Lumbar plexus

The lumbar plexus is a complex network of nerves located in the lower back (lumbar region) that plays a vital role in providing motor and sensory innervation to the lower abdominal wall, anterior thigh, medial thigh, portions of the external genitalia, and specific areas of the lower limb and pelvis. It is primarily formed from the anterior rami of the first three lumbar spinal nerves (L1, L2, and L3) and the majority of the anterior ramus of the fourth lumbar spinal nerve (L4). Additionally, the first lumbar nerve (L1) often receives a contribution from the twelfth thoracic nerve (T12), known as the subcostal nerve.

Location

The lumbar plexus is situated within the psoas major muscle. from where the nerve branches of the lumbar plexus diverge toward their respective targets.

Anterior and posterior parts of the lumbar plexus

Please note that the anterior rami of lumbar spinal nerves L2 – L3 further divide into anterior (ventral) and posterior (dorsal) divisions, forming what we call as the anterior and posterior parts of the lumbar plexus. Some of the nerve branches of the lumbar plexus arise from its anterior part, such as the obturator nerve and genitofemoral nerve, while others arise from its posterior part, including the femoral and lateral femoral cutaneous nerve.

Key Contributions of the lumbar plexus

a) T12, L1 and L2 anterior rami give rise to the following nerves

• Iliohypogastric Nerve: Formed from the upper branch of L1, this nerve provides sensory innervation to the lower abdominal wall and upper buttock region.

• Ilioinguinal Nerve: Also derived from the L1 upper branch, it supplies sensation to the groin and external genitalia.

• Note: Both iliohypogastric and ilioinguinal nerves provide motor innervation to the lower abdominal wall muscles (internal oblique and transversus abdominis muscles).

• Genitofemoral Nerve: The lower branch of L1 combines with a branch from L2 to form this nerve, which divides into the genital and femoral branches to innervate the cremaster muscle (males) and provide sensory input to anterior part of the upper thigh and external genitalia.

b) L2, L3, and L4 anterior rami contribute to the following nerves:

Obturator Nerve: The ventral divisions of L2, L3, and L4 anterior rami join to form this nerve, responsible for motor innervation to the adductor muscles of the medial thigh and sensory innervation to the skin of the medial thigh.

Femoral Nerve: The dorsal divisions of L2, L3, and L4 anterior rami come together to form this nerve, facilitating motor functions in the anterior thigh muscles (e.g., quadriceps femoris and sartorius) and sensory innervation of the anterior thigh and medial leg and foot.

Lateral Femoral Cutaneous Nerve: Smaller dorsal branches of L2 and L3 anterior rami unite to form this nerve, responsible for sensory innervation to the skin of the lateral thigh.

Accessory Obturator Nerve (Variable Presence): This nerve, if present, arises from smaller branches of L3 and L4 and may assist in additional motor innervation of pelvic muscles.

c) Connections to the Sacral Plexus:

A portion of L4 anterior ramus, along with L5 anterior ramus, forms the lumbosacral trunk, which connects the lumbar plexus to the sacral plexus. Through this connection, L4 and L5 fibers contribute to motor innervation of the muscles of the gluteal region via nerve branches such as the superior and inferior gluteal nerves, nerve to obturator internus and nerve to quadratus femoris. And to the posterior thigh and leg muscles via the sciatic nerve.

Branches of the Lumbar Plexus

The branches of the lumbar plexus emerge from three primary locations within the psoas major muscle.

a) Lateral Border Branches:

• Iliohypogastric Nerve

• Ilioinguinal Nerve

• Lateral Femoral Cutaneous Nerve Femoral Nerve

b) Anterior Surface Branch:

• Genitofemoral Nerve, which divides into:

  • Genital Branch: Supplies the cremaster muscle and part of the skin of the external genitalia.

  • Femoral Branch: Provides sensory input to the upper anterior thigh.

c) Medial Border Branch:

• Obturator Nerve Supplies the adductor compartment of the thigh and a portion of its overlying skin.

Sympathetic Contributions

Sympathetic autonomic fibers integrate with the lumbar plexus through the grey and white rami communicantes, forming a connection between the sympathetic trunk and the anterior rami of the lumbar spinal nerves. This essential integration links autonomic nervous system functions to the structures innervated by the plexus.

• White Rami Communicantes: These consist of pre-ganglionic myelinated fibers originating in the lateral gray horns of the spinal cord at the T12-L2 segments, and connecting. These communicantes serve as conduits for these fibers to travel from the anterior rami to the sympathetic trunk.

• Grey Rami Communicants: These serve as conduits for the post-ganglionic fibers to travel from the sympathetic trunk connecting to the lumbar spinal nerves (T12-L4).

The autonomic fibers carried via the lumbar plexus branches regulate critical functions, including vasomotor control (blood vessel tone), sudomotor activity (sweat gland stimulation), and pilomotor responses (hair follicle muscle contraction) in the regions innervated by the plexus.