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Taste pathway

Neural pathway of the taste.

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We'll mostly be using this helpful diagram today highlighting the parts that we're talking about in green as we go along and we'll be looking at a coronal section through the brain and the brainstem from an anterior view. And as you can see in this image, the slice of the brain has been cut just anterior to the pons in a coronal section and we're looking at this image because conveniently the taste pathway runs nicely in this plane. The tongue is also shown in this image and we're looking at the superior surface of the tongue with the front here and the back of the tongue over here leading into the trachea, where we can also see the epiglottis and the vocal folds. Over here the nerves, implicated in the innervation of the tongue, and throughout this tutorial, there'll be other diagrams making an appearance but we'll discuss those when we get to them.

The main topic that we'll be discussing today over the course of this video are the papillae of the tongue, the innervation of the tongue, and the neural pathways to the brain. We'll also be looking at the roles of the other sensations of touch, temperature and pain and smell with regards to how we taste our food. And towards the end of the tutorial, you can look forward to the presentation of a clinical condition called dysgeusia. Therefore, our main and learning point for today are what senses are involved in taste, where taste is sensed, where it is processed within the brain, and how the taste signals are transmitted from the sensory organ to the brain.

So let's begin with an overview of the sensation of taste.

So, taste is a really interesting sense as it is the interaction of several specific signals, and there are four of these and they include the gustatory or taste signals from gustatory cells on the taste buds, touch signals – in other words, information on texture from mechanoreceptors in the oral cavity and this is sometimes referred to as mouth feel, temperature and pain signals from bare nerve endings in the oral cavity are also provided, and olfactory or smell signals from the olfactory epithelium of the cribriform plate in the nasal cavity is our fourth and last signal. There are also some accessory structures assisting with detection of taste which we'll talk about a little bit later. But, first, let's have a look at the gustatory signaling pathway.

So, gustatory information is detected by chemoreceptors on taste buds. Taste buds exist on taste papillae in the oral cavity and gustatory sensation is transmitted through three cranial nerves – the facial nerve, cranial nerve seven; the glossopharyngeal nerve, cranial nerve nine; and the vagus nerve, cranial nerve ten. Through these nerves, signals reach the brainstem where they synapse and are relayed to three main areas of the brain, and we're going to go through these now in a little bit more detail.

So, most lingual papillae are on the upper surface of the tongue, however, there are also some papillae hanging out on the soft palate, the upper esophagus and on the epiglottis. So, there are a few different shapes of papillae found on different areas of the tongue and we're going to go through them now, but keep in mind there's essentially four different types of papillae and these are the vallate papillae, the fungiform papillae, the foliate papillae, and the filiform papillae. Just before we move on to talk about each of these papillae, I just wanted you to note that the filiform papillae do not contain taste buds and rather are accessory structures so we'll talk about them a little bit later.

But, now as promised, we're going to get on to the papillae that are involved in gustatory signaling starting with the vallate papillae. Vallate papillae, also known as circumvallate papillae are arranged in a V-shape with the point of the V towards the throat as you can see on the diagram. They're located immediately anterior to the terminal sulcus which divides the tongue into its anterior two-thirds – that is the body of the tongue – and posterior one third which is the root of the tongue. And there are only seven to twelve vallate papillae on the tongue but each papilla has several thousand taste buds around its base.

A vallate papilla is described as an inverted frustum shape which is a cone with the pointy top chopped off. And to show you this a little bit more clearly, let's consider another diagram which we're going to bring in right now. So, this is a close-up view of the dorsal surface of the tongue showing the different papillae, and as you can see the vallate papillae are highlighted. They have a moat-like structure around them which allows better clearance of detected taste stimuli from the taste buds at the base of the papillae. And, actually, the moat-like structure is where the name of these papillae is derived from. So the word "vallate" comes from the Latin which means surrounded by a wall.

On this image, we can also see a number of von Ebner's glands, and these are minor salivary glands which secrete saliva around the base of the vallate papillae that's helping to clear taste particles from the taste bud receptors. The glossopharyngeal nerve is the nerve that is responsible for taking the taste signals from these taste buds.

The fungiform papillae are the most common papillae found on the tongue with two hundred to four of them spread across the anterior two-thirds of the tongue but concentrated around the edge as demonstrated on the image. So, they're termed fungiform as they are mushroom-shaped which is best displayed here, and as you can see, there are three to five taste buds per papilla highlighted here, and the facial nerve is the nerve that carries gustatory information from these taste buds back to the brain.

The final type of taste papillae that we're going to talk about today are the foliate papillae. As you can see, these are ridge-like folds situated at the edge of the tongue towards the back of the oral cavity, and we have around about twenty foliate papillae in total with each papilla having several hundred taste buds. The more anterior foliate papillae are innervated by the facial nerve whilst the more posterior papillae send taste signals through the glossopharyngeal nerve.

So over the past few slides, we've mentioned the nerves that are involved in the oral detection of taste but let's now take a look at their pathways.

So, if you were listening, you would have noticed that there are three nerves involved in gustation. Number one, the facial nerve; number two, the glossopharyngeal; and number three, the vagus nerve. So, we'll follow taste sensations being picked up in the tongue along each nerve to their synapse in the brainstem and then we'll talk about their common central pathway. And in the course of the following discussion, we'll also talk about some ganglia.

Before we go on to talk about the ganglia though, you might be wondering what a ganglion is, so we'll briefly talk through it right now. So, a ganglion is a collection of nerve cell bodies and these arise at specific anatomical locations throughout the body, and as you can see in the diagram, the ganglia of the taste pathway are highlighted and these are the otic ganglion, the geniculate ganglion, the pterygopalatine ganglion, the petrosal ganglion, and the nodose ganglion. So, let's move on now to the nerves.

The facial nerve is otherwise known as cranial nerve seven and taste from the anterior two-thirds of the tongue is transmitted into the chorda tympani which is a sensory branch of the facial nerve and this nerve passes into the middle ear and crosses the tympanic membrane. A variable degree of taste information can bypass the middle ear via the otic ganglion to hitch a ride on the greater petrosal nerve, and the chorda tympani and the greater petrosal nerve converge at the geniculate ganglion.

Taste from the palate travels along the greater petrosal nerve via the pterygopalatine ganglion where it communicates with the trigeminal nerve. After the convergence of the geniculate ganglion, the afferent fibers form the intermediate nerve which runs alongside but separate to the facial nerve proper. And both of these branches travel in the internal auditory meatus with the vestibulocochlear nerve and do note that the gustatory fibers of the intermediate nerve synapse in the rostral solitary nucleus. The rostral solitary nucleus is synonymous with the gustatory nucleus.

The glossopharyngeal nerve which is our cranial nerve nine is very important in this tutorial because it's responsible for the majority of taste sensation. This is because it innervates the posterior third of the tongue including the vallate papillae which, if you remember back to our previous slides, house the majority of the taste buds. From the taste buds, nerve signals are transmitted in the lingual branches which travel towards the jugular foramen.

The inferior glossopharyngeal ganglia, also known as the petrosal or the petrous ganglion, contains the sensory cell bodies and it is situated just below the jugular foramen. The glossopharyngeal nerve enters the cranium through the jugular foramen with the vagus nerve and the accessory nerve and the afferent fibers travel through the superior glossopharyngeal or the lesser petrosal ganglion. They carry on into the medulla through the cerebellar pontine angle to synapse in the rostral solitary nucleus which is slightly caudal to the synapses of the facial nerve and you can see this on our diagram just here.

The vagus nerve is cranial nerve ten, and we've highlighted superior laryngeal branch of the vagus nerve which carries taste information from taste buds on the laryngeal surface of the epiglottis. So, this branch joins the vagus nerve from the thoracic and abdominal internal organs and their sensory cell bodies form the inferior vagal ganglion. The afferent fibers into the cranium through the jugular foramen with the glossopharyngeal nerve and the accessory nerve and pass through the superior vagal ganglion and they synapse in the rostral solitary nucleus caudal to the synapses of the glossopharyngeal nerve.

Other projections of the vagus nerve such as those responsible for saliva secretion and gastric secretion and motility synapse in the solitary nucleus. And this explains why taste increases salivation and gastric activity. The vagus nerve is also an effector of the vomiting reflex so a bad taste can cause you to vomit. This is important evolutionarily as it's allowed us to recognize and rapidly expel potentially harmful food based on their taste.

By the way, if you're interested, there's another video on Kenhub covering the innervation as well as the blood supply of the tongue and if you're interested, you should check it out on our website. It's called the Neurovasculature of the Tongue.

At the rostral solitary nucleus, the paths of the taste afferents converge as demonstrated. At this point, the fibers from each nerve mix and then they split into three pathways. So, the first pathway goes to the ventral posteromedial nucleus of the thalamus and then it moves onto the taste sensory cortex where we become aware of the sensation. The second lot of fibers travel to synapse in the pontine taste area before going on to terminate in the lateral hypothalamic area. And the third pathway also synapses in the pontine taste area and it runs to the amygdala.

The taste sensory cortex communicates with the lateral hypothalamic area and amygdala and it's generally accepted that the lateral hypothalamic area and amygdaloid body are responsible for appetite, satiety and other homeostatic mechanisms. The fact that the sensory cortex sends signals to these areas could be the reason we feel more satiated after experiencing taste we desire. And it's important to note that the amygdala is involved in the motion and memory formation amongst other functions which is why we attach such strong emotions to food and perhaps why we crave certain foods in certain emotional states, for example, pizza or whatever it is that gives you comfort when you're feeling down.

So, we've seen how the raw sensation of taste is detected and brought to our attention, and now, we'll look at the other senses involved in sensing the flavor of a food starting with somatosensory pathways. And there are two parts of the somatosensory pathway – number one being touch and number two being temperature and pain, which are grouped together as they are transmitted by the same nerve fibers. Of course, let's begin by looking at touch.

So, throughout the oral cavity, the sensation of touch is detected by mechanoreceptors with the same nerve endings that are present in the rest of the body. Signals are carried by the maxillary branch of the trigeminal nerve which is shown here and the mandibular branch which is highlighted here. The branches converged at the trigeminal ganglion and then leave and enter the brainstem through the trigeminal trunk. In the medulla, the fibers decussate to the contralateral dorsal medial lemniscal pathway which carries the information to be registered in the brain. And this gives us information on the shape and on the texture of a food.

Moving on to the other aspect of the somatosensory component of taste which is temperature and pain. So temperature and pain are detected by bare nerve endings in the oral cavity and the peripheral pathway is the same as of that of the touch pathway passing through the maxillary and mandibular branches of the trigeminal nerve through the trigeminal ganglion and into the brainstem via the trigeminal trunk.

In the medulla, the nerve synapse in the trigeminal spinal nucleus. The pathway then decussates to the spinothalamic trunk to ascend into the cortices and we gain information on the temperature of the food and detect dangers causing pain. FYI, spicy food is not a true taste and is, in fact, a sensation from pain and temperature fibers. So actually when you're eating your favorite curry, what you're detecting is not taste per se but the pain from the heat that it's causing you.

So, let's now move on to discuss how the nose helps us taste things and we've changed our diagram for this because we want to be looking at a midline sagittal section through the nasal cavity and the brain and this image is from the medial aspect.

So, taste buds can actually only taste around five flavors – sweet, salty, sour, bitter and umami which is that Japanese taste that you find in miso soup. So, the different combinations of these allow for the detection of a range of different tastes but this does not really account for the many taste that we can experience. So, olfaction – that is, our sense of smell – is actually really vital for the interpretation of taste and it's detected by olfactory epithelium on the cribriform plate on the top of the nose.

Olfactory nerve fibers penetrate through the cribriform plate to take smell signals into the olfactory bulb and from there, the information is relayed along the olfactory tract to synapse in the nuclei of the olfactory cortex. Notes that the olfactory cortex has multiple nuclei in different locations. Firstly, it has the dorsal medial thalamus which is responsible for the conscious component of smell, the amygdala, and the limbic system which is responsible for linking smell to emotions and memory.

So we've been talking about how touch, temperature, pain and smell contribute to the experience of eating a delicious slice of pizza but how do they interact? So, let's talk about the orbitofrontal cortex. The orbitofrontal cortex contains secondary cortices of gustation, sensation, olfaction and sight. And what does this mean? This means that connecting fibers from the primary cortices bring signals to the orbitofrontal cortex. And, here, information from the individual senses is combined to give us an overall impression of the food. The orbitofrontal cortex also has communicating fibers with the limbic system as well as the amygdala which allows us to attach emotion and to reward values to certain food experiences, and it also facilitates memory formation in relation to that food.

There's a couple more things that are involved in the taste pathway if it wasn't complicated enough. Though for things to be tasted, you need to expose the chemical area of the food so that it combine to a taste receptor on the gustatory cells and you need to get the food to the taste receptors. So, there are two main accessory structures which are involved in these and the first one is the filiform papillae which we mentioned earlier and the salivary glands. And, of course, we're going to talk briefly about how each of these contributes to taste.

So, you might remember this diagram from a little bit earlier in our tutorial and it shows a magnified view of the tongue. So, the filiform papillae are these hair like structures and as we mentioned earlier, they have no taste function. Instead, they have mechanical functions. So the filiform papillae are really helpful in assisting with swallowing, with cleaning the mouth and it has a role in spreading saliva around the mouth. These functions are really important because they increase the chances of food particles passing over the taste receptors and it also helps with washing particles that have already been tasted out of the taste buds. Therefore, it can be seen that they work closely with the next accessory structure we'll be discussing which is the salivary glands.

And there are three main pairs of salivary glands – the parotid glands, the submandibular glands and the sublingual glands. The salivary glands assist with taste by acting as a solvent for taste particles allowing them to be washed around the mouth and this increases the chances that each food particle will be tasted. It also facilitates clearance of detected taste particles from taste buds and the other way they help with taste detection is through the enzymes they produce as the enzymes that they produce start to digest food which exposes more molecules to bind with taste receptors.

There are also a couple of minor salivary glands such as von Ebner's glands which we mentioned earlier when we spoke about the vallate papillae, and these glands assist with the clearance of detected food particles from taste buds and it folds around the vallate papillae and between the foliate papillae.

Before we finish, let's give a mention to the clinical relevance of taste. So, if you remember at the beginning of the tutorial, we mentioned that we're going to talk about a condition called dysgeusia which is a condition when taste perception is lost or distorted – lost meaning a complete loss or decreased ability to taste, distorted meaning anything from abnormal perception of a taste or perception of a taste in the absence of a taste stimulus also known as phantom taste.

So, according to some sources, around seven percent of people in the U.S.A. have a problem with taste or smell. And there are a few causes some of which include chemotherapy drugs, zinc deficiency, oral thrush, antibiotics and head injury. Dysgeusia can be very distressing and it can reduce a patient's quality of life to a huge degree. Imagine, not being able to taste your favorite dinner or instead of tasting it as it's meant to be, it tastes metallic.

So, the mainstay of managing this condition is to change the taste of the food eaten by, for example, adding more spices or condiments and drinking more water to rinse away bad taste. Unfortunately, there are no drug therapies to help alleviate the symptoms and it's not really clear why taste is affected with any of these causes but hopefully with greater knowledge of the pathways involved in taste, we'll be able to understand this soon. And understanding the factors contributing to taste will us to think of other ways to replace taste sensation if the detection in the mouth is damaged.

Alright, thanks for sticking with me throughout this tutorial. It was a little bit complicated but I'm sure you're stuck with me. So, we're going to just go over a summary of what we discussed today. And, today, we talked about the aspects of taste which include gustation, somatosensation and olfaction. We chatted about the pathways involved in each and mentioned that the sensations are combined and processed in the orbitofrontal cortex.

For gustation, we saw how a taste is detected by taste buds on the taste papillae in the oral cavity and then we looked at how the facial nerve, the glossopharyngeal nerve and the vagus nerve work together to carry taste sensation to the rostral solitary nucleus in the brainstem. From there, signals are passed superiorly by three different pathways to terminate in the taste sensory cortex, the amygdala and the lateral hypothalamic area.

Next, we talked about the somatosensory pathway which is divided into two parts – touch and temperature and pain – then we went over olfaction and its pathway and, finally, we mentioned dysgeusia which is a condition where knowledge of the taste pathway may be relevant in discovering more understanding of what's going on and developing ways to help those afflicted.

So, thanks for watching this Kenhub tutorial. All the best and happy studying!

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