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Chapter 15

ANP1106 Chapter 15: Smell and Taste

4 Pages

Anatomy and Physiology
Course Code
Jackie Carnegie

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ANP1106 C Dr. Jackie Carnegie & Dr. W Staines 1. Physiology of Smell and Taste 03.21.2017 Chemical senses – gustation (taste) and olfaction (smell); Their chemoreceptors respond to chemicals in aqueous solution; Taste – to substances dissolved in saliva; Smell – to substances dissolved in fluids of the nasal membranes  Complement each other and respond to different classes of chemicals Smell Odorant bind to olfactory receptor neuron and generate AP Olfactory Receptor Location and Structure Olfactory Epithelium: organ of smell, pseudostratified epithelium in roof of nasal cavity covering superior nasal conchae on each side of nasal septum Contains millions of olfactory sensory neurons (receptor cells) cushioned by supporting cells  Unique bipolar neurons each with a thin apical dendrite terminating in knob where many long olfactory cilia radiate – increase receptive surface area, covered by coat of mucus to capture and dissolve airborne odorants, largely nonmotile  Smell is the only input pathway to the cerebral cortex that doesn’t pass through thalamus  Superficial location therefore damaged easily – life span of 30-60 days; olfactory stem cells are at the base of epithelium and replace olfactory sensory neurons Specificity of Olfactory Receptors – an odor is made of hundreds of different chemicals  Mucus captures airborne odorants (chemicals capable of detection by the receptors); there are ~1000 unique receptors capable of differentiating ~10 000 odors o It may take activation of 10-20 odorant receptors fitting together to build the ‘picture’ of one unique odor o just a few molecules are sufficient to activate an olfactory receptor cell (highly sensitive) Physiology of Smell - odorant must be volatile (gaseous state entering nasal cavity) and dissolve in fluid Activation of Olfactory Sensory Neurons – dissolved odorant stimulate olfactory sensory neurons by:  binding to the receptor proteins in olfactory cilium membranes  opening cation channels and generating receptor potential  upon threshold stimulation, AP is generated and conducted to first relay station in olfactory bulb Transduction – uses receptor linked to a G-protein and follows the events of a G-coupled protein receptor  odorant binds to receptor which initiates G proteins to activate enzymes (Adenylate cyclase) to synthesize cAMP as a second messenger o cAMP acts as an intracellular ligand to activate ligand gated ion channels to made an EPSP  cAMP opens Na and Ca channels to enter cell and cause a depolarization in receptor membrane of olfactory sensory neurons which triggers an AP in these neurons o Ca influx causes the transduction process to adapt, decreasing response in a sustained stimulus –2+lfactory adaptation; constant smell becomes odorless after a while; Ca potential is 140 mV The Olfactory Pathway  Axons of olfactory receptor cell ascend through tiny openings in cribriform plate and synapse in olfactory bulbs (distal ends of olfactory tracts) into cranial cavity  Filaments of olfactory nerves (bulbs) synapse with neurons called mitral cells which are second-order sensory neurons o the nonmyelinated axons of mitral cells form the olfactory nerve (not a true nerve, more of olfactory tracts)  When mitral cells activated, impulses flow from olfactory bulbs via the olfactory tracts to the piriform lobe of the olfactory cortex; from there, two major pathways take info to various parts of the brain: i. To olfactory cortex (inferior frontal lobe, above orbit) where smell is consciously interpreted and identified; or ii. To hypothalamus, amygdala, and other regions of limbic system where emotional responses to odors occur; i.e odors associated with danger = smoke, cooking gas, skunk  localization of smell is just anterior the auditory cortex, deep in temporal lobe o temporal lobe seizure causes ppl to smell things of olfactory stimulus that aren’t present  main clusters are seen bilaterally in the amygdala and neighboring cortex and in the right insula and orbital gyrus upon olfactory stimulus (memory and emotion) Physiology of Taste Location and Structure of Taste Buds (Taste organ) - ~10 000 taste buds are located on papillae on the surface of the tongue each taste bud is composed of >50 epithelial cells consisting of:  gustatory epithelial cells – receptor taste cells o gustatory hairs (sensitive part, receptor membrane) are long microvilli that project from tips of all gustatory epithelial cells and extended through a taste pore to surface of epithelium where they are bathed in sal
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