Taste and olfaction

• Humans can recognise around 10,000 different odors through the olfactory receptors on chemoreceptors in the nasal cavity
• The older an individual is, the less sensitive is their sense of smell as olfactory receptors may die as the individual ages; this may affect satiety and the ability to detect danger in a timely manner
• Humans have around 1000 genes related to olfactory receptors; more than 400 are psuedogenes (function yet to be found) but there are around 390 coded genes known to be actively transcribed and translated into olfactory receptors
• The olfactory bulb is found at the basal side of the brain and is responsible for sending signals from the nasal cavity to the brain
• Mice have a proportionately much larger olfactory bulb, so they are more sensitive to smell
• The olfactory epithelium is found lining the nasal cavity, only in the upper region for humans while the entire nasal cavity is covered by olfactory epithelium in dogs (much more sensitive to smell)
• Olfactory chemoreceptor cells contain olfactory receptors on their membranes and are located in the olfactory epithelium in the upper region of the nasal cavity
• Each olfactory cell contains only one type of olfactory receptors that is receptive to only one type of odor (there are at least 390 types of olfactory cells found in the olfactory epithelium)
• All cells with a particular receptor will converge on a olfactory glomerulus in the olfactory bulb
• Olfactory nerves pass through the cribiform plate and reach the olfactory bulb, which sends signals to other parts of the brain to help remember the odor and take corresponding action
• The olfactory bulb sends signals to the lateral olfactory stria, which then relays the signal to the primary olfactory cortex, then to the amygdaloid complex and hippocampus, which send out secondary neurons to other parts of the brain
• The first step in olfaction is for odorant molecules to enter the nasal cavity and dissolve into the mucous that covers the epithelium
• Dissolved odorant molecules bind to specific odorant receptors on the cilia of olfactory chemoreceptor cells (each type of chemoreceptor cells only expresses one type of olfactory receptor)
• One odor may be recognised by more than one type of chemoreceptors, depending on the epitope of the odorant chemical
• Detection of a particular odorant is encoded by the firing of a distinct combination of specific sensory cells
• Binding of the odorant molecule to the receptor causes the membrane to depolarise, sending signals through the projection of cells and through the cribriform plate to the olfactory bulb
• Each glomerulus is a convergence of one type of chemoreceptor cell (around 100), and each is connected to a mitral cell (second-order neuron) that will transmit impulses to other regions of the brain
• Because there are many different glomeruli, the glomeruli that fires forms the spatial map
• Different odorants are detected by different combinations of receptors and have different receptor codes, and the brain translates these codes into diverse odor perceptions; the number of possible receptor combinations is the basis for the ability to recognise over 10,000 odors
• The frontal cortex is responsible for the conscious perception of smell
• Smell may trigger a pleasant or unhappy memory, and the hypothalamus and amygdala is responsible for the motivational/emotional aspects of smell
• The hippocampus is trained to memorise the smell so the correct response can be elicited the next time the smell is encountered
• Sympathetic nerve endings can release norepinephrine, which binds to chemoreceptor cells and alters the signals it sends to the brain (adrenergic stimulation)
• Sympathetic stimulation increases odorant contrast, filters out weaker responses, amplifies strong ones and focuses on the most prominent and salient olfactory cues
• Parasympathetic nerves release acetylcoholine, which binds to the receptors expressed on chemoreceptor cells and alters the intensity of the signal sent to the mitral cell (cholinergic modulation)
• Parasympathetic stimulation increases response to many odorants, enhances appreciation for the richness and complexity of olfactory cues
• Flavor is a complex sensation involving smell (olfaction), taste (gustation), and texture (tactile sensation)
• There are five basic taste qualities: sweet, umami, salty, sour, bitter
• Sweet: identifies energy-rich nutrients
• Umami: a compound that gives an umami taste is glutamate, which acts as a flavor enhancer in the form of monosodium glutamate (MSG)
• Salty: ensures proper dietary balance of electrolytes
• Sour and bitter: warns against intake of potentially noxious and/or poisonous chemicals
• Taste receptor cells are assembled into taste buds, which are located in the papillae (there are thousands of taste receptors in each bud)
• Papillae types: foliate, circumvallate, filiform, fungiform
• Not all papillae contain taste buds (filiform papillae does not have taste buds)
• Lingual papillae

  A) circumvallate

  B) fungiform

  C) filiform

  D) foliate

  
• In fungiform papillae, the taste buds are found on the surface, while the taste buds are ound inside the cleft for foliate and circumvallate papillae (less sensitive)
• The taste bud contains a taste pore at the top (for food chemicals to enter) and the apical region of taste receptor cells project into the pore, which is in contact with saliva; the basal end of the taste cell forms synapses with primary sensory neurons
• Each taste bud contains thousands of receptor cells and each receptor cell has its own receptors to detect for a particular taste (so at least 5 different types of taste receptor cells)
• Taste cells regenerate every ~10 days