Nerves of the small intestine seen from an anterior view of the abdomen.
Did you know that the small intestine is responsible for about ninety percent of digestion of the food that we eat every day? And if we were to lay our small intestine out straight, it would measure approximately six meters long which is about the same of the combined height of three NBA basketball players. Or around the same height as your average male giraffe. The small intestine is an impressive digestive organ by any means but have you ever wondered how does the body tell the small intestine when it's time to start digesting the food or slow that process down in order to produce more energy for other bodily functions? Or what causes the muscular wall of the small intestine to contract and relax, keeping the digestive food moving along its length? We're going to find out the answers to these questions right now by examining the innervation of the small intestine.
Let's get straight into it and begin by reminding ourselves of the anatomy of the small intestine. As you know, the small intestine consists of three parts – the duodenum, the jejunum and the ileum. As you can see in the image here, the small intestine is proceeded in the gastrointestinal tract by the stomach and terminates at the beginning of the large intestine also known as the caecum. The small intestine receives signals from external or extrinsic sources which come from the brain as well as internal or intrinsic sources which come from within the small intestine itself. These neural inputs allow for the function of the gastrointestinal tract to be achieved and this includes motility, secretomotor function of glands in the GI tract, regulation of blood flow and immunomodulation which is the regulation of the immune system within the small intestine.
We're first going to look at the extrinsic innervation of the small intestine. Like other organs of the gastrointestinal tract, the small intestine receives its external or extrinsic innervation from the autonomic nervous system. And the autonomic nervous system is the part of the peripheral nervous system responsible for controlling bodily functions which are not consciously directed such as breathing, our heart rate and our course of digestion. The autonomic nervous system is a division of the peripheral nervous system that can be divided into two parts – the sympathetic nervous system and the parasympathetic nervous system.
Before looking at the specific anatomy involved here, let's first familiarize ourselves with some basic points about the sympathetic and parasympathetic nervous systems.
The sympathetic nervous system consists of two neuron chain – one shorter known as the preganglionic neuron and one longer which is known as the postganglionic neuron. Preganglionic neurons arise from the thoracolumbar spinal cord and their axons then travel to a ganglion which is a cluster of neuronal cell bodies where then they synapse. And from here, the postganglionic neuron continues to supply the target structure. When considering the sympathetic nervous system in relation to the small intestine, the ganglia in question are known as the prevertebral ganglia and are located anterior to the abdominal aorta. And we will discuss these later in this tutorial.
In general, sympathetic stimulation inhibits digestion – meaning it slows down gastrointestinal secretion and motor activity as well as causing contraction of the gastrointestinal sphincters and blood vessels. Like the sympathetic nervous system, the parasympathetic nervous system is a two-chain pathway. The difference being that the preganglionic neuron is long and the postganglionic neuron is then short. The origins of the preganglionic neurons are set to arise from the craniosacral spinal cord while the synapses or the ganglia as well as the postganglionic axons are generally located in the target structure which, of course, in our case today, is the small intestine.
The effect of parasympathetic stimulation on the small intestine is effectively the opposite caused by the sympathetic innervation – meaning that it stimulates digestion resulting in an increase of gastrointestinal secretions and motor activity. So let's get into the fine detail of the anatomy involved here starting with the nerves which transmit sympathetic nerve fibers. There is quite a few structures involved but don’t be overwhelmed at all. We'll have this down by the end of this tutorial.
The first set of structures we're going to meet are the greater splanchnic nerves which you can see here highlighted in green. The greater splanchnic nerves consist of preganglionic sympathetic axons that arise from the fifth to ninth and sometimes the tenth thoracic spinal cord levels. These neurons emerge from the spinal cord and pass directly through this structure right here which is known as the sympathetic chain and they do this without synapsing. Once formed, the greater splanchnic nerves make their way into the abdomen by piercing the ipsilateral crus of the diaphragm.
In this image here, we are looking at the right greater splanchnic nerve. The preganglionic sympathetic axons of the greater splanchnic nerves terminate or synapse at the celiac ganglia which are these two groups of neuronal cell bodies here. The celiac ganglia are one of the major prevertebral ganglia where the postganglionic sympathetic neurons arise. These ganglia are the largest in the autonomic nervous system and lay on either side of the coeliac trunk which is the first major branch of the abdominal aorta. The celiac ganglia project fibers to some other ganglia and plexuses which we are about to look at as well as postganglionic sympathetic fibers to the duodenum and to several other organs such as the stomach, liver, gallbladder and pancreas. It's worth mentioning that the celiac ganglia also receive some preganglionic parasympathetic fibers but we'll look at them a little later on.
Another pair of ganglia which we need to be aware of in this region are the aorticorenal ganglia. These are considered as the inferior part of the celiac ganglia but as a distinct subdivision. These ganglia house the synapses of some sympathetic axons from nerves known as the lesser splanchnic nerve and occasionally from the least splanchnic nerve, both of which we will talk about shortly. The aorticorenal ganglia project nerve fibers to the celiac plexus which is also coming up as well as the kidney via the renal plexus and the suprarenal gland.
If you're feeling just a tad confused now, don't worry. Stick with me and this will all come together. We're almost, almost getting there.
The next structures we're going to look at are the lesser splanchnic nerves. These nerves arise from the tenth and eleventh thoracic level of the spinal cord just inferior to the greater splanchnic nerves and then traveling lateral to them also piercing the crura of the diaphragm. The lesser splanchnic nerves deliver axons to the ipsilateral celiac and aorticorenal ganglia which we have just met, and we can see the lesser splanchnic nerve here synapsing in the left aorticorenal ganglion.
I'm going to add one more nerve to our image here which is the left least splanchnic nerve. The least splanchnic nerves arise variably from the eleventh and twelfth thoracic level of the spinal cord and may deliver axons to the ipsilateral aorticorenal ganglion as you can see in our image as well as occasionally to the celiac ganglia. These nerves are more closely linked to innervation of the kidney, however, it's good to know of their presence in this area. The celiac and aorticorenal ganglia are considered to be part of the celiac plexus and, if you remember, a nerve plexus is a complex network of intersecting nerves.
The celiac plexus is an autonomic plexus that is found at the level of the twelfth thoracic and first lumbar vertebrae. Its dense network of fibers connect the two celiac ganglia and the two aorticorenal ganglia. Like these ganglia, the celiac plexus not only houses sympathetic neurons but it also integrates parasympathetic neurons which we are going to look at very soon. The celiac plexus will give off several other smaller plexuses that will supply the small intestine, the stomach, liver, the pancreas, the gallbladder, the spleen and the kidneys. We won't look at all these plexuses today but let's focus on one related to the small intestine which is the superior mesenteric plexus.
This plexus is considered to be an inferior continuation of the celiac plexus and it accompanies the superior mesenteric artery. At the origin of the superior mesenteric plexus, there is a ganglion which is known as the superior mesenteric ganglion. Like the celiac ganglia, this is one of the prevertebral ganglia where the preganglionic axons synapse with the postganglionic axons. Some preganglionic sympathetic nerve fibers from the greater and lesser splanchnic nerves synapse here. The superior mesenteric plexus gives off several secondary plexuses or branches which we can see here. This follow the branches of the superior mesenteric artery and therefore innervate notably the inferior portions of the duodenum, the jejunum and the ileum as well as the caecum and ascending colon of the large intestine.
We have spoken a lot about the sympathetic extrinsic innervation of the small intestine, now it is time for us to turn our attention to the parasympathetic nerve supply. You might remember when we looked at the celiac plexus earlier, we mentioned that in addition to receiving sympathetic fibers, it also has parasympathetic contributions. These are derived from the vagus nerve also known as the cranial nerve number ten. Originating from the brainstem, the left and right vagus nerves make their way down through the neck and thorax running laterally on either side of the esophagus before turning anteriorly and posteriorly to become the anterior and posterior vagal trunks just before they cross the diaphragm entering then your abdomen.
The anterior vagal trunk which we can see here is a branch of the left vagus nerve but it does contain some fibers from the right vagus nerve. Once in the abdomen, it supplies the stomach and may give off a celiac branch to the celiac ganglia as we can see in this image.
Another branch of interest to us is this nerve now highlighted in green. This is known as the hepatic branch of the anterior vagal nerve. And this nerve primarily supplies the parasympathetic innervation to the liver seen here. However, before reaching its target, it gives off a pyloric branch as we can see here. This branch follows along the gastroduodenal artery and innervates the pylorus of the stomach and more relevant to this tutorial – the superior part of the duodenum.
We're now going to return our focus to the vagus nerve for just a moment and to explain the location of our next nerve of interest. I'm first going to now add on the inferior half of the esophagus to our image. We have already mentioned this nerve which is the anterior vagal trunk which runs anterior to the esophagus before piercing the diaphragm. Now you may remember we mentioned another branch of the vagus nerve which is the posterior vagal trunk.
This nerve is primarily a branch of the right vagus nerve although it does carry some fibers from the left vagus nerve and as you can see in our image, runs posterior to the esophagus before entering the abdomen. One important branch of the posterior vagal trunk which we need to know is this one shown in the image called the celiac branch of the posterior vagal trunk. This provides parasympathetic contributions to the celiac ganglion and plexus which by now I'm really hoping that you are quite familiar with them and from the celiac plexus, the parasympathetic fibers follow the same pathways as the sympathetic fibers. And, again, don't worry if you're feeling boggled. We're going to go over all of this again at the end of this tutorial.
I hope you're still with me wherever you're watching this video. It's been a long road to get this far as we're almost, almost there.
Everything which we have just examined is what we call the extrinsic innervation of the small intestine – meaning the innervation by the autonomic nervous system which comes from your brain to then the small intestine. But we also know that the small intestine as well as much of the GI tract has its own intrinsic or internal innervation. Sometimes overlooked, this intrinsic innervation of the small intestine is what's commonly known as the enteric nervous system which I'm sure you've heard it before. The term "enteric" comes from the Latin enteron which means intestine.
The enteric nervous system is primarily composed of two plexuses which run within the wall of the gut. The first is known as the myenteric plexus or plexus of Auerbach and is found between the circular and longitudinal layers of the muscular layer of the gut sometimes known as the muscularis externa. This plexus controls the tone of the smooth muscle of the gut wall as well as the speed and intensity of muscle contractions. The second plexus which runs along the walls of the small intestine is known as the submucosal plexus or Meissner's plexus and is found in the – you guessed it – the submucosa of the gut. The submucosal plexus is involved with local conditions within the intestines and controls processes such as local intestinal secretions of enzymes to digest food, absorption of food as well as submucosal muscle movements and blood flow to the gut wall.
The enteric nervous system in the gut is also sometimes known as the "little brain" because unlike the rest of the peripheral nervous system, it's able to operate independently of the brain and spinal cord. It's kind of cool, right? And this means it is able to monitor and react to conditions within the small intestine on its own without any input from the central nervous system. It does this by sensory neurons which are found in the mucosa of the gut wall which relay conditions back to both enteric plexuses. These sensory neurons also relay information back to the central nervous system. That being said, most of the time, the enteric nervous system works in unison with the sympathetic and parasympathetic innervation of the autonomic nervous system which we looked at earlier.
And that completes our examination of the anatomy involved with the innervation of the small intestine. Before we recap what we've just covered, let's quickly look at one of the many relevant clinical correlates of interest here.
We will hopefully remember that when we looked at the celiac ganglia and plexus, we mentioned that the plexus gives off branches to several areas of the upper abdomen – for example, the duodenum and pancreas. Patients who suffer with advanced cancer of these areas very often suffer from chronic pain which cannot be treated effectively with medication. To deal with this, doctors can conduct what's known as a coeliac plexus block which blocks off the nerve pathways to the organs innervated by the celiac plexus including the duodenum. Coeliac plexus block is also known as coeliac plexus neurolysis.
Now that's been quite a lot of information to take in, I must admit, but before we finish this tutorial, I think it's important to quickly recap what we just covered will hopefully help you connect all the dots here on this tutorial.
First, we looked at the sympathetic innervation of the small intestine. The preganglionic nerve fibers emerge from the spinal cord as the greater splanchnic nerves and travel into the abdomen to the celiac ganglia which continues inferiorly as the aorticorenal ganglia which also receive preganglionic nerve fibers from the lesser splanchnic nerves. All of these contributions come together as the celiac plexus which continues inferiorly towards the superior mesenteric ganglion and superior mesenteric plexus. From here, the postganglionic sympathetic nerve fibers are then distributed to the inferior part of the duodenum, jejunum and ileum of the small intestine.
We then looked at the parasympathetic innervation of the small intestine. The parasympathetic innervation comes from the left and right vagus nerves which branch into the anterior and posterior vagal trunks. The anterior vagal trunk innervates the small intestine via its celiac branch to the celiac ganglia as well as via its pyloric branch which supplies the superior part of the duodenum. Then, we looked at the posterior vagal trunk which sends parasympathetic contributions via its celiac branches to the celiac ganglia. Parasympathetic fibers which go through the celiac ganglia go on to follow the same pathway as the sympathetic nerve fibers to the small intestine.
Remember, sympathetic input into the small intestine is responsible for inhibition or slowing down of digestion while parasympathetic input typically stimulates digestion. Last, but not least, don’t forget that the gastrointestinal tract has its own internal independent nervous system known as the enteric nervous system which consists mainly of the myenteric plexus and the submucosal plexus.
So that brings us to the end of this tutorial – one of the trickier lessons on anatomy – but I hope you enjoyed it and thank you for using Kenhub today and we'll see you on the next tutorial.