What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Lumbosacral Plexus: Nerves, Roots, and Comparative Anatomy

Name: Lumbosacral Plexus: Nerves, Roots, and Comparative Anatomy
Creator: Dr. Zubair Khalid

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

The lumbosacral plexus is a network formed by the ventral rami of the lower lumbar and sacral spinal nerves, and in the dog it draws from L4 through S3. Its named terminal branches, including the femoral, obturator, sciatic (with tibial and common fibular divisions), cranial and caudal gluteal, and pudendal nerves, supply the pelvic limb, the gluteal muscles, and the perineum.

This article explains how those roots reorganize into named nerves, why the pattern differs between dogs, cats, horses, and people, and what goes wrong when the lumbosacral plexus nerves are injured or compressed.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

What the Lumbosacral Plexus Is and Where It Sits

A nerve plexus is a region where ventral rami of spinal nerves exchange fibers before they continue as named peripheral nerves. The lumbosacral plexus is the pelvic counterpart of the brachial plexus. It sits in the lumbosacral region, the area spanning the last lumbar vertebrae and the sacrum, buried within and beneath the psoas and iliacus musculature in the dog.

Each spinal nerve divides shortly after leaving the intervertebral foramen. The dorsal ramus runs backward to supply the epaxial muscles and skin of the back. The ventral ramus runs forward and downward into the body wall or limb. The lumbosacral plexus is built entirely from these ventral rami. That detail matters clinically because a lesion that damages only the dorsal rami spares limb function, while a lesion of the ventral rami can produce dramatic limb deficits.

The plexus is not a single anatomical object with a fixed outline. It is a functional grouping of roots that share a destination. In the dog, the roots that contribute are L4, L5, L6, L7, S1, S2, and S3, with the largest contributions coming from L6, L7, and S1. These roots divide into a cranial group that supplies the ventral and medial thigh and a caudal group that supplies the gluteal region, the caudal thigh, and everything below the stifle.

The Ventral Rami That Build the Plexus

Root-by-Root Contributions in the Dog

The dog pattern is the reference standard for veterinary anatomy, and it is also the pattern against which wild canids are compared. In the dog, the femoral nerve arises mainly from L4, L5, and L6. The obturator nerve arises from L4, L5, and L6 as well, with some variation between individuals. The sciatic nerve is the largest branch of the plexus and forms from L6, L7, and S1, with a variable contribution from S2. The cranial gluteal nerve comes from L6 and L7, and the caudal gluteal nerve from L7 and S1. The pudendal nerve arises from S1, S2, and S3.

A study of two South American wild canids, the pampas fox and the crab-eating fox, found a plexus formed by whole ventral branches of L5 through L7 and S1, plus a contribution from S2, organized into three trunks [1]. The cranial trunk, built from L5, L6, and S1, divided into lateral cutaneous femoral, femoral, and obturator branches. The caudal part of the plexus formed a thick ischiadic branch from L6, L7, S1, and S2 that gave rise to the cranial and caudal gluteal nerves, the caudal cutaneous femoral nerve, and the sciatic nerve. The pudendal nerve arose from S2 with contributions from L7 and S1 [1]. That arrangement closely mirrors the dog model, which is why the dog is used as the comparative baseline.

Why Roots Overlap

A single muscle usually receives fibers from more than one spinal root. The quadriceps, for example, is supplied by the femoral nerve, which itself carries fibers from three roots. This overlap is protective. If one root is damaged, the remaining roots can often keep the muscle working, at least partially. It also explains why a single-root lesion may produce subtle weakness rather than complete paralysis, while a lesion that destroys the whole plexus produces a limb that cannot bear weight.

The Named Terminal Branches

Femoral Nerve

The femoral nerve is the major nerve of the cranial thigh. It runs with the femoral artery and vein through the femoral triangle and supplies the quadriceps group, the iliopsoas, and the sartorius. Its sensory branch, the saphenous nerve, runs down the medial side of the stifle and supplies skin over the medial crus and tarsus.

Femoral nerve block is a common component of pelvic limb anesthesia in dogs, and its anatomy has been studied in detail. One cadaver study used the superficial circumflex iliac artery as an ultrasound landmark and confirmed dye placement directly on the femoral nerve in all eleven limbs where the artery could be visualized [2]. Another study evaluated blind perineural injection techniques for the saphenous, obturator, and lateral cutaneous femoral nerves and found that 0.2 mL/kg stained the obturator and lateral cutaneous femoral nerves well, while 0.3 mL/kg was needed for the saphenous nerve [3]. These numbers describe dye spread in cadavers, not clinical dosing, but they show how much volume it takes to coat a nerve along a useful length.

Obturator Nerve

The obturator nerve leaves the pelvis through the obturator foramen and supplies the adductor muscles of the medial thigh. It is a pure motor nerve in most respects, with little or no cutaneous territory. Injury produces weakness in adduction, which owners may notice as a limb that splays outward or slips on smooth floors.

Sciatic Nerve and Its Two Divisions

The sciatic nerve is the largest nerve in the body. It leaves the pelvis through the greater ischiatic notch, passes over the hip joint, and runs down the caudal thigh between the biceps femoris and the adductor. Near the stifle it divides into the tibial nerve and the common fibular nerve.

The tibial nerve supplies the caudal muscles of the crus, including the gastrocnemius, and continues into the paw as the plantar nerves. The common fibular nerve wraps around the head of the fibula and divides into superficial and deep fibular branches that supply the cranial muscles of the crus and the dorsal paw. A study of the sciatic nerve paraneural sheath in dogs described the gross and microscopic anatomy of the sheath and demonstrated that a subparaneural injection spreads along the nerve [4]. Another cadaver study compared two ultrasound-guided approaches to the sciatic nerve at mid-thigh and found that an out-of-plane approach produced no needle-nerve contact in twelve injections, while the conventional in-plane approach produced contact in nine of twelve [5]. That is a useful reminder that technique affects nerve trauma, not just nerve blockade.

The sciatic nerve is also a target in regenerative research. A canine study created a 40 mm sciatic nerve defect bridged by a polyglycolic-polypropylene mesh and compared scaffold alone with scaffold seeded with adipose-derived stem cells, platelet-rich plasma, or both [6]. This kind of work is experimental and does not yet define standard care, but it shows how much research effort the sciatic nerve attracts because of its size and its importance for standing and walking.

Cranial and Caudal Gluteal Nerves

The cranial gluteal nerve supplies the gluteal muscles, which extend the hip and stabilize the pelvis during the stance phase. The caudal gluteal nerve supplies the caudal gluteal muscles and the tensor fasciae latae in some species. Both arise from the lumbosacral trunk region, and both are at risk during procedures near the greater ischiatic notch. A cadaver study comparing two ultrasound-guided approaches to the lumbosacral trunk found that the greater ischiatic notch plane approach stained the trunk in 100% of injections and the parasacral approach in 93.3% [7]. The study also assessed staining of the cranial gluteal nerve and the pelvic cavity, which shows how closely these structures sit together.

Pudendal Nerve

The pudendal nerve arises from the sacral roots and supplies the perineum, the external anal sphincter, and the external genitalia. In the dog it is often grouped with the caudal rectal nerves and the pelvic nerve as the pudendal plexus. The pudendal nerve is the somatic counterpart to the autonomic pelvic nerve, which supplies the pelvic viscera. Injury to the pudendal nerve can affect continence and perineal sensation, though the clinical picture depends on which branches are involved.

Comparative Anatomy Across Species

The lumbosacral plexus varies more between species than most veterinary students expect. The table below summarizes the contributing roots and the notable differences.

Species	Contributing spinal roots	Notable differences
Dog	L4 to S3	Reference pattern. Sciatic from L6, L7, S1 with variable S2. Femoral and obturator from L4 to L6. Pudendal from S1 to S3.
Cat	L4 to S3	Similar root range to the dog. The posterior femoral cutaneous nerve arises from the bigeminal nerve type in cats, unlike the pudendal transitional type in dogs [8].
Horse	L4 to S2	Dominant sciatic nerve. No distinct lumbosacral trunk pattern as seen in humans. Femoral nerve is large and clinically important.
Human	L1 to S4	Lumbar plexus and sacral plexus form separately, joined by the lumbosacral trunk. The posterior femoral cutaneous nerve is the sciatic transitional type [8].

Dog and Cat

Dogs and cats share the same root range, L4 through S3, and the same major branches. The differences are in the details. A comparative study of the posterior femoral cutaneous nerve across mammals classified its origin into seven types. The horse, rat, bird, frog, and salamander show the sciatic nerve type, where the nerve arises from the sciatic nerve and its roots. Humans and monkeys show the sciatic transitional type, arising from the sciatic nerve, its roots, the junction of the sciatic and bigeminal nerves, and the bigeminal nerve itself. Cats and great apes show the bigeminal nerve type. Dogs show the pudendal transitional type, arising from the junction, the bigeminal nerve, and the pudendal nerve [8]. That single nerve is a good example of how much variation exists in a structure that most clinicians never think about.

Horse

The horse plexus is built from L4 through S2, one segment shorter than the dog. The sciatic nerve dominates the picture. It is large, and it supplies most of the caudal thigh and all of the crus and foot through its tibial and common fibular divisions. The horse does not have a distinct lumbosacral trunk in the human sense, where a separate lumbar plexus and sacral plexus are joined by a defined trunk. Instead, the roots blend into a single continuous formation. The femoral nerve is still present and still supplies the quadriceps, and it is large enough to be a clinical concern in horses with pelvic limb weakness.

Human

The human arrangement is different enough that it is usually taught as two plexuses. The lumbar plexus forms from L1 through L4 and gives rise to the femoral, obturator, and lateral cutaneous femoral nerves. The sacral plexus forms from L4 through S4 and gives rise to the sciatic, gluteal, and pudendal nerves. The two are connected by the lumbosacral trunk, which carries fibers from L4 and L5 into the sacral plexus. That trunk is the structural feature the horse lacks. A magnetic resonance tractography study of healthy human volunteers reconstructed the major lumbosacral plexus bundles using the cauda equina as seed points and processed diffusion data in individual L3 through S2 nerve fibers [9]. That level of imaging detail is not yet routine in veterinary patients, but it shows how the human plexus is studied.

How the Plexus Is Imaged and Studied

Magnetic Resonance Imaging in Dogs

The normal canine femoral and sciatic nerves have been characterized on 3 Tesla MRI. A prospective study of ten healthy Beagle dogs found that both nerves were visible in all images, were symmetric, and had homogeneous signal intensity. They were iso- to mildly hyperintense to muscle on T2-weighted images, mildly hyperintense on T2-SPIR, and iso- to mildly hypointense on T1-weighted images, with no contrast enhancement [10]. That baseline is useful because it tells clinicians what a normal nerve looks like, which is the first step in recognizing an abnormal one.

Plastination and Teaching

Dissection and plastination of the complete nervous system of a dog has been described as a teaching tool. The specimen included the brain, spinal cord with meninges, cervical plexus, brachial plexus, intercostal and abdominal nerves, lumbosacral plexus, sympathetic trunk, vagus nerve, and phrenic nerves [11]. Plastinated specimens let students trace the lumbosacral plexus nerves in three dimensions without the time pressure of a wet dissection.

Ultrasound and Regional Anesthesia

Ultrasound guidance has transformed how the lumbosacral plexus is approached for nerve blocks. A prospective clinical study of psoas compartment and sacral plexus block for pelvic limb amputation in dogs identified the lumbar plexus within the psoas compartment using electrolocation lateral to the lumbar vertebrae at the fourth-fifth, fifth-sixth, and sixth-seventh vertebral interspaces, and identified the sacral plexus ventrolateral to the sacrum [12]. That study is one of the clearest demonstrations that the lumbosacral plexus is a practical target, not just a dissection exercise.

Clinical Relevance, Limitations and Common Mistakes

Femoral Nerve Injury

Femoral nerve injury weakens the quadriceps. The quadriceps is the main extensor of the stifle, so a dog with a femoral nerve lesion may stand with the stifle slightly flexed, may be unable to extend the limb fully, and may have a reduced patellar reflex. The saphenous sensory branch may also be affected, producing numbness over the medial crus. Because the femoral nerve carries fibers from three roots, a partial lesion can look like subtle weakness rather than obvious paralysis.

Sciatic Nerve Injury

Sciatic nerve injury produces a characteristic dropped hock. The tibial nerve supplies the gastrocnemius and the other caudal crural muscles that extend the hock. When the tibial division is lost, the hock cannot be extended against gravity, and the dog walks with the hock dropped toward the ground. The common fibular division supplies the cranial crural muscles that flex the hock and extend the digits, so a complete sciatic lesion also produces a dropped hock with knuckling of the paw. The combination of a dropped hock and a dragged paw is one of the most recognizable pelvic limb nerve deficits in dogs.

Tumors of the Plexus

Malignant peripheral nerve sheath tumors can arise from a plexus nerve or nerve root and cause significant morbidity. A retrospective study of thirty dogs treated for a naive malignant peripheral nerve sheath tumor of the brachial or lumbosacral plexus found that surgery was amputation alone in 17 cases and amputation or hemipelvectomy with laminectomy in 13 cases. Four dogs had an intraoperative complication, 11 had postoperative complications, and 13 had recurrence. The median disease-free interval was 511 days, and the median disease-specific overall survival time was 570 days, with 1-year and 2-year survival rates of 82% and 22% [13]. Those numbers describe a specific surgical population and should not be generalized to every dog with a nerve sheath tumor.

Common Mistakes

The most common mistake is assuming that a dropped hock means a hock problem. A dropped hock is a nerve sign until proven otherwise. Another common mistake is attributing pelvic limb weakness to hip dysplasia when the history and neurologic examination point to a nerve lesion. A third is forgetting that the lumbosacral plexus sits close to the lumbosacral spine, so a dog with a lumbosacral disc or vertebral lesion can present with nerve signs that look like a peripheral nerve problem. The reverse is also true. A plexus tumor can present with back pain before it presents with limb weakness.

Individual cases need a veterinarian. The pattern of deficits, the rate of progression, and the imaging findings all matter, and no article can replace an examination.

Frequently Asked Questions

What is the lumbosacral plexus?

The lumbosacral plexus is a network of ventral rami from the lower lumbar and sacral spinal nerves that gives rise to the named nerves of the pelvic limb, gluteal region, and perineum. In the dog it draws from L4 through S3.

Which spinal roots form the lumbosacral plexus in a dog?

The dog plexus forms from L4, L5, L6, L7, S1, S2, and S3. The largest contributions come from L6, L7, and S1, which build the sciatic nerve.

What are the main branches of the lumbosacral plexus?

The main branches are the femoral, obturator, sciatic, cranial gluteal, caudal gluteal, and pudendal nerves. The sciatic nerve divides into the tibial and common fibular nerves.

How does the horse lumbosacral plexus differ from the dog?

The horse plexus forms from L4 through S2, one segment shorter than the dog, and the sciatic nerve dominates the pattern. The horse also lacks a distinct lumbosacral trunk like the one that connects the lumbar and sacral plexuses in humans.

Why are the human lumbar and sacral plexuses taught separately?

In humans the lumbar plexus forms from L1 through L4 and the sacral plexus from L4 through S4, joined by the lumbosacral trunk. The separation is a human anatomical convention that reflects the distinct clinical presentations of lumbar versus sacral lesions.

What happens if the femoral nerve is injured?

Femoral nerve injury weakens the quadriceps, which extends the stifle. The dog may stand with a flexed stifle, may not extend the limb fully, and may lose the patellar reflex.

What does a dropped hock mean?

A dropped hock usually means the tibial division of the sciatic nerve is not working. Without the gastrocnemius and other caudal crural muscles, the hock cannot be extended against gravity.

Can the lumbosacral plexus be imaged?

Yes. MRI can show the normal femoral and sciatic nerves in dogs, and ultrasound is used routinely to guide nerve blocks in the lumbosacral region. MR tractography of the human lumbosacral plexus has been described in healthy volunteers.