The sacral plexus is a network of nerves formed by the lumbosacral trunk (L4, L5) and sacral spinal nerves (S1 - S4).
The plexus gives off numerous branches including the anterior, posterior branches and one terminal branch). These branches provide the motor and sensory innervation for the posterior thigh, most of the lower leg, the entire foot, and part of the pelvis.
This article will discuss the anatomy and function of the sacral plexus as well as some of its important clinical relations.
|Origin||L4, L5, S1, S2, S3, S4|
Anterior branches: Nerve to quadratus femoris, nerve to obturator internus, pudendal nerve, nerves to levator ani and external anal sphincter
Posterior branches: Nerve to piriformis , superior gluteal nerve, inferior gluteal nerve, posterior femoral cutaneous nerve, perforating cutaneous nerve, pelvic splanchnic nerves
Terminal branch: Sciatic nerve (divides into tibial and common fibular/peroneal nerves)
Motor: tensor fasciae latae, gluteus maximus, gluteus medius, gluteus minimus, hamstings, adductor magnus, sphincters, levator ani, muscles of the leg and foot
Sensory: skin over the medial surface of the buttock to the middle of the calf muscles, skin of the external genitalia, leg and foot
Organs: urinary bladder, large intestine, external genitalia
|Mnemonic for the major branches (SIIPS)||Superior gluteal nerve, inferior gluteal nerve, posterior cutaneous nerve of thigh, pudendal nerve, sciatic nerve (SIPPS)|
The sacral plexus is a nerve network comprised of the lumbosacral trunk and sacral spinal nerves. The lumbosacral trunk is formed by the lumbar spinal nerves L4 and L5. The trunk then descends into the pelvis to meet the roots of sacral spinal nerves S1 - S4, as they emerge from the spinal cord. Note that S4 root only partially contributes to the formation of the sacral plexus.
Branches and supply
The sacral plexus provides a large number of branches which can be divided into posterior, anterior and terminal branches. The posterior branches are derived from posterior divisions of the anterior rami of spinal nerves, while the anterior branches are derived from the anterior divisions. The largest and only terminal branch is the sciatic nerve that provides two divisions: tibial nerve and common fibular (peroneal) nerve.
Superior gluteal nerve
The superior gluteal nerve is formed from the posterior divisions of the anterior rami of L4, L5, and S1 spinal nerves. It gains access to the gluteal region by coursing over the superior border of piriformis muscle through the greater sciatic foramen. Afterwards, the superior gluteal nerve travels laterally underneath the gluteus medius and innervates the tensor fasciae latae, gluteus medius and gluteus minimus.
Inferior gluteal nerve
The inferior gluteal nerve receives contributions from the posterior divisions of the anterior rami of the L5, S1, and S2 spinal nerves. It takes a similar route through the greater sciatic foramen, but inferior to piriformis muscle. It travels superficial to the sciatic nerve and innervates the gluteus maximus muscle.
Perforating cutaneous nerve
The perforating cutaneous nerve is also formed of purely posterior divisions, this time originating from the fibers of the anterior rami of spinal nerves S2 and S3. It passes through the greater sciatic foramen, superior to the gemelli and obturator internus, piercing the sacrotuberous ligament and gluteus maximus. This nerve provides cutaneous supply to a small area of skin on the inferior medial part of the buttock.
Pelvic splanchnic nerves
The pelvic splanchnic nerves originate from the anterior rami of spinal nerves S2, S3 and S4. They provide both parasympathetic and motor innervation to muscles of the pelvic cavity and pelvic floor. 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 participates in clitoral (or penile) erection.
Nerve to piriformis
The nerve to piriformis usually arises from the dorsal branches of the
anterior rami of spinal nerves S1 and S2 (sometimes only S2) and
enters the anterior surface of piriformis muscle and supplies it.
Nerve to quadratus femoris
The nerve to quadratus femoris originates from the anterior divisions of the anterior rami of the spinal nerves L4, L5 and S1. The nerve travels inferiorly underneath the sciatic nerve and passes towards the quadratus femoris. Along its course, it supplies the inferior gemellus muscle and provides an articular branch to the hip joint.
Nerve to obturator internus
The nerve to obturator internus arises from the anterior divisions of the anterior rami of spinal nerves L5, S1 and S2. The nerve travels inferior to piriformis leabes the pelvis through the greater sciatic foramen, where it gives off a branch to innervate the superior gemellus muscle. It then courses around the ischial spine and reenters the pelvis through the lesser sciatic foramen and pierces obturator internus.
Posterior cutaneous nerve of the thigh
The posterior cutaneous nerve of the thigh (also known as the posterior femoral cutaneous nerve) is the only other branch of the sacral plexus that receives fibers from both anterior and posterior divisions of anterior rami.
Its fibers originate from:
- Posterior divisions of the anterior rami of spinal nerves S1 and S2.
- Anterior divisions of the anterior rami of spinal nerves S2 and S3.
The nerve enters the posterior thigh by way of the greater sciatic notch, passing inferior to piriformis, deep to the inferior gluteal artery and superior to the gemelli, obturator internus and the sciatic nerve.
The nerve has an anteroposterior and superoinferior range of cutaneous and fascial supply between the anterior and posterior axillary lines from the buttock to the middle of the calf muscles.
The posterior femoral cutaneous nerve also gives rise to gluteal branches that provide cutaneous supply to the contour of the buttock and perineal branches that innervate skin of the lateral perineum (two thirds of the posterolateral aspect of the scrotum in males, or labium majus in females) and adjacent region of the upper medial thigh.
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The anterior divisions of the anterior rami of spinal nerve 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 travels inferiorly around the sacrospinous ligament to enter the pudendal canal with the pudendal vessels. The pudendal nerve gives rise to:
- The inferior rectal nerve that innervates the external anal sphincter, anal canal and perianal skin.
- The perineal nerve that provides 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.
- The dorsal nerve of the clitoris/penis that provides cutaneous innervation to this region.
Nerve to levator ani and ischiococcygeus muscles
The last two branches of the sacral plexus are the nerve to levator ani and ischiococcygeus muscle. It provides motor innervation to the pelvic part of the muscles for which they are named.
The sciatic nerve is the terminal and largest branch of the sacral plexus formed from both anterior and posterior divisions of the anterior rami of spinal nerves L4 to 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 the common peroneal and tibial nerves. These two nerves are responsible for innervating the muscles and joints of the leg and foot. It is noteworthy that the sciatic nerve receives vasa nervorum (small artery providing arterial blood to peripheral nerves) from the inferior gluteal artery.
To master the knowledge of the sacral plexus, take a look at the videos, illustrations and quizzes included in the following study unit:
Chances are you won't need to remember every single branch of the sacral plexus. But just in case you do, we have you covered! The phrase ' Some Irish People Came Traveling Quickly On Perfect Paper Planes Laughing Proudly' will help you remember the following list of structures:
Superior gluteal nerve
Inferior gluteal nerve
Nerve to Piriformis muscle
Common fibular division of sciatic nerve
Tibial division of sciatic nerve
Nerve to Quadratus femoris
Nerve to Obturator internus
Posterior cutaneous nerve of thigh
Perforating cutaneous nerve
Nerve to Levator ani
Perineal branch of S4
Now onto the main branches of the plexus. Although there are only seven, 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 sacral plexus arise from three nerve roots EXCEPT the sciatic nerve, which has five nerve roots.
- The superior gluteal nerveand 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 six P’s:
- Nerve to Piriformis
- Posterior cutaneous nerve of thigh
- Perforating cutaneous nerve
- Pudendal nerve
- Perineal branch of the 4th sacral nerve
- Pelvic sphlancnic nerves
- 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
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