pectineus, adductor longus, adductor brevis, adductor magnus, gracilis.

Sensory Functions: Innervates the skin over the medial thigh.

Question 8

Describe the Femoral Nerve

Answer 8

Roots: L2, L3, L4.

Motor Functions: Innervates the muscles: Illiacus, pectineus, sartorius, all the muscles of quadriceps femoris.

Sensory Functions: Innervates the skin on the anterior thigh and the medial leg.