welcome back to dirty medicine this video is going to be part of the anatomy series and in this video we're going to be discussing all of the high guild lower extremity nerves now if you're unfamiliar with how my channel is organized i'd like to take this moment to tell you that i already have a video about the upper extremity nerves and in that video what i focused on was the anatomical relationship in terms of where the nerve is running and what type of mechanism of injury might cause that nerve to manifest as some type of clinical sequelae so in other words for example if the radial nerve runs through a certain area in the upper extremity what might you have to do to damage that radial nerve and then if damaged what are the clinical findings that was the upper extremity nerve video and if you haven't seen it yet i suggest that you start there before watching this video and the reason for that is because the lower extremity nerves are a lot more complex not only are there more lower extremity nerves but you need to know more information so in this video what we're going to be focusing on is what is the origin of the nerve so what nerve root gives rise to each of these individual nerves what type of injury typically leads to this nerve being damaged what is the sensory distribution of this nerve what is the motor function of this nerve and what other high-yield findings or branches of these nerves do you need to know now let me pause for a second and explain why i'm going to approach this video in this fashion so that you understand where i'm coming from in most of the questions about the lower extremity nerves if they're going to ask you a question about what's the name of the nerve what's the origin of the nerve what muscle does it control on its motor end what cutaneous distribution does it control on its sensory end a lot of times in the clinical vignette they'll give you some sort of hint and it'll be any of the information that i'm going to discuss today but oftentimes it's going to be the mechanism of injury that leads to problems with the nerve so as i go through each of these nerves what i need you to pay very special attention to is what mechanism of injury damages these nerves and that's why i went on that little tangent a few moments ago to explain how i approached the upper extremity nerve video and how that will differ differ from this video where we're talking about the lower extremity nerves so i'll point out all of these high yield tidbits as we go i'll throw in some mnemonics for the nerves that don't make a lot of sense that you could use some type of memory hook to memorize but with that said let's just dive right in so our first nerve is the iliohypogastric nerve now you see the red arrow in the picture pointing to the sensory distribution and this nerve originates from the t12 to l2 roots an injury to the hypo to the iliohypogastric nerve typically comes about when a patient is undergoing abdominal surgery and i'll show you on the next slide why that is this nerve provides sensory to the suprapubic region which you do see in the picture here where the red arrow is pointing now in terms of motor control the iliohypogastric nerve is responsible for the transverse abdominis muscle as well as the internal obliques now the reason that this is classically damaged in abdominal surgery is because of where the ileal hypogastric nerve runs in relation to the abdominal musculature now as you see on this slide that iliohypogastric nerve sits right embedded in the abdominal musculature so if a surgeon needs to gain access to some type of local structure then there is a very high chance that when they go in they can damage the iliohypogastric nerve and if damaged you're going to have problems of course in the t12 to l2 distribution providing sensory innervation to the suprapubic area and motor control to the transverse abdominis and internal oblique so the way that you should memorize this is that the abdominal surgery damages the ill eo so ilio for iliohypogastric and abdominal the ill reminds me of illeohypogastric so abdominal surgery damages the ill-eo hypogastric nerve and that's my mnemonic for memorizing this now how is this going to be useful to you well let's pretend for a second that you're taking us emily or comlex and they give you a vignette where somebody gets some type of abdominal surgery maybe it's a hernia repair maybe it's an appendectomy in either way it doesn't matter but they're getting an abdominal surgery and then all of a sudden they have paresthesias in the distribution of the suprapubic region what the test writer is hinting at is that it's damage to the iliohypogastric nerve which you could have known either because you understand that the sensory innervation is going to the suprapubic region or they've given you the buzzword that there was abdominal surgery done and you used my mnemonic that abdominal surgery damages the ill leo hypogastric nerve okay so you can see how you need to know all of this information because the test writer might only give you one or two of these four facts and expect you to be able to recall the other remaining facts that are not in the vignette so to summarize abdominal surgery damages the ill eo-hypogastric nerve t12 to l2 suprapubic for sensory and transverse abdominis and internal oblique for motor so that's our first nerve let's go into the genito femoral nerve the genito femoral nerve arises from l1 to l2 and this mechanism of injury is classically laparoscopic surgery the sensory distribution of the genitopharmal nerve goes to the scrotum and the labia and you see that depicted here on the slide where the red arrow is pointing motor control for the genito femoral nerve is the cremaster muscle now this is really easy to remember if you use my pneumonic and the way that we're going to remember what the genito femoral nerve does is we're going to call it the genital femoral nerve and the reason that we use the term genital is because take a look at what the sensory and motor distribution is well for sensory it's things that are classically found around the genitals so the scrotum and the labia and motor control is the cremaster muscle and hopefully everybody's familiar with what a cremaster reflex is which is something that is very closely tied to the genitals so instead of saying genito femoral nerve i want you to think genital femoral nerve to remind you that this is sensory and motor in the genital area so that's our genito femoral nerve it's the second nerve a lot easier to memorize but just remember genital femoral nerve our next nerve is the lateral femoral cutaneous nerve now this is a nerve that shows up on tests a lot and the reason that this shows up on tests a lot is because its sensory distribution is so wide now look at this picture here we'll start by talking about sensory it does the anterior and lateral thigh region which you see as that really huge area with the light green shading so the lateral femoral cutaneous nerve is responsible for a very large large cutaneous sensory distribution it originates from l2 to l3 and the injury to the lateral femoral cutaneous nerve is typically either someone who wears a way too tight clothing so it squeezes that nerve as it comes off or obese patients with a very similar mechanism of there's just not enough space there so the nerve which sits somewhat superficially can be pinched very easily so how do you memorize this what's the dirty mnemonic here well lateral femoral cutaneous is lfc so i want you to memorize lfc nerve femoral cutaneous nerve again when you get a buzzword about someone who's obese or somebody who's a mega hipster and wears really tight clothing think the lfc so we're cruising guys you already know three nerves you're an expert in anatomy you're going to make all your fellow med students very jealous our next nerve is the obturator nerve the obturator nerve comes from the l2 to l4 roots and it it's classically injured due to pelvic surgery the obturator nerve provides sensory innervation to the medial thigh which you see depicted on the slide with the red arrow so it's sort of that mucus colored patch on the medial thigh now motor for the obturator nerve is the obturator externus and the adductors of the thigh now let's pause for a second before i even tell you the mnemonic it should be pretty obvious that the obturator nerve innervates the obturator externus after all it has obturator in the name but just in case you need a mnemonic to memorize the mechanism of injury what i want you to memorize is that the obturator is damaged by the operator so somebody who's getting an operation or getting surgery that is pelvic surgery they can classically damage the obturator nerve and have sensory issues on their medial thigh or motor issues with the adductors of the thigh or the obturator externus and again obtrader externus is fairly easy to memorize because it has obturator in the name so just memorize that the obturator is damaged by the operator and that is someone who's getting pelvic surgery or they're being operated on for pelvic surgery so four nerves down you guys are experts gunners valedictorians of your medical school class i'm so proud of you let's talk about the femoral nerve the femoral nerve is a very high yield nerve and you absolutely need to know a lot about this so we're going to spend a little bit more time here the femoral nerve originates from l2 to l4 now pelvic fracture is classically the injury associated with femoral nerve damage and why the femoral nerve is really important is because it provides sensory to both the anterior thigh but also the medial leg and let me pause for a second and explain this so the femoral nerve itself provides sensory innervation to the anterior thigh from the anterior femoral cutaneous nerve which is a direct extension of the femoral nerve but as that femoral nerve goes down further it actually gives off a branch which is called the saphenous nerve and the saphenous nerve is what provides sensory innervation to the medial leg which is that bottom red arrow that you see on the side of this slide now as far as its motor function the femoral nerve goes to the quads the iliacus the pectineus and the sartorius and if you instead don't want to memorize those four different muscles just remember that the femoral nerve is responsible for thigh flexion and leg extension so if you memorize those two functions and you already know the muscles then there's no need for you to memorize quads iliacus pectineus and sartorius because after all if you're taking a test and you get a question on the femoral nerve and you know that the femoral nerve does thigh flexion and leg extension then you can say okay well what are the thigh flexors and you can select the right answer or you can say what are the leg extenders and you can pick the right muscles so if you want to save yourself a little bit of brain space just memorize the function of the motor distribution of the femoral nerve instead of the actual muscles themselves but what i really want to harp on here once more so i'm going to repeat myself because this is extremely high yield is that the femoral nerve does the anterior thigh in and of itself but then the branch of the femoral nerve the saphenous nerve covers the medial leg and why this is so high yield as you'll see a little bit later on in this video is because the rest of the leg and the foot is all innervated by one nerve and the exception to that rule is the saphenous branch of the femoral nerve gets the medial part of the leg so we'll come back to this idea but i want to plant the seed now because it's extremely high yield now how do you memorize the function of the femoral nerve all right what's really important to understand is what it's doing and where it's doing it so how i want you to memorize this is femoral fem so f stands for flexors of the thigh e stands for extension and m stands for of the medial leg this is really important because your it's it the motor function of the femoral nerve is thigh flexion and leg extension and specifically it's the medial leg now of course that leg extension is just the whole leg but the reason that i made the m stand for medial leg is to actually remind you of the sensory distribution of the saphenous branch of the femoral nerve so the medial portion of the leg is your saphenous branch of your femoral nerve but the femoral nerve itself in terms of motor function does thigh flexion and leg extension via the quads iliacus pectineus and sartorius so if you just memorize thigh flexion leg extension and medial leg to remind you of sensory medial leg saphenous you guys should have all of that information that you need the really high yield stuff that shows up on usmle and comlex beautifully wrapped into my dirty mnemonic so that's the femoral nerve fem flexion extension medial leg so you guys are crushing it right now if you've been keeping up with me you know all of the high yield tidbits that you need to know on test day let's talk about a really high yield nerve the sciatic nerve so the sciatic nerve originates from l4 to s3 the injuries that cause damage to the sciatic nerve are usually two different ones that show up very often on tests you'll either have a disc herniating into the sciatic nerve or you'll have damage to a muscle called the piriformis now look at the picture on the right side of this slide the sciatic nerve is really two different nerves that are bundled together and in this picture they're depicted as the green part and the yellow part now both of those together is referred to as the sciatic nerve and that sciatic nerve in most people anatomically exits from underneath the piriformis so if the piriformis becomes injured whether it's tight and hypertonic whether it's torn whether it doesn't matter what the mechanism of the injury to the muscle is but when the muscle's damaged it pushes on the sciatic nerve right underneath it and then you get what's you know classically referred to as sciatica so damage to the sciatic nerve causing paresthesias in the distribution of the sciatic nerve now in terms of the motor function the sciatic nervous goes to the semi-membranosis the semitendinosus the biceps femoris and the adductor magnus and you should memorize that if you have the brain space but if you don't it's probably not a huge deal instead what's more important to understand is how the sciatic nerve is branching and giving off sensory distribution so the sciatic nerve drops down and splits into two other nerves and we're going to talk about each of these nerves individually after we talk about the sciatic nerve but just to give you a little appetizer of what's to come the sciatic nerve splits into the tibial nerve and the common perineal nerve now each of those two nerves will carry on their own functions and give off sensory distributions and motor distributions separate of the sciatic nerve but they'll actually come back together and both give rise to a nerve called the sorrel nerve and that's really important because the sural nerve in and of itself has its own distribution as well so what i want you to take away from this is a couple things is that first damage to the sciatic nerve is done either through disc herniation or injury to the piriformis muscle which sits right on top of the sciatic nerve that's incredibly important but the next really high yield bit is that all of the leg and the foot is innervated by the sciatic nerve with the exception of the sapherous bran the saphenous branch of the femoral nerve which i already talked about when we talked about the femoral nerve so look at this picture on the right side of the slide you can see that the common perineal sometimes referred to as common fibular as it's shown on the slide and the tibial is innervating all of the sensory distribution of the leg and the foot and the only exception is where the saphenous branch of the femoral nerve gets the medial part of the leg so if you can memorize that the sciatic nerve covers everything sensory except for the saphenous branch of the femoral on the medial leg you'll have a lot of success on test day when you have to answer questions about the sensory distribution of either the sciatic nerve the tibial nerve the common perineal nerve or the saphenous branch of the femoral nerve now that we've talked about the sciatic nerve and all of its high yield information let's take a moment to discuss the common perineal nerve and the tibial nerve individually so first we'll begin with the common perineal nerve the common perineal nerve is from l4 to s2 and again it comes from the sciatic nerve injury specific to the common perineal nerve will be due to damage to the lateral aspect of the leg at the fibular neck so the fibular neck is the area where the common perineal nerve wraps right around it and then dives down as it descends the tibia and you can see in the picture on the right side of this slide that if you're walking and you suffer a lateral blow to the lateral aspect of your fibula also known as the fibular neck there's a chance that you can damage the common perineal nerve and you know simply it's because that common perineal nerve sits right above the fibular neck so there's a classic test question where somebody suffers a lateral blow to the fibula specifically at the fibular neck and they fracture the fibular neck and then the question will ask you either what's the nerve that's damaged and it's pretty easy it's the common perineal nerve oral or they'll ask you for the clinical sequelae of common perineal nerve damage in which case you need to understand which muscles aren't going to work so with that let's talk about where the common perineal nerve goes to so the common perineal nerve drops down and then gives off two branches one is the deep perineal nerve which you see on the right side of the slide that just dives deep into the leg and the other is the superficial perineal nerve now the superficial peroneal nerve will go to the peroneus longus and brevis and you can see those two branches on the right side of the slide they're the two little yellow branches that come off the superficial perineal the deep perennial as it dives deep into the leg will go to the tibialis anterior the left side of the slide is showing you the nerves in red giving going to their motor distribution which are the muscles in blue and on the right side of the slide you can not only trace the nerves as they come down but you can also see the sensory or cutaneous distribution of all of these different nerves on the rightmost part of the slide so it's really high yield to understand how the common perineal nerve dives down how it branches and that it originates from the common perineal nerve which in and of itself originates from the sciatic nerve so something that's really important not only on us emily and comlex but also in your you know uh classes themselves where you know say you're taking anatomy or something is to understand how these nerves can continue to branch as you go more distal toward the foot now we said that the common perineal was one branch off of the sciatic nerve and the other branch was the tibial nerve the tibial nerve originates from l4 to s3 again one of two major branches from the sciatic nerve now injury to the tibial nerve is usually one of two things the first occurs in the popliteal fossa and it's what's known as a popliteal or baker cyst now if you look at this image it's basically a little cyst that collects on the back of the knee in the popliteal fossa and because the tibial nerve runs right through that fossa if that cyst gets big enough it pushes on the tibial nerve and when it pushes on the tibial nerve it'll create some clinic some clinical sequelae and we'll talk about the clinical problems with the tibial and common peroneal at the end of uh this discussion the other mechanism of injury occurs in the tarsal tunnel and it's what's known as tarsal tunnel syndrome so in the foot just like in the hand you have a flexor retinaculum underneath of which you've got the tibial nerve and the posterior tibial artery and some veins that pass right underneath it so if you create damage or stress in that tunnel which is a small closed space with a finite amount of space for nerves and vasculature to pass through then you're going to squeeze down or clamp down if you will on the tibial nerve and cause problems with whatever the sensory or motor distribution of that tibial nerve is so it's really high yield to take away from this that you've got tarsal tunnel syndrome due to stress or damage or repeated use or overuse of the flexor retinaculum as well as in the popliteal fossa you can get the baker or popliteal cyst both of these will push on the tibial nerve and cause the clinical sequelae that we'll talk about in just a second now just for completeness sake i do want to put on this slide where the tibial nerve is responsible for in terms of the muscle so it goes to the popliteus the plantaris the triceps suray the biceps femoris and the flexors of the foot and that's really not important to know in terms of what's high yield on usmle or comlex what is really high yield is to know what's on this slide so let's differentiate our two major branches of the sciatic nerve we've got the tibial nerve which will be on the left shown in red and the common perineal nerve shown on the right in blue now we've already talked about where it originates from and the mechanisms of injury all of that is extremely high yield the last bit of information that you need to know is what happens clinically if you have an injury to the tibial or common perineal nerve so if you have a baker cyst or tarsal tunnel syndrome what happens and if there's a lateral injury to the fibular neck what happens the way that you memorize this is with tip and ped so tip stands for tibial inverts and plantarflexes and ped stands for the perineal everts and dorsiflexes and what what this means is that typically the tibial nerve is responsible for inversion and plantar flexion and the common perineal is responsible for eversion and dorsiflexion so therefore if there's an injury to either of these nerves you lose those functions and you can figure out on your test what clinical sequelae you'll see and what the answer will be so that's it you guys are nerve masters here's a chart summarizing the origin and mechanism of injury for your studying pleasure again just to quickly summarize we talked about the iliohypogastric nerve the genito femoral nerve the lateral femoral cutaneous nerve the obturator nerve the femoral nerve giving off its very high-yield saphenous branch the sciatic nerve which courses down and splits into two major nerves which are the common perineal and the tibial nerve you guys are now lower extremity nerve experts i hope that this was helpful to you on test day you should only dedicate a very small portion of your brain to to anatomical concepts so if you just don't want to learn the full extent of this topic then i really do think that this video will help you out