Chemcial Sense: Encoding

1. Compared to other senses, the sense of smell is…?

1. Slow and sensitive to detect and clear odors

What does the olfactory epithelium have?

1. Olfactory neurons and cilia in mucous

What is the small, thin sheet of cell high up in the nasal cavity called?

1. Olfactory epithelium

   1. Exudes a thin coating of mucus that flows constantly and is replaced about every 10 minutes

If an odor is to "disappear" from being smelled, what has to occur in the nasal cavity?

Mucous needs to be cleared

1. What does mucus consist of?

1. Water base with dissolved mucopolysaccharides, proteins (antibodies, enzymes, and odorant binding proteins) and salt

   1. Antibodies critical because olfactory cells can be direct route by which some viruses and bacteria enter the brain 

1. What occurs on the olfactory receptor neuron prior to transduction?

1. Waving from the knob (on epithelium), within mucus layer, are several long, thin cilia

2. Odorants dissolved in the  mucus bind to the surface of the cilia 

What constitutes the olfactory nerve (cranial nerve I)?

1. Olfactory axons 

   1. However, not all come together to form a single nerve bundle. After leaving the epithelium, small clusters of the axons penetrate a thin sheet of bone called the cribriform plate, then into the olfactory bulb

What are the parts of the cribriform plate and olfactory epithelium?

1. Olfactory nerve

2. Cribriform plate

3. Basal cell

   1. Source of new receptor cells 

4. Olfactory receptor cell

   1. Sites of transduction

5. Supporting cell

   1. Help to produce mucus

6. Cilia of olfactory cells

7. Mucus layer

Which part of the section connecting the cribriform plate and olfactory epithelium only live for about 4-8 weeks?

1. Olfactory nerves and olfactory receptor cells

1. What is unique to the nose/ olfactory system?

1. New neurons have to regrow axons throughout life

   1. Doesn’t occur in adult mammal brian apart from nose 

1. What are the  broad/main structures involved in the olfactory system?

1. Olfactory bulb

2. Cribriform plate

3. Olfactory epithelium

What received information from olfactory epithelium and is relatively small in humans compared to dogs?

Olfactory bulb

1. Where do the variabilities between species for the olfactory system reside?

1. Numbers of olfactory receptor neurons (ORNs)

2. Numbers of olfactory receptor genes

   1. Tuned for chemicals and differentiating smalled

3. Size of olfactory bulb in brain

   1. 1st major stage of processing smells

1. What is generally consistent across animals?

1. Olfactory behaviors

How can the perception of odors be affected?

1. Perception of smells can depend on concentration 

   1. Change of odor character can change depending on concentration

   2. Some chemicals are attractive at low concentrations and aversive at high concentrations

2. Different receptors for different enantiomers of molecules, creating different perceptions

Do all odors activate the same brain regions?

1. No, pleasant and unpleasant odors can activate different regions

   1. All smells go through the primary pathway but if associated with memory pleasant/aversive then can have different activation

How is olfactory sensitivity affected by age?

1. We gradually lose our sense of smell with age

   1. Stem cells dividing and generating does not continue at same rate like skin losing plasticity 

1. What is anosmia?

1. The inability to detect odors

What is hyposmia?

Decreased ability to detect odors

What is dysosmia?

1. Distorted identification of smell

   1. Apple → think peanut butter

What is parosmia?

1. Altered perception of smell in the presence of an odor, usually unpleasant

What is phantosmia?

1. Perception of small without an odor present

Might be a psych disorder

What is agnosia?

1. Inability to classify or contrast odors, although able to detect odors

What are the risk factors for smell impairment?

1. Epilepsy

2. Stroke

3. Nasal congestion or upper respiratory tract infection

4. Current smoking vs. never

5. Past smoking vs. never

6. sex, male

7. Age, per 5 year increment

1. What is the very loose processing of smell?

1. Smell goes through nose

2. Goes to glomerulus to encode the smell

3. Go to axon

1. What does the glomerulus do?

1. Responds to just 1 chemical to encode

1. What has axons and generate their own action potentials in response to chemical stimuli?

1. Olfactory Receptor Neurons (ORNs)

1. Olfactory receptor neurons are ___________ within the pseudostratified olfactory epithelium.

1. Bipolar cells

1. What kind of receptors are odorant receptors (on cilia or ORNs)?

1. Metabotropic receptors

   1. Each one has a unique binding site for a different chemical in the environment

   2. Each ONR usually expresses only one receptor type

1. What are the molecular mechanisms of odorant transduction?

1. Odorants

2. Bind to membrane odorant receptor proteins

3. Stimulate G-protein (G) → olf

4. Activate adenylyl cyclase 

5. Form cAMP

6. Bind cAMP to a cyclic channels and allow influx of Na+ and Ca 2+

7. Open Ca 2+ → activated Cl- channels

8. Cause current flow and membrane depolarization (receptor potential)

1. Why is the current negative with collecting information on intracellular responses of receptor neurons to selected odorants?

1. Because it is an inward depolarizing current

1. How is broad tuning of single olfactory receptor cells done?

1. Population encoding 

What is the olfactory bulb composed of?

1. Glomeruli

1. Each glomeruli receives input from all the ORNs that encode ______?

1. For the same chemical stimuli receptor gene

   1. Therefore, each glomeruli represents a specific chemical component of a smell

1. What are labeled lines coding?

1. Very specific neurons for smells

1. What does lateral inhibition do?

1. Sharpens discrimination between scents

   1. Done through exciting a neuron and suppressing activity of others

   2. Also known as inhibitory interneurons

1. Describe odor maps in olfactory bulbs?

1. Each color represents the area activated by a different smell

   1. Appears similar on both sides of the brain indicating genetically predetermined organization of smells

1. What is population coding?

1. Complex odors are represented by a spatial map of multiple activated stimuli

2. Smell is activated by a distributed network of brain regions

1. What is unique about the olfactory system as opposed to other systems?

1. No primary olfactory cortex or region

   1. The closest thing would be the piriform cortex

1. What is the reason which olfaction can evoke emotions?

1. Olfactory bulb output diverges to entorhinal cortex

1. What is the main odor processor?

1. Piriform cortex

What links smells to reward?

1. Olfactory tubercle

1. What links smells to emotions and fear?

1. Amygdala

What links smells to memory

1. Entorhinal cortex

1. What parts of the brain are activated in response to pleasant smells?

1. Orbitofrontal cortex

2. Pyriform cortex

3. Amygdala

__________ neurons in Pyriform Cortex display preferences (tuning) for select chemical cues.

1. Cortical IV 

1. Complex smells are represented by ___________ of neural activation.

1. Complex spatial patterns

   1. Called “spatial coding” or population coding 

1. What are pheromones?

1. Chemicals released from the body and are important signals for reproductive behaviors, and may be used to mark territories, identify individuals, and indicate aggression or submission

What system do animals use to detect pheromones and mediate social behaviors involving mother, mating, territory and food?

1. Accessory olfactory system

   1. Runs in parallel to the primary olfactory system

   2. Consists of separate chemically sensitive regions in the nasal cavity (vomeronasal organ)

1. What appears to be absent or vestigial in most mature people?

1. Vomernasal organ

   1. Does not appear to have functional receptor proteins or direct connects to the brain 

      1. Doesnt mean cant detect pheromones

Summarize the olfactory pathway.

1. Once the cation selective cAMP gated channels open

2. Current flows in and membrane of the olfactory neuron depolarizes

3. cAMP-gated channel allows substantial amounts of Ca2+ to enter the cilia

4. Ca 2+ triggers a Ca 2+ activated Cl- current that may amplify the olfactory receptor potential

   1. Usually inhibits neurons 

What may terminate the olfactory response?

1. Odorants diffuse away

2. Scavenger enzymes in the mucus layer often break down (enzyme degredation)

3. increase in cAMP in the receptor cell

1. All other sensory systems first pass information through the ______ before projecting it to the cerebral cortex.

1. Thalamus

1. Conscious perceptions of smell may be mediated by the path that goes through…?

From the olfactory tubercles, to the medial dorsal nucleus of the thalamus, and to the orbitofrontal cortex

What are the important concepts of the spatial and temporal representations of the Olfactory information?

1. Each odor is represented by the activity of a large population of neurons

2. The neurons responsive to particular odors may be organized into spatial maps

3. The timing of the action potentials may be an essential code for particular odors

1. How do we perceive the countless flavors of food?

1. Each food activates different combination of the basic tastes

2. Most foods have a distinctive flavor as a result of their combined taste and smell occurring simultaneously.

3. Other sensory modalities contribute to a unique food-tasting experience.

   1. Texture and temperature and pain sensations are essential to the hot, spicy flavor of foods laced with capsaicin

4. Brain combines sensory information about taste, its smell, and its feel

Odors from the food we are eating can also pass via pharynx into ________ where they can be detected by olfactory receptors.

the nasal cavity

1. What are the different structures involved in the gustatory system?

1. Ventral posterior medial nucleus of thalamus

2. Tongue

   1. Tip of the tongue is most sensitive to sweetness

   2. The back to bitterness

   3. The sides to saltiness and sourness

   4. Most of the tongue is sensitive to all basic tastes

3. Larynx 

4. Nucleus of the solitary tract

   1. Gustatory nucleus

5. Ventral posterior medial nucleus thalamus

6. Axons from the nucleus of the solitary tract

7. Insular taste cortex 

8. Frontal taste cortex 

   1. Frontal operculum

What are the scattered small projections on the surface of the tongue shaped like ridges, pimples, or mushrooms on the tongue called?

Papillae

Taste buds also have ____________that surround the taste cells, plus a set of gustatory afferent epithelium.

1. basal cells

Which papillae are the largest and most posterior?

Vallate papilla

Which papillae are elongated?

1. Foliate

1. Describe fungiform papillae.

1. Relatively large toward the back of the tongue and much smaller along the sides and tip

1. What is a taste bud?

A cluster of taste cells (the receptor cells), gustatory afferent axons and their synapses with taste cells and basal cells

What are microvilli?

1. At the apical end of the taste cells extend into the taste pore, the site where chemicals dissolved in saliva can interact directly with taste cells

What is threshold concentration?

1. Concentrations too low will not be tasted but at critical concentration, the stimulus will evoke a perception of taste

Are taste receptors neurons?

1. No

   1. They do form synapses with the endings of the gustatory afferent axons near the bottom of the taste bud

   2. Also make electrical and chemical synapses on some basal cells

1. Describe the path of excitation for a taste receptor cell.

1. Membrane potential changes through depolarization

2. Depolarization of the receptor membrane cause voltage gated calcium channel top open

3. Ca 2+ enters the cytoplasm, triggering the release of transmitter molecules

   1. If the taste cells are salty or sour, they release serotonin on gustatory axons

   2. If the taste cells are sweet, bitter, and umami taste cells release ATP

What are the mechanisms to taste transduction?

1. taste stimuli

   1. Directly pass through ion channels (salt or sour)

   2. Bind to and block ion channels (sour)

   3. Bind to G protein couples receptors in the membrane that activate second messenger systems that open ion channels (bitter, sweet, and umami)

What is Amilodride?

1. A diuretic (a drug that promotes urine production) used to treat hypertension and heart disease

2. Blocks Na+ channels on salt sensitive taste cells

What are the transduction mechanisms of salt and sour tastants?

1. Tastants can directly interact with ion channels either by passing though them (Na+ or H+) or by blocking them (H+ blocking the potassium channel)

1. Where and what does the ventral posterior medial nucleus (VPM) do?

1. A portion of the thalamus that deals with sensory information from the head.

2. VPM taste neurons send axons to the primary gustatory cortex

1. Taste pathways to the thalamus and cortex are ___________ to the cranial nerves that supply them.

1. Ipsilateral

1. What is the path of the cranial nerve x (vagus nerve) in the gustatory system?

1. To the taste buds (epiglottis)

2. To nucleus of the solitary tract 

3. To the VPM of thalamus, hypothalamus, amygdala

4. From the VPM to the Insular and frontal taste cortices to the motivational centers

   1. Hypothalamus 

   2. Amygdala

1. What is the path of the cranial nerve 9 (glossopharyngeal) in the gustatory system?

1. To the taste buds (one third of the tongue (posterior))

2. To nucleus of the solitary tract 

3. To the VPM of thalamus, hypothalamus, amygdala

4. From the VPM to the Insular and frontal taste cortices to the motivational centers

   1. Hypothalamus 

   2. Amygdala

1. What is the path of the cranial nerve VII (facial nerve) in the gustatory system?

1. To the taste buds (two third of tongue and palate)

2. To nucleus of the solitary tract 

3. To the VPM of thalamus, hypothalamus, amygdala

4. From the VPM to the Insular and frontal taste cortices to the motivational centers

   1. Hypothalamus 

   2. Amygdala

Where is the gustatory cortex?

1. In the insular cortex region

Where does cellular transduction happen?

1. Apical microvilli

What is one major difference between the olfactory and gustatory system?

1. Taste cells do not have axons that regenerate

_____ of cell has voltage-gated Ca 2+ channels to initiate neurotransmitter release.

Base

1. What occurs for salt to go through taste transduction?

Sodium leak ion channels open

What occurs for acid to go through taste transduction?

1. H+ ion binds to ion channels and block K+ channels from leaving 

2. When K+ permeability is decreased, it depolarize

3. H+ may also activate or permeate transient receptor potential channels (TRP) 

   1. Cation current through TRP channels can depolarize sour receptor cells

1. What occurs for bitter to go through taste transduction?

1. Quinone binds to ion channel and blocks K+ channels

2. Relies on T1R and T2R g coupled protein receptors 

Which receptors are poison detectors?

Bitter

____________receptors use a second messenger pathway to carry their signal to the gustatory afferent axon.

Bitter

What is the general pathway for the processing of bitter, sweet, and umami foods?

1. Tastant binds to receptor which activated G-proteins

2. Stimulates the enzyme phospholipase C

3. Increase production of intracellular IP3

   1. IP3 pathways are ubiquitous signaling systems in cells 

   2. Actiates specific ion channel 

4. Opens and allow Na+ to enter, depolarizing cell

5. IP3 also triggers release of Ca 2+

   1. Taste cells for bitter, sweet, and umami lack filled vesicles

6. Activates special membrane channel that allows ATP to flow out of cell

7. ATP serves as a synaptic transmitter and activates purinergic receptors on postsynaptic gustatory axons.

What are the sweet receptors?

1. T1R family

   1. T1R2

      1. All carnivores lack the genes encoding T1R2

   2. T1R3

2. If missing, might not be able to taste sweet

What is the difference between the transduction of umami as compared to sweets?

1. Composed of members of T1R protein family

   1. T1R1 

      1. Determined whether the receptor is sensitive to amino acids

      2. When removed mice unable to taste glutamate and other amino acids 

   2. T1R3

Protein receptors for what senses of taste are considered to be dimers?

Bitter, sweet, and umami

What receptors are needed for sweet, umami, bitter to go through taste transduction?

1. Metabotropic receptors

_____________ in peripheral taste coding support the labeled line hypothesis

Specificity

What is the main flow of taste information?

From taste buds to the primary gustatory axons into the brain stem, up the thalamus, and into the cerebral cortex