Chemosensation

Question 1

“Loss of taste” =

“Loss of smell” =

Answer 1

“ageusia”

“anosmia”

Question 2

Chemosensory systems that make up “taste” sensation

Answer 2

• olfaction = smell; olfactory neuroreceptors @ roof of nasal cavity
• gustation = taste buds @ oral cavity/pharynx to detect sweet, sour, bitter, salty, umami
• chemesthesis = detection of irritant/noxious chemicals by sensory processes of tigeminal extending into nose/mouth

Question 3

Smell: receptor cell, CN, primary sensory nucleus @ CNS, morphology of sensory epithelium

Answer 3

• ciliated, bipolar neuron
• CN I
• nucleus = olfactory bulb

Question 4

Gustation: receptor cell, CN, primary sensory nucleus @ CNS, morphology of sensory epithelium

Answer 4

• modified epithelial cell ==> synapses onto nerve fiber from cranial ganglion cell
• CN VII, IX, X
• Nucleus of the solitary tract

Question 5

Chemesthesis: receptor cell, CN, primary sensory nucleus @ CNS, morphology of sensory epithelium

Answer 5

• free nerve ending of cranial ganglion cell
• CN V
• Spinal trigeminal nucles

Question 6

Types of papillae

Answer 6

• Taste papillae = contain taste buds
  
  • fungiform = @ anterior end of tongue
  • foliate papillae = @ sides of the tongue
  • cirumvallate papillae = @ posterior part of tongue
• filiform papillae = non-taste = tactile organs

Question 7

Innervation of the tongue

Answer 7

• chorda tympani (facial nerve) ==> fungiform taste papillae @ anterior 2/3
• glossopharyngeal ==> circumvallate papillae
• superior petrosal (facial nerve) ==> palate
• vagus ==> extreme posterior tongue, oropharynx, epiglottis

Question 8

Types of taste receptor cells stimuli-response

Answer 8

• salty/sour (protons) detected via ion-gated channels
• sweet, bitter, umami rely on G-protein coupled receptors

Question 9

Central taste pathways (conscious)

Answer 9

• afferents from tongue (via CN VII, CN IX, CN X) ==> ipsilateral nucleus of the solitary tract
  
  • organized “orotopically”
• ==> bilateral medial portion of ventrobasal thalamus (VPMpc) ==> insular cortex = primary gustatory area = conscious appreciation of taste
• integration/perception of flavor = @ secondary gustatory cortex (receives projections from primary gustatory and olfactory areas of insula

Question 10

Central taste pathways (sub-conscious)

Answer 10

• NST ==> hypothalamus and amygdala for regulation of food intake and visceral reactions to ingested foods
• NST ==> reflex connections to brainstem nuclei involved in gagging, swallowing and salivation

Question 11

Variations in taste sensitivities and impacts on health

Answer 11

• bitter = evoluntionarily important in avoidance of noxious/harmful chemicals and foods
• bitterness of drugs ==> decreased compliance; use-limiting factor
• sensitivity to bitterness ==>
  
  • eating less vegetables
  • nutritionnally-related pathology (e.g. colonic neoplasms)

Question 12

Peripheral anatomy of olfactory system

Answer 12

• olfactory neuroepithelium is covered w/a thin layer of mucus
• olfactory neurons extend cilia into mucus layer
  
  • cilia ==> expand SA of olfactory membrane exposed to mucus
• olfactory neurons = bipolar = dendrite @ one end & single, thin unmyelinated axon from other end ==> olfactory bulb ==> CN I

Question 13

Characteristics of odorant interaction with olfactory neurons

Answer 13

• odorants dissolve in mucus ==> interact w/olfactory receptor proteins
  
  • clia contain most of biochem machinery necessary to tranduce chemical to electrical signal

Question 15

Peripheral mechanisms of olfactory transduction

Answer 15

• recognition of odors via one or more olfactory receptor proteins on ciliary membrane
  
  • 7TM g-coupled receptor proetins
  • subfamilies probably bind distinct odorants
  • each receptor predominantly expresses one type of receptor
• G_olf ==> activates adenylate cyclase ==> increase cAMP ==> opening of ion channels ==> amplified depolarization ==> AP @ cell body

Question 16

Olfactory sensation @ the olfactory bulb

Answer 16

• overall: olfactory neurons ==> olfactory bulb ==> central olfactory system
• olfactory neurons ==> excitatory synapses w/apical dendrites of mitral (tufted) cells
  
  • olfactory neurons w/similar ORproteins proect to same glomerulus ==>
  • odor-related map of glomeruli @ olfactory bulb
• identification of odors entails recognitions of the pattern of activity across all glomeruli

Question 17

Central olfactory pathways

Answer 17

• olfactory bulb projects directly to cortex
• olfactory bulb axons ==> lateral olfacotry tract ==>
  
  • olfactory cortex = lateral olfactory gyrus + part of uncus
    
    • olfactory cortex divided into:
    • piriform cortex ==> orbitofrontal (secondary gustatory) cortex directly + via MD thalamus
    • entorhinal cortex ==> hippocampus
  • olfactory tubercle ==> hypothalamus
  • amygdala ==> hypothalamus