The sacral plexus is known as the plexus of the lower limb. It is comprised of lumbosacral outflow from nerve roots L4 to S4. The nerve fibers from the anterior and posterior division of L4 and L5 cross the arcuate line anteroinferiorly to meet similar fibers from S1 – S4 that also travel anteroinferiorly, across the anterior surface of piriformis. Branches of the triangular shaped plexus pass through the greater sciatic foramen and continue caudally to supply the muscles of the gluteal region along with the entire lower limb. This article will evaluate the branches of the sacral plexus and their relationship to other anatomical structures.
Branches & Relations
The superior and inferior gluteal nerves are the first two sets to branch from the posterior division of the rami. The superior gluteal nerve (L4, L5, and S1) has the responsibility of innervating tensor fasciae latae, gluteus medius, and gluteus minimus. It gains access to the gluteal region by coursing over the superior border of piriformis through the greater sciatic foramen. Afterwards, it travels under the cover of gluteus medius laterally as it innervates the aforementioned muscles. The inferior gluteal nerve (L5, S1, and S2) takes a similar route through the greater sciatic foramen, but inferior to piriformis. It travels superficial to the sciatic nerve and recurs to innervate gluteus maximus.
The third branch to be formed purely from posterior rami is the nerve to piriformis (S1 and S2). These nerve fibers have a short course posteriorly to innervate the muscle. The perforating cutaneous nerve is also formed of purely posterior rami fibers of roots S2 and S3. It provides cutaneous supply to a small area of skin on the inferior medial part of the buttock after passing through the greater sciatic foramen (superior to the gemelli and obturator internus) and piercing the sacrotuberous ligament and gluteus maximus.
Pelvic splanchnic nerves of roots S2, S3 and S4 provide both parasympathetic and motor innervation to muscles of the pelvic cavity. The parasympathetic outflow joins the inferior hypogastric (pelvic) plexus. Its motor component supplies the bladder and the distal part of the large intestines (from the left colic flexure, onwards). It also causes clitoral (or penile) erection.
As the largest branch of the sacral plexus, the sciatic nerve is formed from both anterior and posterior rami from nerve roots L4 to S3. The nerve can be functionally divided into a common fibular (peroneal) component that is formed by the posterior rami of L4, L5, S1 and S2; and a tibial component that is formed by anterior rami of L4 – S3. The nerve exits the pelvic cavity by way of the greater sciatic foramen. In the gluteal region, it travels deep to gluteus maximus and the inferior gluteal artery. It courses over the posterior surface of the gemelli, piriformis, quadratus femoris and the ischial fibers of adductor femoris. The sciatic nerve begins branching at about the midpoint between the ischial tuberosity and the greater trochanter to supply the hamstring muscles and the ischial fibers of adductor magnus. At the apex of the popliteal fossa, the nerve typically divides into its common peroneal and tibial components. These two nerves are responsible for innervating the muscles and joints of the leg and foot. It is noteworthy that the sciatic nerve receives a vasa nervorum (small artery providing arterial blood to peripheral nerves) from the inferior gluteal artery.
The nerve to quadratus femoris also has the responsibility of supplying the inferior gemellus. It takes anterior rami fibers from L4, L5 and S1 and travels inferiorly, under cover of the sciatic nerve, towards quadratus femoris. On its way, the nerve also provides a branch to the hip joint.
Similarly, the nerve to obturator internus also innervates the superior gemellus. Its nerve roots come from the anterior division of L5, S1 and S2. To gain access to obturator internus, the nerve travels inferior to piriformis and courses around the ischial spine.
The posterior femoral cutaneous nerve is the only other branch of the sacral plexus that receives fibers from both the anterior and posterior rami. Its fibers originate from the S1, S2 and S3 nerve roots. The nerve has a ventrodorsal and craniocaudal range of cutaneous and fascial supply between the anterior and posterior axillary lines from the buttock to the middle of the calf muscles. The nerve enters the posterior thigh by way of the greater sciatic notch, inferior to piriformis, deep to the inferior gluteal artery and superior to the gemelli, obturator internus and the sciatic nerve. The posterior femoral cutaneous nerve also gives rise to gluteal branches that give cutaneous supply to the contour of the buttock and a long peroneal branch that innervates two thirds of the posterolateral aspect of the scrotum (or labium majus).
The anterior rami of S2, S3 and S4 merge on the anterior surface of piriformis (posterior to the inferior gluteal artery) to form the pudendal nerve. The pudendal nerve then travels inferiorly around the sacrospinous ligament to enter the pudendal canal with the pudendal vessels. It gives rise to:
- an inferior rectal nerve that innervates the external anal sphincter, anal canal and perianal skin,
- a terminal perineal nerve that gives cutaneous supply to part of the posterior scrotum (vulva), mucus membrane of the urethra and vagina, and motor innervation to the muscles of the perineum,
- and a terminal dorsal nerve of the clitoris (penis) that gives cutaneous innervation to this region.
The last two branches of the sacral plexus are the nerve to levator ani and the ischiococcygeus muscle, and the perineal branch of S4. The former provides motor innervation to the pelvic part of the muscles for which they are named. The latter gives muscular innervation to pubourethralis, puborectalis and the pubovaginalis portions of levator ani. It also gives cutaneous supply to the region above the ischioanal fossa.
Although there are only seven major branches of the sacral plexus to remember, keeping the nerve roots in order can be tricky. Here are a few tips that can assist in keeping things straight:
- All the nerves of the lumbar plexus arise from three nerve roots EXCEPT the sciatic nerve, which has five nerve roots.
- The superior gluteal nerve and the nerve to quadratus femoris/inferior gemellus share the same nerve roots (L4, L5 and S1).
- Similarly, the inferior gluteal nerve and the nerve to obturator internus/superior gemellus also share the same nerve roots (L5, S1 and S2).
- There are five P’s:
- Nerve to Piriformis
- Posterior femoral cutaneous nerve
- Perforating cutaneous nerve
- Pudendal nerve
- Perineal branch of the 4th sacral nerve
5. Finally, the major branches can be remembered as SIPPS:
- S – Superior gluteal nerve (nerve to quadratus femoris/inferior gemellus may be associated here since they share nerve roots)
- I – Inferior gluteal nerve (nerve to obturator internus/superior gemellus may be associated here since they share nerve roots)
- P – Posterior cutaneous nerve of thigh
- P – Pudendal nerve
- S – Sciatic nerve