The Lumbar Plexus

Original Author: Kristen Davies
Last Updated: January 24, 2017
Revisions: 26

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.

The plexus is formed by the anterior rami (divisions) of the lumbar spinal nerves L1, L2, L3 and L4. It also receives contributions from thoracic spinal nerve 12. In this article, we shall look at the anatomy of the lumbar plexus – its formation and major branches.


Spinal Nerves

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 intervertebral 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.

Fig 1.0 - The spinal cord outflow at each vertebral level. The anterior rami of vertebral levels C5-C8 and T1 make up the roots of the brachial plexus

Fig 1.0 – The spinal cord outflow at each vertebral level. The anterior rami of vertebral levels L1-L4 make up the roots of the lumbar plexus


The Branches

Fig 1.0 - The right lumbar plexus, and the left sacral plexus.

Fig 1.1 – The right lumbar plexus, and the left sacral plexus.

The anterior rami of the L1-L4 spinal roots divide into several cords. These cords then combine together to form the six major peripheral nerves of the lumbar plexus. These nerves then descend down the posterior abdominal wall to reach the lower limb, where they innervate their target structures.

We shall now consider the branches of the lumbar plexus. (Note: In this article we shall include only brief notes on the function of these nerves –  for more detailed information click on the title to visit their respective pages)


Iliohypogastric Nerve

The iliohypogastric nerve is the first major branch of the lumbar plexus. It 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. (Tip: an easy way to remember that the IlioHypogastric comes before the IlioInguinal is that H comes before I in the alphabet!)

Fig 1.1 - Derivation of the iliohypogastric nerve.

Fig 1.2 – Derivation of the iliohypogastric nerve.


Ilioinguinal Nerve

The ilioinguinal nerve 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 labia majora.

Fig 1.1 - Derivation of the ilioinguinal nerve.

Fig 1.3 – Derivation of the ilioinguinal nerve.


Genitofemoral Nerve

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 labia majora (in females). The femoral branch innervates the skin on the upper anterior thigh.

Fig 1.4 - Derivation of the genitofemoral nerve.

Fig 1.4 – Derivation of the genitofemoral nerve.


Lateral Cutaneous Nerve of the Thigh

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.

Fig 1.5 - Derivation of the lateral cutaneous nerve of thigh.

Fig 1.5 – Derivation of the lateral cutaneous nerve of thigh.


Obturator Nerve

See more detailed information here

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.

Fig 1.6 - Derivation of the obturator nerve.

Fig 1.6 – Derivation of the obturator nerve.


Femoral Nerve

See more detailed information here.

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.

Fig 1.7 - Derivation of the femoral nerve

Fig 1.7 – Derivation of the femoral nerve

Note: A useful memory aid for the branches of the lumbar plexus is: I, I Get Leftovers On Fridays. This stands for the Iliohypogastric, Ilioinguinal, Genitofemoral, Lateral cuteanous nerve of the thigh, Obturator and Femoral.

Clinical Relevance – Lumbosacral Plexopathy

A lumbosacral plexopathy is a disorder affecting either the lumbar or sacral plexus of nerves. They are rare syndromes, caused by damage to the nerve bundles.

A plexopathy is suspected if the symptoms cannot be localised to a single nerve. Patients may complain of neuropathic pains, numbness or weakness and wasting of muscles.

One of the main causes of lumbosacral plexopathy is diabetic amyotrophy, also known as lumbosacral radioplexus neurophagy. In this condition, the high blood sugar levels damage the nerves. Idiopathic plexopathy is another cause, being the lumbosacral equivalent of Parsonage-Turner syndrome (which affects the brachial plexus). Tumours and other local invasions can cause the plexopathy due to the compression of the plexus.

Treatment depends on what is causing the symptoms. For tumours and space-occupying lesions, they should be removed if possible. For diabetic and idiopathic causes, treatment with high-dose corticosteroids can be useful.

 

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Quiz

Question 1 / 5
Which of these options show the nerves which make the basis of the lumbar plexus?

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Question 2 / 5
Which two nerves have motor functions in the lower limb?

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Question 3 / 5
Damage to the L2-L4 spinal roots will NOT affect which of these nerves

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Question 4 / 5
Where does the obturator nerve provide sensory innervation?

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Question 5 / 5
Which nerve of the lumbar plexus innervates the internal oblique?

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