The lumbar plexus is a network of nerves that arises from the anterior rami of spinal nerves L1-L4, along with a contribution from the anterior ramus of spinal nerve T12. It is located on the posterior abdominal wall, anterior to the transverse processes of the lumbar vertebrae and within the posterior portion of the psoas major muscle.
The lumbar plexus gives rise to several branches which supply various muscles and regions of the posterior abdominal wall and lower limb. These branches include the iliohypogastric, ilioinguinal, genitofemoral, lateral femoral cutaneous, femoral and obturator nerves. In addition, the lumbar plexus gives off muscular branches from its roots, a branch to the lumbosacral trunk and occasionally an accessory obturator nerve.
This article will discuss the anatomy and functions of the lumbar plexus.
- Origin and location
- Branches and innervation
Origin and location
The lumbar plexus originates from the anterior rami of spinal nerves L1-L4 and is formed largely within the posterior aspect of the psoas major muscle. The anterior ramus of spinal nerve T12 contributes to the formation of the lumbar plexus via the dorsolumbar nerve, which joins the anterior ramus of spinal nerve L1. Together, these roots (T12, L1) form a common trunk which gives rise to the iliohypogastric and ilioinguinal nerves. The anterior rami of L1 and L2 each give rise to a branch, which go on to merge with one another to form the genitofemoral nerve.
As the spinal rami of spinal nerves L2 - L4 course away from the lumbar vertebrae, they divide into anterior (ventral) and posterior (dorsal) divisions. The anterior divisions merge with one another to form the obturator nerve. Sometimes, the anterior divisions of L3 and L4 anterior rami may give branches that unite to form an accessory obturator nerve. Fibers from the posterior divisions, on the other hand, mainly unite to form the femoral nerve. The posterior divisions of L2 and L3 also give rise to branches that merge to form the lateral femoral cutaneous nerve.
The anterior ramus of L4 gives rise to an additional branch which goes on to merge with the anterior ramus of L5 spinal nerve to form the lumbosacral trunk. The lumbosacral trunk joins the anterior rami of sacral spinal nerves S1-S4 in the pelvis to form the sacral plexus. Thus, this branch to the lumbosacral trunk serves as the point of union between the lumbar and sacral plexuses.
Branches and innervation
Unlike the brachial plexus that undergoes several clear mergers and splits, the lumbar plexus is not as straightforward. However, the main branches of the lumbar plexus can be organised into groupings relative to the psoas major muscle. These branches emerge either lateral, anterior or medial to the muscle:
- Lateral: Iliohypogastric nerve, ilioinguinal nerve, lateral cutaneous nerve of the thigh and femoral nerve
- Anterior: Genitofemoral nerve
- Medial: Obturator nerve, accessory obturator nerve and nerve to lumbosacral trunk
In addition, the lumbar plexus gives off several small motor muscular branches from its roots that innervate the psoas major and quadratus lumborum.
The iliohypogastric nerve is formed from the anteriorl ramus of spinal nerve L1 but may receive a contributory branch from the anterior rami of T12. It runs anterolaterally across the lower posterior abdominal wall after emerging from the superolateral border of the psoas major muscle. This nerve is a mixed nerve that provides both motor and sensory innervation to the internal oblique and transversus abdominis muscles and the skin of the posterolateral gluteal region and suprapubic region respectively.
The ilioinguinal nerve, like the iliohypogastric, arises from the anterior ramus of spinal nerve L1 and may also receive contributions from the subcostal (T12) nerve. It runs inferior to the iliohypogastric nerve in a similar inferolateral direction. The ilioinguinal nerve is a mixed nerve and similar to the iliohypogastric nerve, it provides motor innervation to the internal oblique and transversus abdominis muscles. The sensory fibers of the ilioinguinal nerve innervate the skin over the upper medial thigh, and skin over parts of the external genitalia.
Lateral cutaneous nerve of the thigh
The lateral cutaneous nerve of the thigh, also called the lateral femoral cutaneous nerve, is formed by fibers of the posterior divisions of the anterior rami of spinal nerves L2 and L3. It emerges from the lateral border of the psoas major muscle and courses inferolaterally to enter the iliac fossa, ultimately reaching the thigh. This nerve provides sensory innervation to the peritoneum of the iliac fossa and iliac fascia, and the skin of the anterior and lateral thigh (along the iliotibial tract) to the knee.
The femoral nerve is the largest branch of the lumbar plexus and is formed from the posterior divisions of the anterior rami of spinal nerves L2-L4. It emerges from the lower lateral border of the psoas major muscle and passes below the inguinal ligament lateral to the femoral vessels to reach the thigh. This is a mixed nerve that provides motor and sensory innervation to the thigh and leg through several branches.
The femoral nerve provides motor supply to the following muscles:
- Flexors of the hip: pectineus, iliacus, sartorius
- Extensors of the knee: quadriceps femoris (rectus femoris, vastus medialis, vastus lateralis, and vastus intermedius)
The femoral nerve provides sensory supply to the following regions:
- Skin of anteromedial thigh
- Skin of medial leg and foot
The genitofemoral nerve originates from the anterior rami of the spinal nerves L1 and L2. The nerve emerges on the anterior surface of the psoas major muscle and runs inferiorly, dividing into two branches:
- The genital branch runs through the inguinal canal and supplies motor fibers to the cremasteric muscle and sensory fibers to the skin of the upper anterior scrotum in males. In females, this branch provides sensory innervation to the skin of the mons pubis and labium majus.
- The femoral branch courses posterior to the inguinal ligament and provides cutaneous innervation to the skin of the upper anterior thigh.
Do you know about these common mistakes that hinder your anatomy learning? Be sure to avoid them when revising the lumbar plexus.
The obturator nerve emerges from the medial border of the psoas major muscle and is formed from the anterior divisions of the anterior rami of spinal nerves L2-L4. This nerve is a mixed nerve that enters the medial compartment of the thigh through the obturator canal. The obturator nerve provides motor supply to the obturator externus, and adductors of the hip (adductor longus, adductor brevis, adductor magnus, gracilis, and pectineus). It also provides sensory supply to the skin of the medial thigh.
Accessory obturator nerve
The accessory obturator nerve, when present, also emerges from the medial border of the psoas major muscle. It is typically formed by the anterior rami of spinal nerves L3 and L4. This nerve innervates the pectineus muscle and the hip joint.
To test your knowledge about the lumbar plexus, try out our quiz below:
Remembering the main branches of the lumbar plexus may initially seem daunting. However, it can be made easier using the phrase 'Interested In Getting Lunch On Friday?', which will help you remember the following list of structures:
- Iliohypogastric n. (L1)
- Ilioinguinal n. (L1)
- Genitofemoral n. (L1, L2)
- Lateral cutaneous nerve of thigh (L2, L3)
- Obturator n. (L2, L3, L4)
- Femoral n. (L2, L3, L4)
The following trick will also help remind you of the nerve roots associated with each nerve:
- 2 nerves arise from one root: iliohypogastric and ilioinguinal from L1
- 2 nerves arise from 2 roots: genitofemoral from L1 & L2, and lateral femoral cutaneous from L2 & L3
- 2 nerves arise from 3 roots: femoral and obturator from L2, L3 and L4
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