[0:03]So, we are going to look at uh sensory receptors and uh sensory organs and sensory pathways with emphasis particularly on the receptors of special sensations and their related organs and pathways. This is in light of the fact that uh we have already done the first part of the lecture where we focused on some introductory concepts about uh sensory systems. And then we also talked about receptors of general sensations. Perhaps if you need to remember what we did last in the previous lecture. We defined what sensation is and we classified them into different modalities. We also defined what a sensory receptor is and we classified them according to their different classifications based on the stimulus. We also defined what a sensory organ is and we gave examples. Then in particular we named the receptors of general sensations and uh we stated the sensory modality for each of those receptors. And finally, we described the pathway that general sensations take from the receptors all the way to the cerebral cortex. So that is what we did last time. In this particular lecture, we are going to look at the following. First, maybe we'll review that classification of sensory modalities in brief. We'll also review the structural outline of sensory relay pathways because inasmuch as we did it under general sensation, we use the same template even for special sensation, albeit some few differences and so it will be important to have that in mind. Then in particular we'll focus now on the olfactory sensation. As we do that we look at the structure of the nose and the structure of the olfactory epithelium as well. We'll also look at gustatory sensation which is taste as you do that we look at the structure of the mouth, the structure of the tongue, the epithelium of the tongue and the receptors of the tongue. We'll also look at the structure of the ear and explain the pathways of hearing as well as equilibrium. And finally, we look at anatomy of the eye, retinal structure, visual pathway, extraocular muscles and the lacrimal apparatus.
[3:05]Now, I'll do this in perhaps in bits because it's long. We can't do that in one continuation. So maybe the first part I will handle the first four objectives. And that's what I want us to do in this first part. In the second part, we'll talk about objective number five and in the third part we'll talk about objective number six. I've divided it that way according to the weight of each of the objectives. Let's start with the first objective there which is basically to review the classification.
[3:51]Before that, remember some definitions, we defined sensation as that state of awareness of a stimulus, which can be from external or internal environment, classified as special and general sensations.
[4:10]And I'm going to release that shortly. We also defined a sensory receptor and we said that sensory receptors are structures that detect stimuli. And fundamentally they should be able to convert physical energy that is present in a stimulus whether mechanical, chemical, thermal, whatever kind of physical energy. A sensory receptor must be able to convert that physical energy into nerve impulses. And then we classify those receptors according to what really stimulate them. So we talked about mechano-receptors, thermo-receptors, chemo-receptors, photo- and nociceptors. We also defined what sensory organs are. Said an organ that houses a receptor. So things like the skin, the eye, the tongue, etcetera are sensory organs.
[5:16]So I hope you remember that definition. As we regard to the classification of sensations into general and special, the special sensations are the ones which are housed by the head region. So they're contained within the head region. And so importantly they are relayed by cranial nerves exclusively. And so these five sensations are the ones we're calling special: vision, smell, taste, hearing, and equilibrium of the head. General sensations are not limited to the head region, which means they can be picked even from the rest of the body and they are also not limited to being relayed by cranial nerves, which means they can also be picked by spinal nerves.
[6:08]There are several general sensations. So that idea of they're only five common senses may not be very accurate to this extent. Last time we looked at the right hand side of the column, the general sensations. So in this particular lecture series we are going to focus on this side, the special sensations. And so that brings me to the other thing about sensory relay pathways. We need to review something about the sensory relay pathways.
[6:56]The sense relay pathway we defined, we defined it as the sequence of neuronal fiber processes that convey sensory signals from a particular group of sensory receptors towards the brain cerebral cortex.
[7:23]When we describe a sensory relay pathway, we talk about some entities. The first one is the sensory receptor which detects the stimulus.
[7:44]After it has detect the stimulus, it will carry that to now the first order sensory neuron, the neurons which are now able to relay the impulses to the central nervous system. Now, remember, receptors here could be many varieties. We talked about the receptors of general sensations, are many, with each sensory modality. In this particular lecture, we're going to look at the receptors of special sensations again according to their sensory modality. Now, the primary sensory neuron or the first order sensory neuron is the neuron that is in contact, or sometimes part, of the sensory receptor. It carries information to the central nervous system. So it will terminate either at the level of the brainstem or at the level of the spinal cord.
[8:46]In most cases therefore the cell body of the first order sensory neuron will be outside the CNS and it'll form what we call the sensory ganglion. It synapses with the second order sensory neuron. Second order sensory neuron is usually within the central nervous system. This second order sensory neuron is therefore most likely a multipolar neuron.
[9:20]Now two things that the second order sensory neuron must do. One is it must decassate, which is to cross over. So from right to left or from left to right. And two second order sensory neuron reaches the thalamus. So as the first order and the second order neurons are traversing the spinal cord and the brainstem, you know they run in form of groups of axons of neurons.
[9:52]Those groups of axons of neurons with a similar function is what we call tracts. And so these tracts are carrying information to the brain. We call them sensory tracts or ascending tracts or sensory pathways. So remember, those pathways are formed largely by axons of either first order or second order neurons as they pass through spinal cord or brainstem. Anyway, the second order neuron synapses with the third order sensory neuron. The third order sensory neuron come from the thalamus all the way to the cerebral cortex. Remember it will be now on the opposite side because the second order neuron decassated. The last entity on that pathway is actually the cerebral cortex and in particular the region of the cerebral cortex that is involved in perception of that particular sensation. For example, if it is pain, then you know which region of the cerebral cortex for perception of pain. If it is touch, again you know that. How about if it's hearing? If it is smell, if it is vision, they have different cortical regions for perception of those sensations. So that's the general outline of how sensations are relayed. These images were in the previous class and uh you remember like in this one, maybe the receptor there is a free nerve ending. First order neuron enters the CNS. You can see it as a unipolar neuron so the cell body is outside. It synapses the second order neuron at the level of the spinal cord in this case. The second order neuron decassates and climbs all the way to the thalamus. As this second order neuron is climbing all the way to the thalamus, several of them will form ascending tracts of the spinal cord and brainstem. It synapses the third order neuron which then goes to the cerebral cortex, that region is shaded in that color highlighted. And so if you extrapolate that from this one, we notice that actually that corresponds to the postcentral gyrus, the primary somatosensory cortex. Most likely that's a general sensation. Yes it is. Free nerve endings so pain, temperature, light touch, they go to that region. This other one showing us corpuscles. So maybe the sensation regarding vibration. The first order neuron enters again. In this case, the first order neuron climbed all the way to the brainstem before synapsing. So it will be on the same side. It will still form tracts there. The decassation will take place on the second order neuron and then it will reach the thalamus again. And so even the second order neuron here will also form another tract just like the first order neuron also forming another tract. The third order neuron terminates at the level of the cerebral cortex, the region of perception which again is postcentral gyrus. So the concept here is that we need to know that there are three levers of neurons all the way to the cortex. First order, second order and third order neuron. At the cortex, the region where the third order neuron terminates will depend on the sensation coming in. For example, the sensations which are going to the postcentral gyrus are the ones which are for general sensation. But we know from our neuroanatomy, neurophysiology lectures that we have different cortical regions for different sensations and especially the special sensations that we're going to look at today. Remember, as the axons of first and second order neuron traverse the brainstem and spinal cord, they form ascending tracts. And the image on your left uh captures the different tracts of the spinal cord. I hope you do remember them for general sensations. So those ones are general sensations. I'm bringing this up so that you know that there are some things which are basically a common template. But we've reviewed that enough. So I want us to focus on the first two special sensations. I want us to focus on the olfactory sensation and gustatory sensation in this first part of the lecture. Let's begin with the olfactory sensation. So we're going to look at the structure of the nasal cavity. We'll also look at the structure of the olfactory epithelium and we'll describe the pathway of smell. This is how the nasal cavity looks like. If you were to cut in coronal section, if you take a coronal section of the nose, we'll see something like this. Usually the nasal cavity consists of two nasal airways, the right and the left side. Well, evident by, when you look at the anterior part of your nose, you will see two openings. Those two openings are the ones we call the nostrils and they lead you to each of the nasal airway. There's a septum that divide the right and the left nasal airway. So that we have the nasal septum there. That septum can either have cartilaginous parts and bony parts. So it is both cartilaginous and bony septum.
[15:56]Of course, it's lined by mucosa either way. The lateral wall of the nose has these projections. They're still bony projections.
[16:09]We call them turbinates or conchae. The purpose of these projections is to increase the surface area over which the mucosa of the nasal epithelium will lie. And so, this is the superior nasal turbinate, this is the middle nasal turbinate and that's inferior nasal turbinate. We have three of them.
[16:34]The grooves between the turbinates are the ones we call the meatuses. So this is the inferior nasal meatus. This is the middle nasal meatus. And that's superior nasal meatus. Remember this is the roof of the nose, the upper part. And this is the lower part. So this is the palate here. The oral cavity below this point. Now, the cavity of the nose, whether which chamber is lined by mucosa. And based on how that mucosa looks like, based on the how that epithelium looks like, we can describe three histological regions of the nose. We have a region we call the nasal vestibule, anterior parts. We have a region that we call the respiratory region. And we have a region that we're terming the olfactory region.
[18:05]So the nasal vestibule is this region here, just behind the nostril. That region of the nasal cavity just behind your nostril is lined by epithelium similar to that one of the skin. The skin has what we call stratified squamous keratinized epithelium. When you say epithelium is keratinized, it means that it has several layers of cells. When you say epithelium is squamous it means that the cells on top are flattened. So stratified squamous epithelium is what, the type of epithelium we usually find in regions that are more subject to friction, like the skin. And that extends also into the nasal vestibule. So the epithelium lining of the skin and that of the nasal vestibule is similar.
[19:10]This is how that epithelium will look like. Um that region is the region of the epithelium. It's a lower magnification but we can see some keratin and we can see the epithelium. This is connective tissue corresponding to the dermis of the skin basically. So, in simple terms, the nasal vestibule is lined by skin.
[19:33]That skin has a lot of hair and that's what you're seeing here. These are hair follicles as they are within the dermis. It contains a lot of hair. The hair of the nose is what we call the vibrissae. Other than the hair, usually hair also, usually associated with sebaceous glands. And so there are also several sebaceous glands in that region. We're just saying it's a hairy skin. Now, important to note is that that region of the nose does not contain the receptors of smell.
[20:17]It doesn't. So that is it, the nasal vestibule. The second part is the respiratory region. The respiratory region refers to the lower two-thirds of the rest of the nasal chamber. If you remove the vestibule, the lower two-thirds of the rest of the nasal chambers. This region of the nose is lined by a different type of mucosa. It is lined by the respiratory mucosa. We define respiratory mucosa as pseudostratified columnar ciliated epithelium with goblet cells.
[21:03]And uh beneath it is usually what we call the lamina propria. So let me say that slowly again. When you say epithelium is pseudostratified, it means that you basically have a mixture of short and tall cells.
[21:24]And so histologically it will look like some nuclei are at different levels. So some nuclei are on the basal side and others somewhere. And generally the top of the epithelium is usually lacking in the nuclei. That's how pseudostratified columnar epithelium looks like. So this one, those, that epithelium is ciliated and that's the ciliary lining. We can see that one there. And it usually has a lot of goblet cells. Goblet cells are mucous cells. So that's how the epithelium looks like. Or in this image, we can capture that. You see it's a mixture of short and tall cells. Beneath the epithelium lining is a loose connective tissue layer, which usually call the lamina propria. So when you talk of respiratory mucosa, we are referring to the epithelium lining and its lamina propria.
[22:31]This epithelium and this lamina propria is the one that aderes onto the bones of the nose. It's the one that line to the different bones of the nose. Is the one that lines the respiratory region of the nasal chamber. Important to note is that this epithelium does not have again the olfactory receptors, but instead it has several mucous cells. Maybe it's important to remind ourselves the cell types that are usually found on this epithelium. This is how the respiratory epithelium look like at a low magnification or relatively lower. So the ones you're seeing here, large non-staining parts are the mucous cells. The cell types of the respiratory mucosa are the following: The most abundant are the ciliated columnar cells. These are the cells that contain cilia. They're the ones that form the protective barrier, but as we know, other than forming the protective barrier, the cilia they contain help to propel substances on the surface of the epithelium. Goblet cells are these ones which produce mucus. So the mucus produced by the goblet cells trap particles in the nose and then that mucus is propelled by the cilia to a particular direction, usually posteriorly. That is what we call the mucociliary rejection current. So the ciliated columnar cells and the goblet cells work in collaboration. That collaboration is termed the mucociliary rejection current that help to trap and extrude substances away from the respiratory tree. The third cell type are the basal cells. They are the, they're the visual, they're the short ones. These ones are stem cells basically. They give rise to the other cell types. We have the small granule cells. You can call them small granule cells or the Kulchitsky cells. These cells produce hormones. They are part of what we call the diffuse neuroendocrine system. Cells which produce hormones in response to neuronal stimuli. Diffuse neuroendocrine system.
[25:00]Lastly, we have the brush cells. The brush cells are chemoreceptor cells. They detect concentration of oxygen perhaps within the respiratory tree. So these ones are the cell types of the respiratory region. Important to note the respiratory region has a lot of mucous cells and does not have the olfactory receptors. The last and perhaps the most important one for this lecture is the olfactory region which is now the upper one third of the nasal chambers. The olfactory region is lined by the olfactory mucosa, as opposed to respiratory mucosa. Once again, when we talk of olfactory mucosa, we are referring to a pseudostratified columnar ciliated epithelium but now without goblet cells. It has pseudostratified columnar ciliated epithelium that does not have goblet cells. Deep to that epithelium is the lamina propria. Now what's unique about the lamina propria for olfactory epithelium? The lamina propria is still of loose connective tissue, true, but there's something else. That lamina propria contains several glands, like that one or those ones. These glands are known as the Bowman's glands. In addition to those glands, the lamina propria of olfactory mucosa also contain nerve bundles, like this large thing is a nerve bundle. So these nerve bundles are, of course, the nerve bundles of olfactory nerves.
[27:00]The olfactory region of the nasal cavity is lined by olfactory mucosa, which has that epithelium and that kind of lamina propria. So this image captures for you how the olfactory epithelium would look like and how the lamina propria for the olfactory epithelium look like. Importantly, we have the Bowman's glands in the lamina propria and we have nerve bundles in the lamina propria. In the epithelium itself it has pseudostratified columnar ciliated epithelium that does not have goblet cells. And you can see the pseudostratification here. All the cells reach the base, but we have a mixture of short and tall cells. Now there are three major cell types on the olfactory epithelium.
[27:56]The olfactory receptor cells, which is this one or that one. The olfactory receptor cells are basically chemoreceptor cells. They detect chemicals.
[28:11]As we can see in this image, these olfactory receptor cells are actually bipolar neurons because you can see this is the cell body. The axon goes to form the optic nerve. The dendrite is on this side on the surface with those ciliary processes. So the cilia of the olfactory mucosa come from the bipolar neurons of the olfactory epithelium, rather than from the supporting cells as you may have expected in the respiratory mucosa. So one would look like that with the surface there being an open space that's the test pore and looking around like that. There are some cell types of the test buds that I want us to talk about. We have the taste receptor cells. You can call them gustatory cells. So the gustatory cells are the taste receptor cells. This taste receptor cells are chemoreceptor cells. They detect chemicals within the epithelium of the mouth. Other than that, we also have the sustentacular cells, which are basically supporting cells. They provide structural support to the test buds. Lastly, we have the basal cells.
[29:46]As you mentioned, even for olfactory epithelium, the basal cells here are for regeneration. They give rise to the other cell types. So those are the three types of cells which we find in the test buds.
[50:35]Having said so, let's finish this first part of the lecture by talking about the pathway of taste sensation. So once taste has been detected by the taste buds and in particular the chemoreceptor cells within the taste buds, we've called them the gustatory receptor cells.
[51:00]That information is relayed through first order neurons that carry taste. The first order neurons that carry taste are many depending on the region they come from. The test buds in the anterior two-thirds of the tongue will relay their sensations through the seventh cranial nerve which we call facial nerve. The specific branch of facial nerve that run to the tongue for taste sensation is called the chorda tympani nerve. The cell bodies of those neurons will be within a ganglion within the facial nerve found somewhere in the ear which we call the geniculate ganglion. Geniculate ganglion. The test buds from the posterior one-third of the tongue are relayed by glossopharyngeal nerve. Similarly the sensations of taste from the test buds on the epiglottis are carried by vagus nerve, the tenth cranial nerve. And they also have their own cell body location, a ganglion somewhere. Whichever the taste has come from, the information carried by the first order sensory neuron for taste goes to the brainstem. It will terminate in a nucleus within the brainstem. Remember a nucleus is a collection of cell bodies of neurons within the white matter of the CNS. So that will be containing cell bodies of the second order sensory neurons. We call that nucleus the gustatory nucleus. It is found in the brainstem. From the gustatory nucleus we have second order sensory neurons for taste. They will decassate and climb all the way to the thalamus, as we said. So after decassation, they go to the thalamus.
[53:18]From the thalamus, we'll have synapse in the third order neurons, which will take information from the thalamus to the cortical region for perception of taste. What we call the primary gustatory cortex, Brodmann's area number 43, which is found somewhere in the parital operculum. If you remember the operculum. So the parital operculum, that's where you find the primary gustatory cortex. And that's where the sense of smell is perceived. So that is the pathway of olfaction. And that summarizes for us what I wanted us to capture in the first part of the lecture, focusing on some reviews that we'll just said, then in particular, we've looked at olfactory sensation and taste sensation.
