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Unit 14 Week 2 (Tommy) > 14.2 Neurobiology of smell & taste > Flashcards

14.2 Neurobiology of smell & taste Flashcards

1
Q

What is smell & taste known as?

A
  • Smell = olfaction
  • Taste = gustation

They are examples of visceral senses due to their close association with GI function

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2
Q

What type of receptors are smell & taste and how are they stimulated?

A
  • They are chemoreceptors
  • Stimulated by chemical molecules in solution in mucus in the nose (odorants) and saliva in the mouth (tastants) and the sensations of smell and taste likely evolved as protective mechanisms to avoid the intake of potentially harmful substances
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3
Q

Briefly describe the olfactory epithelium

A
  • Yellowish pigmented
  • Is a specialized portion of the nasal mucosa that covers an area of 10 cm2 in the roof of the nasal cavity near the septum in humans
  • Is in an area where the nervous system is closest to the external world
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4
Q

What are the 3 types of cells in the olfactory epithelium?

Label the diagram

A

There are three cell types:

  1. Olfactory sensory neurons (receptor)
  2. Supporting cells (sustentacular)
  3. Basal stem cells at the base of the epithelium
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5
Q

How is smell picked up?

A
  • Each olfactory sensory neuron (bipolar olfactory sensory neurons - do olfactory transduction) has a dendrite (short & thick dendrites) that projects to the epithelial surface
  • Numerous cilia protrude into the mucus layer lining the nasal lumen
  • Odorants bind to specific odorant receptors on the cilia and initiate a cascade of events leading to generation of action potentials in the sensory axon
  • Each olfactory sensory neuron has a single axon that projects to the olfactory bulb, a small ovoid structure that rests on the cribriform plate of the ethmoid bone
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6
Q

Label this diagram (to do with smell)

A
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7
Q

What is the function of supporting cells?

A

Secrete the mucus that provides the appropriate molecular and ionic environment for odor detection in the olfactory epithelium

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8
Q

How do odor-producing molecules (odorants) get picked up?

A
  • Odor-producing molecules (odorants) dissolve in the mucus and bind to odorant receptors on the cilia of the olfactory sensory neurons
  • Odorant- binding proteins in the mucus may facilitate the diffusion of odorants to and from the odorant receptor
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9
Q

Label the diagram of the nose

A
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10
Q

How do olfactory sensory neurons get replaced?

A
  • By basal stem cells undergoing mitosis (into olfactory sensory neurons) e.g. from damage from environmental exposure
  • Olfactory sensory neurons ~ survive 1-2 months
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11
Q

Explain the olfactory mucosa from a histological view

A
  • The olfactory mucosa is a pseudostratified ciliated columnar epithelium located in the superior-most region of the nasal cavity, and contains bipolar olfactory cells whose cilia are embedded in mucus
  • Chemicals which dissolve in the mucus trigger responses in these cilia which initiate a nervous impulse, interpreted in the brain as an odor
  • Supporting cells surround the olfactory cells
  • Mucus-producing Bowman’s glands are embedded in the lamina propria
  • This connective tissue is richly vascularized
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12
Q

How can the olfactory system discriminate between different odors?

A

It can discriminate between different smells thanks to the existance of many different functional odorant receptors

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13
Q

Explain the genetics behind olfaction

A
  • 1000 olfactory genes
    • 400 of these are for olfactory receptors
  • Amino acid sequence of odorant receptors = DIVERSE
    • BUT are ALL G protein-coupled receptors (GPCRs)
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14
Q

On a microscopic level what happens when an odorant molecule binds to its receptor? (steps)

A
  1. G-protein subunits (α, β, γ) dissociate
  2. The α-subunit activates adenylyl cyclase to catalyze the production of cAMP which acts as a second messenger to OPEN cation channels, increasing the membrane permeability to Na+, K+, and Ca2+
  3. The net effect is an inward-directed Ca2+ current which produces the GRADED receptor potential
  4. This then opens Ca2+ activated Cl− channels, further depolarizing the cell due to the high intracellular Cl− levels in olfactory sensory neurons
  5. If the stimulus is sufficient for the receptor potential to exceed its threshold, an action potential in the olfactory nerve (1st cranial nerve) is triggered

6.

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15
Q

What happens in olfaction in the olfactory bulb?

A
  1. Axons of the olfactory sensory neurons synapse on the primary dendrites of the mitral cells and tufted cells to form olfactory glomeruli
    • Each olfactory sensory neuron projects to only one or two glomeruli
  2. The mitral cells with their glomeruli project to different parts of the olfactory cortex
    • ​The central olfactory system is able to decode the pattern of receptor-cell activity that signals the identity of the odorant
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16
Q

What are the inhibitory & excitatory neurotransmitters in the olfactory system?

A
  • INHIBITION = GABA
  • EXCITATORY = GLUTAMATE
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17
Q

Label the diagram in the olfactory system

A
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18
Q

Explain briefly about odorants and the odor detection threshold

A
  • Odorants are generally small, containing between 3 and 20 carbon atoms; molecules with the same number of carbon atoms but different structural configurations have different odors
  • Relatively high water and lipid solubility is characteristic of substances with strong odors
  • The odor detection threshold is the lowest concentration of a chemical that can be detected
    • ​Is NOT the same in ALL individuals
      • Usually woman have a BETTER sense of smell than men & especially during OVULATION
  • Some toxic substances are essentially odorless; they have odor detection thresholds higher than lethal concentrations
    • E.g. carbon dioxide (will die before detected)
  • To detect a change in concentration of odor producing substance must be about 30% difference before detected
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19
Q

What is anosmia & hyposmia/hypersthesia & congenital anosmia?

A
  • Anosmia = inability too smell
  • Hyposmia/hypersthesia = diminished olfactory sensitivity

These can be due to simple nasal congestion, nasal polyps, OR prolonged use of nasal decongestants

Can also be due to more serious problem such as damage to the olfactory nerves due to fractures of the cribriform plate or head trauma, tumors (eg, neuroblastomas or meningiomas), and respiratory tract infections

Olfactory dysfunction = often early clinical symptom of Alzeihmer’s disease

Congenital anosmia = rare disorder where someone is BORN without the ability to smell

20
Q

What is hyperosmia & dysosmia? (& common causes)

A
  • Hyperosmia = ENHANCED olfactory sensitivity
    • Causes:
      • In pregnant women
  • Dysosmia = DISTORTED sense of smell
    • Causes:
      • Sinus infections
      • Partial damage to the oflactory nerves
      • Poor dental hygiene
21
Q

What is the organ of taste known as and where is it located?

A

The organ for taste (gustation) consists of about 5000 taste buds located primarily on the papillae of the dorsal surface of the tongue in humans

22
Q

What are the different types of papillae?

A
  • Fungiform papillae: rounded structures most numerous near the tip of the tongue
    • Up to 5 taste buds (on top)
  • Foliate papillae: are on the posterior edge of the tongue
    • Contain up to 100 taste buds (sides)
  • Circumvallate papillae: are prominent structures arranged in a V on the back of the tongue
    • Contain up to 100 taste buds (sides)
23
Q

Explain the innervation of the tongue

A

Taste buds on the anterior two-thirds of the tongue are innervated by the chorda tympani branch of the facial nerve; those on the posterior one-third of the tongue are innervated by the lingual branch of the glossopharyngeal nerve

24
Q

What are taste buds composed of?

A
  • Taste buds are composed of basal stem cells and three types of taste cells:
    1. Dark
    2. Light
    3. Intermediate
  • Taste cells extend from the base of the taste bud to the taste pore, where microvilli contact tastants dissolved in saliva and mucus
  • Each taste bud contains 50–100 taste receptor cells and numerous basal cells and support cells
25
Q

What are taste receptors and how do they work?

A
  • The taste receptor cells are modified epithelial cells that respond to chemical stimuli* or *tastants
  • The apical ends of taste cells have microvilli that project into the taste pore, a small opening on the dorsal surface of the tongue where tastes cells are exposed to the oral contents
26
Q

How does saliva function in taste?

A
  • Acts as a solvent for tastants; after dissolving, the chemical diffuses to the taste receptor sites
  • Function to cleanse the mouth to prepare the taste receptors for a new stimulant
  • Each taste bud is innervated by about 50 nerve fibers, and conversely, each nerve fiber receives input from an average of five taste buds
27
Q

How are taste cells produced and how long do they live for?

A
  • Basal cells (from epithelial cells aroud taste bud) into taste cells
  • Survive ~10 days
28
Q

What happens to taste buds when sensory fibres are cut?

A

Taste buds that it innervates degenerate and then suddenly disappear.

29
Q

Explain the gustation area with the 3 nerves

A

On each side, the myelinated but relatively slowly conducting taste fibers in these three nerves unite in the gustatory portion of the nucleus of the tractus solitarius (NTS) in the medulla oblongata

30
Q

Explain the nerve pathway from the tongue to brain

A
  1. On each side, the myelinated but relatively slowly conducting taste fibers in these three nerves unite in the gustatory portion of the nucleus of the tractus solitarius (NTS) in the medulla oblongata
  2. From there, axons of second-order neurons ascend in the ipsilateral medial lemniscus and project directly to the ventral posteromedial nucleus of the thalamus
  3. From the thalamus, the axons of the third-order neurons pass to neurons in the anterior insula and the frontal operculum in the ipsilateral cerebral cortex
  4. This region is rostral to the face area of the postcentral gyrus, which is probably the area that mediates conscious perception of taste and taste discrimination
31
Q

Explain where the gustation cortex is?

A
32
Q

Explain the TRPV1 receptors of taste buds

A
  • Sensory fibers in the trigeminal (5th cranial) nerve also innervate the tongue and contribute to the burning sensation experienced when we eat foods containing capsaicin
  • Taste buds are surrounded by TRPV1 receptors (a protein that, in humans, is encoded by the TRPV1 gene) on trigeminal nociceptive fibers that are activated in response to eating spicy foods
33
Q

What are the basic taste modalities?

A
  • Humans have five basic taste modalities are:
    • Salt (common stimuli: sodium chloride)
    • Sweet (common stimuli: sucrose)
    • Sour (common stimuli: hydrochloric acid)
    • Bitter (common stimuli: quinine)
    • Umami (common stimuli: monosodium glutamate)
  • These have 2 major types of receptors:
    1. Ligand-gated channels (ionotropic receptors)
      • Salt & sour
    2. GPCRs (metabotropic receptors)
      • Sour, bitter & umami
34
Q

What is salt-sensitive taste mediated by?

A

Salt-sensitive taste is mediated by an epithelial sodium channel (ENaC) and the entry of Na+ into the salt receptor depolarizes the membrane, generating a receptor potential

35
Q

Explain the mechanism of sour tasting?

A
  • The sour taste is triggered by protons (H+ ions) as ENaCs(epithelial sodium channel) permit the entry of protons and may contribute to the sensation of sour taste
  • The H+ ions can also bind to and block a K+ sensitive channel
  • The fall in K+ permeability can depolarize the membrane
  • Also, a hyperpolarization-activated cyclic nucleotide-gated cation channel (HCN) and other mechanisms may contribute to sour transduction
36
Q

What picks up sweet taste?

A

2 GPCRs:

  • T1R2
  • T1R3
37
Q

Process of bitter taste explain

A
  • Bitter taste is produced by a variety of unrelated compounds
  • Many of these are poisons, and bitterness serves as a warning to avoid them
  • Some bitter compounds (eg, quinine) are membrane permeable and bind to and block K+-selective channels
  • Many bitter tastants (eg, strychnine) bind to GPCRs (T2R family) that couple to the heterotrimeric G-protein, gustducin
  • Gustducin lowers cAMP and increases the formation of inositol phosphates (IP3), which releases Ca2+ to trigger depolarization
38
Q

Process of umami taste explain

A
  • Umami tastants activate a receptor comprised of T1R1 and T1R3
  • Umami taste may also involve the activation of a truncated metabotropic glutamate receptor, mGluR4, in the taste buds
39
Q

What concentration change of tastant is nercessary to detect a difference?

A

30%

40
Q

What is a taste threshold?

A

Minimum concentration at which a substance can be perceived (bitter taste usually lower threshold)

41
Q

What can damage to linguinal/glossopharyngeal nerve cause (taste)?

A
  • Ageusia (absence of the sense of taste)
  • Hypogeusia (diminished taste sensitivity)
42
Q

What can problems with taste sensitivity be due to?

A
  • Neurological disorders such as vestibular schwannoma
  • Bell palsy
  • Familial dysautonomia
  • Multiple sclerosis
  • Certain infections (eg, primary ameboidmeningoencephalopathy)
  • Poor oral hygiene
  • When serotonin &/or norepinephrine levels are altered (conditions - taste disturbances)
43
Q

What can diminished taste be due to?

A
  • Aging
  • Tabacco use
44
Q

What is dysgeusia/parageusia?

A
  • Unpleasant perception of taste causes a metallic, salty, foul, or rancid taste and in many cases, dysgeusia is a temporary problem
45
Q

Where are the taste types found in a taste bud?

A

Unit 14 Week 2 (Tommy) (5 decks)

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