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Group of neurons organised and located outside the Central Nervous System.
Hey everyone! It's Nicole from Kenhub and welcome to this histology tutorial on the ganglia of the nervous system. In this tutorial, we'll cover what nerve ganglia are including where you can find them and what they do. We'll also look at the different types of neurons so you can better understand what the ganglia are followed by a closer look at the ganglion cells themselves and finally, give a quick note to the clinical relevance of the ganglia. So, pull up a chair and let's begin.
So what exactly is a ganglion, I hear you ask. Well, highlighted in this image here are some examples known as the spinal sensory ganglia. If we take a closer look at this image, I can show you that they are collections or bundles of the cell bodies of afferent neurons. The efferent neurons of cranial nerves three, seven and nine also form a ganglia. The cell body of a neuron, also known as the soma or a ganglion cell, houses the nucleus and most of the organelles of the neuron. From the cell body, arise one or two axons and in some cases, dendrites.
Microtubules, some of which we can see circled here, exists within the axons and dendrites and are used by vesicles to transport neurotransmitters to and from the synapses.
The ganglia act as a connection potion for the nerves which innervate a particular tissue and you can think of them as being like a telephone switchboard as they allow the coordination of signals between areas of an organ that are distant from each other. And the best example of this is the ganglia of the intestine coordinating peristalsis. But we'll talk about that a little bit later in the tutorial.
The location of a ganglion is dependent on the type of neurons employed by its respective nerve system. So talking about the types of neuron you need to know for this topic seems like an appropriate place to begin.
As you probably are already aware, nerves are made up of a bunch of neurons wrapped in a bundle as shown in this histology image taken from a parasympathetic nerve. Our illustration here shows the axon of a nerve which is what makes up our nerve here. When neurons are of the same type, their cell bodies exist at the same point along the nerve, therefore, they collect in a clump like the one highlighted here otherwise known as our friend, the ganglion.
There are four types of major neuron that we're going to be talking about in this tutorial. First one being unipolar, the second being bipolar, the third being the pseudounipolar neuron and finally, the multipolar neuron. And, of course, we're going to go through each of these in turn, starting with the unipolar neuron.
So, unipolar neurons are very scarce in humans being found almost exclusively in the cerebellum. They have just one axon protruding from their cell body. In this histology section from the cerebellum, this point is demonstrated perfectly with the cell body being here and the single axon being here. These neurons vie intrinsically and one of their functions is to amplify signals from the vestibular nerve to the cerebellum, and in doing so, this helps to coordinate balance.
In contrast to the unipolar neurons, bipolar neurons are pretty common. A bipolar neuron cell body as you can tell by the name has two axons – one from the synapse to the cell body and the other from the cell body to the next synapse. And in this image, we're looking at some in a cross-section of the retina of a human eye and we're viewing a bundle of bipolar neurons in cross-section. An important terminology point to mention is that you shouldn't refer to these fibers as pre- or postganglionic as these terms are only used to describe the efferent fibers of cranial nerves three, seven and nine.
Now to look at a type of neuron that's a sort of crossover between the bipolar and unipolar neuron. Now that you know a little bit about the bipolar neuron, let's have a look at a type of neuron that's a sort of crossover between the bipolar and the unipolar neuron.
Pseudounipolar neurons have just one axon coming from the cell body, so in this respect, they can be considered to be unipolar, however, this single axon splits into two fibers. A signal must go through the cell body to go from one end of the neuron to the other so in this way, the neuron is more bipolar. Thus you can look at them as fake or pseudounipolar neurons. This histology slide shows a bundle of pseudounipolar cell bodies in a parasympathetic ganglion.
And the last type to mention is the multipolar neuron. In this type, the cell body has one axon and multiple dendrites. These are the primary type of motor neurons. The dendrites bring signals to the cell body with the axon taking the signal to the muscle. Having multiple dendrites allows the neurons to catch a signal from multiple synapses which increases the amplitude of the signal in the neuron and allows us to more actively activate our muscles. So, the next time you're squatting heavy or even lifting a shopping bag, thank these guys. The dendrites are short and the cell body and therefore the ganglion is located centrally. As a result, efferent neurons do not have distinct ganglia with the exception of cranial nerves three, seven and nine which have ganglia where they have an additional synapse out in the peripheral nervous system.
Now let's give a brief shout out to each of our major nerve systems in turn, mentioning what type of neuron they primarily use and where you can find those cheeky little ganglia starting with these sensory ganglia.
Sensory neurons come in two types – pseudounipolar and bipolar. These nerves bring sensory information from the body to the central nervous system entering the spinal cord via the dorsal root seen here and highlighted here in green are the sensory ganglia themselves positioned dorsally. Ganglia are also present in each of the divisions of the autonomic nervous system. Sympathetic ganglia are found associated with afferent components of the cranial nerves and peripheral nerves and are present in the sympathetic trunk like we can see in green just here.
And there's three distinct peripheral ganglia and these are the coeliac ganglion, the superior mesenteric ganglion, and the inferior mesenteric ganglion which is not visible on this image.
Parasympathetic ganglia are situated in the tissue that they innervate. The efferent neurons of these are, of course, the exceptions – cranial nerve three, cranial nerve seven, and cranial nerve nine. Cranial nerve three – the oculomotor nerve – sends signals to the eyes via the ciliary ganglion, cranial nerve seven – the facial nerve – travels to the sphenopalatine and submaxillary ganglia, and cranial nerve nine – the glossopharyngeal nerve – shoots into the optic ganglion.
In the small intestine which is highlighted on this image, the parasympathetic ganglia are found in the intestinal wall. They integrated within the myenteric plexus which is a network of nerves within the intestine wall and will use the parasympathetic nervous system as an example to examine nerve ganglia more closely. Let's now use the microscope and zoom in on the myenteric plexus.
The myenteric plexus, also known as Auerbach's plexus, innervates the two layers of the muscularis propria. Although the plexus contains parasympathetic and sympathetic fibers, the ganglia in the myenteric plexus are solely parasympathetic. The plexus can be identified as nerve tissue on histological section as a paler area with a well-defined edge caused by the presence of the epineurium.
Another image from our atlas shows the plexus more clearly. The epineurium being this pink line encircling a bundle of nerve fibers. The muscle fibers are also very visible with their elongated nuclei, and let's just isolate the parasympathetic nerve cutted along its length and increase the magnification a little bit further to show an image of our previous view of a bundle of parasympathetic nerve fibers running within a nerve.
So the term nerve fiber is synonymous with neuron and connective tissue stains blue or purple on Ladewig staining which is the staining method used here. With that in mind, let's point out a couple of things.
So, here is the epineurium which encloses the entire nerve and the perineurium which envelops each nerve fascicle and over here, we have the endoneurium which exists around each nerve fiber enclosing the axon and the Schwann cell. Sliding along to the edge of the image, we can observe a parasympathetic ganglion and the nerve cell bodies have been captured well here so we'll use this image as our base for the next section of the tutorial.
So we know that this is a parasympathetic ganglion from context. As we know that this is a histological section of an afferent parasympathetic nerve and each neuron has an axon running within the nerve has its cell body in the ganglia and has a fiber, i.e. the other axon, continuing on its merry way of the right hand side of the image.
Alright, now, let's focus in a little bit more on the ganglion so we can look a little bit more in detail at a ganglion cell.
So here we have an image of a ganglion cell direct from our atlas on Kenhub and we know that this is a ganglion cell because of its location. The cells are in close proximity to the nerve fibers and are within a bulge enclosed by a capsule and the capsule is continuous with the epineurium and the perineum. This bulge is, therefore, a ganglion.
The second reason we can tell it's a ganglion cell is by the nearby cells. So each ganglion cell is surrounded by satellite glial cells which are known to support ganglion cells. Now let's have a look at the parts of a ganglion cell.
So, at the end of the day, a neuron is just a cell so it has the same basic components as most cells. These are, of course, located within the ganglion cell, so let's highlight some of the features of our histology slide just here.
So, here is the cell body containing the cytosol and organelles of the neuron. Strictly speaking, the cell body does not include the nucleus or the nucleolus and on that note, here is the nucleus keeping our DNA safe. And now here's the nucleolus which is the factory-producing ribosomes and ribosomal RNA to be used in protein synthesis. There are also the other usual organelles but they're probably a bit too small to be seen in this image. So examples of these are the mitochondria, the ribosomes, the endoplasmic reticulum, and the Golgi apparatus.
There are a couple of neuron-specific structures to note which again are not visible here but we're going to talk about them anyway. The first are the Nissl bodies which can be demonstrated best by staining with a basophilic substance. Nissl bodies are equivalent to rough endoplasmic reticulum and sometimes appear as large granules within the cell body. The second to mention are neurofibrils also known as neurofilaments and these are believed to provide structural support to the neuron and extend from the cell body all the way along the axon.
One component which can be seen on this slide is the presence of lipofuscin granules and these are a brownish pigment which stains to show this sort of maroon color on Ladewig stain as in the image here and lipofuscin is formed as a byproduct of metabolism and accumulates as the cell ages. Abnormally high quantities can indicate disease so this group of diseases are known as lipofuscinosis. Lipofuscin granules are also formed in other cells with high metabolic turnover such as cardiac and endocrine cells.
Okay, so we've analyzed the inside of a ganglion cell. Let's now briefly look at a cell in the ganglion that isn't a ganglion cell.
Satellite glial cells, as with all glial cells, have a supportive role in the nervous system – kind of like the fans cheering on their favorite team. We can see them highlighted in this slide surrounding the ganglion cells. Satellite glial cells form a continuous sheath around ganglion cells and they're responsible for controlling the microenvironment around neurons, however, their exact role has not really been described in detail.
So, thanks for sticking with me this far. So far, you've learned what we need to know about ganglia and now we're going to talk about why we need to know about them with two clinical examples.
The first condition we'll discuss is something called achalasia. This is a failure of the oesophagus to relax resulting in a disability or inability to swallow. Swallowing of solids is more affected than swallowing of liquids or soft foods and this is a feature to pay attention to as it's the one that's used to differentiate it from other swallowing conditions. Achalasia is thought to be down to autoimmune destruction of the myenteric plexus in the oesophageal wall. This means the gastroesophageal sphincter cannot relax so food troubles to pass down into the stomach.
Reflux and regurgitation of undigested food are common symptoms and retrosternal, which is behind the sternum, chest pain may also be present. The gold standard for diagnosing achalasia is manometry – that is, pressure measurement of the esophagus. However, it's important to exclude cancers in patients who have difficulty swallowing and this is done through a barium swallow or an endoscopy.
Barium swallow is where the patient has a series of x-rays taken after swallowing a radiopaque barium dye. We end up with x-ray pictures that look like this with the barium dye appearing as white allowing us to see the outline of the oesophageal lumen. The images are taken before the swallow, at one minute after the swallow, and at two minutes after the swallow. And the top three images were taken before an operation for achalasia and the bottom three were taken postop. The operation was an esophageal myotomy of the lower esophageal sphincter, circled here, and this is also known as Heller's myotomy. This is where the lower esophageal sphincter is cut to keep it open and is considered to be the most effective treatment for achalasia.
Balloon dilatation and Botox injections of the oesophagus are alternative treatment options. If untreated, aspiration can follow and this is where unswallowed contents of the oesophagus reflux back up into the pharynx and are inhaled into the lungs which is not very nice sounding and potentially a life-threatening complication.
Another clinical example to demonstrate the necessity of ganglia is a condition of the large intestine called Hirschsprung's disease. The image here is of a nerve of the myenteric plexus in the intestinal wall which we showed you a little bit earlier. Hirschsprung's disease is a developmental condition and is usually diagnosed in infancy in which the distal colon - i.e. the last portion of the large intestine and the anus – are aganglionic. This means that the myenteric plexus hasn't formed and results in an absence of parasympathetic innervation to this region of bowel.
The smooth muscle can't relax in the distal colon and anus and coordination of peristalsis is impaired. Therefore, feces cannot be excreted and this is known as functional obstruction. Untreated, the large bowel will fill up with gas and feces and dilate becoming a megacolon, and this will eventually perforate which is again often fatal. And there's really only one definitive treatment for Hirschsprung's disease and that's surgical bowel reconstruction.
And we're done! Well done for staying with me. Histology can feel tricky sometimes. Before we finish though, let's summarize what we've talked about today.
In this tutorial, we talked about how a ganglion is a collection of nerve cell bodies as well as satellite glial cells. We also talked about how a ganglion's location along a nerve is determined by the type of neuron and also that there are four different types of peripheral neuron including unipolar, bipolar, pseudounipolar, and multipolar. This information provided the basis for explaining some of the types of ganglia around the body including the sympathetic ganglia and the parasympathetic ganglia which tend to be situated in the tissue that they innervate and following that, we looked at the parts of the ganglion cell which included it's cell body or soma, the nucleus and the nucleolus along with several other organelles that don't tend to be visible on histology slides. And finally, we provided the clinical scenarios of achalasia and of Hirschsprung's disease as an example of why ganglia are vital.
Thanks for watching this Kenhub histology tutorial. See you next time!