Lumbar Plexus

A nervous plexus is a network or web of nerve fibers coming from the spinal cord. The spinal nerve fibers coming from different segments of the spinal cord join one another in a nerve plexus. As a result, a single peripheral nerve is formed that contains nerve fibers from different segments of the spinal cord.

This nerve then provides motor or sensory supply to the peripheral parts of the body. There are four important nervous plexus in the human body that supply the peripheral areas of the body. These include cervical plexus, brachial plexus, lumbar plexus, and sacral plexus. The lumbar and sacral plexus join to form a single plexus called lumbosacral plexus.  

Lumbar plexus is present in the lumbar region of the body. Lumbar plexus forms a part of the larger lumbosacral plexus. It is formed by the nerves originating from the lumbar segment of the spinal cord. The lumbar spinal nerve join to form the lumbar plexus.

The terminal branches of this lumbar plexus then supply different areas of the lumbar region, pelvic region and lower limbs. In this article, we will study the location, formation, and distribution of the lumbar plexus. We will also study some clinical correlates of the lumbar plexus.

Location

As evident from the name, it is present in the lumbar region, just above the pelvic region. It is embedded in the posterior part of psoas major muscle, a muscle that originates in the lumbar region and inserts in the pelvic region.

Lumbar plexus is present just in front of the transverse processes of the lumbar vertebrae. The terminal branches of lumbar plexus then run downwards anterior to the quadratus lumborum muscle.

Formation

The anterior rami of lumbar spinal nerves L1 to L4. It also receives contributions from the T12 nerve, the last spinal nerve arising from the thoracic segments of the spinal cord. The fifth lumbar nerve (L5) does not participate in the formation of lumbar plexus. Rather, it joins communicating branches from the L4 nerve to form lumbosacral trunk.

Just like brachial plexus, lumbar plexus consists of roots and divisions. It does not have cords or trunks. The terminal branches arise from the divisions of lumbar plexus. Let us have a look at these aspects of lumbar plexus.

Roots

The roots of lumbar plexus are formed by the anterior rami of lumbar spinal nerves L1 to L4. A communicating branch form T12 also joins the L1 nerve to form the first root of the plexus.

Divisions

After the roots join to form trunks or cord of lumbar plexus, the plexus divides into two divisions;

  • Anterior division
  • Posterior division

These divisions give rise to terminal branches of the lumbar plexus. Both types of nerves, sensory as well as motor nerves arise from the two divisions of the lumbar plexus.

Branches and Distribution

In this section of the article, we will discuss terminal branches of the lumbar plexus and their distribution.

The branches arise from the two divisions of the plexus.

Anterior Division

The anterior division of the plexus gives two types of terminal branches, sensory as well as motor branches.

Sensory branches

The following sensory branches arise from the anterior division of lumbar plexus;

  • Ilioinguinal Nerve: This nerve arises from the L1 root of lumbar plexus. The ilioinguinal nerve enters the iliac region of the pelvic cavity. Here it enters the inguinal canal. This nerve provides sensory innervation to the skin at the root of penis and upper part of the scrotum in males, and to the skin over the mons pubis and labia majora in females.
  • Iliohypogastric Nerve: This nerve also arises from the L1 root of lumbar plexus. It enters the pelvic cavity and passes behind the kidneys. Upon reaching the iliac crest, it pierces the transversus abdominis muscle and divided into two branches. These branches supply the skin over the gluteal region and the skin of the hypogastric region.
  • Genitofemoral Nerve: This nerve arises from the first two roots of the lumbar plexus (L1-L2). After originating from the plexus, it runs deep to the psoas major muscle in the pelvic cavity. Lateral to the iliac arteries, the genitofemoral nerve divides into two branches;
  • Genital branch supplies the skin of scrotum in males. In females, it supplies the skin of mons pubis and labia majora. It also supplies a muscle called cremaster in males.  
  • Femoral branch supplies the skin on the upper anterior and medial sides of thigh.

Motor branches

Only one motor nerve arises from the anterior division of the lumbar plexus. It is called the obturator nerve. It arises from the L2 to L4 roots of lumbar plexus.

After passing through the lesser pelvic cavity, it enters the medial thigh. Here, it supplies the adductors of thigh. It also provides innervation to the knee joint and hip joint. Obturator externus and gracilis muscles are also supplied by the obturator nerve.

Posterior Division

The posterior b also gives rise to motor and sensory branches.

Sensory branches

Only one sensory branch arises from the posterior division of lumbar plexus. It is known as the lateral cutaneous nerve of the thigh. This nerve arises from the roots L2 and L3. The lateral cutaneous nerve enters the thigh after passing deep to the inguinal ligament. As the name suggests, it provides cutaneous innervation to the lateral half of thigh on the anterior side.

Motor branches

The posterior division of lumbar plexus gives only one motor branch, the femoral nerve. This nerve arises from L2, L3, and L4 roots of lumbar plexus. This nerve enters the thigh after passing deep to the inguinal ligament. This nerve provides motor innervation to the flexor muscles of the hip and extensors of the knee. It also supplies innervation to the hip joint and knee joint.

Clinical Correlates

Like every other plexus in the body, lumbar plexus also lies much protected. However, it may get injured in a number of cases like wounds of the lumbar region, infection, stab wounds etc.

Most of the injuries sustained by the lumbar plexus are due to automobile accidents, highspeed car accidents, or fall from a height, etc. Any injury to the lumbar plexus will result in loss of sensory as well as motor functions.

Sensory loss in the areas of skin supplied by the sensory branches of the plexus. These include skin around the root of the penis, scrotum and cremasteric muscle in males. Sensory loss in females occurs in the skin covering mons pubis and labia majors.

In addition to this, sensory loss in the upper medial and upper intermediate areas of the thigh as well as the entire lateral side of front of thigh is seen in both genders when lumbar plexus is injured.

Motor loss includes loss of functionality of the adductors of the thigh, the flexors of hip and the extensors of the knee joint. In addition, proprioception from the hip joint and knee joint is also impaired as both these joints are supplied by the branches of the lumbar plexus.

An important clinical correlate of lumbar plexus is referred pain. We have already mentioned in the article that both the hip joint and the knee joint are supplied by the obturator nerve and the femoral nerve. Any disease of the knee joint produces pain in both the knee joint and the hip joint and vice versa due to the same nerves supplying both joint. This phenomenon is known as referred pain.

Conclusion

Lumbar plexus is a network of nerve fibers located in the lumbar region just in front of the transverse processes of the lumbar vertebrae.

The roots of the lumbar plexus are formed by the anterior rami of lumbar spinal nerves L1 to L4 along with contributions from the last thoracic spinal nerve T12.

As the roots join to form the plexus, it immediately divides into two divisions:

  • Anterior Division
  • Posterior Division

Both these divisions give rise to the terminal branches of the plexus which include both sensory and motor nerves.

Three sensory nerves arise from the anterior division of lumbar plexus. These include:

  • Ilioinguinal Nerve (L1)
  • Iliohypogastric Nerve (L1)
  • Genitofemoral Nerve (L1, L2)

These sensory nerves supply the skin around the external genitalia as well as the skin of the upper medial and lateral parts of the anterior thigh.

Only one motor nerve originates from the anterior division of the lumbar plexus known as Obturator Nerve (L2-L4). This nerve supplies the adductors of the thigh, hip joint, and knee joint.

One sensory and one motor nerve arises from the posterior division of lumbar plexus.

The sensory branch is called the lateral cutaneous nerve of thigh (L2, L3). It supplies the skin on the anterolateral half of the thigh.

The motor branch is the Femoral Nerve (L2-L4). This nerve supplies the flexors of the hip, extensors of the knee, hip joint, and knee joint.

Injury to lumbar plexus is mostly due to automobile accidents, highspeed car accidents or falls from height, etc. Any injury results in loss of the functions performed by the above-mentioned nerves. An important clinical correlate of the lumbar plexus is referred pain. Any pain in the knee joint is referred to as the hip joint and vice versa due to the same nerve supply.

References

  1. Anatomy, descriptive and surgical: Gray’s anatomy. Gray, Henry. Philadelphia: Courage Books/Running Press, 1974
  2. Clinically Oriented Anatomy. Moore, Keith L. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, 2010 (6th ed)
  3. Human Neuroanatomy. Carpenter, Malcolm B. Baltimore: Williams & Wilkins Co., 1976 (7th ed)