The Chemical Senses: Smell and Taste

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Transcription:

The Chemical Senses: Smell and Taste

Chemical senses: function is to monitor chemical content of the environment olfaction (smell): airborne gustation (taste): mouth Roles in nature: Finding food sources Judging nutritional value and safety of foods Avoiding predators and hazardous environments Social communication, mating ( pheromones ). Olfaction and Gustation www.brainfacts.org

Olfactory Receptor Cells Olfactory receptor cells are located in the upper part of the nose (nasal epithelium) Receptors are regularly replaced (neurogenesis) There are many different kinds of receptor proteins; each type is scattered throughout the epithelium: mice ~1,500; humans ~1,000 Cribriform Plate embopress.org

Olfactory Receptor Sensitivity to Odors Receptor cells express only one type of protein molecule. However, each protein responds to a variety of odors. Odor is encoded by component processing; that is, by the pattern of activity across receptor types.

Transduction of Olfactory Stimuli Odor receptors are metabotropic (G proteins) Activation opens ion channels (Na +, Ca ++ ) frontiersin.org Graded depolarization depending on strength and duration of odor Rapid adaptation (can respond quickly to new odor in next inhalation)

Chemotopic Organization: Bulb The axons of olfactory receptors (1 st cranial nerve) terminate on mitral cell dendrites in olfactory glomeruli within the olfactory bulb Each glomerulus receives input from many receptors, all with the same receptor protein. Glomeruli are arranged in a consistent, systematic way: chemotopic embopress.org

Projections from the Bulb The olfactory tract projects bilaterally to medial temporal lobe structures including the piriform cortex and the amygdala Only system that does NOT first pass through thalamus before cortex Two pathways from medial lobe: limbic system: emotional response to odors thalamus-orbitofrontal cortex: conscious perception Diffuse projections to the limbic system

Taste refers to the sensations relayed by taste receptor cells Foods activate combinations of at least 5 basic tastes: sweet, sour, salty, bitter and umami (meaty) Nutrients vs anti-nutrients Taste Flavor is a multisensory percept depending on taste, smell, texture, temperature, and pain

Taste is primarily a function of the tongue. Taste buds are grouped in three of the four accessory structures called papillae: vallate, foliate and fungiform There are subtle regional differences in sensitivity to different tastes over the lingual surface, but most of the tongue is sensitive to all tastes. The Tongue

Taste Buds and Cells Each papilla has from 1-200 taste buds A person typically has 2000-5000 taste buds. Each bud has between 50-150 taste receptor cells. Receptors are regularly replaced (neurogenesis)

Taste Receptor Cell Transduction Direct transduction: some taste stimuli are ions that carry currents through ion channels. Indirect transduction: other (non-ionic) taste stimuli bind selectively to specific G protein-coupled membrane receptors. Individual taste cells respond to one or two tastes

Direct Transduction: Salty and Sour Salty: Na + ions enter through Na + channels, depolarizing membrane. Sour: H + ions (protons) enter Na + or H + channels AND block K + channels, depolarizing membrane Depolarization opens voltage-gated Na + and Ca ++ channels, leading to AP s (sometimes) and transmitter release, respectively. Salt Sour

Indirect Transduction: Bitter, Sweet, Umami Bitter, sweet, or umami tastes are transduced by G proteincoupled pathways. binding to receptor activates G protein/plc/ip 3 cascade IP 3 causes release of Ca 2+ from internal stores. opens unique Ca 2+ -activated Na + channel, depolarization opens voltage-gated Ca ++ channels transmitter release

Gustatory afferent neurons leave the mouth as part of the 7 th, 9 th, and 10 th cranial nerves to the solitary nucleus of the medulla. Medulla ventral posterior nucleus of the thalamus primary and secondary gustatory cortex (ipsilateral). Neurons in cortex respond to a wide range of tastes. Chemical senses first merge in orbitofrontal cortex Central Taste Pathways

Brain Damage and the Chemical Senses Anosmia: inability to smell Causes: blows to the head that damage the olfactory nerves; tumors on the olfactory tract Results: can lead to lose of interest in eating (weight loss and malnutrition) or depression Ageusia: inability to taste Rare due to multiple pathways carrying taste information from mouth

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