Lumbosacral Plexus: Definition, Description, Function, Branches and Distribution of the Nerve

It is a network of nerve fibers that supplies the skin and muscles of the lower limb.

It is located in the lumbar region, within the substance of the psoas major muscle and anterior to the transverse processes of the lumbar vertebrae.

The plexus is made up of the anterior rami (divisions) of the L1, L2, L3, and L4 lumbar spinal nerves. It also receives contributions from the thoracic spinal nerve 12.

Description and function

The lumbar spinal nerves are in the intervertebral foramina and are numbered according to the vertebra under which they lie.

The L1 spinal nerve is located below the L1 vertebrae in the L1-2 intervertebral foramen, L2 is located in the L2-3 intervertebral foramen, and so on.

Each spinal nerve is connected to the spinal cord through a ventral and dorsal root. Peripherally, the spinal nerve divides into a larger ventral branch and a smaller dorsal branch.

The spinal nerve roots join the spinal nerve at the intervertebral foramen, and the ventral and dorsal branches form just outside the foramen.

Spinal nerves are short, and no longer than the width of the intervertebral foramen in which they are located. The spinal nerve roots distribute their fibers directly to the ventral and dorsal branches without actually forming a spinal nerve.

The dorsal root of each spinal nerve carries sensory fibers from the spinal nerve to the spinal cord . The ventral root is largely responsible for the transmission of motor fibers from the cord to the spinal nerve, but it can transmit some sensory fibers.

The spinal cord ends at the vertebral canal, opposite the level of the L1-2 intervertebral disc, although it can sometimes end at T12-L1 or as low as L2-3.

The angle that each pair of nerve roots leaves the dural sac varies. The L1-2 roots leave the dural sac at an obtuse angle, but the dural sleeves of the lower nerve roots form increasingly acute angles.

The angles formed by the L1 and L2 roots are approximately 80 degrees and 70 degrees, while the angles of the L3 and L4 roots are approximately 60 degrees.

Lumbar 1 to Lumbar 4 donate their anterior branches to create the lumbar plexus. The lumbar plexus innervates the structures of the lower abdomen. The anterior and medial segments of the lower limb are also innervated.

The smallest part of the fourth lumbar nerve joins with the fifth to form the lumbosacral trunk , which forms the sacral plexus. The fourth nerve is called the nervus furcalis because it is subdivided between the two plexuses.

Remaining

The branches of the lumbar plexus form the following nerves:

  • L1-Iliohipogástrico e Ilioinguinal.
  • Divisiones dorsal L1, L2-Genitofemoral.
  • L2, L3-Lateral femoral cutaneous.
  • L2, L3, L4-femoral, ventral divisions.
  • L2, L3, L4-Obturador.
  • Accessory shutter L3, L4.

The lumbar spine has an extensive innervation system. Subsequently, the branches of the lumbar dorsal rami are distributed to the zygo-physical joints and the muscles of the back.

Anteriorly, the ventral rami supply the quadratus lumbar and psoas major. The vertebral bodies and intervertebral discs are surrounded by plexuses that accompany the longitudinal ligaments and are derived from the lumbar sympathetic trunks.

Within the posterior plexus, the largest branches make up the synovertebral nerves. Short branches innervate the vertebral periosteum, while long branches enter the vertebral body from all aspects of its circumference.

Nerves enter the outer third of the annulus fibrosus from the longitudinal plexuses anteriorly, laterally, and posteriorly. The posterior plexus innervates the dura and nerve root sleeves.

The posterior division also produces the femoral nerve. This is the most important nerve in the lumbar plexus. It innervates the muscles of the anterior and lateral thigh, as well as the medial leg and ends in the foot.

The femoral nerve also innervates the quadratus femoris, iliopsoas, and sartorius muscle with motor coordination. The anterior division of the lumbar plexus produces the obturator nerve. It innervates the cutaneous areas of the medial thigh and the abductor muscles of the hip.

It is crucial that physical therapists have a strong knowledge base and understanding of the lumbar plexus when treating our orthopedic patients.

To accurately determine the cause of pain or dysfunction in our patients, analyzing which level is affected can help us make better intervention decisions, which in turn will lead to better results.

Many times patients present deficits in the lower extremities, however these many times start from the lumbar plexus, and not from the extremity.

Being able to use our anatomy and physiology in treating patients with musculoskeletal dysfunctions can help our patients progress more effectively and faster, which will help lower the costs of long-term treatment due to misdiagnosis.

Lumbosacral plexus nerve distribution

The cranial iliohypogastric nerve emerged between the psoas major and minor muscles and divided into 3, forming the medial, intermediate, and lateral branches on the medial side of the abdominal wall and the peritoneum.

The caudal iliohypogastric nerve supplied the paralumbal fossa, the craniolateral part of the femur, and the ventral abdominal wall.

The ilioinguinal and genitofemoral nerves, after emerging between the lumbar muscles, continued subperitoneally and caudo-ventrally towards the abdominal inguinal ring.

These nerves innervated the internal abdominal oblique muscle and the cremaster muscle, the testicular fascia, the spermatic funiculus and the foreskin in men, the nipples in women, and the skin on the medial side of the femoral region in both sexes.

The lateral cutaneous femoral nerve arose from the LSP, reaching the caudal iliac region crossing the abdominal muscles. It extended over the external oblique muscles of the abdomen, iliac muscles, and tensor fascia lata, the skin of the femoral region, and the craniomedial side of the knee joint.

The femoral nerve originated from the LSP with the obturator nerve. It was the thickest nerve in the plexus.

After emerging between the psoas major and minor muscles and innervating them, together with the deep lumbar muscles, it reached the femoral space running caudo-ventrally and emitting 2 branches of the quadriceps femoris muscle.

The saphenous nerve emerged lower. The gracilis, pectineus and sartorius muscles were divided into 3 branches and a branch to irrigate the skin and fascia in the medial femoral region.

The obturator nerve had a similar origin to the femoral nerve. After exiting the pelvic cavity through the obturator foramen, it supplies branches to the adductor, pectineus, gracilis, internal and external obturator muscles.

The ischial plexus was a nerve trunk arising from the junction of spinal nerves L5-6 and S 1, supplying the iliac region dorso-caudally and extending caudally into the femoral region.

This plexus branched and gave rise to 4 nerves: cranial gluteal, caudal gluteus, caudal cutaneous femoral and ischiadic. The gluteal cranial nerve, after exiting the pelvic cavity through the greater ischiadic notch, supplied the branches of the deep gluteal, piriformis, and tensor fascia lata muscles.

The caudal gluteal nerve was a motor nerve that arose from the caudal portion of this common trunk and innervated the superficial gluteus, the cranial part of the biceps femoris, the abductor crural caudalis, and the vertebral head of the semitendinosus and semimembranous muscles.

The gluteus medial muscle was innervated by the 2 gluteal nerves. The caudal cutaneous femoral nerve innervated the biceps femoris and semitendinosus muscles and, after passing between these muscles, supplied the skin over the ischial tubercle, the caudal femur, and a junction with the pudendal nerve, the perineum.

The common trunk continued as the sciatic nerve after giving rise to the caudal cutaneous femoral nerve. The sciatic nerve branched into the proximal muscular branch for the internal obturator, gemelli, and quadratus femoris muscles.

This branch extends to the skin after innervating the caudal portion of the biceps femoris, semitendinosus and semimembranosus muscles.

Near the knee joint, the ischial nerve divides into the sura of the caudal cutaneous, lateral cutaneous sural, common peroneal, and tibial nerve.

The cutaneous sural lateralis nerve, the first branch of the sciatic nerve, innervated the subcutaneous tissue and the skin in the surae cranial region, while the sural caudal rose through the caudal region and innervated the skin of this region and the common tendon of the calcaneus.

The tibial nerve ran at the level of the tarsal joint, producing thin branches in the skin in this region, and divided into the dorsal digital nerve and the common plantar nerve V.

After this branch, the tibial nerve detached a distal muscular branch that extended to the flexor muscles, ran between the 2 heads of the gastrocnemius and the popliteal muscles, continued as branches to the muscles of the plantar metatarsal surface and divided into the common plantar digital nerves II, III and IV.

The common fibular nerve passed over the gastrocnemius muscle and entered the groove between the peroneus longus and extensor digitorium lateralis muscles, then divided into the superficial fibula and deep peroneal nerves.

The superficial fibular nerve innervated the skin of the dorsal tarsus and metatarsus and delivered the muscular branches to the tibial cranial, long digital extensor, and lateral digitalis extensor muscles, and formed the common III and IV digital dorsal nerves.

The deep peroneal nerve supplies the fibularis longus and fibularis brevis muscles.

The pudendal nerve crossed into the caudal pelvic opening where it divided into the dorsal nerves of the penis or clitoris and the superficial and deep perineal nerves.

The former innervated the ischiocavernosus, bulbospongiosus, and retractor muscles of the penis and foreskin in the male and the vulva, clitoris, and vulva constrictor muscle in the female. Superficial and deep pudendal nerves innervated the skin and muscles of the anal and perianal regions.

The caudal rectal nerve innervated the region to the end of the rectum, the sphincter muscles of the anus, and the skin of the anal region.