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Sacral plexus

The sacral plexus is a nerve plexus which supplies the pelvis and lower limb.

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Transcript

Oh, what have we got here? Looks like a big plate of pasta, maybe? Ahhh, yum! Let’s add some pesto to that or maybe arrabiata. Time to eat! Wait – what do you mean this is not my lunch, but rather my neuro exam? I have to be able to identify this stringy mess as nerves? You’re joking, right?

Nerves can be tricky to identify especially when they're all bunched together and look more like your student-friendly bowl of spaghetti than a nerve plexus. But, don't worry, that's why we're here. In this tutorial, we'll be making sense of this complex network and supply you with some tips and tricks to identify them. Let’s get started learning about the sacral plexus.

Before we get started, here's a quick overview of what we'll cover today. We’ll start by talking about the sacral plexus as a whole, learning where it is, and what it is composed of. We’ll then look at the spinal nerves that contribute to the sacral plexus. Next stop will be the main big nerves that travel into the lower limb, and lastly, we'll finish up with nerves of the gluteal region and the perineum including any other small branches we may have missed. So, to start, what is the sacral plexus?

The sacral plexus is a network of nerves that is formed from the interconnections between the anterior rami of spinal nerves from L4 to S4. This plexus mainly serves the lower limb. The largest of its nerves traveling into the posterior thigh. The sacral plexus also innervates muscles of the gluteal region and the perineum, in addition to providing sensation to those areas as well.

In this image, we're looking into a female pelvis from the right side and we can see much of the left pelvic muscles and the neurovasculature. Highlighted in green hiding in this area is much of the sacral plexus. It lies against the posterior aspect of the pelvic wall, anterior to the piriformis muscle, and posterior to the internal iliac artery and vein. This is the sacral plexus image that we're going to get really familiar with today. It may look like spaghetti now, trust me, but you'll be able to identify each nerve in no time.

So we know the spinal nerves L4 to S4 contribute to the sacral plexus, but let's have a look at these in a bit more detail so we know exactly which parts of these nerves are involved here.

As the spinal cord travels inferiorly through the vertebral canal, it gives rise to a pair of spinal nerves at every vertebral level. These spinal nerves exit the vertebral column through the intervertebral or sacral foramina and provide innervation to muscles, skin, and organs throughout the body. When spinal nerves exit through the intervertebral foramina, they split into two rami – an anterior ramus and a posterior ramus.

The posterior ramus is smaller, usually traveling immediately posteriorly to provide innervation to structures such as the deep muscles and the skin of the back. The anterior ramus, however, is much larger and is the part of the spinal nerves involved in creating the sacral plexus. So, this is what we're more interested in for this tutorial.

The sacral spinal nerves, however, take a bit of a different path since the sacral vertebrae are fused together to form the sacrum. At the sacrum, we'll see the spinal nerves emerging separately as either the anterior rami or the posterior rami. The anterior rami will travel anteriorly through the anterior sacral foramina while the posterior rami will travel posteriorly through the posterior sacral foramina. Makes sense, right?

Because the sacrum is fused, we would have a hard time seeing the full spinal nerve, but rather we can see it separate rami as they emerge anteriorly or posteriorly respectively. Remember, it's these anterior rami that we’re interested in for the sacral plexus.

In our network image of the sacral plexus, the anterior rami of the spinal nerves are these nerves here, on the right side of the image. In this image, we're looking at the right sacral plexus from an anterior view. The vertebral column would be here medially and what we see here are the anterior rami of all these spinal nerves which have exited through the intervertebral foramina or anterior sacral foramina to come together and form the sacral plexus. The merging and separating of nerves occurs within the pelvis – in this area – before exiting the pelvis to travel to their respective destinations within the lower limb, the gluteal region, or the perineum.

So which spinal nerves contribute to the sacral plexus? Well, even though it's called the sacral plexus, we do have contributions from some lumbar spinal nerves, notably the anterior rami of the L4 and L5 spinal nerves. Keep in mind that even though we get contributions from L4 and L5 to the sacral plexus, they also contribute to other nerves, especially L4 whose fibers help to make up both the femoral and obturator nerves, which belong to the lumbar plexus.

Inferior to the lumbar spinal nerves is where we find the anterior rami of the sacral spinal nerves. Contributing directly to the sacral plexus, other nerves we can see highlighted now: S1, S2, S3, and S4. The final nerves we can see here are the anterior rami of the S5 and coccygeal spinal nerves. These nerves don't contribute to the sacral plexus, but contribute to another small plexus called the coccygeal plexus.

As you can see in this image, many of the anterior rami divide quickly as they course distally from the spinal cord. We have these divisions in the anterior rami of L4, L5, S1, S2, and S3 – an anterior or ventral division and a posterior or dorsal division. If we follow the L4 spinal nerve from proximal to distal, you can see here the split into an anterior division and a posterior division. Similarly, let's follow S2.

Now, S2 gives off some other small branches right away, but then splits into two main components – its anterior and posterior divisions. We’ll see the significance of these divisions later on when we look at the sciatic nerve. Now, these divisions are not so clearly defined when looking at the nerves in situ. The nerves often stay bound together until they branch off to form different terminal nerves. The divisions are important for knowing contributions to various nerves rather than actually being able to see the anterior and posterior parts of S1, for example.

The largest nerve that arises from the sacral plexus is the one highlighted now. This is the sciatic nerve. The sciatic nerve comes from the spinal nerve roots L4, L5, S1, S2, and S3. The sciatic nerve exits the pelvis by traveling through this space here – the greater sciatic foramen. Usually, the sciatic nerve exits below the piriformis which is this muscle here. This nerve travels inferiorly along the posterior thigh innervating the hamstring muscles before it divides into the tibial and common fibular nerves which continue distally into the leg.

So looking back at our sacral plexus network image, the sciatic nerve is highlighted again. Can you see, though, that it is formed by the coming together of two distinct parts? The more anterior and medial portion is the tibial division of the sciatic nerve while here we have the common fibular division of the sciatic nerve which is more lateral and posterior. These divisions travel through the posterior thigh together with the common sheath as the sciatic nerve before splitting around the level of the popliteal fossa. However, as the sciatic nerve innervates the hamstring muscles, the nerve fibers specifically come from either the tibial division or the common fibular division. For example, the long head of the biceps femoris is innervated by the tibial division of the sciatic nerve whereas the short head of the biceps femoris is innervated by the common fibular division.

So let's take a look at these divisions a bit more closely. Now, we can see the tibial division of the sciatic nerve highlighted in green including the divisions of the anterior rami which contribute to it. Tibial nerve arises from spinal nerves L4 to S3. Specifically, though, it arises from the anterior divisions of the anterior rami of L4 to S3.

The tibial nerve proper arises in the inferior aspect of the thigh where it splits from the common fibular nerve and travels into the posterior leg. Remember, in the posterior thigh, split between the tibial and common fibular nerves isn't usually seen but they do distinctly innervate different muscles in this area before they split.

The other component of the sciatic nerve is the common fibular division, also known as the common peroneal division. It arises from spinal nerves L4 to S2, so one less spinal nerve than the tibial division which also includes S3. To be more specific, the common fibular nerve receives contribution from the posterior divisions of the anterior rami L4 to S2.

In the posterior thigh, the only muscle innervated by the common fibular division of the sciatic nerve is the short head of the biceps femoris. The remaining hamstring muscles are innervated by the tibial division. The common fibular division is lateral. Remember on the network image, it is the lateral and posterior part of the formed sciatic nerve.

In the posterior thigh, it remains lateral, and when it splits from the tibial division, it continues on a more lateral trajectory than the tibial nerve to reach the lateral and anterior compartments of the leg.

Now that we've seen the large terminal components of the sacral plexus, let's have a look at some of the deeper intricacies of the network. The parts of the lumbar nerves that travel to and become involved in the sacral plexus form what's called the lumbosacral trunk. The lumbosacral trunk is formed by parts of the anterior rami of spinal nerves L4 and L5. In this image, we're looking into the pelvis from the right side and can see the lumbosacral trunk highlighted in green. The part of L4 that contributes to the trunk is here and you can see L5 exiting the vertebral canal just above the sacrum and joining the lumbosacral trunk as well.

So this is the right lumbosacral trunk traveling inferiorly to the sacral plexus. Because these nerves are coming from the lumbar region, the lumbosacral trunk has to pass into the pelvis traveling over this anterior prominence of the sacrum. Notice how in this image, we can't see the anterior and the posterior divisions of the anterior rami of the spinal nerves. The parts of the lumbosacral trunk all merge together from a macroscopic view.

And, now, it's time to tackle all of these smaller nerves that are coming off the plexus as it moves from anterior rami of spinal nerves to the large terminal sciatic nerve. There are a lot of them, but will go through them one by one and make sure you're a pro by the end.

The first nerve we'll look at is the superior gluteal nerve. This nerve arises from the posterior divisions of the anterior rami of L4, L5, and S1 spinal nerves. The superior gluteal nerve exits the pelvis through the greater sciatic foramen usually superior to the piriformis muscle. Identifying piriformis is a very helpful way to find and identify the superior gluteal nerve. This nerve innervates three muscles – the gluteus medius, the gluteus minimus, and the tensor fasciae latae, all of which are in the superior gluteal region.

So if we have a superior gluteal nerve, we also must have an inferior gluteal nerve. This nerve arises from the posterior divisions of the anterior rami of L5, S1, and S2 spinal nerves. The inferior gluteal nerve also exits the pelvis through the greater sciatic foramen; however, it will usually exit inferior to the piriformis muscle. We can see the superior gluteal nerve in this image exiting superior to the piriformis. The inferior gluteal nerve has only one muscle target which is the largest of the gluteal muscles – the gluteus maximus. Therefore, this nerve will stay in the gluteal region as you can see in this image.

The next nerve that arises from the sacral plexus is the nerve to the piriformis. This nerve receives contributions from the posterior divisions of the anterior rami of S1 and S2. This nerve, as its name suggests, innervates the piriformis muscle which we can see highlighted in this image now. This muscle is what we've been using as a landmark for nerves exiting the pelvis through the greater sciatic foramen. The nerve to the piriformis dives directly into the muscle so when identifying the nerves, hopefully, this one is quite straightforward.

We’ll now work our way medially across the sacral plexus to identify the remaining nerves.

Next up is the nerve to the quadratus femoris which arises from the anterior divisions of the anterior rami of the L4 to S1 spinal nerves. This nerve exits the pelvis through the greater sciatic foramen inferior to the piriformis, deep to the sciatic nerve. The nerve to the quadratus femoris innervates both the quadratus femoris, as its name suggests, as well as the inferior gemellus muscle which is immediately superior to the quadratus femoris.

Next up is the nerve to the obturator internus which arises from the anterior divisions of the anterior rami of L5 to S2. Another nerve named for the muscle it innervates – lucky! So, we know this nerve innervates the obturator internus and it also innervates the superior gemellus which is immediately superior to the tendon of the obturator internus. Although this nerve is innervating the lateral rotators of the thigh, it stays a bit more medial than the nerve to the quadratus femoris to reach its muscles. It starts by exiting the pelvis through the greater sciatic foramen inferior to the piriformis just like many of the other nerves. At this point, it gives off a branch to the superior gemellus muscle before curving around the ischial spine to reenter the pelvis through the lesser sciatic foramen. Once it gets here, it has access to the pelvic surface of the obturator internus and can innervate it from there.

Moving more medially still, the next nerve arising from the sacral plexus is a relatively large nerve. This is the posterior cutaneous nerve of the thigh that we can now see highlighted in green. It’s also known as the posterior femoral cutaneous nerve. This nerve comes from posterior divisions of the anterior rami of the spinal nerves S1 and S2 and the anterior divisions of the anterior rami of spinal nerves S2 and S3. It travels on a similar path to the sciatic nerve exiting the pelvis through this greater sciatic foramen inferior to the piriformis. When identifying this nerve, look for a nerve doing the same thing as the sciatic nerve. It’s just much thinner. It travels down the posterior aspect of the thigh and, as its name suggests, provide sensory cutaneous innervation to the posterior thigh.

Next up is the perforating cutaneous nerve which most often arises from the posterior divisions of the anterior rami of the S2 and S3 spinal nerves. This nerve can be variable arising from other parts of the sacral plexus including the pudendal nerve which we'll look at next or even being absent altogether. This image shows the area that the perforating cutaneous nerve provides sensory innervation to the inferomedial aspect of the gluteal region.

The last of the large more significant nerves is highlighted now. This is the pudendal nerve which, as you can see, arises from the anterior divisions of the anterior rami of spinal nerves S2, S3, and S4. Let’s follow the course of this nerve. It exits the pelvis through the greater sciatic foramen, also inferior to the piriformis. However, it then takes a turn around the sacrospinous ligament to travel through the lesser sciatic foramen and enter the perineum. So any guesses as to what this nerve does? It provides motor and sensory innervation to much of the perineum. In terms of identifying this one, make sure to use its unique path. Remember, it comes from S2, S3, and S4, and although it exits through the greater sciatic foramen, it quickly wraps around the ischial spine and sacrospinous ligament to enter the perineum.

We’re almost at the end. The nerve that we can see highlighted now is the nerve to the levator ani and the coccygeus muscles arising from spinal nerves S4 and, sometimes, S5. Another nerve named for what it innervates, it travels directly to those muscles from the sacral plexus. First, the nerve will come across the coccygeus and will then continue to supply primary innervation to the levator ani muscles. Different to many of the nerves we've looked at so far, this nerve stays within the pelvis to reach its target muscles.

Last but not least is the smallest nerve of the day. This is the perineal branch of S4. This branch travels to the ischioanal fossa which is the space we can see highlighted in green next to the anus, and sometimes provides some motor innervation to the external anal sphincter. As it's traveling there, it gives off branches that provide sensory cutaneous innervation to the skin between the anus and the coccyx. Do take note of something important though. The perineal branch of S4 is not the same as the perineal nerve. The perineal nerve is a branch of the pudendal nerve that we looked at earlier.

And there they are - the nerves of the sacral plexus. If you're feeling like it's an information overload and it's a lot to remember, here are a few tricks that may help.

The five main nerves of the sacral plexus can be remembered by the acronym SIPPS – superior gluteal nerve, inferior gluteal nerve, posterior cutaneous nerve of the thigh, pudendal nerve, and sciatic nerve. In terms of nerve roots, most of the nerves of the sacral plexus have three. The main exception is the sciatic nerve which hopefully is easy to remember since it's so big.

There are two pairs that share the same nerve roots that are worth remembering – the superior gluteal nerve and the nerve to the quadratus femoris. Both have contributions from spinal nerves L4, L5, and S1. The inferior gluteal nerve and the nerve to the obturator internus both of contributions from spinal nerves L5, S1, and S2.

So there you have it – the nerves of the sacral plexus. But if you'll bear with me, I just want to have a quick look at what could go wrong with one of the largest of these nerves – the sciatic nerve.

If a patient presented with the following symptoms, what would you think was wrong? Pain, tingling, numbness or weakness in the buttocks, the back of the leg, or the foot? The patient may also have back pain, but the pain in the lower limb is greater. If these symptoms are unilateral, it's possible the patient has sciatica. Sciatica results when there is pressure on the sciatic nerve causing the symptoms such as pain and tingling to be felt along the path the sciatic nerve takes and innervates. The most common cause of sciatica is due to a slipped disc. If a disc slips, it can protrude towards and put pressure on the spinal nerve that is exiting the vertebral column next to it. If this spinal nerve is from L4 to S3, it contributes to the sciatic nerve and, therefore, the pressure on it will cause pain, numbness, and other uncomfortable feelings along the path of the sciatic nerve leading to sciatica.

And now you're an expert on the sacral plexus. Before I let you go, let's quickly review what we looked at today.

The sacral plexus is formed by contributions from the anterior rami of spinal nerves L4, L5, S1, S2, S3, and S4. In this image, we also identified the anterior rami of S5 and coccygeal spinal nerves. We saw that after these nerves emerge from the intervertebral and anterior sacral foramina, the anterior rami formed the sacral plexus which is against the posterior pelvic wall anterior to the piriformis.

Following the anterior rami of the spinal nerves distally, we found some nerve splitting into anterior or ventral and posterior or dorsal divisions. The largest nerve to arise from the sacral plexus is the sciatic nerve. We noted that the sciatic nerve has two divisions within itself – the tibial division and the common fibular division. The tibial division arises from the anterior divisions of the anterior rami of L4 to S3 while the common fibular division arises from the posterior divisions of the anterior rami of L4 to S2.

Traveling back proximally, we saw the lumbosacral trunk as a combined structure formed from components from the anterior rami of the L4 and L5 spinal nerves, traveling to become part of the sacral plexus.

Next up, we started looking at smaller nerves that arise from the sacral plexus. First, we identified the superior gluteal nerve. This nerve exits the greater sciatic foramen superior to the piriformis muscle. We then looked at the inferior gluteal nerve which innervates the gluteus maximus muscle. Next up was the nerve to the piriformis and this nerve does exactly what it says – it innervates the piriformis muscle.

The next two nerves were both also named for the muscles they innervate. Firstly, the nerve to the quadratus femoris is highlighted on the sacral plexus image. This nerve innervates both the quadratus femoris and the inferior gemellus muscles. And, secondly, the nerve to the obturator internus innervates both the obturator internus and the superior gemellus muscles.

We then hit up a couple of cutaneous nerves starting with the posterior cutaneous nerve of the thigh and, as its name suggests, it provides sensory innervation to the posterior aspect of the thigh. The other cutaneous nerve was the perforating cutaneous nerve and this nerve provides sensory innervation to the inferomedial aspect of the gluteal region. Continuing medially along the plexus, we found the pudendal nerve which arises from spinal nerve roots S2, S3, and S4, and the pudendal nerve provides sensory and motor innervation to the perineum.

Next up, we identified a small nerve called the nerve to the levator ani and the coccygeus muscles. Again, as its name suggests, this nerve innervates the muscles of the levator ani and coccygeus. Last but not least, we identified the smallest branch of the sacral plexus – the perineal branch of the fourth sacral nerve.

Finally, we looked at a potential consequence of a slipped disc putting pressure on a spinal nerve root that contributes to the sciatic nerve and therefore causes pain, tingling, numbness, and weakness along the path of the sciatic nerve resulting in sciatica.

And that brings us to the end of our tutorial on the sacral plexus. I hope you enjoyed it. Thanks for joining me. Happy studying!

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