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This article is closely associated with the article on the Lumbar Plexus, as both the lumbar and sacral plexuses supply peripheral nerve innervation to the lower limb.
The sacral plexus is a convergence of spinal nerves that occurs lateral to the sacrum. It includes input from six spinal nerves (L4 [lumbar nerve 4], L5, S1 [sacral nerve 1], S2, S3 and S4) and has five peripheral nerve outputs (superior gluteal, inferior gluteal, sciatic, posterior femoral and pudendal).
Each of these peripheral nerves will be discussed individually throughout the article.
A mnemonic can be useful when trying to remember the order that the peripheral nerves of the sacral plexus occur: ‘Sassy Irish Sailors Pester Parrots’ can be useful, representing ‘Superior (gluteal), Inferior (gluteal), Sciatic, Posterior (femoral), Pudendal’.
It is also useful to know that all of the peripheral nerves that originate in the sacral plexus leave the pelvis via the greater sciatic foramen.
Diagram - The sacral plexus, with the six spinal nerves on the left and the five peripheral nerves on the right (and the bottom). It is important to note that although in this diagram the sciatic nerve is shown to have two different portions with different roots, they make up the same nerve
SimpleMed original by Maddie Swannack
Peripheral Nerves of the Sacral Plexus
The superior gluteal nerve is the first branch of the sacral plexus.
It leaves the pelvis through the greater sciatic foramen, superior to the piriformis muscle.
It has three spinal nerve contributions: L4, L5 and S1.
The motor functions of the superior gluteal nerve include the gluteus minimus, gluteus medius and the tensor fascia lata.
It has no sensory functions.
The inferior gluteal nerve is the second branch of the sacral plexus.
It leaves the pelvis through the greater sciatic foramen, inferior to the piriformis muscle.
It has three spinal nerve contributions: L5, S1 and S2.
The motor function of the inferior gluteal nerve is the innervation of gluteus maximus.
It has no sensory functions.
The sciatic nerve is the largest nerve in the body, approximately 2cm wide, and is the third peripheral nerve to exit the sacral plexus. It can be considered to be two separate nerves within the same fibrous sheath: the tibial nerve and the common peroneal nerve (also known as the common fibular nerve). In most people, the sciatic nerve separates into the two constituents just superiorly to the popliteal fossa, but they may separate at any point superior to this.
It leaves the pelvis through the greater sciatic foramen, emerging inferiorly to the piriformis muscle (in most people). It passes over the muscles of the deep gluteal region and enters the posterior thigh by passing deep to the long head of biceps femoris. Within the posterior thigh, it gives off the branches to the hamstring muscles, and the adductor magnus (which is half innervated by the sciatic nerve and half innervated by the obturator nerve). When the sciatic nerve reaches the superior border of the popliteal fossa, it divides into the tibial and common peroneal nerves.
Spinal Nerve Contributions
- Sciatic – to the sciatic nerve as a whole, the following nerves contribute: L4, L5, S1, S2, and S3.
- Tibial – to the tibial portion of the sciatic nerve, the following nerves contribute: L4, L5, S1, S2, and S3 (all of the spinal nerves that contribute to the sciatic nerve branch into the tibial nerve).
- Common Peroneal – not all of the spinal nerves that contribute to the sciatic nerve contribute to the common peroneal branch: L5, S1, and S2 only.
- The sciatic, tibial and common peroneal nerves all have individual motor functions:
- Sciatic – posterior compartment of the thigh (biceps femoris, semitendinosus, semimembranosus) and the hamstring portion of adductor magnus.
- Tibial – posterior compartment of the leg (gastrocnemius, soleus, plantaris, popliteus, tibialis posterior, flexor digitorum longus and flexor hallucis longus) and some of the intrinsic muscles of the foot.
- Common Peroneal – anterior compartment of the leg (tibialis anterior, extensor digitorum longus, extensor hallucis longus), the lateral compartment of the leg (fibularis longus and fibularis brevis), and the rest of the intrinsic muscles of the foot.
- In summary the sciatic nerve and its derivatives supplies the posterior thigh and the entire lower leg.
- The sciatic nerve itself does not provide any sensory innervation, but the tibial and common peroneal branches do.
- Tibial – sole of the foot.
- Common Peroneal – lateral side of the lower leg.
The posterior femoral nerve is the fourth branch of the sacral plexus and is also known as the posterior cutaneous nerve.
It has three spinal nerve contributions: S1, S2, and S3.
It leaves the pelvis via the greater sciatic foramen, inferior to the piriformis. It then travels inferiorly, deep to gluteus maximus, and down the posterior thigh to the knee.
The posterior femoral nerve has no motor functions.
Its sensory innervation includes the posterior surface of the thigh and lower leg, as well as the perineum.
The pudendal nerve is the fifth and final branch of the sacral plexus.
It leaves the pelvis via the greater sciatic foramen, then re-enters the pelvis via the lesser sciatic foramen. It moves between the ischium and the rectum.
It has contributions from three spinal nerves: S2, S3, and S4.
The motor functions of the pudendal nerve include the skeletal muscles of the perineum, the external urethral sphincter, the external anal sphincter and the levator ani muscles of the pelvic floor. This gives rise to the saying ‘S2, S3 and S4 keep sh*t off the floor’ referencing the roots of the pudendal nerve.
The sensory functions of the pudendal nerve include the penis, the clitoris and the skin of the perineum.
Other Branches of the Sacral Plexus
Along with the five major peripheral nerves produced by the sacral plexus, there are a number of smaller branches that are not shown on the diagram. They are generally nerves that are directly supplying muscles:
- Nerve to piriformis
- Nerve to obturator internus
- Nerve to quadratus femoris
Except for the Pelvic Splanchnic Nerves, which provide parasympathetic innervation to the hindgut.
Pathologies of the Sacral Plexus
Damage to the Lumbosacral Plexuses
Damage to the lumbar or sacral plexuses (or both) can be very serious and are suspected when the symptoms shown cannot be localised to a single peripheral nerve region.
It can present with neuropathic pain, numbness, weakness or wasting of muscles.
One of the main causes is diabetic amyotrophy, also known as lumbosacral radiculoplexus neuropathy. It occurs when the nerves are damaged by high levels of blood sugar. High dose corticosteroids may be useful in treatment to reduce the inflammation surrounding the nerves and ease symptoms.
Another cause is local invasions, such as tumours. Unfortunately, the only treatment for this is surgery to remove the obstruction, but this can be dangerous as there is a high risk of further damage to nerves occurring.
Piriformis Syndrome
Piriformis syndrome is a clinical syndrome caused compression of the sciatic nerve by the piriformis muscle resulting in; pain, numbness and muscle weakness. Treatment options can range from analgesia and physiotherapy, to corticosteroid injections, to intraoperative relief of pressure.
Edited by: Dr. Ben Appleby
Reviewed by: Dr. Thomas Burnell
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