Sacral Nerves

The sacral nerves are five pairs of spinal nerves designated S1 through S5. They arise from sacral segments of the spinal cord and provide neural pathways to extensive regions of the pelvis and lower limb. Their fibers contribute to major peripheral nerves supplying the gluteal region, posterior thigh, leg, foot, pelvic organs, and perineum.

The anterior rami of the upper sacral nerves participate prominently in the sacral plexus, while lower sacral nerves contribute to pathways supplying the pelvic floor and perineum. Sacral spinal segments also have major autonomic importance because S2-S4 contain the spinal origin of pelvic parasympathetic outflow.

Sacral nerve anatomy is therefore important not only for lower limb movement and sensation but also for continence, pelvic organ function, sexual function, and sensory innervation of the perineum.

Number of Sacral Nerves

There are five pairs of sacral spinal nerves:

• S1
• S2
• S3
• S4
• S5

Unlike the lumbar nerves, which leave through intervertebral foramina between separate vertebrae, the sacral nerves are associated with foramina in the fused sacrum.

Origin of the Sacral Nerve Roots

The sacral spinal cord segments are located considerably superior to the corresponding sacral vertebral levels because the adult spinal cord is shorter than the vertebral canal.

Sacral nerve roots therefore descend for a substantial distance within the vertebral canal before reaching their respective exit points. These descending roots form part of the cauda equina.

Sacral Nerves and the Cauda Equina

The cauda equina consists of lumbar, sacral, and coccygeal nerve roots descending below the conus medullaris. The sacral roots travel inferiorly within the lumbar cistern before continuing into the sacral canal.

This arrangement results from differential growth of the spinal cord and vertebral column during development.

Formation of a Sacral Spinal Nerve

Each sacral spinal nerve is formed by the union of an anterior root and a posterior root.

The anterior root contains predominantly efferent fibers, while the posterior root carries afferent sensory fibers and is associated with a spinal ganglion containing primary sensory neuron cell bodies.

Anterior Roots

The anterior roots of sacral spinal nerves contain somatic motor fibers destined for skeletal muscles. At S2-S4, they also provide pathways for preganglionic parasympathetic fibers arising from neurons within the sacral spinal cord.

These different functional components are redistributed after the roots join and enter spinal nerve and plexus pathways.

Posterior Roots

The posterior roots carry sensory information toward the spinal cord from the lower limb, pelvis, perineum, and visceral structures.

Their fibers include somatic afferents from skin, muscles, and joints as well as visceral afferent fibers associated with pelvic organs.

Spinal Ganglia

The posterior roots are associated with spinal ganglia, also called dorsal root ganglia. These contain the cell bodies of pseudounipolar sensory neurons.

The peripheral processes of these neurons extend toward sensory receptors, while their central processes enter the spinal cord.

Division into Anterior and Posterior Rami

After formation, each sacral spinal nerve divides into an anterior ramus and a posterior ramus. Both rami contain mixed fibers.

The anterior rami contribute to plexuses and supply structures of the pelvis and lower limb. The posterior rami primarily supply posterior sacral structures and overlying skin.

Anterior Sacral Foramina

The anterior rami of the first four sacral spinal nerves emerge through the anterior sacral foramina on the pelvic surface of the sacrum.

These rami then participate in plexus formation and other neural pathways within the pelvis.

Posterior Sacral Foramina

The posterior rami of the upper sacral nerves emerge through the posterior sacral foramina. They supply intrinsic posterior muscles, joints, ligaments, and portions of the skin overlying the sacral and gluteal regions.

S5 Spinal Nerve

The S5 spinal nerve is the lowest sacral spinal nerve. Its branches emerge near the inferior end of the sacral canal and sacral hiatus rather than through a complete pair of sacral foramina comparable to the upper sacral levels.

S5 contributes particularly to sensory and motor pathways of the lowest pelvic and perianal regions and participates in the coccygeal plexus.

Sacral Plexus

The sacral plexus is formed primarily by the lumbosacral trunk and the anterior rami of S1-S4. The lumbosacral trunk contains fibers from L4 and L5, creating continuity between the lumbar and sacral plexuses.

The plexus lies on the posterolateral pelvic wall, anterior to the piriformis muscle. Its branches provide much of the innervation of the gluteal region, posterior thigh, leg, foot, pelvis, and perineum.

Root Contributions to the Sacral Plexus

Component	Contribution
Lumbosacral trunk	L4-L5
S1	Major contribution to upper sacral plexus
S2	Contribution to lower limb, pelvic, and perineal nerves
S3	Important pelvic and perineal contribution
S4	Contribution to sacral and coccygeal plexus pathways

Major Branches of the Sacral Plexus

Nerve	Typical Root Values	General Distribution
Superior gluteal nerve	L4-S1	Gluteus medius, gluteus minimus, tensor fasciae latae
Inferior gluteal nerve	L5-S2	Gluteus maximus
Sciatic nerve	L4-S3	Posterior thigh and most motor and sensory pathways to leg and foot through its branches
Posterior femoral cutaneous nerve	S1-S3	Skin of posterior thigh and adjacent regions
Pudendal nerve	S2-S4	Perineum and external genital region
Nerve to obturator internus	L5-S2	Obturator internus and superior gemellus
Nerve to quadratus femoris	L4-S1	Quadratus femoris and inferior gemellus

Sciatic Nerve

The sciatic nerve is the largest major peripheral nerve arising from the sacral plexus. It contains fibers from L4 through S3.

It leaves the pelvis through the greater sciatic foramen, usually inferior to piriformis, and descends through the posterior thigh. Its two major components are the tibial and common fibular divisions.

Tibial Component

The tibial component of the sciatic nerve contains fibers derived predominantly from anterior divisions of the anterior rami contributing to the sacral plexus.

It ultimately supplies the posterior compartment of the leg and most intrinsic muscles of the plantar foot through its terminal branches.

Common Fibular Component

The common fibular component contains fibers predominantly from posterior divisions of the contributing anterior rami.

Its branches ultimately supply the anterior and lateral compartments of the leg and provide sensory pathways to substantial regions of the leg and dorsum of the foot.

Superior Gluteal Nerve

The superior gluteal nerve contains fibers from L4, L5, and S1. It exits the pelvis through the greater sciatic foramen superior to piriformis.

It supplies the gluteus medius, gluteus minimus, and tensor fasciae latae, muscles that are important for stabilization of the pelvis during walking.

Inferior Gluteal Nerve

The inferior gluteal nerve contains fibers from L5, S1, and S2. It leaves the pelvis inferior to piriformis and supplies the gluteus maximus.

The gluteus maximus is a powerful extensor of the hip, particularly during activities such as rising from a seated position and climbing.

Posterior Femoral Cutaneous Nerve

The posterior femoral cutaneous nerve is formed primarily by S1-S3 fibers. It descends through the gluteal region and posterior thigh.

Its branches provide cutaneous innervation to the posterior thigh and contribute sensory fibers to adjacent inferior gluteal and perineal regions.

Pudendal Nerve

The pudendal nerve arises from the anterior rami of S2-S4. It is the major somatic nerve of the perineum.

It leaves the pelvis through the greater sciatic foramen, curves around the region of the ischial spine and sacrospinous ligament, and enters the perineum through the lesser sciatic foramen.

Branches of the Pudendal Nerve

Major branches of the pudendal nerve include:

• Inferior rectal nerves
• Perineal nerve
• Dorsal nerve of the penis or clitoris

Through these branches, the pudendal nerve provides important motor and sensory innervation to the perineum.

Inferior Rectal Nerves

The inferior rectal nerves supply the external anal sphincter and provide sensory innervation to the perianal region and lower anal canal.

Their somatic innervation is important in voluntary control of defecation.

Perineal Nerve

The perineal nerve provides motor innervation to several muscles of the superficial and deep perineal regions and sensory innervation through posterior scrotal or labial branches.

Its fibers are important for the somatic function of the pelvic outlet and external genital region.

Pelvic Splanchnic Nerves

The pelvic splanchnic nerves arise from anterior rami of S2-S4 and carry preganglionic parasympathetic fibers into pelvic autonomic plexuses.

They are distinct from the somatic branches of the sacral plexus even though they originate from the same general spinal levels.

Sacral Parasympathetic Outflow

Preganglionic parasympathetic neurons associated with S2-S4 provide the spinal component of parasympathetic innervation to pelvic viscera and portions of the distal gastrointestinal tract.

Their fibers travel through pelvic splanchnic nerves and enter autonomic plexuses before reaching target organs.

Pelvic Organ Function

Sacral parasympathetic pathways contribute to functions involving the urinary bladder, distal bowel, and erectile tissues. These pathways operate together with sympathetic and somatic nervous systems.

Consequently, lesions involving sacral spinal segments or roots can affect both somatic and visceral functions.

S1 Spinal Nerve

The S1 spinal nerve is a major contributor to the sacral plexus. Its fibers enter numerous peripheral nerves supplying the gluteal region, posterior thigh, leg, and foot.

S1 has particularly important contributions to plantar flexion, hip extension and abduction pathways, and sensory innervation of portions of the lateral and plantar foot.

S2 Spinal Nerve

The S2 spinal nerve contributes to both lower limb and pelvic pathways. It supplies fibers to the sciatic nerve, posterior femoral cutaneous nerve, pudendal nerve, and several muscular branches of the sacral plexus.

S2 also contributes to the pelvic splanchnic nerves and therefore participates in parasympathetic innervation of pelvic viscera.

S3 Spinal Nerve

The S3 spinal nerve contributes less extensively to distal lower limb motor pathways than S1 and S2 but is highly important in pelvic and perineal innervation.

Its fibers contribute to the pudendal nerve, pelvic splanchnic nerves, posterior femoral cutaneous nerve, and other pelvic pathways.

S4 Spinal Nerve

The S4 spinal nerve contributes to the lower portion of the sacral plexus and to the coccygeal plexus. It also contributes fibers to the pudendal and pelvic splanchnic nerves.

S4 therefore participates prominently in pelvic floor, perineal, and visceral innervation.

S5 Spinal Nerve

The S5 spinal nerve contributes to the coccygeal plexus and supplies structures near the inferior sacrum, coccyx, and perianal region.

Although relatively small, its sensory distribution is clinically important when evaluating lower sacral neurological function.

Coccygeal Plexus

The coccygeal plexus is a small neural network formed by contributions from S4, S5, and the coccygeal nerve.

It supplies portions of the pelvic floor and skin over and around the coccygeal and perianal regions.

Sacral Dermatomes

A dermatome represents an area of skin supplied predominantly by sensory fibers associated with a particular spinal nerve root. Sacral dermatomes extend along the posterior lower limb and into the foot, perineum, and perianal region.

There is substantial overlap between adjacent dermatomes, and exact boundaries vary among individuals.

Approximate Sacral Dermatome Landmarks

Root	Common Clinical Landmark
S1	Lateral foot and little-toe region
S2	Posterior thigh and upper posterior leg
S3	Buttock and medial perineal region
S4	Perianal region
S5	Skin immediately surrounding the anus

Sacral Myotomes

A myotome is a group of muscles or movement receiving a predominant contribution from a particular spinal cord segment or nerve root.

Sacral roots participate extensively in movements of the hip, knee, ankle, toes, and pelvic floor.

Common Sacral Myotome Associations

Root	Common Functional Association
S1	Ankle plantar flexion and foot eversion, with contributions to hip movements
S2	Knee flexion and contributions to hip extension
S2-S4	Pelvic floor and external sphincter pathways

Ankle Plantar Flexion

Ankle plantar flexion receives a strong contribution from S1 and S2 through the tibial nerve. Muscles such as gastrocnemius and soleus are major plantar flexors.

Repeated heel raising can be used clinically as one method of assessing plantar-flexion strength.

Knee Flexion

The hamstring muscles responsible for much of knee flexion receive substantial innervation from lower lumbar and upper sacral roots through the sciatic nerve.

S1 and S2 are particularly important contributors, although the movement is multisegmental.

Achilles Reflex

The Achilles tendon reflex, also called the ankle jerk, assesses a reflex arc involving the tibial nerve and predominantly the S1 spinal segment, with contribution from S2.

Contraction of gastrocnemius and soleus produces plantar flexion of the ankle.

Sacral Root Reflexes

Lower sacral segments participate in several reflex pathways associated with pelvic and perineal function. Clinical examination of these pathways can provide information about the integrity of S2-S4 and related peripheral nerves.

Such testing may be particularly relevant when cauda equina or spinal cord pathology is suspected.

Sacral Nerves and Pelvic Floor Muscles

Sacral spinal nerve fibers provide important innervation to the pelvic floor. Direct branches from sacral plexus levels and pudendal pathways contribute to muscles involved in supporting pelvic organs and controlling pelvic outlets.

The exact root contributions and nerve pathways vary among individual pelvic floor muscles.

Sacral Nerves and External Sphincters

Somatic fibers derived primarily from S2-S4 contribute to voluntary control of the external urethral and external anal sphincters through pudendal and related pathways.

This somatic control works together with autonomic pathways controlling smooth muscle in pelvic organs.

Sacral Nerves and Bladder Function

Bladder function requires coordination among parasympathetic, sympathetic, and somatic pathways. Sacral parasympathetic fibers from S2-S4 participate prominently in contraction of the detrusor muscle during voiding.

Somatic pathways associated with the pudendal nerve contribute to voluntary control of the external urethral sphincter.

Sacral Nerves and Bowel Function

Sacral parasympathetic pathways contribute to motor and secretory functions in portions of the distal gastrointestinal tract. Somatic sacral pathways provide voluntary control over the external anal sphincter.

Damage to lower sacral neurological pathways can therefore disturb continence and defecatory function.

Sacral Nerves and Sexual Function

Sacral nerve pathways contribute to both autonomic and somatic aspects of sexual function. Pelvic splanchnic nerves carry parasympathetic fibers important in erectile responses, while pudendal pathways provide somatic sensory and motor innervation of the external genital region.

These functions depend on coordinated activity with other autonomic and central neural pathways.

Sacral Radiculopathy

Sacral radiculopathy results from dysfunction or irritation of one or more sacral nerve roots. Symptoms can include pain, altered sensation, weakness, or reflex abnormalities according to the affected level.

S1 is particularly relevant clinically because it is commonly affected by lower lumbar intervertebral disc disease.

S1 Radiculopathy

S1 radiculopathy may produce sensory symptoms extending toward the posterior or lateral lower limb and lateral foot. Motor findings can include weakness of plantar flexion, depending on severity.

The Achilles reflex may also be diminished when the S1 reflex arc is significantly affected.

S2-S4 Root Dysfunction

Damage involving S2-S4 can affect perineal sensation, pelvic floor function, sphincter control, sexual function, and parasympathetic pathways to pelvic viscera.

Because these functions are clinically important, lower sacral deficits require careful neurological assessment.

L5-S1 Disc Herniation

A typical posterolateral L5-S1 intervertebral disc herniation commonly compresses or irritates the traversing S1 nerve root.

This can produce an S1-pattern radiculopathy involving characteristic sensory, motor, and reflex abnormalities.

Sciatica

Sciatica describes pain occurring along the distribution associated with the sciatic nerve or its contributing roots. It is commonly associated with lumbosacral nerve root irritation rather than direct injury to the sciatic nerve itself.

The exact distribution depends on the neural structures involved.

Sacral Plexopathy

A sacral plexopathy affects multiple neural components after the spinal roots have entered the plexus. Depending on the location of the lesion, several peripheral nerves may be affected simultaneously.

The resulting pattern can differ from both an isolated radiculopathy and an individual peripheral nerve lesion.

Sciatic Nerve Injury

Injury to the sciatic nerve can affect motor and sensory pathways derived from multiple lumbar and sacral roots. The pattern depends on the level and extent of the lesion and whether tibial, common fibular, or both components are affected.

Because the sciatic nerve contains fibers from L4-S3, its deficits cannot be attributed to a single sacral spinal nerve.

Pudendal Nerve Injury

Pudendal nerve dysfunction can affect perineal sensation and motor control of muscles supplied by its branches. Because the nerve contains fibers from S2-S4, its distribution differs from that of a single sacral dermatome.

Knowledge of its pelvic course is important for distinguishing peripheral pudendal pathology from sacral root lesions.

Root Lesions versus Peripheral Nerve Lesions

A sacral nerve root lesion can affect fibers that later enter several different peripheral nerves. A lesion of an individual peripheral nerve affects only fibers that have already been redistributed into that nerve.

For example, an S1 root lesion can affect functions mediated through multiple branches of the sacral plexus, whereas an isolated tibial nerve lesion has a more restricted peripheral distribution.

Sacral Nerves and the Piriformis

The sacral plexus lies on the anterior surface of the piriformis muscle. Several major branches leave the pelvis through the greater sciatic foramen in close relationship to piriformis.

The superior gluteal nerve usually passes superior to piriformis, while the sciatic, inferior gluteal, posterior femoral cutaneous, pudendal, and several other nerves typically leave inferior to it.

Greater Sciatic Foramen

The greater sciatic foramen is a major route through which sacral plexus branches leave the pelvis. Piriformis passes through the foramen and provides an important landmark for describing the position of the nerves.

Several nerves subsequently enter the gluteal region or travel toward the perineum.

Sacral Nerves and the Gluteal Region

Sacral plexus branches provide the principal motor innervation to the major gluteal muscles. The superior gluteal nerve supplies gluteus medius and minimus, while the inferior gluteal nerve supplies gluteus maximus.

Other sacral plexus branches pass through the gluteal region en route to the thigh or perineum.

Sacral Nerves and the Posterior Thigh

The sciatic nerve provides motor branches to most hamstring muscles, while the posterior femoral cutaneous nerve supplies much of the skin of the posterior thigh.

These nerves contain substantial contributions from upper sacral spinal nerves.

Sacral Nerves and the Leg

Sacral nerve fibers reach the leg primarily through branches derived from the sciatic nerve. The tibial nerve supplies the posterior compartment, while the common fibular nerve divides into superficial and deep fibular nerves supplying lateral and anterior compartments.

These pathways also carry sensory fibers from extensive regions of the leg and foot.

Sacral Nerves and the Foot

Sacral spinal nerve fibers contribute extensively to motor and sensory innervation of the foot. S1 and S2 fibers travel through branches of the tibial and common fibular nerves to reach intrinsic foot muscles and cutaneous territories.

The S1 dermatome is particularly associated clinically with the lateral aspect of the foot.

Sacral Nerves and Perineal Sensation

The lower sacral roots provide sensory innervation to the perineum through pudendal and other branches. The dermatomes become progressively concentrated around the perineal and perianal regions at lower sacral levels.

This arrangement contributes to the characteristic saddle region used during neurological examination.

Saddle Sensory Region

The term saddle region refers to the perineal, perianal, and adjacent medial buttock or upper thigh regions that would contact a saddle.

Altered sensation in this region can indicate dysfunction of lower sacral roots and is particularly important when evaluating possible cauda equina pathology.

Cauda Equina Syndrome

Cauda equina syndrome results from severe dysfunction or compression of multiple nerve roots below the conus medullaris. Sacral root involvement is responsible for many of its most clinically significant features.

Possible findings include saddle sensory loss, bladder or bowel dysfunction, sexual dysfunction, and lower limb motor or sensory abnormalities. It is a neurological emergency requiring urgent clinical assessment.

Conus Medullaris and Sacral Function

The conus medullaris is the tapered terminal portion of the spinal cord containing lower spinal cord segments. Lesions in this region can affect sacral neural functions even though the lesion is located near the upper lumbar vertebral levels.

This distinction between spinal cord segment and vertebral level is essential when localizing neurological disease.

Sacral Nerves on MRI

MRI can demonstrate the cauda equina, lumbosacral intervertebral discs, sacral canal, neural foramina, sacral plexus region, and surrounding soft tissues depending on the imaging protocol.

It is particularly useful when evaluating structural causes of lumbosacral nerve root compression.

Sacral Nerves on CT

CT provides detailed visualization of the sacrum, sacral foramina, lumbosacral junction, and surrounding osseous structures.

It can be especially useful when trauma, fracture, or bony narrowing may affect sacral nerve pathways.

Sacral Fractures and Nerve Injury

Because sacral nerve roots pass through and around the sacrum, fractures involving the sacral canal or foramina can potentially injure neural structures.

The neurological pattern depends on the location and severity of the injury and which sacral roots are involved.

Clinical Examination of Sacral Nerve Function

Evaluation of sacral nerve function may include:

• Testing sensation along posterior lower limb and foot territories
• Testing ankle plantar flexion
• Testing selected knee and hip movements
• Assessing the Achilles reflex
• Evaluating perineal sensation when clinically indicated
• Assessing relevant sphincter function when lower sacral dysfunction is suspected
• Comparing right and left lower limb findings
• Distinguishing root patterns from peripheral nerve distributions

Sacral Nerve Anatomy Summary

Feature	Anatomy
Number	5 pairs, S1-S5
Roots	Descend within the cauda equina before reaching sacral levels
Sacral plexus	Lumbosacral trunk plus primarily S1-S4
Major peripheral nerve	Sciatic nerve, L4-S3
Major perineal nerve	Pudendal nerve, S2-S4
Parasympathetic outflow	Pelvic splanchnic nerves, S2-S4
Major S1 reflex	Achilles tendon reflex
Lower sacral functions	Perineal sensation, pelvic floor, sphincter, bladder, bowel, and sexual pathways

Clinical Importance

The sacral nerves provide essential neural connections between the spinal cord and the lower limb, pelvis, pelvic floor, and perineum. Their fibers are distributed through the sacral plexus into major nerves such as the sciatic, gluteal, posterior femoral cutaneous, and pudendal nerves.

The sacral spinal levels are also distinguished by their major role in pelvic autonomic function. Parasympathetic fibers from S2-S4 travel through the pelvic splanchnic nerves and contribute to regulation of pelvic viscera.

Knowledge of sacral nerve anatomy is particularly important when evaluating lumbosacral radiculopathy, sciatic symptoms, sacral plexus lesions, pudendal nerve disorders, sacral fractures, pelvic neurological dysfunction, and cauda equina syndrome.

Functional Importance

The sacral nerves integrate somatic motor, somatic sensory, and autonomic functions. Upper sacral fibers contribute extensively to lower limb movement and sensation, while lower sacral fibers become increasingly important for the pelvic floor, perineum, sphincters, and pelvic viscera.

The sacral plexus redistributes fibers from several spinal levels into peripheral nerves. Consequently, individual muscles and cutaneous territories usually receive contributions from more than one spinal root, while a single sacral root may contribute fibers to several different peripheral nerves.

The S2-S4 levels have a particularly important dual role. Somatic fibers participate in pudendal and pelvic floor pathways, while parasympathetic fibers leave through the pelvic splanchnic nerves. This close anatomical relationship explains why significant lower sacral neurological lesions can simultaneously affect sensation, voluntary motor control, and visceral function.

Mastery of sacral nerve anatomy provides a foundation for understanding the sacral plexus, sciatic nerve, pudendal nerve, pelvic splanchnic nerves, sacral dermatomes, sacral myotomes, pelvic floor innervation, lumbosacral radiculopathy, cauda equina, and lower limb innervation.