The Anatomy of the Lumbar Plexus

The lumbar plexus is a network of nerves in the lumbar region of your body (the abdominal segment of your torso). It's a part of the larger structure called the lumbosacral plexus. The lumbar plexus is made up of branches of the first four lumbar nerve along with contributions from the subcostal nerve, which is the lowest nerve of the thoracic region (the one just above the lumbar area.) The plexus' major function is to supply nerve function to the front part of the thigh.

Anatomy

The nerves in your body emerge from the spinal cord and exit to the rest of the body between the vertebrae.

Structure

The spine is divided up into five sections. The lumbar region of the spine is the third section, with the sacral region below it and the thoracic region above. Even so, the lumbar region is low in your back, where it naturally curves inward. (The sacral and coccygeal regions below it are short, while the thoracic region is the longest.)

Once the nerves emerge from the spinal column, they branch out like trees so they can travel to the various muscles, joints, and other tissues they innervate (provide nerve function to). All of the spinal nerves are considered mixed, providing both motor function (having to do with movement) and sensory function (having to do with sensation, such as touch and temperature).

Shortly after exiting the spine, the nerve splits into three parts. Each part is called a ramus (plural is rami). The three rami are:

• Dorsal rami (rear division)
• Ventral rami (front division)
• Rami communicans (which form connections between nerves so they can communicate)

You have several nerve plexuses, which are made up of branches of intersecting nerve fibers. The major ones and the areas they serve are:

• Cervical plexus: Head, neck, and shoulders
• Brachial plexus: Chest, shoulders, arms, and hands
• Lumbar plexus: Back, abdomen, groin, thighs, knees, calves
• Sacral plexus: Pelvis buttocks, genitals, thighs, calves, feet
• Coccygeal plexus: A small region over the coccyx (your "tailbone")

Location

The lumbar plexus contains the ventral rami (front portions of spinal nerves) that emerge from between the five lumbar vertebrae (L1-L5). In addition, it's joined by a portion of the lowest thoracic nerve, which emerges from the T12 vertebra just above the lumbar region.

This plexus forms alongside the spine and passes through the psoas major muscle, which connects to the lumbar region of the spine and stretches down to the bottom of your pelvis, near where your thigh meets your torso. Small motor branches of the plexus innervate the psoas major, the quadratus lumborum muscle, and the lumbar intertransverse muscle. Meanwhile, the larger branches continue on, exiting the psoas major muscle through various places.

After that, they travel down in front of your hip joint then through and out of the pelvis to reach the front part of the thigh. It gives off branches as it goes.

Function

The five main branches of the lumbar plexus are responsible for much of the movement and feeling in your legs. Most of them have both motor and sensory roles.

Iliohypogastric Nerve

The first major branch of the lumbar plexus, the iliohypogastric nerve runs to the iliac crest (the top and outer edge of your hip bones) and across the quadratus lumborum muscle before perforating the transversus abdominis muscle. There, it divides into its terminal branches.

The iliohypogastric nerve provides motor function to the internal oblique and transversus abdominis muscles. It provides sensory function to a portion of skin in the pubic region.

Ilioinguinal Nerve

The ilioinguinal nerve has a close relationship with the iliohypogastric. It follows the same course for much of its run and joins the larger iliohypogastric in supplying motor function to the internal oblique and transversus abdominis muscles in the abdominal wall.

From there, it travels a separate course and provides sensory function to the skin on the upper-middle thigh, then continues on to the genitalia. In men, it's responsible for feeling in the skin at the root of the penis and forward part of the scrotum. In women, it innervates the skin over the mons pubis and labia majora.

Genitofemoral Nerve

The genitofemoral nerve divides just outside the psoas major muscle to form a femoral branch and a genital branch.

The femoral branch supplies feeling to the skin on the front of the upper thigh.

The genital branch is mixed, supplying motor function to the cremaster muscle of the scrotum and inguinal canal. Along with the ilioinguinal nerve, it provides sensory innervation to the skin on the front of the scrotum in men and the mons pubis and labia majora in women.

Lateral Cutaneous Nerve of the Thigh

The location of this nerve—in the thigh—must be specified because there's also a lateral cutaneous nerve of the arm.

The lateral cutaneous nerve of the thigh is purely a sensory nerve. It provides feeling to the skin down the front and outer side of the thigh to the knee.

Obturator Nerve

The obturator nerve is an especially important motor nerve, as it provides function to numerous muscles on the front and inside of the thigh. They are:

• Obturator externus
• Pectineus
• Adductor longus
• Adductor brevis
• Adductor magnus
• Gracilis

Also a sensory nerve, it innervates the skin over the front and inside of the thigh.

Femoral Nerve

The femoral nerve is another important one, supplying motor nerves to muscles in the thigh and down the inside of the knee and calf, as well. These include:

• Illiacus
• Pectineus
• Sartorius
• All the muscles of the quadriceps femoris

It also provides sensation to the skin on the front of the thigh and down the inside of the entire leg.

Associated Conditions

A couple of conditions can interfere with the function of the lumbar plexus.

Lumbosacral Plexopathy

Lumbosacral plexopathy is a rare syndrome that affects either the lumbar or sacral plexus. It's caused by damage to the nerve bundles; your healthcare provider may begin to suspect it if symptoms can't all be tied to a single nerve.

Symptoms of lumbosacral plexopathy can impact any area innervated by the affected nerves. They include:

• Neuropathic pains (electric, shooting, or "zinging" pains)
• Numbness
• Weakness and muscle wasting

A primary cause of this painful and potentially debilitating condition is diabetic amyotrophy, which is caused by high blood sugar levels damaging the nerves.

Tumors that compress one or more of the nerves of the plexus can also cause plexopathy, as can other growths that invade the spaces the nerves pass through and cause compression.

In some cases, no cause can be found. This is called idiopathic plexopathy. (Idiopathic is the medical term for no known cause.)

Lumbar Radiculopathy

Lumbar radiculopathy is similar to plexopathy except that it's caused by the compression of nerve roots themselves as they leave the spine in the lumbar region. This can be from chemical irritation, injury (including repetitive stress injury), compression from herniated discs or bone spurs, spinal stenosis, or the thickening of nearby ligaments. It can also be caused by scoliosis, infection, or, rarely, tumors. Some people develop radiculopathy due to congenital abnormalities, as well.

Symptoms of lumbar radiculopathy include:

• Tingling
• Radiating pain
• Numbness
• Paresthesia (abnormal, sometimes painful nerve sensations)
• Shooting pains
• Loss of motor function in the muscles innervated by the damaged nerve
• Loss of sensation in the tissues innervated by the damaged nerve

While radiculopathy can occur in any spinal nerves, it's more common in the lumbar, sacral, and cervical regions. Even so, lumbar radiculopathy only accounts for about 3% to 5% of lower back pain diagnoses.

Rehabilitation

The rehabilitation is dependent on the condition that is affecting the function of the lumbar plexus.

Treatment of Lumbosacral Plexopathy

Treatment of lumbosacral plexopathy depends on what's determined to be the cause.

Unfortunately, no treatment has proven to be consistently effective for diabetic amyotrophy or idiopathic plexopathy. In diabetics, better control of blood sugar levels is always recommended. Additionally, a multidisciplinary treatment regimen may include tricyclic antidepressants, anti-seizure medications, physical and occupational therapy.

In the case of a tumor or other growth causing nerve compression, the offending growth should be removed whenever possible.

Treatment of Lumbar Radiculopathy

The treatment of lumbar radiculopathy also depends on the cause. In an acute setting, analgesics such as nonsteroidal anti-inflammatory drugs (NSAIDs) or acetaminophen and activity modification are the main treatments. A herniated disc may be a contributing factor and surgery might be recommended for some patients with chronic pain.

The treatment course is most often conservative and can include:

• Physical therapy focusing on core stabilization exercises
• Pain killers
• Nonsteroidal anti-inflammatory drugs (NSAIDs)
• Muscle relaxants
• Steroids

Less common options include:

• Traction
• Chiropractic manipulation
• Ultrasound
• Hot packs
• Acupuncture
• Bed rest
• Wearing a corset