Neurobiology of smell and taste Flashcards
LOs:
- Describe the structure and function of the neural elements in the olfactory epithelium and olfactory bulb.
- Identify the significance of the family of olfactory receptor genes.
- Explain how odorant receptors are activated and the mechanism by which signal transduction occurs in these receptors.
- Label the components of the pathway by which impulses generated in the olfactory epithelium reach five regions of the olfactory cortex.
- Describe the location and cellular composition of taste buds.
- Name the five major taste modalities and compare the signal transduction mechanisms in the receptors mediating these different taste modalities.
- Label the components of the pathways by which impulses generated in taste receptors reach the gustatory region of the insular cortex.
- Name and discuss abnormalities in odor and taste sensations.
Relationship between smell and taste
Both help us interpret the chemical world.
examples of visceral senses because of their close association with GI function
Physiologically, they are related to each other as the flavor of food is a combination of its taste and smell
Olfactory epithelium
what kind if tissue is it
what is it a part of?
where is it?
which 3 cells make it up?
pseudostratified epithelium
yellowish pigmented olfactory epithelium
specialised part of he nasal mucosa in the roof of the nasal cavity near the septum in humans
- Olfactory sensory neurones (receptor)
- Supporting cells (sustenacular)
- Basal stem cells (at the base of the epithelium
Olfactory sensory/ receptor neurone
Responsible for olfactory transduction
Each has a dendrite that projects to the epithelial surface, at the end of each one is a knob from it numerous cilia protrude into the mucous 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 sensory neurone has a single axon that projects to the olfactory bulb thorugh the cribiform plate of the ethmoid bone to enter the olfactory bulb.
olfactory bulb
small ovoid structure that rests on the cribiform plate of the ethmoid bone
sends olfactory information to be further processed in the amygdala, the orbitofrontal cortex (OFC) and the hippocampus
supporting cells in the olfactory epithelium
they secrete the mucous that provides the appropraite molecular nad ionic environment for odor detection in the olfcatory epithelium
odor producing molecules and odorant binding proteins
odor producing molecules (ordorants)- dissolve in the mucous and bind to odorant receptors on the cilia of the olfactory sensory neurones
odorant binding proteins- in the mucous may facilitate the diffusion of odorants to and from the odorant receptor
basal stem cells
undergo mitosis to undergo new olfactory sensory neurones as needed to replace those damaged by exposure to the environment
odorant receptors and signal transduction
The amino acid sequences of odorant receptors are very diverse, but all are Gprotein-coupled receptors (GPCRs)
- odorant binds to its receptor
- the G-protein subunits dissociate
- the a subunit activates adenylyl cyclase
- adenylyl cyclase catalyzes the production of cAMP
- cAMP acts a second messenger to open cation channels
- membrane permeabilty to Na+, K+ and Ca2+ increases
- net effect: inward directed Ca2+ current creating a graded potential
- Ca2+ activated Cl- cahnnels open further depolarising cell as Cl- kleaves neurone
if the stimulus is great enough and the threshold potential is overcome then an action potential in CNI is generated
olfactory sensory pathway
Odor detection threshold
odorants are generally small (3-> 20 Carbons) molecules with the same number of carbons but different structural configurations have different odours
relaticely high water and lipid solubility is characteristic of substances with strong odours
odour detection threshold- lowest concentration of a chemical that can be detected (this is not the same in everyone)
takes a big change to perceive a change in smell intensity
abnormalities in odour detection
Asomnia-
hyposmia
Dyosmia-
serious head trauma to the cribiform plate or head, tumours and resp. tract infections can casue serious damage to what
Asomnia- inability to smell
hyposmia and hypesthesia- diminished olfactory sensitivity
Hyperosmia- enhanced olfactory sensitivity (pregnant women)
Dyosmia- distorted sense of smell
olfactory nerve
what is the primary organ of taste (gustation)
where are they found
what are the types
5,000 taste buds located on papillae on the dorsal aspect of the tongue
- fungiform pupillae- rounded structurees near tip of tongue
- foliate papillae- posterior edge of the tongue
- circumvallate papillae- prominent structrues arranged in a V in the back of the tongue
taste buds can also be found in the soft palate, epiglottis and pharynx
taste buds on the anterior 2/3 are innervated by
posterior a
/3 innervated by
chorda tympani branch of the facial nerve
lingual branch of the glossopharyngeal nerve
taste buds are composed of 2 groups of cells
stem cells
taste cells
taste cells
3 kinds:
dark, light. intermediate
extend from the base of the taste bud to the taste pore, where microvilli contact tastants dissolved in mucous and saliva
Each taste bud contains 50–100 taste receptor cells and numerous basal cells and support cells
taste receptors
modified epithelial cells that respond to chemical stimuli or tastants
Apical ends have microvilli that project into the taste pore, a small opening on the dorsal surface of the tongue where taste cells are exposed to the oral contents.
role of saliva in taste-
Acts as a solvent for tastants; after dissolving, the chemical diffuses to the taste receptor sites
cleanse the mouth to prepare the taste receptors for new stimulant
basal cells
Basal cells arise from the epithelial cells surrounding the taste bud and differentiate into new taste cells
If the sensory nerve to a taste bud is cut
the bud will degnerate and eventually disappear
taste pathways
The sensory nerve fibres from the taste buds on the anterior 2/3 of the tongue travel in the chorda tympani branch of the facial nerve
those from the posterior 1/3 of the tongue reach the brainstem via the glossopharyngeal neve.
Fibres from areas other than the tongue (e.g., pharynx), reach the brainstem via the vagus nerve (CNX)
these 3 nerve fibres unite in the gustatory portion of the nucleus of the tractus solitarius (NTS) in the medulla oblongata
from there seconf order neurones ascend in the ipsilateral medial lemniscus and project to the ventral posteromedial nucleus of the thalamaus
third order neurones pass to neurones in the anterior insula and the frontal operculum in the postcentral gyrus
primary gustatory cortex- located in the parietal lobe within the insular and opercular cortex of the frontal lobe
which receptors and nerve is involved in transmission of spicy food taste info
Sensory fibres in the trigeminal contribute to the burning sensation experienced when we eat foods containing capsaicin.
Taste buds are surrounded by TRPV1 receptors
what are the 5 basic modalities of taste
give an example of a common stimuli for each
Salt (common stimuli- sodium chloride)
Sweet (-sucrose)
Sour (-hydrochloric acid)
Bitter (-quinine)
Umami (-monosodium glutamate)
can a taste cell respond to more than one type of tastant?
yes
CNS can distinguish tastes from one another because each one connects to a particular gustatory axon
what are the 2 major types of receptor involved in taste receptors
which flavours trigger each
Ligand gated (ionotropic)- salt and sour trigger them
GPCRs (metabolic)- sour, bitter and umami trigger
salt sensitive taste which channels
mediated by ENaC’s (epithelial sodium channel)
entry of Na+ into the salt receptor depolarizes the membrane, generating a receptor potential
sour taste
triggered by H+ protons
- ENaCs permit entry of H+ so they may be involved
- also bind to and block a K+ sensitive channel, the fall in K+ permeability can depolarise the membrane
- HCN and other mechanisms also
bitter
produced by a variety of unrelated compounds, many of which are poisonous
Some bitter compounds (e.g., quinine) are membrane permeable and bind to and block K+ selective channels
Many bitter tastants bind to GPCRs that couple to the G-protein, gustdicing.
Gustducin lowers cAMP and increases the formation of inositol phosphates, which releases Ca2+ to trigger depolarisation.
umami
tastants activate a receptor comprised of T1R1 and T1R3
may involve the activation of a truncated metabotropic glutamate receptor, mGluR4, in the taste buds.
intensity discrimination
30% change in tastant concentration is necessary before a difference can be detected
taste threshold
minimum concentration at which a substance can be perceived
Damage to the lingual or glossopharyngeal nerve may cause
Ageusia (absence of the sense of taste)
Hypogeusia (diminished taste sensitivity)
causes for problems with taste sensitivity
Neurological disorders such as vestibular schwannoma
▪ Bell palsy
▪ Familial dysautonomia
▪ Multiple sclerosis
▪ Certain infections (eg, primary ameboid meningoencephalopathy)
▪ poor oral hygiene
Diminished taste:
▪ Aging
▪ Tobacco abuse
occur under conditions in which serotonin (5-HT) and norepinephrine (NE) levels are altered (eg, during anxiety or depression)
Dysgeusia or Parageusia
unpleasant perception of taste) causes a metallic, salty, foul, or rancid taste and in many cases, dysgeusia is a temporary problem
