Physiology Unit 4

• Gustation and olfaction are stimulated by chemicals, with gustatory (taste) and olfactory (smell) receptors being involved
• Gustation influences eating behavior by providing information about what a material contains, allowing us to decide whether to ingest it or not
• We are sensitive to five (and perhaps six) qualities of taste: salt, sweet, bitter, sour (acid), umami, and possibly fat
• Detecting salt in food is important for regulating sodium levels, while sweetness often indicates safe-to-eat foods like fruits and some vegetables
• Bitter and sour tastes signal foods to avoid, with sourness often indicating spoilage and bitterness signaling potentially poisonous substances
• Umami, derived from the Japanese word for "deliciousness," is produced by monosodium glutamate (MSG) and indicates foods high in proteins
• Fat may have its own taste quality, detected through mouthfeel rather than specific taste receptors, with evidence suggesting taste receptors respond to fatty acids
• To taste a substance, it must dissolve in saliva, stimulating taste receptors found in taste buds located on the tongue, palate, pharynx, and larynx
• Each taste bud contains around 20-50 taste receptors, which are replaced every 10 days due to constant exposure to stimuli
• Taste receptors convert taste sensations into electrical impulses through transduction, with different molecules binding to different receptors to produce various taste sensations
• Sweet, umami, and bitter molecules interact with taste receptors, altering their membrane potential and stimulating a bipolar sensory neuron
• Salt and sour molecules stimulate taste receptors by passing ions directly through ion channels, causing depolarization and the production of an action potential
• Taste sensations are detected by taste buds on the tongue, with different types of taste buds responsible for detecting sweet, sour, salty, bitter, and umami tastes
• Taste signals from cranial nerves VII, IX, and X are sent to the nucleus of the solitary tract in the medulla, then to the thalamus and the primary gustatory cortex in the frontal lobe
• The primary gustatory cortex processes taste sensations, with different parts responding to different tastes like sweet, bitter, etc., and works with the secondary gustatory cortex to process taste sensations
• Individual differences in taste sensitivity can be influenced by factors like hormone levels, genetics determining the number of taste buds, leading to supertasters and nontasters
• Olfaction, like gustation, guides eating behavior by detecting safe and unsafe foods, with olfactory receptor cells in the nasal cavity detecting odorants that bind to receptor proteins
• Olfactory receptor cells send impulses along axons to the olfactory bulbs at the base of the brain, where mitral neurone axons project to the rest of the brain along the olfactory tract
• Humans possess 339 types of olfactory receptor cells, each containing one type of receptor protein, allowing recognition of up to 10,000 different odorants
• Olfactory receptor cells can produce different patterns of activation across olfactory receptor cells when stimulated by different odorants, leading to different olfactory sensations
• Olfactory receptor cells send signals to the olfactory bulb, which is organized into a map where glomeruli in specific regions become activated by odorants with similar molecular structures
• Despite similar molecular properties, odorants do not necessarily produce similar olfactory sensations; for example, octanol smells "sweet and fresh," whereas octanoic acid smells "rancid and sour"
• Smells are perceived when olfactory signals are sent from the olfactory bulbs to the primary olfactory cortex, also known as the piriform cortex
• The piriform cortex sends signals to the orbitofrontal cortex in the frontal lobe, which plays a crucial role in combining gustatory and olfactory information to create flavor
• In the olfactory bulb, activity associated with some odorants can overpower or mask the activity of other odorants, a phenomenon utilized by the deodorant industry
• Research suggests that the sense of smell varies between people, mainly according to age and gender; women appear to be generally more sensitive to odorants compared to men, independently of age and culture