Sacral plexus

Posterior branches

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.

Anterior branches

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 leaves 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.

Ready to test your knowledge on the sacral plexus? Try a quiz! They're the secret to success. 

Pudendal nerve

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.

Sciatic nerve

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: