Sacral plexus

The sacral plexus is a network of nerves formed by the union of the ventral rami of the L4, L5, S1, S2, S3, and a part of S4 spinal nerves. It is responsible for supplying nerves to the pelvis, lower limb, and parts of the genital and urinary systems. The sacral plexus gives rise to several important nerves, such as the sciatic nerve, superior and inferior gluteal nerves, and the nerve to obturator internus.

Location

The sacral plexus is located on the posterior pelvic wall in front of the piriformis muscle. It lies anterior to the sacrum and is situated deep within the pelvic cavity, near the lower lumbar vertebrae and sacral bones. The plexus runs across the pelvic region toward the greater sciatic foramen, where many of its nerves exit the pelvis to supply the lower limb.

Structure and Anatomy

Formation

The sacral plexus is formed by the ventral rami of the spinal nerves from L4, L5, S1, S2, S3, and a portion of S4. It is often referred to as being part of the lumbosacral plexus, as the ventral rami of L4 and L5 contribute to both the lumbar and sacral plexuses. The part of L4 and L5 that contribute to the sacral plexus form the lumbosacral trunk, which then merges with the sacral nerves.

Structure and Components

The sacral plexus is composed of both sensory and motor fibers, and it gives rise to several important peripheral nerves. These nerves exit the pelvis and innervate the lower limbs and certain pelvic structures.

Roots

L4-S4: The spinal nerves contributing to the sacral plexus are the ventral rami of L4, L5, S1, S2, S3, and a portion of S4. These nerves unite to form the plexus, with contributions from L4 and L5 forming the lumbosacral trunk, which combines with the sacral nerves.

Divisions

The ventral rami of the spinal nerves forming the sacral plexus are organized into anterior and posterior divisions, from which different nerves arise.

• Anterior division: Primarily gives rise to nerves that innervate flexor muscles and supply sensory fibers to the posterior part of the lower limb.
• Posterior division: Mainly gives rise to nerves that innervate extensor muscles and supply sensory fibers to the anterior part of the lower limb.

Major Nerves of the Sacral Plexus

The sacral plexus gives rise to several important nerves, some of which exit the pelvis through the greater sciatic foramen to reach the lower limb:

• Sciatic Nerve: The largest and most important nerve from the sacral plexus. It is made up of fibers from L4, L5, S1, S2, and S3 and exits the pelvis via the greater sciatic foramen below the piriformis muscle. It then travels down the posterior thigh.
• Superior Gluteal Nerve: Composed of fibers from L4, L5, and S1, this nerve exits the pelvis through the greater sciatic foramen above the piriformis muscle.
• Inferior Gluteal Nerve: Made up of fibers from L5, S1, and S2, this nerve exits the pelvis through the greater sciatic foramen below the piriformis muscle.
• Pudendal Nerve: Originates from S2, S3, and S4. It exits the pelvis through the greater sciatic foramen, loops around the ischial spine, and re-enters through the lesser sciatic foramen to supply the perineal region.
• Nerve to Obturator Internus: Arises from L5, S1, and S2, exiting the pelvis through the greater sciatic foramen and re-entering through the lesser sciatic foramen to innervate the obturator internus muscle.
• Nerve to Quadratus Femoris: Originates from L4, L5, and S1, supplying the quadratus femoris and inferior gemellus muscles.
• Posterior Femoral Cutaneous Nerve: Originating from S1, S2, and S3, this nerve exits the pelvis through the greater sciatic foramen and provides sensory innervation to the skin of the posterior thigh and leg.

Relations to Surrounding Structures

• Piriformis muscle: The sacral plexus lies anterior to this muscle, and many of the nerves exit the pelvis either above or below it through the greater sciatic foramen.
• Greater sciatic foramen: Most of the nerves from the sacral plexus exit the pelvis through this foramen to reach the lower limb.
• Pelvic Viscera: The plexus lies near important structures such as the rectum, uterus (in females), and bladder, but does not directly innervate these organs, except for certain branches like the pudendal nerve, which supplies structures in the perineal region.

Anatomical Variations

The arrangement of the sacral plexus can vary among individuals, particularly in how the sciatic nerve forms and exits the pelvis. In some cases, the nerve may pass through, rather than below, the piriformis muscle (a condition known as piriformis syndrome).

Vascular Supply

The blood supply to the sacral plexus and surrounding nerves is provided by branches of the internal iliac artery, particularly the superior and inferior gluteal arteries, and the internal pudendal artery. These vessels accompany the nerves as they exit the pelvis and supply the lower limb and pelvic structures.

Function

The sacral plexus has a critical role in providing both motor and sensory innervation to the lower limbs, pelvic region, and parts of the perineum. Its complex network of nerves serves various functions related to movement, sensation, and autonomic control in these regions.

Motor Functions

The sacral plexus supplies motor innervation to a variety of muscles in the lower limbs, buttocks, and pelvis. These muscles are essential for movement and stability of the hips, thighs, and legs.

Hip Movement

The sacral plexus controls many muscles involved in hip movement, particularly those responsible for extension, abduction, adduction, and rotation of the thigh.

• Hip Extension: The sciatic nerve, via its tibial component, innervates the hamstring muscles (semitendinosus, semimembranosus, and biceps femoris), which are responsible for extending the hip joint during activities like walking or running.
• Hip Abduction: The superior gluteal nerve innervates the gluteus medius, gluteus minimus, and tensor fasciae latae muscles, which abduct the hip and stabilize the pelvis during walking.
• Hip Adduction: Branches from the sacral plexus, including the nerve to obturator internus and nerve to quadratus femoris, innervate muscles like the obturator internus and quadratus femoris, which assist in rotating and stabilizing the hip joint.

Knee Movement

• Knee Flexion: The sciatic nerve, particularly its tibial division, supplies the hamstring muscles responsible for knee flexion. These muscles contract to bend the knee during activities like sitting, climbing stairs, or jumping.
• Knee Extension: Although mainly supplied by the lumbar plexus, the sacral plexus indirectly supports knee extension through its stabilization of the lower limb muscles that work with the quadriceps.

Ankle and Foot Movement

The sacral plexus also plays a significant role in controlling movement in the ankle and foot.

• Plantarflexion: The tibial nerve, a branch of the sciatic nerve, supplies the gastrocnemius and soleus muscles, which are responsible for plantarflexion (pointing the foot downward). This is essential for actions such as walking and standing on the toes.
• Dorsiflexion: The common fibular nerve, another branch of the sciatic nerve, innervates the muscles involved in dorsiflexion (lifting the foot upward), such as the tibialis anterior.
• Foot Eversion/Inversion: The superficial fibular nerve and deep fibular nerve, branches of the common fibular nerve, control muscles that evert (turn outward) and invert (turn inward) the foot, important for balance and stability during walking and running.

Pelvic Floor and Perineal Muscles

The pudendal nerve, a branch of the sacral plexus, innervates the muscles of the pelvic floor and perineum, which are critical for maintaining continence and supporting pelvic organs. It controls the external anal sphincter and external urethral sphincter, as well as muscles like the ischiocavernosus and bulbospongiosus, involved in reproductive and urinary functions.

Sensory Functions

The sacral plexus is also responsible for transmitting sensory information from the skin, joints, and muscles of the lower body to the central nervous system.

Sensory Innervation of the Lower Limb

• Posterior Thigh and Leg: The posterior femoral cutaneous nerve, a branch of the sacral plexus, supplies sensory innervation to the skin of the posterior thigh, buttocks, and parts of the perineum. This nerve is essential for sensing pressure, temperature, and pain in these areas.
• Leg and Foot: The sciatic nerve, via its tibial and common fibular branches, provides sensory innervation to large portions of the leg and foot. The tibial nerve supplies sensation to the sole of the foot, while the common fibular nerve provides sensation to the lateral leg and dorsum (top) of the foot.

Pelvic Sensation

The pudendal nerve is responsible for sensory innervation of the perineum, external genitalia, and anus. This includes:

Perineal sensation: The pudendal nerve transmits sensory information from the skin around the anus, genitalia, and perineum, which is essential for sexual function, bladder control, and defecation.

Reflex Functions

The sacral plexus also plays a role in certain reflex arcs, where sensory stimuli from the lower limbs trigger involuntary motor responses.

Patellar Reflex (Knee Jerk)

While primarily involving the lumbar plexus, the stability and functioning of the knee reflex are indirectly influenced by the sacral plexus, which helps stabilize the lower limbs.

Plantar Reflex

The sacral plexus is involved in the plantar reflex (or Babinski reflex), which tests for normal response in the foot. The tibial nerve, part of the sacral plexus, is responsible for the motor component of this reflex.

Autonomic Functions

While primarily somatic in nature, the sacral plexus has autonomic components that contribute to visceral control within the pelvis.

Parasympathetic Contributions

The sacral plexus contains parasympathetic fibers from S2-S4 spinal levels that help control involuntary functions in the pelvic organs, such as bladder contraction and bowel movements. These fibers contribute to the pelvic splanchnic nerves and influence smooth muscle contraction in the bladder and rectum.

Pelvic Organ Innervation

The pudendal nerve and other branches from the sacral plexus play a key role in autonomic control over the pelvic organs, including the bladder, rectum, and reproductive organs. These nerves help regulate processes like urination, defecation, and sexual function.

Clinical Significance

The sacral plexus is crucial for both motor and sensory functions of the lower limbs, pelvis, and perineum. Damage or injury to this plexus can result in a variety of clinical issues, depending on the location and extent of the damage.

Common Causes of Injury

• Trauma: Pelvic fractures or direct trauma to the sacrum or pelvis can injure the sacral plexus, leading to loss of function in the lower limbs or pelvic organs.
• Surgical Complications: Surgeries involving the pelvis or spine, such as hip replacement or pelvic tumor removal, can inadvertently damage the sacral plexus.
• Compression: Conditions like piriformis syndrome, tumors, or hematomas can compress the plexus, resulting in pain, weakness, or sensory deficits in the lower limbs.

Symptoms of Sacral Plexus Damage

• Motor deficits: Weakness or paralysis in the muscles of the hip, thigh, leg, and foot. This can affect activities like walking, running, or climbing stairs.
• Sensory loss: Numbness, tingling, or loss of sensation in the lower limbs, perineum, or genital region.
• Bladder and bowel dysfunction: Injury to the autonomic fibers of the sacral plexus can lead to incontinence or difficulty with bladder and bowel control.

Clinical Conditions

• Sciatica: Compression or irritation of the sciatic nerve (a major branch of the sacral plexus) can lead to pain radiating down the leg, known as sciatica.
• Cauda equina syndrome: Compression of the lower spinal nerves, including those of the sacral plexus, can result in a medical emergency with symptoms like severe lower back pain, leg weakness, and loss of bladder or bowel control.