Next Lesson - The Sacral Plexus
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This article is closely associated with the article on the Sacral Plexus, as both the lumbar and sacral plexuses supply peripheral nerve innervation (nerve supply) to the lower limb.
The lumbar plexus is a convergence of spinal nerves that occurs lateral to the lumbar spine, within the psoas major muscle (a thick muscle on the back wall of the abdomen that flexes the hip). It includes input from five spinal nerves (T12 [thoracic nerve 12], L1 [lumbar nerve 1], L2, L3 and L4). It has six peripheral nerve outputs (iliohypogastric, ilioinguinal, genitofemoral, lateral cutaneous nerve of the thigh [referred to in this article for brevity as lateral cutaneous nerve], obturator, and femoral).
Each of these peripheral nerves will be discussed individually throughout the article.
A mnemonic can be useful when trying to remember the order of the nerves in the lumbar plexus:
‘I, I Get Leftovers On Fridays’ is a very common one that is listed at this point, but I find it a lot more useful to go down a more interesting route with the ‘Incredible Imogen Gets Laid On Fridays’.
Diagram - The lumbar plexus, with the five spinal nerves on the left and the six peripheral nerves on the right (and the bottom)
SimpleMed original by Dr. Maddie Swannack
Peripheral Nerves of the Lumbar Plexus
The iliohypogastric nerve is the first branch of the lumbar plexus.
Its path runs from the lumbar plexus (just lateral to the lumbar spine) to the iliac crest (the upper edge of the hip bone), running posterior to the kidneys.
It has contributions from two spinal nerves: T12 and L1.
Its motor functions include the internal oblique and transverse abdominis muscles (two of the muscles of the abdominal wall).
Its sensory functions include the lateral gluteal region (lateral hip joint area) and the suprapubic area (the skin just above the pubic bone).
A tip for remembering that iliohypogastric comes before ilioinguinal is that H comes before I in the alphabet.
The ilioinguinal nerve is the second branch of the lumbar plexus.
It follows the same path as the iliohypogastric nerve, and after supplying the muscles of the abdominal wall, it passes through the superficial inguinal ring (the outer opening of the inguinal canal, a passage through the lower abdominal wall).
It has contributions of one spinal nerve: L1.
Its motor functions include the internal oblique and transverse abdominis muscles (as for the iliohypogastric nerve).
Its sensory functions include the superior anteromedial thigh (the upper front and inner side of the thigh) and either the anterior scrotum (the front of the sac of skin holding the testes) in males or the labia majora (the outer skin folds of the female external genitalia) in females.
The genitofemoral nerve is the third branch of the lumbar plexus.
It passes inferiorly out of the lumbar plexus, penetrating the psoas major muscle, and continuing downwards.
It has contributions from two spinal nerves: L1 and L2.
Its motor functions include the cremasteric muscle, responsible for the cremasteric reflex (when the skin of the inner thigh is stroked, the testis on the same side is pulled upwards).
Its sensory functions include the superior medial thigh (referenced above in relation to the cremasteric reflex) and the anterior scrotum or labia majora.
Lateral Cutaneous Nerve of the Thigh
The lateral cutaneous nerve is the fourth branch of the lumbar plexus.
It enters the thigh at the lateral side of the inguinal ligament (the band running from the front of the hip bone to the pubic bone).
It has contributions from two spinal nerves: L2 and L3.
This nerve has no motor functions.
Its sensory functions include the anterior and lateral thigh.
The obturator nerve is the fifth branch of the lumbar plexus. This is the case even though the diagram above shows it as the sixth branch. Its path runs parallel to the midline, which makes it hard to draw as the fifth.
It emerges from the medial wall of the psoas major muscle, travelling along the pelvic wall with the obturator vessels to the obturator canal.
It has contributions from three spinal nerves: L2, L3, and L4.
It innervates multiple muscles in the medial compartment of the thigh (the inner side of the thigh), meaning it is primarily responsible for adduction of the thigh. It supplies the three adductor muscles (longus, brevis and magnus), which pull the thigh towards the midline, as well as gracilis and obturator externus. Only the adductor part of adductor magnus is supplied by the obturator nerve: its hamstring part is supplied by the tibial nerve (part of the sciatic nerve).
Its sensory function includes the middle part of the medial thigh. This small section is between the superior portion of thigh innervated by the genitofemoral nerve and the inferior portion which is supplied by the femoral nerve.
The femoral nerve is the sixth and final branch of the lumbar plexus.
Its path sees it pass lateral to the psoas major muscle, under the inguinal ligament into the femoral triangle (a triangular space at the top of the front of the thigh). Within the triangle, about 4 cm below the inguinal ligament, it divides into two sections:
- The anterior division includes the anterior cutaneous branches, the branch to sartorius and the branch to pectineus.
- The posterior division includes the saphenous nerve and the branches to quadriceps femoris.
The femoral nerve has contributions from three spinal nerves: L2, L3, and L4.
The motor functions of the femoral nerve include muscles of the anterior thigh: this means it supplies the hip flexors (muscles that lift the thigh forwards at the hip: pectineus, iliacus, sartorius) and the knee extenders (the quadriceps femoris - rectus femoris, vastus lateralis, vastus medialis and vastus intermedius).
The sensory functions of the femoral nerve are dependent on the branch:
- The anterior cutaneous branch supplies the inferior aspect of the anteromedial thigh.
- The saphenous nerve supplies the medial aspect of the lower leg and the medial foot.
Damage to the Lumbosacral Plexuses
Damage to the lumbar or sacral plexuses (or both) can be very serious. Plexus damage is suspected when the symptoms shown cannot be localised to a single peripheral nerve region.
Damage can present with neuropathic pains (pain caused by damaged nerves), numbness, weakness or wasting of muscles.
One of the main causes is diabetic amyotrophy (muscle wasting linked to diabetes), also known as diabetic lumbosacral radiculoplexus neuropathy (disease of the nerve roots and nerve network in the lower back and pelvis). It typically starts suddenly with pain in the thigh and hip on one side, which may spread to the other side over weeks to months, followed by progressive weakness of the leg muscles. It is thought to involve inflammation of the small blood vessels that supply the nerves (microvasculitis), which damages the nerves by reducing their blood supply. Corticosteroids have been used in some cases to reduce inflammation, but evidence for benefit is limited. Management is usually supportive (for example pain relief and physiotherapy), and most people recover at least to some degree, although recovery can be slow.
Another cause is a tumour that grows into (invades) or presses on the plexus. Treatment depends on the type of tumour. It may include radiotherapy (treatment with high-energy radiation) to the plexus, chemotherapy (anti-cancer drugs), pain control and, in selected cases, surgery to remove the tumour.
Edited by: Dr. Ben Appleby
Reviewed by: Dr. Thomas Burnell and Dr. Marcus Judge
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