smell and taste

• taste and olfaction are chemoreceptors
• olfaction detects chemical in solution
• olfactory epithelium is the organ of smell; located in roof of nasal cavity
• olfactory sensory neurons are unusual bipolar neurons and each has a dendrite that terminates in a knob where cilia radiate
• olfactory cilia increase receptive surface area, lie flat in nasal epithelium and covered in mucus made by supporting cells and glands
• nonmyelinated axons of olfactory sensory neurons gather into small fascicles collectively to from filaments of olfactory nerve
• smell is a mix of chemicals so cannot be fit into specific categories like taste
• 400 "smell genes" in nose and each gene encodes unique receptor protein

  • each protein respond to one or more odors
  • each odor binds to several different receptor types
  • each receptor cell has only ONE type receptor protein
• nasal cavities contain pain and temp receptors
• odorant must be volatile to be smelled and must dissolve in fluid coating olfactory epithelium
• dissolved odorants stimulate olfactory sensory neurons by binding receptor proteins in olfactory cilium membranes
• olfactory cells synapse with mitral cells , second-order sensory neurons, in glomeruli
• mitral cells refine signals, amplify it and then relay it
• taste buds are sensory organs for taste
• few taste buds are scattered on soft pallets, inner cheek surface, pharynx and epiglottis but most in papillae
• fungiform papillae scattered all over entire tongue surface
• foliate papillae side walls of tongue
• vallate papillae are largest and least numerous papillae; inverted V at back of tongue
• taste buds consist of gustatory and basal epithelial cells
• gustatory cells receptor for taste
• basal cells stem cell that replaces dead gustatory cells
• all areas w/ taste buds can detect all modalities of taste
• taste likes and dislike have homeostatic value
• for a chemical to be taste it must be dissolved in saliva, diffuse in taste pore and contact gustatory hairs
• tastant binds to receptors, graded depolarizing potentials cause neurotransmitter release
• salty taste due to Na influx

  sour mediated by H ion

  bitter, sweet and umami gustducin and release of Ca
• facial nerve, chorda tympani, transmits impulses of taste receptors in anterior 2/3 of tongue
• glossopharyngeal nerve, lingual branch, posterior 1/3 of tongue
• afferent fibers synapse in solitary nucleus of medulla to thalamus to gustatory cortex in insula
• hypothalamus and limbic systems determine appreciation for taste
• taste also trigger digestion and increased salivation and gastric juices break down foods
• taste is 80% smell and the mouth contains thermoreceptors, mechanoreceptors and nociceptors
• when a tastant binds to receptors it induces graded depolarizing potentials that causes neurotransmitter release
• thermoreceptors, mechanoreceptors, and nociceptors are in the mouth; temp and texture of foods enhance and detract from their taste
• spicy foods excite pain receptors in mouth
• taste receptors disorders are less common than smell disorders, taste receptors are served by three different nerves