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Major nerves of the thorax and abdomen.
Hey everyone! It's Nicole from Kenhub, and in today's tutorial, we're going to be talking about the main nerves of the trunk. So, in this tutorial, we're going to be talking about the main nerves of the thorax and the abdomen which include the spinal nerves and the autonomic plexuses. However, before we go on to talk about these topics in depth, let's just start this tutorial by answering some basic questions about the nervous system and the body. Let's, of course, begin by defining what is a nerve?
And a nerve is a cord-like structure that conveys information and impulses from cell bodies in the brain and the spinal cord to and from the peripheral body. Nerves are considered to be part of the peripheral nervous system or the PNS. A nerve acts much like a telephone cable carrying messages along a wire carrying fibers from the brain and the spinal cord or the central nervous system – the CNS – to an organ or structure in the periphery. And, in this image on the right, you can see the nerves in the peripheral nervous system highlighted in green.
We're also going to be talking about the trunk in this tutorial so it's best if we define what a trunk is. So, a trunk or the torso is the part of the body that does not include the head or the appendages. And, of course, in this tutorial, we'll be discussing the nerves and the nerve plexuses that pass through this region. So, in this image that you see of this lady here, the part overlaid by the rectangle is the part of the body that is considered the trunk.
Let's just talk a little bit more about one more important structure to do with nerves before we move on to our topic for today, and that structure is a nerve plexus. So, what is a nerve plexus? A nerve plexus is a network or a group of nerves that come together and then redistribute. Examples of nerve plexuses include the brachial plexus or the autonomic plexuses, and on this image, you can see the cervical plexus highlighted in green. Lots of nerves arise from plexuses and then go on to innervate different parts of the body. And later on this tutorial, we'll be talking about a lot of nerves that arise from these plexuses.
Now, this is a fairly complex tutorial so I'm just going to do a little bit more explaining at the beginning here. As you can see within the trunk, there are two types of nerve structures and these are separated by function. So, first, we have the spinal nerves which belong to the somatic nervous system or the SoNS, and the somatic nervous system is the part of the peripheral nervous system that is associated with voluntary skeletal movements and usually either carry motor innervation or sensory innervation – motor, of course, being related to movement and sensory, of course, being related to the senses. And usually with spinal nerves, both motor and sensory fibers are carried.
The trunk also houses nerves involved with the autonomic nervous system, and the autonomic nervous system, otherwise known as the visceral nervous system, is the system that is involved with innervation of involuntary smooth muscle. The autonomic nervous system itself can be divided into two systems – one called the sympathetic system which is associated with fight or flight and the parasympathetic system which is associated with the rest and digest response. And this is just a very brief overview, we're going to talk a little bit more about this later in the tutorial. So just keep the sympathetic and parasympathetic systems in mind for later on. But, for now, let's move on and talk about the spinal nerves.
And just because as I mentioned that this topic is a little bit complicated, we're going to start our section on the spinal nerves with a few more definitions – there won't be too many more, I promise – but I just wanted to define the spinal nerve a little bit more clearly because a spinal nerve as we mentioned earlier is a mixed nerve which carries motor and sensory innervation between the spinal cord and the body and it's, therefore, formed by the dorsal or posterior root arising from the spinal nerve which is made up of sensory axons and cell bodies – and you can see it highlighted in green in this image just here coming off the spinal cord – and the ventral or anterior root which is a motor root and that's made up of motor neurons – and that's highlighted in green on our right.
So, you've got these two roots arising from the spinal cord and in this image, you can see how these two roots join and leave the spinal cord through the intervertebral foramen to create the spinal nerve which is now highlighted in green, and the intervertebral foramen is just marked out by my blue circle just here. And then, after this point, the spinal nerve immediately branches off into what we would call the anterior rami and the posterior rami which we see on the right. So, if you think about it this way, you have the posterior and ventral roots which come off directly from the spinal cord and the posterior is sensory while the anterior is motor and then they come together to form a spinal root which then again splits off into an anterior branch or rami and a posterior branch or rami. So try not to get these two confused but it can be a little bit confusing. And then from these rami, the spinal nerves divide from that.
So, I hope that's clear because now we're going to talk about the spinal nerves and how they arise from the dorsal and ventral spinal roots. And all of these dorsal and spinal roots arise from the spinal cord all along down the vertebral column. And in order to be able to understand them a bit more clearly, we generally re-divide them into regions.
So within the medical field, we usually divide them into five regions – the first one being the region of the cervical vertebrae which is from C1 to C8, the second one being the thoracic region from T1 to T12, the third one being the lumbar region from L1 to L5, and then the region of the sacral area which is S1 to S5 and, finally, in the coccygeal region which is down the bottom.
So, these are just the regions but to make things a little bit more complicated, what happens after the spinal nerves arise from these anterior and posterior rami is that they come together to form plexuses which as we mentioned are networks of nerves that come together from nerves and then split off into different nerves again. And if we're going to say it a little bit more formally, the spinal plexuses are formed by intersecting subdivisions of the anterior rami of spinal nerves.
And like the regions, there are five plexuses but they are made up of slightly different rami than the regions. So, the cervical plexus is formed by the ventral rami of C1 to C4, the brachial plexus is formed by the ventral rami of C5 to T1, the lumbar plexus is formed by L1 to L4, the sacral plexus is formed by the ventral rami of S1 to S4, and the coccygeal plexus is formed by the ventral rami of S4 and S5. Again, to make things slightly more complicated, not all spinal nerve ventral rami form plexuses. For example, the ventral rami of the spinal nerves arising between the level of the second to twelfth thoracic vertebrae, or T2 to T12, do not form a nerve plexus. So, just keep that in mind as we go through all the nerves beginning, of course, with the cervical spinal nerves.
So, let's have a look again at our cervical spinal nerves, and the cervical spinal nerves, as we mentioned, arise from C1 to C8. And the cervical spinal nerves branch into many different nerves and these include the greater occipital nerve, the lesser occipital nerve, the medial and lateral pectoral nerves and the left and right phrenic nerves. And there are more than these but these are the ones that we're going to be talking about in this tutorial.
So let's begin with the greater occipital nerve. And in this image of the greater occipital nerve, we can see the nerve running superiorly along the posterior aspect of the skull, and the greater occipital nerve originates from the medial branch of the posterior ramus of the second cervical spinal nerve, C2, which is down here circled in blue. The greater occipital nerve then runs posteriorly behind the articular process of the cervical vertebrae which you can see it doing just here – that little bony bit is the cervical vertebrae – and then penetrates the semispinalis capitis and trapezius muscles to innervate the occipital part of the scalp. And here you can see the nerve as it pierces the semispinalis capitis which is dotted in blue and the trapezius muscle now dotted in blue.
The corresponding nerve of the greater occipital nerve is the lesser occipital nerve and – a small note – the lesser occipital nerve is a cutaneous spinal nerve so it runs near to the skin. The lesser occipital nerve arises from the anterior ramus of C2 and it emerges between the first and second cervical vertebrae with the greater occipital nerve – so this point just here circled in blue – and, of course, our greater occipital nerve is running alongside it and is now highlighted in blue. As the lesser occipital nerve is a cutaneous nerve, it will innervate the skin posterior and superior to the ear.
Before we go on to talk about the lateral and medial pectoral nerves, I just want to have a brief chat about the brachial plexus, and you may recall that we mentioned the nerve plexuses early in the tutorial which is a collection of branches that come together to form a network before branching off again. So, the brachial plexus if you remember, is the second of the major plexuses of the body and is made up of the ventral branches of C5 to T1.
So, in this image, you can see how these nerves come together from the spinal cord to form a series of what we call trunks. And there are three trunks within the brachial plexus – the superior trunk which consists of nerve fibers from C5 and C6, a middle trunk highlighted in green which consists of nerve fibers from C7, and an inferior trunk which consists of fibers from C8 and T1. What happens in this plexus is that these trunks then join to form the lateral, posterior and medial cords. And, again, in this image, you can see how the lateral cord is made up of superior and middle and inferior trunks and the posterior cord is formed from superior and inferior trunks and, lastly, the medial cord is formed by the inferior trunk.
So I know that was a little bit of a side note in the tutorial but this is going to help us explain the lateral and medial pectoral nerves which arise from these cords. So let's begin with the lateral pectoral nerve. And in this ventral image of the rib cage, you can see the lateral pectoral nerve highlighted in green. And the lateral pectoral nerve is a branch of the lateral cord of the brachial plexus which I've highlighted for you in blue which, if you remember, is made up of the superior, middle and inferior trunks of the brachial plexus.
As you can see in the picture, the lateral pectoral nerve runs along the anterior part of the upper chest before innervating the pectoralis major muscle – and you can see the pectoralis major muscle here dotted in blue just here – and the nerve also forms a communicating branch with the medial pectoral nerve – and we've just dotted that in blue as well over here.
Now, let's have a look at the medial pectoral nerve which arises from the medial cord of the brachial plexus – and that I've highlighted for you in blue – and remember the medial cord is made up of fibers from the inferior trunk of the brachial plexus, and the medial pectoral nerve then descends lateral to the lateral pectoral nerve running on the anterior surface of the chest and it innervates the pectoralis minor which is outlined for you in blue and the sternocostal part of the pectoralis major which is also dotted in blue.
Let's move on now to the left and right phrenic nerves, and these nerves follow a similar pathway before the thoracic cavity originating from the C4 spinal root and they also receive nerve fibers from C3 and C5 from the cervical plexus. Both right and left phrenic nerves begin at the lateral border of the anterior scalene muscle – and let's not forget the anterior scalene muscle is this one just here highlighted in blue – and then it continues superficial to it which you can see in this image. The left and right phrenic nerves then pass superficial to the subclavian artery and posteriorly to the subclavian vein – and the subclavian artery is just here in red and the subclavian vein is here in blue – before entering the thoracic cavity.
Now, let's have a look at the pathways of the right and left phrenic nerves when they are within the thoracic cavity. So, once inside the thoracic cavity, the right phrenic nerve descends superficial to or in front of the root of the right lung – which I've circled for you in blue – and down the right side of the pericardium which encloses the heart – and the pericardium here is pointed out by my blue arrow. The left phrenic nerve – as you can now see on the image highlighted in green – crosses the aortic arch and the vagus nerve and over here, we can see the aortic arch pointed out by my blue arrow and over here, the phrenic nerve crossing over the vagus nerve which is running behind the left side of the heart just here. The left phrenic nerve then descends anteriorly to the root of the left lung down the left side of the pericardium.
Let's come back to our discussion of both the left and right phrenic nerves. In this image, we can see the anterior aspect of the thoracic cavity with the ribs and the lungs pulled back and the phrenic nerves, highlighted in green, are running along the anterior aspect of the pericardium. and as you can see in the image, the phrenic nerve supplying motor innervation to the diaphragm – which you can see down here pointed out by my arrow – and the main respiratory muscles which are the intercostal muscles that you can see between the ribs here – and again I'm pointing them out with my blue arrows. And they also supply the sensory innervation to the central part of the diaphragm and the surrounding pleura and the pericardium. In this image, we can't see the pleura but we can see the pericardium which is now highlighted in blue.
Okay, now that we're finished the cervical spinal nerves, let's move on to talk about the thoracic spinal nerves. And in the thoracic segment of the spinal cord, there are twelve pairs of spinal nerves which innervate the thoracic wall of our body and you can see them highlighted in green on our right just here. In this section, we'll talk about two groups of nerves – the intercostal nerves which are comprised of T1 to T11 and the subcostal nerve from T12.
In this posterior view of the rib cage with the spinous processes of the thoracic vertebrae here, you can see the eleven intercostal nerves highlighted in green. And as I mentioned at the beginning of the video, the dorsal and the ventral roots from the spinal cord join and exit the intervertebral foramen as a mixed spinal nerve. The spinal nerve then immediately divides into anterior and posterior rami. So, the intercostal nerves are formed by the anterior rami from T1 to T11 and run between the ribs in the intercostal spaces as you can see in this image.
There are two types of intercostal nerves – typical and atypical. The typical nerves which include the third to sixth intercostal nerves usually run within the intercostal space and divide into two branches – the lateral cutaneous branch which pierce the internal and external intercostal muscles and, in turn, divide into an anterior and a posterior branch – and we can't see any of these on this particular image but do take note of these divisions. And these innervate the lateral thoracic skin and the abdominal walls, and there are other branches that the typical intercostal nerves give off but we'll discuss them in another tutorial.
Atypical intercostal nerves include the first and second intercostal nerves which run along the internal surface of the first and second ribs and occasionally give rise to a large lateral cutaneous branch. And the seventh to eleventh nerves, after giving off lateral cutaneous branches, become the thoraco-abdominal nerves of the anterior abdominal wall. Now, that's a lot of detail to take in, but the main thing to take away from the intercostal nerves is that there are eleven of them, T1 to T11, and that they can be divided into typical and atypical nerves.
We're now going to talk about the subcostal nerve, which is the nerve that arises from the anterior ramus of T12 where it runs along the lower border of the twelfth rib. And, of course, our twelfth rib is this rib just here. And the subcostal nerve gives off four branches. It often gives off a communicating branch to the first lumbar nerve so let's see, you can see the communicating branch here in green running along down to the first lumbar nerve. Subcostal nerve also communicates with the iliohypogastric nerve of the lumbar plexus which I've dotted for you in blue just here. It gives off a branch to the pyramidalis muscle – but as the pyramidalis muscle is below this image, we can only see the dotted line just running down here. And the subcostal nerve also gives off a lateral cutaneous branch that supplies sensory innervation to the skin over the hip – and we'll draw that coming off the nerve over here. The subcostal nerve also innervates the transverse abdominal muscle which you can see here in blue.
Okay, now that we've finished the thoracic spinal nerves, let's move on to the lumbar spinal nerves. Now, if you recall, we mentioned how some of the spinal nerves arise from plexuses and some of them don't. In this particular case, the lumbar spinal nerves actually all arise from the lumbar plexus. And the lumbar plexus is formed by the anterior rami of L1 to L4 and receives fibers from T12. And as you can see, there are six main lumbar spinal nerves that arise from this plexus and they are the iliohypogastric nerve, the ilioinguinal nerve, the genitofemoral nerve, the lateral femoral cutaneous nerve, the obturator nerve and the femoral nerve. Let's, of course, go from top to bottom, so let's begin with the iliohypogastric nerve.
And in this ventral view of the abdomen with the transverse abdominal muscle visible on the left part of the body but on our right visually, you can see the iliohypogastric nerve highlighted in green. And the iliohypogastric nerve arises from L1 with fibers from T12. It pierces the transverse abdominal muscle near the iliac crest and, ironically, you probably can't see it on the side where the muscle lies but you can see the nerve curving around the level of the iliac crest to pierce an invisible transverse abdominal muscle in dark gray. It then divides into the lateral and anterior cutaneous branches. The iliohypogastric nerve innervates the lower fibers of the transverse abdominal muscles which I've highlighted for you in blue and the internal oblique muscle which I've outlined for you in blue. And, as you can see, it overlies the transversus abdominal muscle.
The next nerve from the lumbar plexus we'll look at today is the ilioinguinal nerve which arises from the fusion of T12 and L1, and it can be found running across the lateral border of the psoas major muscle – and we're going to draw that in for you and just to remind you, the psoas major muscle is this one just here – and you can see the ilioinguinal nerve running just across it. It then crosses to the iliac crest which I've highlighted for you in blue inferior to the iliohypogastric nerve which is shown just here just above the ilioinguinal nerve – and I'm drawing the iliohypogastric nerve for you in blue. The nerve then pierces the transverse abdominal muscle and the internal oblique muscle which you should remember from the previous slide, and the ilioinguinal nerve also provides these two muscles with motor innervation. The ilioinguinal nerve also has a sensory component and it gives sensory branches to structures such as the pubic symphysis, superior and medial aspect of the femoral triangle, the penis and the anterior scrotum in males or the mons pubis and the labia majora in females.
Let's move on now to the genitofemoral nerve and, of course, in this ventral view of the abdomen with the viscera cut away, we can see the aorta in red running down the center of the image and, of course, the genitofemoral nerve highlighted in green. And the genitofemoral nerve arises from the anterior rami of L1 and L2. It pierces the psoas major which you can see highlighted in blue before running along the anterior surface and descending into the retroperitoneal area where it divides into femoral and genital branches. And you can see the genital branch just here and the femoral branch just here. In terms of innervation, the genital branch in males provides motor supply to parts of the genitalia and sensory supply to the upper portion of the scrotum while in females, the nerve supplies sensation to the skin of the mons pubis and the labium majus. The femoral branch supplies the skin of the groin below the inguinal ligament in both sexes.
So moving on to the lateral femoral cutaneous nerve and as its name suggests, the lateral femoral cutaneous nerve is an entirely sensory nerve supplying the skin of the thigh. It arises from the lumbar plexus specifically from the posterior division of the anterior rami of L2 to L3 and from the psoas major, it courses along the anterior surface of the iliacus muscle – which you can see outlined in blue – superior to the lateral surface of the iliac fossa, the lateral border of which I've highlighted for you in blue. And from there, it pierces the inguinal ligament. Once it emerges from the inferior border of the inguinal ligament, it passes superior to the sartorius and divides into an anterior branch which is this branch here and a posterior branch which is this branch down here. The lateral femoral cutaneous nerve innervates, via the anterior branch, the anterolateral surface of the thigh and, via the posterior branch, the posterior lateral surface of the thigh.
The obturator nerve is a large nerve arising from the lumbar plexus specifically from the anterior divisions of the level of L2 to L4. It emerges from the medial border of the psoas major muscle – and so in this image, you can see the left psoas major muscle colored in on the right with the right psoas major muscle outlined on the left – then passes through the obturator foramen on the thigh which I've highlighted for you just down here. The obturator nerve innervates the adductor muscles.
Let's move on now to the femoral nerve, and the femoral nerve is also a large nerve arising from the lumbar plexus and also arises from L2 to L4. The femoral nerve, unlike the obturator nerve, emerges from the lateral border of the psoas major muscle and enters the thigh deep to the inguinal ligament along with the femoral artery and vein. And in this image, you can see the femoral artery in red and the femoral vein in blue. And the femoral nerve innervates the flexors of the hip and the extensors of the knee.
Let's move on now to look at the sacral spinal nerves. Like the lumbar spinal nerves on how they arise from the lumbar plexus, the sacral spinal nerves arise from the sacral plexus. The sacral plexus is formed by the ventral rami of L5, S1 and S2 and, partly, the ventral rami of L4 and S3, and it's responsible for the innervation of the gluteal region and the lower limbs. The sacral plexus gives rise to five major nerves – the superior gluteal nerve, the inferior gluteal nerve, the sciatic nerve, the posterior femoral cutaneous nerve and the pudendal nerve. And, of course, we'll go over these over the next few slides starting with the superior gluteal nerve.
So, the first nerve to arise from the sacral plexus is the superior gluteal nerve, and the superior gluteal nerve conveys fibers from the posterior divisions of the ventral rami of the fourth lumbar, fifth lumbar and first sacral spinal nerves. And, of course, we can't see these rami because they're situated anteriorly and we're looking at a posterior view of the pelvis – the right pelvis – and on this posterior aspect, we can see the right pelvis with the gluteus medius muscle in blue attaching the pelvis to the femur which is also in blue. The superior gluteal nerve then travels through the greater sciatic foramen superior to the piriformis which is this muscle in blue on its way out of the pelvis. And, of course, in this image, you can see that it innervates the gluteus medius, the gluteus minimus and the tensor fascia lata.
The inferior gluteal nerve arises from the posterior division of the ventral rami of the fifth lumbar nerves as well as the first and second sacral nerves. And the inferior gluteal nerve also leaves the pelvis via the greater sciatic foramen, however, you can see it passing inferior to the piriformis in this image. The inferior gluteal nerve follows and runs superficial to the sciatic nerve out of the pelvis and, of course, our sciatic nerve is this one in blue which I've outlined for you just here. The inferior gluteal nerve innervates the gluteus maximus muscle which is outlined in blue.
Let's move on now to the sciatic nerve which is highlighted in green on our image, and the sciatic nerve is the longest nerve arising from the sacral plexus and is also the largest and broadest nerve of the human body. It arises from the anterior and posterior rami of the fourth and fifth lumbar nerves as well as the first to third sacral nerves and, like the gluteal nerves, it passes through the greatest sciatic foramen inferior to the piriformis. It then descends into the posterior compartment of the thigh before it divides into two terminal branches at the popliteal fossa, and these branches are the tibial nerve and the common peroneal nerve. And the sciatic nerve innervates the posterior compartment of the thigh, the muscles of the leg and the muscles of the foot.
Like the sciatic nerve, the posterior femoral cutaneous nerve arises from the anterior and posterior rami, however, the fibers of the posterior femoral cutaneous nerve arise from the first three sacral nerves. The posterior femoral cutaneous nerve also exits the pelvis through the greater sciatic foramen deep to the piriformis before descending into the posterior thigh. And the posterior femoral cutaneous nerve has three branches – a cutaneous branch, a gluteal branch and a peroneal branch which carries components of S3. And the posterior femoral cutaneous nerve innervates the skin of the upper medial and posterior thigh, the skin of the buttock and the perineum, the skin of the posterior scrotum and the labia and the skin of the calf.
Moving on to the pudendal nerve, the pudendal nerve arises from the anterior division of the ventral rami of the second to fourth sacral spinal nerves and exits the greater sciatic foramen with the internal pudendal artery beneath piriformis. It enters the perineum through the lesser sciatic foramen and enters the pudendal canal – and let's just pause here a second to explain the pudendal canal. So what you can't see on this image is that the pudendal canal is formed by the fascia of the obturator internus muscle which is this muscle just here and the canal runs along the surface of it which I've outlined in blue. And the pudendal canal carries three structures – the pudendal nerve which you can see in green, the pudendal artery in red and the internal pudendal vein in dark blue.
So coming back to the pudendal nerve, we can see that the pudendal nerve gives rise to three branches within the pudendal canal – the inferior rectal nerve which is this nerve here, the terminal peroneal nerve which is this nerve down here and the terminal dorsal nerve – and the pudendal nerve terminates as this nerve which we haven't illustrated on this image. The pudendal nerve innervates the skin of the perineum, the muscles of the perineum and the skin of the penis or the clitoris.
And there are six additional nerves that arise from the sacral plexus and we'll just mention them on this slide. There's the nerve to the piriformis which arises from the ventral rami of S1 and S2 and I've drawn in the little piriformis to remind you of what it looks like and where it sits, the perforating cutaneous nerve which arises from the posterior rami of S2 and S3 and innervates the lower part of the medial buttock, the pelvis splanchnic nerves which arise from the S2 to S4 nerve roots and the pelvic splanchnic nerves are part of the parasympathetic system within the pelvis which we'll talk about a little bit later.
There's also the nerve to the quadratus femoris and the inferior gemellus which arise from the ventral divisions of L4 to S1 and, of course, they innervate the quadratus femoris which you can see outlined in blue and the inferior gemellus which is now outlined in blue. The nerve to the obturator internus and the superior gemellus is another one of these nerves and this arise from the ventral divisions of L5 to S2 and, of course, this innervates the obturator internus and the superior gemellus. And, lastly, we have the nerve to the levator ani and the coccygeus which arises from the anterior rami of S3 and S4, and here you can see the nerve innervating the levator ani while here it's innervating the coccygeus.
Now that we're finished talking about the sacral spinal nerves, let's move on to talk about the coccygeal spinal nerves. The coccygeal spinal nerves arise from the coccygeal plexus which is formed by the anterior rami of S4 and S5 and the coccygeal nerve. The coccygeal plexus supplies several structures including the coccygeus, the levator ani and the sacrococcygeal joint. The coccygeal plexus also gives rise to the anococcygeal nerve which we'll talk about on the next slide.
In this image of the inferior aspect of the female genitalia where the anus over here and the coccyx down here, you can see the anococcygeal nerve highlighted in green down the bottom. As we mentioned, the anococcygeal nerve arises from the coccygeal plexus and it supplies a small area of the coccygeal region.
So that was the last part of our discussion on the spinal nerves. Now that we're finished talking about them, we're going to move on to talk about the autonomic plexuses which innervate the voluntary smooth muscle within the thorax. Now before we go on to talk about autonomic plexuses, I'm just going to have a brief chat about the autonomic system and a few other important structures. So, let's begin with the autonomic system.
Now if you remember at the beginning of the tutorial we talked a bit about how the autonomic system was divided into two systems – so the sympathetic system which is responsible for the fight or flight responses while preparing your body to deal with a stressful situation, for example, increasing the heart rate when kickboxing like our little image on the right here or when going for a very fast run, and, of course, the other system was the parasympathetic system which is responsible for the rest and digest responses within the body, for example, decreasing the heart rate during sleep.
So over the next few slides, we're going to be talking about the parts of the body within the thorax that are related to firstly, the sympathetic system and then, secondly, the parasympathetic system.
So the sympathetic system. The sympathetic nervous system can otherwise be referred as to the thoracolumbar component of the autonomic system as most of its components are found within the thorax and the lumbar region. And it has its presynaptic neurons in the brain. Its postsynaptic neurons, however, are found in two locations. The first one is the paravertebral ganglia and these are two chains that lie on either side of the spinal cord and essentially accompany its entire length, and in the image, you can see the right sympathetic chain highlighted in green on the left side of the image. And, of course, this is mostly contained within the thorax.
The other place we would find sympathetic postsynaptic neurons would be in the prevertebral ganglia, and they are plexuses that are found and located in the abdominal aortic area. And on our right, we can already see the coeliac ganglia highlighted in green. We also have the superior mesenteric ganglia which is now highlighted in green. And also we have the inferior mesenteric ganglia which is not visible in this image as its located behind the omentum here but I've roughly outlined the spot where they would be in blue.
So they're the sympathetic ganglia that are found within the thorax and another group of sympathetic structures we want to have a look at are the sympathetic nerves which are the splanchnic nerves which are just located a little bit lower than the sympathetic chains. And there are five splanchnic nerves beginning with the cardiopulmonary nerve which innervates the heart, lungs and the oesophagus; the thoracic splanchnic nerves which in this image are highlighted in green and blue and these divide into the greater splanchnic nerve which is highlighted in green on the image, the lesser splanchnic nerve and you can see this now highlighted on the image – and we're looking at both the left greater splanchnic and lesser splanchnic nerves just as a remainder – and finally, there are the least splanchnic nerves which are not visible in this image, so again, we're going to draw it as a blue dotted line down the bottom. These nerves provide sympathetic innervations to the abdominal organs.
The other splanchnic nerves we want to look at are the lumbar splanchnic nerves which innervate the smooth muscles of the distal colon, the sacral splanchnic nerve which innervates the pelvic organ and the vessels and this nerve is not highlighted in this image so I've just drawn it for you in blue over here, and the pelvic splanchnic nerve which innervates the pelvic and genital organs. However, do note that the pelvic splanchnic nerve carries parasympathetic fibers.
So now that we're finished talking about the sympathetic structures that are found within the trunk, so let's just briefly talk about the parasympathetic structures that are found within the trunk. And as we mentioned on the previous slide, the pelvic splanchnic nerves carry parasympathetic fibers and together with the fibers arising from the ventral roots of the spinal nerves S2 to S4, they innervate the viscera of the pelvic cavity. In addition to these splanchnic nerves, there are also autonomic nervous plexuses that are important within the trunk and the autonomic nerve plexuses, of course, are involved in involuntary actions of the human body and are formed by sympathetic and parasympathetic nerve fibers.
And there are several autonomic plexuses which surround the abdominal aorta and its major branches, and different textbooks will tell you different things about which ones are the most important but today we're just going to focus on five plexuses. The first one being the cardiac plexus which is found in the region within the green box on our anatomical woman on the right, the oesophageal plexus which is in this second box just here, the aortic plexus or the thoracic aortic plexus, the superior hypogastric plexus, and the inferior hypogastric plexus. The coeliac plexus which surrounds the coeliac trunk and the pulmonary plexus which innervates the pulmonary area are also important but that's for another tutorial. So let's, of course, begin with the cardiac plexus.
The cardiac plexus is formed by the superior cardiac branch of the sympathetic trunk and the lower superior cervical cardiac branch of the vagus nerve. And as you can see on our image on our right with the green highlight indicating the plexus, the plexus surrounds the heart making it responsible for the innervation of the heart. In this image of the ventral aspect of the thorax with the viscera cut away which is a more close-up version of what we saw in the previous slide, we can see the oesophageal plexus highlighted in green as it surrounds the lower half of the oesophagus. The oesophageal plexus otherwise known as the thoracic autonomic plexus is formed by the right and left branches of the vagus nerve – and I've sketched these out for you in blue just here – and the visceral branches from the thoracic sympathetic trunk and I've highlighted some of these for you in blue. The oesophageal plexus innervates the lower two-thirds of the oesophagus which, of course, is this region that I've highlighted for you down here in blue.
The aortic plexus or the thoracic aortic plexus refers to the plexus surrounding the aortic arch. The thoracic aortic plexus surrounds the thoracic aorta and is formed by fibers from the upper thoracic ganglia of the sympathetic trunks – one of which I've highlighted in blue – and the greater splanchnic nerves which we can't see in this image as they're behind the heart. It's important to note at this point that parts of the spermatic, inferior mesenteric and hypogastric plexuses arise from this plexus. Some further notes on its pathway through the thorax are that caudally, it passes through the aortic opening in the diaphragm which if we move this image up a little bit – there we go – we can see it down here outlined in blue. And the thoracic aortic plexus is continuous with the coeliac plexus superiorly and the abdominal aortic plexus inferiorly.
Moving down along the aorta, we'll now talk about the plexuses found in the abdominal cavity. In this sagittal image of the pelvic region, you can see the superior hypogastric plexus highlighted in green. The superior hypogastric plexus is a plexus of nerves found on the vertebral bodies below the bifurcation of the abdominal aorta and you can see that abdominal aorta just here. And as the superior hypogastric plexus descends into the pelvis, it splits into a left and right hypogastric nerve to join the inferior hypogastric plexus below.
The right and left inferior hypogastric plexuses also called the pelvic plexuses are located on either side of the rectum, the cervix and the bladder. And if you look at our sagittal section of our female in the region of the pelvis, our rectum is the structure pulled back just down here, and the cervix and the uterus is this structure down here, and the bladder is hanging out just in front just here. And the inferior hypogastric plexuses divide into the middle rectal plexus and on our right here if we just swap out the female pelvis for a male pelvis, we can see the middle rectal plexus highlighted in green on the lateral aspect of the rectum and the vesical plexus. And now if we come back to our image of the female pelvis, we can see the vesical plexus highlighted in green. Additionally, in females, there's also the uterovaginal plexus which we can see highlighted in green on the image and in males, we have the deferential plexus which again is highlighted in green. The inferior hypogastric plexus innervates the viscera of the pelvic cavity and the urogenital structures found in the pelvis.
So, now, we've reached the end of our tutorial and I want to just have a brief talk about some clinical notes, so the main thing I want to talk about with regards to the main nerves of the thorax is spinal cord disorders. So as you can tell from this tutorial, the spinal cord is very important and very involved in the innervation of different structures of the body. So if we have any damage to the spinal cord, there can be a lot of severe consequences. And so one of these well-known consequences is tetraplegia or quadriplegia which is the total loss of all four limbs due to spinal damage at around C1 to C7. Paraplegia is the loss of function of the lower extremities due to spinal damage at the thoracic, lumbar or sacral regions, and basically any point below the spinal cord injury will lose its function.
So thanks for sticking with me for this tutorial. It was quite long and a little bit complicated. So just to go over a few things again, let's go through our summary. The major nerve structures found within the thorax are the spinal nerves which are involved with motor innervation and the autonomic plexuses which are involved with involuntary movement.
And if you remember our spinal nerves are divided into regions, and the first region we went through were the spinal nerves within the cervical spinal region which goes from C1 to C8. The first nerve that we looked at was the greater occipital nerve which innervates the occipital part of the scalp, the lesser occipital nerve which is a cutaneous nerve and innervates the area over the ear, the lesser pectoral nerve arises from the lateral cord of the brachial plexus while the medial pectoral nerve arises from the medial cord of the brachial plexus. We also had the right phrenic nerve and the left phrenic nerve, both of which are involved with the innervation of the diaphragm.
After the cervical nerves, we looked at the thoracic spinal nerves which mainly consist of the intercostal nerves which are the nerves from T1 to T11 and the subcostal nerve which arises from T12. We then looked at the lumbar spinal nerves which includes the iliohypogastric nerve which innervates the transverse abdominal muscle and the ilioinguinal nerve which also supplies the transverse abdominal muscle. We looked at the genitofemoral nerve which, of course, has a genital branch and a femoral branch. We also looked at the lateral femoral cutaneous nerve which is an entirely sensory nerve. We looked at the obturator nerve which is a large nerve that innervates the adductor muscles and we also looked at the femoral nerve which is a major nerve of the leg.
Following the lumbar plexus, we looked at the nerves that arise from the sacral plexus which include the superior gluteal nerve which supplies the muscles of the hip, the inferior gluteal nerve which supplies the gluteus maximus muscle, the sciatic nerve which is the largest nerve of the body, the posterior femoral cutaneous nerve which like the sciatic nerve arises from the anterior rand posterior rami but in this case arises from the first three sacral nerves, and the pudendal nerve which arises from the anterior division of the ventral rami. We also looked at the coccygeal plexus which only has one nerve and that's the anococcygeal nerve.
After we finished talking about the spinal nerves, we then moved on to talk about the autonomic system. And we, of course, talked about the sympathetic system and the structures found within this region namely the paravertebral ganglia which are otherwise known as the sympathetic chains and the prevertebral ganglia which divide into the coeliac, the superior mesenteric and the inferior mesenteric ganglia. We then moved on to talk about the splanchnic nerves which are situated a little bit lower in the thorax. the first one of these being the cardiopulmonary nerve followed by the thoracic splanchnic nerve three divisions – the greatest splanchnic nerve, the lesser splanchnic nerve and the least splanchnic nerve – and this is followed by the lumbar splanchnic nerve which innervates the smooth muscles of the distal colon, the sacral splanchnic nerve which innervates the pelvic organ and the vessels, and the pelvic splanchnic nerve which innervates the pelvic and genital organs and is also the one nerve that carries parasympathetic fibers.
In addition to the splanchnic nerves, we talked about the autonomic plexuses of which, for this tutorial, there are five – cardiac plexus which innervates the heart, oesophageal plexus which innervates the lower two-thirds of the oesophagus, the aortic plexus which supplies the area around the thoracic aorta followed by the superior hypogastric plexus which is one of the two abdominal plexuses and gives rise to the left hypogastric nerve and the right hypogastric nerve, finally followed by the inferior hypogastric plexuses which supply the pelvic region.
Finally, for our clinical notes, we talked about spinal cord disorders and tetraplegia which is a lesion in the cervical area and paraplegia which is a lesion within the thoracic, lumbar or sacral regions.
And that's all we have time for today. Thanks for sticking with us throughout this tutorial. Thanks for watching!