Video: Nerves of the abdominal wall

Okay y'all! Time to do some vacuums – stomach vacuums, that is. Okay, yeah, bring it on. Now out…bring it in y'all! Keep going. That's great, y'all!

It's not actually going to give you a six-pack, but it's sure going to look good on TikTok! Ah, yes, the things we do for social media. If only influencers were so quick to show the aftereffects of using stomach vacuums to squeeze their abdominal organs a gazillion times. How are our abdominal muscles hardwired into such torture? Well, it's all about what innervates them, right, and that's exactly what we're going to be looking at today as we explore the nerves of the abdominal wall.

First of all, let's take a run through of what we're going to cover in this tutorial. We'll start by looking at the abdominal wall and explore its functions and subdivisions. Then we're going to move into the main topic of the tutorial which is the nerves of the abdominal wall starting with the anterolateral wall. After that, we're going to describe the nerves of the posterior wall. Finally, we'll take a look at the clinical notes where we'll talk about an anesthetic technique known as abdominal nerve blocks. So without further ado, let's start talking about the abdominal wall.

So we know that the abdomen is the region of the trunk that lies between the thorax and pelvis. Unlike its neighbors, however, the abdomen is rather unique in that it is largely defined by a thick and strong muscular wall, rather than a bony one. Therefore, an intact abdominal wall is essential for protecting and supporting the abdominal viscera and stabilization of the rotation of the trunk. Besides that, contraction of the muscles of the abdominal wall leads to increased intraabdominal pressure which is essential for physiological processes like voiding the contents of the bladder and rectum, giving childbirth, as well as coughing and vomiting.

As you can see in this cross-section of the abdomen, the abdominal wall can be divided into two main subdivisions – the anterolateral wall and the posterior wall. Muscles of the anterolateral abdominal wall include the three flat muscles – the external and internal oblique and transversus abdominis; and the two vertical muscles – rectus abdominis and pyramidalis. Muscles of the posterior abdominal wall include psoas major and minor, iliacus, and quadratus lumborum.

Now that we have a general picture of the abdominal wall structure, function, and subdivisions, it's time to get to the main subject of this tutorial - the nerves of the abdominal wall.

Let's explore this topic starting with the anterolateral abdominal wall. The structures of the anterolateral abdominal wall such as the skin, muscles, and the peritoneum are supplied by several nerves which are ultimately derived from the T7 to T12 and L1 spinal nerves. The anterior rami of spinal nerves T7 to T11 pass around the body from posterior to anterior in an inferomedial direction. They are initially located between the ribs in the intercostal spaces so they are known as the intercostal nerve 7 to 11. These nerves leave their intercostal spaces, passing deep to the costal cartilages and continue on to the anterolateral abdominal wall between the internal oblique and the transversus abdominis muscles which are not represented in this image.

Throughout their course, these nerves supply the muscles and parietal peritoneum of the anterior abdominal wall. Unlike the rest of the intercostal nerves, these nerves are located both in the thorax and abdomen supplying structures in both locations. Because of that, the intercostal nerve 7 to 11 are also known as the thoracoabdominal nerves.

As these intercostal nerves proceed through the intercostal space about midway through the vertebrae and sternum, they give off lateral cutaneous branches which is what we can observe here in this representation of an intercostal space. These lateral cutaneous branches pierce the muscles to supply the skin of the lower aspect of the lateral thoracic wall and the skin of the lateral abdominal wall. Each one of these lateral cutaneous branches divide into an anterior and posterior branch. When these intercostal nerves reach the distal end of their related rib, they terminate as anterior cutaneous branches which pass through the rectus abdominis muscle and the interior wall of the rectus sheath to supply the skin of most of the anterior abdominal wall as far down as the region just below the navel and the rectus abdominis muscle itself.

The anterior ramus of spinal nerve T12 is also known as the subcostal nerve. Basically, this nerve’s course and branches are very similar to the thoracoabdominal nerves, but because it lies inferior to the lowest rib, it gets a different name. It supplies the muscles of the anterolateral abdominal wall except for the rectus abdominis as well as a band of overlying skin superior to the iliac crest and inferior to the umbilicus.

Emerging just inferior to the first lumbar vertebra below the subcostal nerve is spinal nerve L1. The posterior ramus of the L1 spinal nerve which is not visible in this image runs to supply structures of the back whereas the anterior ramus runs in an inferolateral direction, contributing to the formation of the lumbar plexus – a web of nerves located in the lumbar region of the body.

Arising directly from the anterior ramus of the first lumbar nerve are two small branches which contribute to the innervation of the abdominal wall – the iliohypogastric and ilioinguinal nerves. The iliohypogastric nerve arises together with the ilioinguinal nerve as a single trunk from the anterior ramus of spinal nerve L1. It courses obliquely towards the iliac crest and bifurcates to form the anterior and lateral cutaneous branches. Together with its branches, the iliohypogastric nerve supplies innervation to the internal oblique and transversus abdominis muscles of the anterolateral abdominal wall as well as sensory innervation to the skin of the posterolateral gluteal and suprapubic regions.

Also arising from the anterior ramus of spinal nerve L1 is the ilioinguinal nerve. It separates from the iliohypogastric nerve and follows its course inferior to this nerve. Unlike the intercostal, subcostal, or iliohypogastric nerves, the ilioinguinal nerve does not have anterior or lateral cutaneous branches. Instead, it gives off an anterior scrotal nerve in individuals with male reproductive anatomy or anterior labial nerve in individuals with female reproductive anatomy. Together with its branches, the ilioinguinal nerve supplies motor innervation to the internal oblique muscle as well as sensory innervation to the skin of the anteromedial aspect of the thigh and skin covering parts of the external genitalia. Therefore, it does not supply cutaneous innervation to the abdominal wall.

Now that you know everything about the nerves of the anterior abdominal wall, it's time to move on to the nerves of the posterior abdominal wall.

There are several important components of the nervous system located in the posterior abdominal wall. The lumbar sympathetic trunk is made up of sympathetic trunks and associated splanchnic nerves. These structures belong to the sympathetic nervous system – a component of the autonomic nervous system. The lumbar sympathetic trunk functions to provide sympathetic innervation to organs of the abdominal pelvic regions.

Next, we meet the prevertebral plexus which is a network of nerve fibers and ganglia of the autonomic nervous system that surrounds the abdominal aorta and its branches. These structures play a critical role in the innervation of the abdominal viscera.

Let's start with the previously mentioned lumbar plexus of nerves. This plexus is formed by the anterior rami of nerves L1 to L3 and most of the anterior ramus of L4. It also receives a contribution from the T12 nerve. Important branches of the lumbar plexus are the iliohypogastric and ilioinguinal nerves which we have already discussed earlier in this video as well as the muscular branches which supply the quadratus lumborum and psoas major muscles of the posterior abdominal wall.

As previously mentioned, the lumbar spinal nerves also have posterior branches known as the posterior rami of spinal nerves L1 to L5, also known as the first to fifth lumbar nerves. We can observe the posterior rami of the spinal nerves from this superior view. The posterior rami give off two main branches known as the posterior cutaneous branches of the lumbar spinal nerves and the superior cluneal nerves. Together with its branches, this group of nerves provide motor innervation to longissimus and iliocostalis components of the lumbar erector spinae as well as to the multifidus muscle and sensory innervation to the skin of the back.

Okay, it's time to find out about abdominal nerve blocks in our clinical notes.

Abdominal nerve block, also known as transversus abdominis plane block, is a technique that can be performed to provide anesthesia and analgesia, or pain control, for abdominal wall procedures and surgeries. It is often used as an alternative to an epidural. While both are equally effective in pain control, abdominal nerve blocks have been shown to lead to fewer episodes of postoperative hypertension and often result in a decreased length of stay in hospital.

Unilateral blocks are used for one-sided procedures such as appendectomy while bilateral blocks are used for midline and transverse abdominal incisions such as cesarean section. These blocks are performed by injecting a local anesthetic into interfascial planes through which nerves run in order to deposit the anesthetic agent close to the nerves.

Learning the anatomy of the nerves of the abdominal wall is essential to understand how these nerve blocks are performed. Although anatomical landmarks are important, ultrasound guidance is being increasingly used to assist in the localization of tissue planes and nerves, helping in successful nerve block.

In the anterolateral abdominal wall, these techniques can be applied to the thoracoabdominal nerves, iliohypogastric and ilioinguinal nerves. The thoracoabdominal nerves are blocked as they emerge from beneath the costal margin and enter the abdominal wall between the transverse abdominis and internal oblique muscles. The needle is inserted along the lower border of the costal margin between the xiphoid process and the 10th or 11th rib. The iliohypogastric and ilioinguinal nerves are blocked by inserting the needle about 2.5 centimeters above the anterior superior iliac spine. Although rare, complications have been reported with these techniques including bowel perforation and hematoma and liver laceration.

Okay, we're nearly there. To finish off, let's review what we've covered in this video.

We started by looking at the structure of the abdominal wall – a layered structure that consists partly of bone but mainly of muscle. We also saw that the abdominal wall can be divided into two main subdivisions – the anterolateral and the posterior abdominal wall. Then we moved on to the main topic of this tutorial, the nerves of the abdominal wall, starting with the anterolateral wall. We saw the intercostal nerve 7 to 11 which are also known as the thoracoabdominal nerves that give off the lateral cutaneous branches and the anterior cutaneous branches, the subcostal nerve, the iliohypogastric nerve, and the ilioinguinal nerve. Next, we examined the nerves of the posterior abdominal wall, specifically, the lumbar sympathetic trunk, the prevertebral plexus and ganglia, the lumbar plexus, and the posterior rami of spinal nerves L1 to L5. At the end of our tutorial, we discussed an anesthetic technique known as abdominal nerve blocks.

And that's it for today. Hope you enjoyed this video. Happy studying and see you next time.