Lumbar plexus: Nerves and areas of innervation

In this note, we are going to discuss the anatomy of the lumbar plexus (plexus lumbalis), which innervates various structures of the posterior abdominal wall and lower limb.

The lumbar plexus arises from the anterior rami (rami anteriores) of lumbar spinal nerves L1-L4, and it also receives some contributions from T12.

It is located on the posterior abdominal wall within the psoas major muscle (m. psoas major), anterior to the transverse processes of the lumbar vertebrae.

The lumbar plexus gives rise to several small muscular branches (rami musculares) as well as numerous larger nerves.

They can be classified into three groups relative to the psoas major muscle.

The lateral group includes:

Iliohypogastric nerve
Ilioinguinal nerve
Lateral femoral cutaneous nerve
Femoral nerve

The anterior group includes only the genitofemoral nerve.

The medial group includes:

Obturator nerve
Accessory obturator nerve
Nerve to the lumbosacral trunk

Let’s discuss each of the nerves in greater detail.

Iliohypogastric nerve (nervus iliohypogastricus)

The iliohypogastric nerve is formed mainly by the fibers from L1, but may occasionally receive contributions from T12.

It descends along the posterior abdominal wall. This nerve provides motor innervation to the internal oblique and transversus abdominis muscles, as well as the sensory innervation to the skin of the suprapubic region (ramus cutaneus anterior)

Anterior cutaneous branch (ramus cutaneus anterior)

and posterolateral gluteal region (ramus cutaneus lateralis).

Lateral cutaneous branch (ramus cutanleus laterlalis)

Ilioinguinal nerve (nervus ilioinguinalis)

The ilioinguinal nerve, like the previous one, arises mainly from L1 and may also receive contributions from T12.

It runs almost in a similar direction as the iliohypogastric nerve between the transversus and internal oblique abdominis muscles, innervating them.

It passes through the inguinal canal and eventually gives off anterior scrotal branches (rami scrotales anteriores) in men and anterior labial branches (rami labiales anteriores) in women.

The ilioinguinal nerve innervates the skin of the inguinal and pubic areas, as well as the proximal medial thigh.

Lateral femoral cutaneous nerve (nervus cutaneus femoris lateralis)

It is formed by the posterior divisions of the anterior rami of spinal nerves L2 and L3. It courses downwards along the anterior surface of the iliacus muscle and then under the inguinal ligament to reach the thigh.

This nerve provides sensory innervation to the peritoneum of the iliac fossa, iliac fascia, and the skin of the thigh along the iliotibial tract down to the knee.

Femoral nerve (nervus femoralis)

The femoral nerve is the largest nerve of the lumbar plexus. It is formed by the posterior divisions of the anterior rami of spinal nerves L2, L3 and L4.

It descends under the iliac fascia and passes below the inguinal ligament lateral to the femoral vessels to reach the thigh.

The motor branches of the femoral nerve supply the quadriceps femoris, pectineus, iliacus and sartorius muscles.

The cutaneous branches innervate the skin of the anteromedial thigh down to the knee joint.

The cutaneous branches of the femoral nerve

The terminal branch of the femoral nerve is the saphenous nerve (nervus saphenus), which innervates the skin of the anteromedial leg and foot down to the toe.

It also gives off the infrapatellar branch (ramus infrapatellaris)

Genitofemoral nerve (nervus genitofemoralis)

The genitofemoral nerve originates from L1 and L2, pierces the psoas major muscle and splits into two branches — a genital branch and a femoral branch.

The genital branch (ramus genitalis) runs through the inguinal canal. It innervates the cremasteric muscle, skin of the scrotum in men or round ligament of the uterus and skin of the mons pubis and labium majus in women.

The femoral branch (ramus femoralis) courses along the femoral artery on the thigh, pierces the cribriform fascia and innervates the skin of the anterior thigh below the inguinal ligament.

Obturator nerve (nervus obturatorius)

This nerve is made up of the contributions from L2, L3 and L4.

It runs along the medial border of the psoas major muscle and, together with the obturator vessels, goes through the obturator canal to enter the thigh.

The obturator nerve innervates the obturator externus and adductors of the hip (longus, brevis and magnus) as well as the gracilis and pectineus muscles.

It also provides sensory innervation to the skin of the medial thigh and capsule of the hip joint.

Cutaneous branches of the obturator nerve

Accessory obturator nerve (nervus obturatorius accessorius)

The accessory obturator nerve is present in about 30% of people. It is formed by the fibers from L3 and L4. This nerve innervates the pectineus muscle and the capsule of the hip joint.

In addition, L4 gives off the branch, which together with the fibers from L5 form the so-called lumbosacral trunk (truncus lumbosacralis). It takes part in the formation of the sacral plexus with the anterior rami of the spinal nerves S1-S4.

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