Olfaction and Taste PDF

Olfaction and Taste OLFACTION The receptor organ for olfaction is the olfactory epithelium. In this epithelium, we find different types of cells: 1. Olfactory neurons: these neurons have olfactory cilia, whose surfaces contain the receptors. They have a knob at the most distal part, soma embedded in...

Olfaction and Taste OLFACTION The receptor organ for olfaction is the olfactory epithelium. In this epithelium, we find different types of cells: 1. Olfactory neurons: these neurons have olfactory cilia, whose surfaces contain the receptors. They have a knob at the most distal part, soma embedded in the epithelium, and their axons exiting as “fila” through the cribriform plate. Join to form the I CN. 2. Supporting cells: found around the neurons. They coat them, but also have a role in secreting mucus to form the mucus layer (which is also secreted by Bowman's glands in the lamina propria) 3. Regenerative cells, constantly replacing olfactory neurons and regenerating the epithelium. They also have cilia, but not for olfaction; much less prominent: these cilia will keep the mucus moving (in this way, substances won’t remain in the medium for too long and saturate the receptors. ONly the necessary time to detect smell) “Neurons do not regenerate”. That’s globally true, but there are exceptions, one of them being olfactory neurons. They do regenerate as they each only last 30-45 days. They can easily be regenerated every x time, but most of our cortical neurons (brain, etc) are lost and do not regenerate. Why do we have a mucus layer? Odorant substances need to dissolve in order to be recognized. Contact with the cells and their receptors is much better when the substances in the air are solubilized in fluid. Gives place to a longer contact with the receptors The axons of these olfactory neurons enter small grooves and cross the cribriform plate of the ethmoid bone. After crossing them, they reach the olfactory bulb. Here, they synapse, and the input continues to the brain through the olfactory tract. It's not completely correct to talk about a single olfactory nerve, as there are many fibres crossing the ethmoid which compose the “I CN complex”. The olfactory epithelium is attached to the bony structures in the nasal cavity, in the upper part; however, the olfactory bulb and brain are floating. The filla that are crossing from epithelium to the bulb are therefore between something that can move and something that is fixed, meaning they can easily be broken, cut, upon brute movements. If you get a concussion in your head = brain is floating so it moves within the CSF. But the filla are attached, within the cribriform plate. It causes them to “snap”. We can easily lose olfaction when there’s trauma to the head (boxers, car accidents,..) Olfactory transduction In the case of this system, the receptor IS the neuron! (in the auditory system = hair cells act as the receptors, which then synapse with neurons; in vision = photoreceptors, and then synapse with ganglion cells (neurons). For touch on the other hand, the receptors are neurons too). How is smell turned into electrical impulse (olfactory transduction)? Olfactory neurons have on their surfaces a receptor family associated to G-Proteins. Made up of 7 transmembrane domains, binding in 4 and 5. - The odorant substance contacts this GP receptor, activates it cand causes release of 2nd messengers. Many types of 2ary messengers, mainly cAMP or IP3 in this case. - One way or another, the final effect is the opening of cation channels - Na+ enters the cell, depolarizing the neurons and an AP is generated that will travel through the axon, to reach the next neuron - There’s only one receptor type per neuron How many different olfactory receptors do we have? It’s unknown! The different types of odours follow different paths. What we do know is that every olfactory neuron only has 1 type of receptor and therefore, responds to only one type of odorant. We can see how the fila crosses the cribriform plate and reaches the olfactory bulb. Organization of the olfactory bulb Olfaction is the oldest of all our senses, present even in non-vertebrate animals. This makes structures relating to olfaction quite atypical. Transverse section of the olfactory bulb: - Fibres reach the periphery of the olfactory bulb - There is processing from the periphery to the centre - Then the output of the bulb exits from the centre ○ Input = periphery ○ Output = centre From top to bottom of the image, we go from periphery to centre A dractory cranial nerve ↳ Peri glomerular Cell At the periphery, we have olfactory nerve fibres coming in. They reach and make synapse in a region called the Glomerulus. Here they’ll synapse with 2 cell types: ○ Depicted in red, the main neurons which I CN fibers contact with. These cells will generate the axons that exit the bulb. They have 2 subtypes: - Mitra cells functiona synapse) - Tufted cells Their fibres will form the output of the bulb: the olfactory tract ○ Depicted in blue, the Periglomerular cells, around the glomeruli. ○ Granule cells, not found making synapse with the I CN but with Tufted and Mitral cells. Also dendrodendritic synapses. 3 & function Synapse wirevee CELLS AND MITRAL & with otacony cranial Nerve (e) Fiber * We can see that we’ll find classical synapses (dendro-axonic), of the I CN with the red cells, but there are also atypical synapses between the red cells and periglomerular cells. We have dendrodendritic synapses! This synapse can be bidirectional, which is also very atypical. NT release from both sides. Only happens here. Periglomerular cells have dendrites but also axons participating in synapses. Therefore, they can contact mitral and tufted cells through: - Dendrodendritic synapses on one end, with the mitral and tufted cells - Axodendritic synapses with mitral and tufted cells on the other end Periglomerular and Granule cells are both inhibitory. Regulate the typical synapses of the main path followed by the relay: - Periglomerular cells (contact olfactory neurons + bidirectional contact with mitral cells, using synapses with their dendrites and axons). Inhibition - Granule cells, again dendrodendritic synapses. Inhibition > - GRANULE CELL In summary: the olfactory pathway in the bulb is regulated by interneurons, where we can find special types of dendrodendritic synapses. NO NEED TO KNOW THE LAYERS Centrifugal projections Lastly, we find neurons that are not exiting but entering the olfactory tract and bulb! These descending projections travel through the olfactory tract and reach the glomeruli + granule cells. Known as centrifugal projections. These projections come from structures such as: produced madrenaline - Locus coeruleus Inbesinster produced - Raphe nuclei - Olfactory cortex Input from here goes to glomeruli and granule cells, inhibitory interneurons. Their function will be to modulate the transmission of olfaction by regulating the regulatory interneurons: granule and periglomerular cells. - > - I , In Reticular formation ; here most sustain is * Olfactory cortex here is most - since #is ofactory cortex is interconnected to deas many et PRIMARY The olfactory cortex is a complex association of neurons and regions; it includes very different areas. It’s a very primitive structure, comprises what is known as the “paleocortex” (paleo = old). Additionally, it’s the only sense that doesn't need to go through the thalamus to reach the cortex. * Main areas Anterior olfactory nucleus From the bulb, the neurons exit through the tract to reach the anterior olfactory nucleus, which is located in the tract. This nucleus has bilateral connections, connects with the opposite side and with the next step in the pathway: the olfactory tubercle Olfactory tubercle Processing centre that is contained within the olfactory cortex and ventral striatum and plays a role in reward cognition. Many other cortical areas are reached, located either in the most ventral part of the frontal lobe or tip of the temporal lobe. Piriform cortex On the ventral aspect of the frontal lobe Amygdaloid nucleus Around the tip of the temporal lobe, medially, we find the amygdala, which is related to emotions. Olfaction may have an emotional performance (smell = good reminders, memories,...), has close relation to the limbic system and emotions. Extrinsic projections Hypothalamus The olfactory system contacts the hypothalamus. Why? For generating autonomic responses. Ex: very bad smell = vomit, faint. Autonomic responses Hippocampus There are also connections to the hippocampus. Some smells bring back memories (all that is effect on behaviour, memory,... are subconscious effects) Thalamus (Dorsomedial thalamic nucleus We also have a discriminative interpretation of smell. Conscious. We can detect and associate what a smell corresponds to (ex, to know what type of cheese or wine it is). Specific perception is carried out by the thalamus. If we want conscious perception of olfaction, it has to go through the thalamus, and then reach the cortex. TASTE How many tastes are there? 5 recognized (main), 1 “candidate” 1. Sweet (saccharides: glucose, sacarose,...) 2. Salty (NaCl mostly) 3. Sour (protons, H+) 4. Bitter (many compounds, more complex. Mechanism is similar to the one carried out by the saccharides) 5. Umami (“tasty”. Carried out by Glutamate. Ex: soy, meat has a lot of glutamate,...) Glutamate is added to food, to make it more tasty. Discussion about the existence of “fatty” taste. Spoonful of olive oil, fatty part of the meat,... Transduction mechanisms There are different transduction mechanisms for each substance. For taste perception, receptors are NOT in neurons but in specific receptor cells - Salty = Nacl → sodium channels (ionic). Open upon the presence of NaCl and the cell depolarizes - Sour = H+→ ionic channels. Protons block potassium channel, K+ cannot go out, and the cell slightly depolarizes - Umami = glutamate receptor, metabotropic - Sweet = G protein associated receptor - Bitter = G protein associated receptor The 2 first are simple to understand, and are both activated by ionic channels. The 2nd, umami, as it’s related to Glutamate, works through Glutamate receptors. These last are very similar in transduction, but are each activated by different agonists. Seems to be associated with a G protein receptor, but the effect of it is under discussion. Types of papillae Receptor cells are located in taste buds. The taste bud includes: ○ Receptors, expressed on gustatory cells ○ Supporting cells Receptors will make synapse with gustatory fibres, neurons that will carry gustatory sensation to the brain (different neurons depending on the region) Taste buds may be individual, or grouped in papillae - Tongue = grouped in papillae - Other parts of the oral cavity = scattered, no papillae Within the tongue, we distinguish several types of papillae: Filiform papillae = no taste buds! Just irregularities on the pill type to be hi ired #P (only a surface Fungiform papillae, quite common in the anterior ⅔, quite large, also have taste buds located in the lateral wall (anterolateral) Circumvallate papillae, located in the junction between the posterior ⅓ and anterior ⅔ Foliate papillae, indentations in the posterolateral (⅓) part of the tongue > - There are also glands, secreting saliva. Von Ebner glands are located in foliate and circumvallate papillae. As said, out of the tongue, taste buds are not grouped in papillae, but scattered. Receptor cells make synapses with the nerve fibre that will carry sensation. 3 nerves, each with their ganglion, but all the info will ultimately reach the same place. “3 nerves, 3 ganglia, but a single nucleus (destination)” ○ Anterior ⅔ of the tongue = Chorda tympani, branch of the VII CN, whose soma is located in the geniculate ganglion (however, somatosensory sensation = touch, temperature, of the anterior ⅔ will be conveyed by the lingual nerve, branch of the V) ○ Posterior ⅓ of the tongue (unlike the anterior, both taste and touch go with the same nerve) = glossopharyngeal nerve (IX CN). The cell bodies will be located in the petrosal ganglion of the IX. ○ Oropharynx = Vagus, X CN, through the superior laryngeal nerve. Its soma is located in the Nodose ganglion All this input coming in through cranial nerves will end in the Solitary tract nucleus (NTS). - Rostral part of NTS is the one related to taste - Caudal part is related to visceral info TASTE and FLAVOUR. FLAVOUR INTEGRATION Flavour includes much more than taste! From the NTS, taste inputs will have to travel to the cortex. Before, as always, there is relay in the thalamus. They pass through the thalamus by synapsing at the ventral-posterior nucleus of the thalamus (VPM). From there, to the insula, and then to the orbitofrontal cortex. This is a cortex that receives visual, taste, somatosensory,... and integrates all inputs. Integration will take place in the orbitofrontal cortex. Taste Olfaction. This is important as most of the flavour we interpret from the food is olfaction! When you have a cold, the food tastes different as there is no olfaction. Somatosensory. It provides texture of the food + spicy or minty perception (“pain” without pain) (ex: drinking a cold or hot beer, having cold soup,...). Flavour changes with somatosensory too DISORDERS OF OLFACTION and TASTE TASTE DISORDERS Ageusia = no taste Hypogeusia = reduction of taste Dysgeusia = misinterpretation. Disorders where we perceive for ex sweet as sour,... OLFACTORY DISORDERS Anosmia = No olfaction Hyposmia = reduction of olfaction Parosmia = no correct interpretation of smells - Cacosmia = type of parosmia. Everything smells bad Examination of taste. Tests we can perform - Main tastes. Ask a person to pull out the tongue, put substances in regions corresponding to specific papillae and then ask them to tell us what they perceive. But they cannot put their tongue in the mouth as saliva will mix and spread everything, we wouldn’t be able to individually assess each region. - Electrogustometry Examination of Olfaction - We have to use non-irritating substances, as irritating ones will also convey somatosensory inputs! - Each nostril can be explored separately although there will always be some connection at the top between one and the other SUMMARY