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Lumbar plexus

The structure of the lumbar plexus including its spinal roots and branches, which supply the abdominal wall, pelvis and lower limb.

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The cremaster muscle, a muscle that covers and suspends the testes, is unique to the male human body. Its function is to lower and raise the testis in reaction to certain physiological factors. For example, when a man enters an environment with a rather cold temperature, the cremaster muscle contracts raising the testis nice and snug to get some body heat. This muscle also contracts in reaction to fight-or-flight situations. For example, when a man has to fight or run away from a threat, the cremaster muscle contracts, pulling the testes close to the body to protect it.

Interestingly enough, the cremaster muscle also reacts when the medial aspect of the thigh is stroked or touched causing the testis of the side to be retracted. This is known as the cremaster reflex. My question for you today is, what do you think might be causing this to occur and why might this reflex be important in clinical practice? If you're intrigued to find out the answer to this, why not stay with me now, because the answer lies within the subject material for today's tutorial on the lumbar plexus.

Okay, so what will we actually learn in this tutorial? Let me just give you a quick overview. We’ll start with defining what exactly is the lumbar plexus and what it does. We’ll then look at its origin off the spinal cord and what nerves are formed from it. We’ll then move on to look at the structures supplied by these nerves and their course that they take to get there. There’s quite a lot of variation, so we'll discuss that too where appropriate and, of course, we'll top it off with some related clinical notes right at the end.

So the lumbar plexus is one of the four spinal nerve plexuses found in the body. While the part lumbar quite clearly indicates its location in the lumbar region, you might wonder what a nerve plexus actually is.

So think of it like a little electrical distribution board or network that receives cables or wires which represent spinal nerves. It reorganizes and combines their fibers into wires which then come out of the network to travel to their corresponding areas of the body. But back to the lumbar plexus. It is formed by – you guessed it – the lumbar spinal nerves. To be more specific, it is formed by the anterior rami of the L1 to L4 spinal nerves.

Just in case you've forgotten what I mean by the term anterior rami, let me quickly refresh your memory.

As each spinal nerve leaves the vertebral canal, it divides into an anterior and posterior ramus. The posterior rami of the spinal nerves generally innervate the skin and muscles of the back with the anterior rami contributing to the spinal plexuses. The main function of the lumbar plexus is to innervate the lower abdominal wall, certain muscles of the hip and thigh, and designated areas of the skin of the lower limb. It’s important to note that the L5 spinal nerve is generally not considered to be part of the lumbar plexus and this is because the L5 spinal nerve, together with parts of L4 spinal nerve, forms a structure called the lumbosacral trunk, which contributes to the more inferior sacral plexus.

And today we're going to focus solely on the lumbar plexus, but before we study it in detail, let's see what these nerves are actually made of.

So you might be wondering what actually makes up all the nerves in the lumbar plexus. Here we have a transverse section of the lumbar vertebral column and spinal cord, and we know that sensory fibers from the posterior spinal root and the motor fibers from the anterior root combine to form a spinal nerve. These fibers are then distributed into posterior rami which supply the posterior wall of the trunk and the buttocks and the anterior rami which are distributed to more distal target structures. The anterior rami are therefore much larger than the posterior rami as they supply more structures and regions in the body.

So I've just brought up this anterior image of the lumbar plexus and something you'll also notice if you look closely are tiny connections between the spinal nerves and the ganglia in the sympathetic chain. In this illustration, there are two connections at the L1 and L2 level. They represent white rami communicantes carrying preganglionic sympathetic fibers into the ganglia, which are only found in L1 and L2 levels, and gray rami communicantes carrying postganglionic fibers away from the ganglia and into the body, which are found in all lumbar levels. That’s why we see two connections between the sympathetic ganglia and the nerve in L1 and L2, and only one in L3 and L4. In certain individuals, the sneaky white ramus communicans can pop up in L3 as well.

Now that we know what the plexus is made of and what it does, let's see where it is. As the anterior rami of the L1 to L4 spinal nerves course away from the lumbar vertebral column, they enter the psoas major muscle and interconnect to form the lumbar plexus. Yep, you heard that right – the lumbar plexus is largely contained in a muscle.

The lumbar plexus gives off several nerves which leave the psoas major muscle along its medial and lateral borders as well as its anterior surface. So should we take a look at what these branches are? I'm going to use this simplified image of the lumbar plexus, so, it's really easy to keep track of how all the nerves are formed here. Let me give you a quick overview first of these nerves and then we'll look at each of them individually to get a better idea of their specific anatomy. Let’s begin here with the anterior ramus of the L1 spinal nerve, which gives rise to two nerves – the iliohypogastric nerve and the ilioinguinal nerve.

The iliohypogastric nerve also receives contributions from the subcostal nerve, also known as the anterior ramus of the T12 spinal nerve, through a small branch called the dorsolumbar nerve, or simply, the communicating branch to the iliohypogastric nerve. Then, follows the genitofemoral nerve which is formed from L1 and L2 spinal nerves while the L2 and L3 fibers give rise to the lateral cutaneous nerve of the thigh, also known as the lateral cutaneous femoral nerve.

The anterior branches of the L2, L3, and most of L4 spinal nerves then do something a little bit unusual in that they each split into anterior and posterior divisions. The anterior divisions then join to form the obturator nerve and the posterior divisions make up the femoral nerve. So, it might look a little complicated at first, but there is some order to this if we have a look a little bit closer.

So there's only six main nerves arising from this plexus and they follow a nice little pattern which you may have already noticed. The two most superior nerves originate just from one lumbar vertebral level, two nerves below them originate from two lumber levels, and, of course, the two inferior most nerves have three lumbar vertebral origins. Something else that you'll notice is that the names of the nerves are usually pretty telling about their pathway or the structures that they innervate. So let's take some time now to have a look at each of these nerves in more detail.

Let’s begin once again with the iliohypogastric nerve, which we briefly met a few moments ago. It’s one of the two terminal branches of the anterior ramus of the L1 spinal nerve; however, keep a note that it also receives fibers from the anterior ramus of spinal T12, also known as the subcostal nerve, via this communicating branch seen here.

Switching to a different illustration now, let's take a look at the course of the iliohypogastric nerve from its origin at the lumbar plexus. It arises here with the ilioinguinal nerve as a single trunk which runs anterolaterally parallel to the iliac crest crossing the anterior surface of the quadratus lumborum muscle. It then separates from the ilioinguinal nerve close to the lateral border of the psoas major and pierces the transversus abdominis muscle before continuing along the lateral abdominal wall between the transversus abdominis and internal oblique muscles.

It gives off a lateral cutaneous branch which supplies the posterolateral gluteal skin before piercing the internal oblique and becoming more superficial. As it courses to the anterior aspect of the abdomen, it gives off motor branches to the abdominal musculature as well as an anterior cutaneous branch which pierces the external oblique muscle innervating the skin of the suprapubic region.

We’re moving on to the ilioinguinal nerve now, the iliohypogastric nerve’s little pal, which we mentioned a moment ago. And just like the iliohypogastric muscle, it originates from the anterior ramus of the L1 spinal nerve with contributions from the subcostal nerve also and leaves the lumbar plexus as a common trunk with the iliohypogastric nerve. After it branches, the ilioinguinal nerve takes a more inferior yet roughly parallel course to the iliohypogastric nerve at first before passing more obliquely to the crest of the iliac bone.

At the anterior end of the iliac crest, the ilioinguinal nerve pierces deep to the internal oblique and transverse abdominis muscles giving off motor branches while doing so to enter the inguinal canal. From here, it emerges to exit the inguinal canal via the superficial inguinal ring to innervate the skin of the anterior one-third of the scrotum and the root of the penis in males or anterior one-third of the labia majora and the root of the clitoris in females.

A fun little variation is that these two nerves may travel as a single trunk as far as the anterior superior iliac spine.
Remember how I said the names of the nerves of the lumbar plexus were rather telling. Let’s put that information to the test. So, if we break up the word iliohypogastric, we get the terms ‘ilio’ referring to the ilium, of course; ‘hypo’ meaning under or lower; and ‘gastric’ which, of course, refers to the stomach. So, no surprise it innervates structures of the lower abdominal wall close to the iliac crest. The ilioinguinal nerve also follows its course close to the ilium, hence, ‘ilio’, and it passes through the ‘inguinal’ canal. Anatomy is really not that complicated when you think about it.

Let’s continue on now with our next nerve of interest. The genitofemoral nerve is formed by superior and inferior contributions which originate from the anterior rami of the L1 and L2 spinal nerves respectively. Once formed, it exits through the anterior surface of the psoas major muscle close to its medial border and then descends on the surface of the psoas major beneath the peritoneum. We’ll follow the course of the genitofemoral nerve in this new illustration I've just brought up, and here you can see how the nerve finishes descending on psoas major and dips under the ureter.

The nerve then divides into its genital and femoral branches lateral to the common and external iliac arteries which is right about here. A little variation – it sometimes can divide early and already emerge from the psoas muscle as two separate nerves.

The genital branch of the genitofemoral nerve that you can see highlighted in the image is associated with the anterior rami of the L1 and L2 spinal nerves and travels within the inguinal canal emerging at the superficial inguinal ring, accompanying the spermatic cord in men or the round ligament of the uterus in women. In men, it provides motor supply to the cremaster muscle and sensory innervation to the spermatic fascia, the tunica vaginalis of the testes, and skin over the anterior scrotum and groin region. In women, it provides sensory innervation to the mons pubis and the labia majora.

The femoral branch of the genitofemoral nerve which I've now highlighted in the image is formed by fibers from the anterior rami of L1 and L2 spinal nerves. It descends behind the inguinal ligament and enters the femoral sheath and then pierces the femoral sheath and the fascia lata. The femoral branch of the genitofemoral nerve provides sensory innervation to the skin over the upper part of the femoral triangle.

Quick tip: The genitofemoral nerve is probably the easiest nerve to remember because the name literally gives you its divisions and the general areas that the femoral and genital nerves go to, so all you have to do is learn a couple of landmarks along its pathway and you're all set.

Alright, looking once again at our schematic of the lumbar plexus, our next nerve of interest is the lateral cutaneous nerve of the thigh, also known as the lateral femoral cutaneous nerve. This branch of the lumbar plexus is formed by fibers from the anterior rami of the second and third lumbar nerves. To better see the course of the nerve, we'll switch to this image here and you'll see that the nerve leaves the psoas major muscle at its lateral border. It descends inferolaterally, crossing obliquely over the iliacus muscle. It enters the thigh posterior to the inguinal ligament, medial to the anterior superior iliac spine. Here it mainly supplies the skin over the anterolateral surface of the thigh as far down as the knee and it may also supply the skin of the gluteal region.

Okay, so, maybe I was wrong and this is the easiest nerve to remember. The name literally tells you that the nerve goes to the lateral skin of the thigh. As a little added bonus, the word lateral in the lateral cutaneous nerve of the thigh can help you remember that it emerges from the lateral border of the psoas major muscle. Two birds with one stone, eh?

So we're working nicely down through the nerves of the lumbar plexus. Time to meet the penultimate member of this family, which is the obturator nerve. The obturator nerve is formed by the anterior divisions of the anterior rami of L2 to L4 spinal nerves. It emerges from the medial aspect of the psoas major to travel inferiorly, crosses the pelvic brim at the sacroiliac joint behind the common iliac vessels and enters the thigh through the obturator foramen. Here, it supplies the pelvic parietal peritoneum. After passing through the foramen, the nerve divides into two branches, known as its anterior and posterior divisions.

The anterior division gives motor innervation to the adductor brevis, the adductor longus, and gracilis muscles. Sometimes, it gives off branches to supply the pectineus muscle, the hip joint, and sensory innervation to the skin of the medial thigh. The posterior division carries motor innervation to the obturator externus which we can see here from the posterior aspect and the adductor part of the adductor magnus muscle. It also gives off a small articular branch to supply the knee joint.

I should mention that the obturator nerve also sometimes has a little pal called the accessory obturator nerve and it receives innervation from the anterior rami of the L3 and L4 spinal nerves. It travels anteroinferiorly to dive under the inguinal ligament and give off three branches – one supplies the pectineus muscle, one the hip joint, and one joins or communicates with the anterior division of the obturator nerve.

So we have another ‘kinda’ straightforward nerve over here and this one goes through the obturator foramen and innervates the two obturator muscles. Just remember that it has anterior and posterior divisions and the other muscles that they innervate and you'll be fine.

So we're on to our last division of the lumbar plexus. The femoral nerve is a product of the posterior divisions of anterior rami of the L2 to L4 spinal nerves. And as you can see, it's pretty large. In fact, it's the largest branch of the lumbar plexus. I know it can get a bit confusing with all the anterior and posterior divisions. But if you just take your time and watch this as many times as you need to, you'll figure it out just fine. Maybe if you're a visual learner, it might be helpful to make little drawings as we go along.

So the femoral nerve travels within the psoas major muscle and emerges on its inferolateral border. As it descends, it gives off small branches to the iliacus muscle. It then travels posteriorly to the inguinal ligament. In this image here, you can see it going into the femoral canal just lateral to the femoral vessels, and here, it does a great many things. Quite conveniently, the branches are basically just named after the muscles they supply. So, firstly, we have the nerve to the pectineus which is this muscle right here, right as it enters the femoral canal. Then similarly to the obturator nerve, it splits into anterior and posterior divisions.

The anterior division gives off two nerves to the sartorius muscle which is the strap muscle crossing the anterior aspect of the thigh. One of these nerves becomes the intermediate femoral cutaneous nerve, also known as the anterior femoral cutaneous nerve. It gives sensory innervation to the fascia lata and the skin covering the anterior thigh to the knee. The anterior division also gives another cutaneous branch called the medial femoral cutaneous nerve which innervates the medial aspect of the thigh.

The posterior division supplies quite a few motor branches. It supplies the whole group of the quadriceps femoris muscles. The quadriceps femoris is made up of the rectus femoris right in the middle, the vastus lateralis over here, and then if we remove the rectus femoris, we can see the vastus intermedius and the vastus medialis. Finally, it gives off a teeny-tiny muscular branch to the articularis genus muscle and two articular branches – one to the hip joint and one to the knee joint.

The only cutaneous branch of the posterior division is called the saphenous nerve and it provides cutaneous innervation to the anterior and medial aspects of the knee and the leg. That’s quite a distance to cover so it comes as no surprise that the saphenous nerve is the longest nerve in the body. So the femoral nerve is not as straightforward as the other nerves. While the name rightly suggests it goes to the structures of the thigh, it's such a large nerve with so many branches that it might be useful to spend a little bit more time on this nerve, maybe, even re-watch this section of the video.

So that brings us to the end of our examination of the lumbar plexus. But before we recap, let's go back to the cremasteric reflex we saw at the beginning. I know you thought I'd forgotten, didn't you?

So we saw this animation at the beginning of our tutorial of the cremaster muscle contracting due to stroking of the medial aspect of the upper thigh. Now we get to find out why it happens. So, the cremaster reflex is a type of event known as the superficial reflex due to the fact that it involves motor responses occurring due to the scraping of the skin and these are not the same as muscle stretch reflexes such as the patellar reflex in that the sensory signal must not only reach the spinal cord, but also ascend into the brain before descending the spinal cord again as a motor neuron to a target muscle.

In the case of the cremaster muscle when the inner thigh is stroked, this stimulates a sensory response of the ilioinguinal nerve. Yep, the same one that we mentioned earlier. The motor reflex comes via the motor fibers of the genital branch of the genitofemoral nerve which innervates the cremaster muscle causing the testes to elevate up on contraction. This cremaster reflex is commonly used in clinical practice as it can be a sign of testicular torsion and this is effectively excessive twisting of the spermatic cord leading to restriction of blood flow of the testes. The cremaster reflex can also be used as an indicator of upper and lower order motor neuron disorders, injury of the spinal cord at the L1 or L2 level, or if the ilioinguinal nerve has been damaged during hernia repair.

So, before we wrap up, let's just run through what we learned today. We’ll use this illustration that we used many times throughout this tutorial and we started by recapping on what is a plexus. We then learned that the lumbar plexus was formed by anterior rami of L1 to L4 spinal nerves in the psoas major muscle. We then learned about its branches, their course, and what they innervate. We started with the iliohypogastric and ilioinguinal nerves formed by L1 spinal root with a small contribution from the T12 subcostal nerve innervating transverse abdominis and internal oblique muscles.

The genitofemoral nerve was then formed by L1 and L2 and provided motor innervation to the cremaster muscle. The obturator nerve arises from the anterior divisions of L2 to L4 and innovates the obturator externus, the adductor longus, the adductor brevis, the gracilis, the pectineus, and the adductor magnus.

The posterior divisions of those same L2 to L4 roots form the femoral nerve which is the largest nerve of the lumbar plexus and innervates the iliopsoas, the pectineus, the sartorius, the quadriceps femoris, and the vastus muscles. One branch of the lumbar plexus – the lateral femoral cutaneous nerve – arises from L2 and L3 roots and it only provides sensory information to the skin on the lateral thigh, although it's important to remember that the other nerves of the plexus also provide innervation to the skin mostly over the anteromedial thigh and the general groin region. And finally in the clinical note section, we looked at the cremasteric reflex.

Alright, guys, we've reached the end of this tutorial. Thanks for watching, see you next time, and happy studying!

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