SACRAL PLEXUS: CHARACTERISTICS, PATH, FUNCTION, DISORDERS - ANATOMY AND PHYSIOLOGY - 2024

The sacral plexus is a complex set of spinal nerves distributed and interconnected in the form of a network, responsible for innervating the lower limbs, the buttocks and the pelvis. The sacral plexus comes from the Latin plexus sacralis.

It can be studied dividing it into two parts. A first part is made up of the nerves that run towards the lower limbs and towards the pelvis, called the sacral plexus itself (lumbosacral trunk L4, L5) and sacral nerves (posterior branches S1, S2 and S3).

Graphic representation of the Sacral Plexus. Source: Henry Vandyke Carter. Edited image.

The second part is called the pudendal plexus and is formed by the anterior branches of the nerves S2-S3 and S4) that innervate the perineum, external sexual organs and visceral organs of the pelvis. The pudendal plexus is also closely related to the hypogastric plexus.

The sacral plexus and pudendal due to their intimate relationship are studied as one and to study it they are distributed as follows. It is divided into 2 collateral branches (anterior and posterior) and a terminal branch.

Each collateral branch is divided in turn by several nerves. The anterior collateral branch is made up of 5 nerves, these are: internal obturator, rectal or inferior hemorrhoidal, levator ani and coccygeal muscles, pelvic splanchnic and pudendal.

Whereas, the posterior collateral branch comprises 6 nerves. These are: perforating cutaneous nerve, piriformis, quadratus femoris, gluteus superior, gluteus inferior, and posterior cutaneous thigh.

Finally, the terminal branch is made up of the greater sciatic or sciatic nerve or better known as the "sciatic nerve."

These nerves fulfill several functions, some are responsible for the sensory part of the muscles (tactile), others for the motor part (movement).

Likewise, there are those that have a vasomotor function, that is, they act on the blood vessels (vasodilator and vasoconstrictor function) and finally, others have a proprioception function (they indicate the position of the body).

characteristics

The shape of the sacral plexus can be compared to a triangle, as it contains a vertex and a base. The thickness of the sacral nerves is wider towards the vertex and they become thinner as they approach the base.

The sacral plexus owes its name to the fact that it is formed by the S1, S2, S3 nerves corresponding to the sacral region of the body, together with the L4 and L5 nerves (lumbosacral).

Location and route

Before reaching the sacral plexus is the lumbosacral trunk, this passes exactly in front of the sacroiliac joint and the fin of the sacrum, to later converge in the ischial notch or also called the greater sciatic notch, where it fuses with the first sacral nerve (S1), for its anterior part.

There the sacral plexus is born, then it continues its journey, also fusing with the sacral nerve 2 and 3 (S2 and S3) and relating to the anterior branch of the sacral nerve 4 (S4), corresponding to the pudendal plexus.

These nerves then exit through the anterior sacral foramina. There they are distributed in front of the pyramidal muscle. It should be noted that S1, S2 and S3 are related to the pyramidal muscle. S1 passes through its upper edge obliquely, S2 passes in front and S3 passes through its lower edge, then there is a meeting point where they merge with each other.

Some of the branches (S4) remain in the pelvis to innervate the present structures (pelvic muscle, genital organs and perineum).

While the branches of S1, S2, S3 exit through the sacral foramen or greater sciatic foramen (specifically through the infra-pyramidal space), to innervate both glutes and continue descending, until they merge and form the sciatic nerve, with the exception of the superior gluteal nerve that it exits through the suprapyramidal space.

Relations

During the journey, the sacral plexus is closely linked to the pudendal plexus, also relating to the anterior aspect of the pyramidal muscle. The sacral plexus separates the pyramidal muscle from the hypogastric vessels and from the visceral part of the interior of the pelvis.

Function and distribution

- Anterior collateral branch (includes 5 nerves)

These nerves make up what is known as the pudendal plexus. However, it should be mentioned because of these branches the medial portion of the sciatic nerve is formed. It is formed by the following nerves:

Internal obturator nerve and superior calf

It comes from branches L5, S1, S2 (L5-S2). This nerve exits through the infra-pyramidal space, leaving the greater sciatic foramen to later enter the lesser sciatic foramen, where it innervates the internal obturator muscle and also provides a branch to innervate the superior geminus muscle.

Rectal or lower hemorrhoidal nerves

They arise from the pudendal nerve in most cases, but sometimes from the sacral plexus. It innervates the lower part of the rectum and the external sphincter of the anus and its surroundings, reaching the levator ani muscle.

Nerve of the levator ani muscle and coccygeus

Born from S3 and S4. As its name indicates, it innervates the muscle it refers to. It passes under the supraspinatus ligament.

Pelvic splanchnic nerves

It comes from S2 –S4 and converges in the intrinsic ganglia of the descending and sigmoid colon, rectum and other intrapelvic viscera.

Pudendal nerve

Born of S2, S3, S4 (S2-S4). It is responsible for controlling the sphincters, since it innervates the external urethral and anal sphincter. It also innervates the skeletal muscles of the perineum. This corresponds to its motor function.

It also fulfills a sensitive function. It innervates the penis and the clitoris according to the sex of the individual and of course also a large part of the skin that covers the perineum.

- Posterior collateral branch comprises 6 nerves (sacral plexus)

From these nerves the lateral portion of the sciatic nerve is formed.

Perforating cutaneous nerve

It is born from the sacral nerves S2-S3. It has a sensory function. It innervates the skin corresponding to the lower medial part of the gluteus maximus muscle.

Piriformis nerve

It is the only one that does not come out through the suprapyramidal space, nor through the infrapyramidal space. This comes from the roots of the sacral nerves S1 and S2 (S1-S2) and remains innervating the pyramidal muscle or piriformis.

Nerve of the square femoris or nerve of the inferior geminus

It comes from L4, L5, S1 (L4-S1). It has a sensitive function. It exits through the infrapyramidal space and is directed and internalized in the inferior calf muscle. A branch to the quadratus femoris or femoral muscle also passes through its superficial part. It is responsible for innervating the skin that covers the back of the thigh and leg, as well as the skin that covers the perineum.

Superior gluteal nerve

It comes from L4, L5, S1 (L4-S1) and is the only one that exits the pelvis through the suprapyramidal space next to the superior gluteal vessels. It innervates the gluteus minimus, gluteus medius, and tensor fascia lata (upper and lateral thigh). Its function is motor.

Lower gluteal nerve

It comes from the lumbosacral trunk and the S1 and S2 branch (L5-S2). This nerve supplies the gluteus maximus and its function is purely motor.

Posterior cutaneous nerve of the thigh

Also called the posterior cutaneous femoral nerve or the lesser sciatic nerve. It comes from S1, S2, S3 (S1-S3). It is the most medial nerve of all in relation to its location. It has only a sensitive function. It passes through the lower border of the gluteus maximus, piercing the gluteal fascia and the femoral fascia from the back. It is divided into three branches of crucial importance, they are:

- Lower clunas that are responsible for the cutaneous innervation of the gluteal area.

- Scrotal pudenda that innervate the scrotum.

- The hamstrings, these pass through the thigh and reach the knee joint.

Graphic representation of the origin of the nerves of the sacral plexus. Source: Kareemn2357

- Terminal branch

Sciatic nerve or greater sciatic nerve

It comes from (L4-S3). Specifically, it exits the L4 and L5 and the posterior branch of the sacral nerve S1 and S2 and receives a contribution from the S3 branch, to form the anterior part of the sciatic major.

This nerve is the most important in the area, as well as being not only the thickest, but also the longest in the body. This nerve leaves the pelvis and reaches the popliteal or hamstring, that is, it innervates the back of the thigh and the muscles and integuments of the leg and foot.

The hamstring is subdivided into two branches, called the common peroneal nerve and the tibial nerve or better known as the external popliteal sciatic nerve and internal popliteal sciatic nerve, respectively.

Disorders

Lumbosacral plexus compression syndrome

It can occur in women who have had a difficult and long labor. The risk increases if instruments are used, such as: forceps, suction cup or spatula. It can also appear early in the puerperium, especially if the mother is a newcomer and the baby is large.

However, the incidence is approximately 0.92%, that is, it is not very frequent.

The syndrome is characterized by the sensation of paraesthesia and weakness in the lower limbs and foot drop syndrome, pain, among others. By eliminating the cause of compression, the picture progressively improves.

Lumbosacral plexus nerve injury

This affectation is frequent in people who have suffered a fracture of the sacrum or pelvis (especially in the type C pelvic ring), as well as in posterior osteo-ligament injuries. The fracture together with the hematoma causes damage to the nerves in the retroperitoneal area.

Under these circumstances the patient cannot move the hip, knee and ankle, mainly due to the involvement of the femoral and obturator nerves, as well as there may also be axonotmesis (damage to the axon and the myelin sheath) of the sciatic nerve.

Sacral plexus injuries at the intrapelvic level are the most difficult to treat and heal.

Sciatica

It is the product of some pathology that compresses the sciatic nerve in some part of its trajectory. The most common causes are: bulging or herniated disc, bone spur or narrowing of the spine. Compression causes inflammation and a lot of pain in the patient.

The pain starts from the spine (lumbar area), passes through the gluteal area and radiates to the back of the leg, calf, until it reaches the big toe.

Symptoms can vary as can the intensity of pain. It ranges from mild numbness or tingling in one of the lower limbs, to really excruciating pain.

References

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3. Sforcini C. Anatomical review of the lumbar and sacral plexuses and the nerves of the lower limbs. Arg. Anest, 2004; 62 (6): 468-475. Available at: spa.org.ar/wp
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