Flashcards in Chapter 15 - The Chemical Senses Deck (86):
Three sensory systems are associated with the nose and mouth. Name them.
1. The olfactory system
2. The gustatory system
3. The trigeminal chemosensory system
The ...... system detects airborne molecules called odorants.
The ......... system detects ingested tastants (primarily water- or fat-soluble molecules)
The ...... system provides information about irritating or noxious molecules that come into contact with the skin or the mucous membranes of the eye, nose and mouth.
Airborne molecules detected by the olfactory system is called..
Where is the olfactory epithelium?
It lines the interior of the nose, at the roof of the nasal cavity.
Receptor cells for odourants have axons that project directly to ...
The olfactory bulb.
The olfactory bulb has neurons that are innervated by the receptor cell axons of the olfactory epithelium. The olfactory bulb has axons that project to ..
1. The pyriform cortex in the temporal lobe.
2. Other structures in the forebrain.
Which sensory system does not include a thalamic relay?
The olfactory system!
The pyriform cortex has one defining cytoarchitectonic feature. Which?
It has three layers, not six - therefor not defined as neocortex.
The olfactory system is the only sensory system that does not have a thalamic relay. The thalamus is still involved though.. how?
Although the initial pathway to the pyriform cortex bypass the thalamus, the thalamus does play an important role in subsequent stages of olfaction.
"In humans, olfaction is often considered the least acute of the senses". You've heard this many times. Why do we think so?
It mainly relies on the finding that the surface area of our olfactory epithelium is vastly smaller in contrast with other mammals. A rat has a bigger olfactory epithelium than humans do.
One of the most consistent aspects of human olfactory perception is the ...
The classification of odors as either pleasant and attractive or unpleasant and repulsive.
Clinicians use uniquely scented "probes" - such as coffee grounds or soap - to test the function of the olfactory nerve (cranial nerve I) as a part of the standard cranial nerve examination. Some individuals consistently fail to identify one or more common odors. Such chemosensory deficits are known as ...
What is an interesting feature of anosmia?
It is often restricted to a single odourant, suggesting that a specific element in the olfactory system - either an olfactory receptor gene or genes that control expression or function of specific odourant receptor genes - is inactivated.
Olfactory deficits are known to occur during a broad range of disorders. The cause of this is not known. Mention four disorders in which olfactory deficits are common.
1. Eating disorders
What are pheromones?
Specific odorants that play an important role in behaviour. Especially social, reproductive and parenting-behaviours.
In other animals, including many mammal species, pheromones are detected by a part of the olfactory epithelium called ...
the vomeronasal organs.
Do humans have vomeronasal organs?
Primates, including humans, do not have vomeronasal organs, distinct vomeronasal receptors, or significant numbers of functional vomeronasal receptor genes.
So primates don't have vomeronasal organs. Are there pheromones in the human species?
Despite many attempts to identify pheromones that elicit responses from humans, there is little evidence for them.
The neural portion of the olfactory epithelium includes several cell types. Name four types.
1. Olfactory receptor neurons (ORNs)
2. Bowman's glands
3. Basal cells
4. Sustentacular (supporting) cells.
Give a short description of what the olfactory receptor neurons (ORNs) do.
They are the receptor cells that transmits chemical information to central locations. They have olfactory cilia that extend into a thick layer of mucus.
Give a short description of what the Bowman's glands are.
Bowman's glands are secretory specialisations that produce the mucus in the olfactory epithelium.
Research on the olfactory epithelium is possibly highly relevant for knowledge about more than just olfaction. Why?
Olfactory receptor neuron regeneration relies on maintaining a population of neural stem cells that give rise to new receptor neurons. This naturally occurring regeneration provides an opportunity to investigate how neural stem cells can successfully produce new neurons and reconstitute synaptic connections and function in the mature central nervous system.
The olfactory cells have cilia, but they are slightly different to normal cilia. How?
Despite their external appearance, olfactory cilia do not have the cytoskeletal features of motile cilia. Instead, the actin-rich olfactory cilia more closely resemble microvilli of other epithelia and thus have a greatly expanded cellular surface to which odorants can bind.
The Nobel Prize in Physiology or Medicine was awarded in 2004 to Richard Axel and Linda Buck for their discovery of what?
The odorant receptor gene family
The number of expressed odorant receptors, is correlated with...
The olfactory capacity of different species. Less is not more.
In mammals, the principal pathway for generating electrical activity in olfactory receptors is similar to ...
The processes found in rod photoreceptors.
What happens when an odorant binds to a receptor protein on the cilia on the ORN?
The alpha unit of the G-protein dissociates.
What happens in the ORN when the G-protein has been dissociated by the binding of an odorant?
The alpha unit activates adenyl cyclase III(ACIII), which is an olfactory-specific adenylate cyclase.
What happens in the ORN when the alpha unit activates adenyl-cyclase III (ACIII)?
The ACIII generates cyclic AMP (cAMP).
What happens in the ORN when the ACIII starts generating cyclic amp (cAMP)?
cAMP targets a cation-channel that, when open, permits the influx of Na+ and Ca2+ into the cilia, resulting in depolarization of the cell.
The cAMP gated channel of an ORN has been activated, and the cell is slightly depolarised. What happens thus?
The cAMP-gated channel permits the influx of Ca2+ into the cilia. The increase in intracellular CA2+ opens Ca2+-gated Cl- channels that provide most of the depolarisation because Cl- escapes the cell.
What is the most distinctive feature of the olfactory bulb?
The array of glomeruli.
What are glomeruli?
Glomeruli lie just beneath the surface of the olfactory bulb, and are the synaptic targets of the primary olfactory axons.
In mammals, including humans, within each glomerulus the axons of the receptor neurons contact ....
mitral cell dendrites.
In mice, what is the quantitative relationship between the olfactory receptor neurons and the mitral cells?
In the mouse, whose glomerular connectivity has been studied quantitatively, each glomerulus includes the apical dendrites of approximately 25 mitral cells, which in rerun receive input from approximately 25,000 olfactory receptor axons. Remarkably, most if not all 25,000 of these axons come from olfactory receptor neurons that express the same, single odorant receptor gene.
Each glomerulus also includes dendritic processes from two other classes of local circuit neurons. Which?
1. Tufted cells
2. Periglomerular cells.
What is the function of tufted and periglomerular cells?
We don't know.
What are tufted cells?
A cell class that is found in the olfactory bulb. Has dendritic processes that extend into the glomeruli.
What are periglomerular cells?
A cell class that is found in the olfactory bulb. Has dendritic processes that extend into the glomeruli.
The olfactory bulb features several classes of cells. You need to know four of these. List them.
1. Mitral cells
2. Tufted cells
3. Periglomerular cells
4. Granule cells
What are granule cells?
Granule cells is a class of cells in the glomeruli that synapse primarily on the basal dendrites of mitral cells. They lack an identifiable axon, and instead make reciprocal dendrodendritic synapses with mitral cells. They are thought to establish local lateral inhibitory circuits with mitral cells as well as participating in synaptic plasticity in the olfactory bulb.
Research on glomeruli organisation suggest that...
The relationship between olfactory receptor neurons expressing one odorant receptor and small subsets of glomeruli suggests that individual glomeruli respond specifically to distinct odorants.
Mitral cell axons from each olfactory bulb form a bundle, leaving the bulb as the ...
lateral olfactory tract
Where does the lateral olfactory tract project to?
1. Accessory olfactory nuclei
2. Olfactory tubercle
3. Entorhinal cortex
4. Pyriform cortex
How much of the olfactory tract cross the midline?
Just a small subset of mitral cell axons cross the midline.
What is special about the organisation of inputs in the pyriform cortex?
Apparently, the segregation of information in the olfactory bulb is not maintained in the pyriform cortex. Thus, the nature of the transformation of information from the olfactory epithelium though the bulb to the pyriform cortex remains somewhat difficult to discern.
Receptor proteins for taste in the oral cavity are located in epithelial specialisations called...
Taste buds have special cells called ...
The taste cells are found in which areas of the digestive system?
Taste buds are distributed on the dorsal surface of the tongue, soft palate, pharynx, and the upper part of the esophagus.
The taste buds send their axons to the ...
Nucleus of the solitary tract in the medulla.
The taste buds on the ant. two-thirds of the tongue send their axons along which cranial nerve?
Cranial nerve VII (facial nerve)
The taste buds on the post. one-third of the tongue send their axons along which cranial nerve?
Cranial nerve IX (glossopharyngeal).
The taste buds in the epiglottis send their axons along which cranial nerve?
Cranial nerve X (vagus nerve)
The nucleus of the solitary tract receives axons from the taste buds in the oral cavity and esophagus. It sends projections to ...
1. The VPM (ventral posterior medial nucleus) of the thalamus
2. The Hypothalamus
3. The Amygdala
The VPM (ventral posterior medial nucleus) of the thalamus receives innervation from the nucleus of the solitary tract. It sends projects to the ...
insular and frontal taste cortices.
The insular and frontal taste cortices receives innervation from the VPM (ventral posterior medial nucleus) of the thalamus, and itself sends projections to ...
The nucleus of the solitary tract in the medulla receives innervation from ...
The taste buds.
The nucleus of the solitary tract in the medulla receives innervation from the taste buds. It is also called the ...
Knowing a little bit of neuroscience. Why would you think the information from the gustatory system reaches the hypothalamus?
These projections presumably influence homeostatic responses associated with eating, given that the hypothalamus is the major center governing homeostasis.
Knowing a little bit of neuroscience. Why would you think the information from the gustatory system reaches the amygdala?
These projections presumably influence affective aspects associated with eating (e.g., pleasurable versus aversive experience of food; food-seeking behaviour).
Can you say something about the threshold for tasting certain tastants?
In humans, threshold concentrations for most ingested tastants are quite high. For potentially dangerous substances, the threshold is much lower.
Taste buds are found on receptive specialisations called ...
Define a taste papillae.
Papillae are defined by multicellular protuberances surrounded by local invaginations in the tongue epithelium. These invaginations form a trench to concentrate solubilised tastants.
In humans, approximately X taste buds are distributed throughout the surface of the tongue as well as the palate, epiglottis, and esophagus.
Taste cells have a lifetime of ..
about 2 weeks.
Taste cells have a lifetime of about two weeks. This is similar to olfactory receptor neurons. What is the leading hypothesis for this?
Presumably because they are both reasonably likely to be exposed to infectious agents and environmental toxins. It is also presumed that the best way to ward a system against such constant exposure is simply to renew it.
There are three types of papillae. Which?
There are three types of papillae. Fungiform, circumvallate and foliate. They all have taste buds. Out of the estimated 4000 total taste buds, how many percent are in each type of papillae type?
Fungiform contain about 25% of the total number of taste buds, circumvallate 50% and foliate 25%.
Where on the tongue are the fungiform papillae?
They are found only on the anterior two-thirds of the tongue. The highest density is at the tip.
Where on the tongue are the circumvallate papillae?
Nine circumvallate axillae form a V-shape at the rear of the tongue.
Where on the tongue are the foliate papillae?
Two foliate papillae are present on the posterolateral tongue, each having about 20 parallel ridges with about 600 taste buds in their walls.
All tastes can be detected over the entire surface of the tongue. T/F.
All tastes can be detected over the entire surface of the tongue. But....
But there are different thresholds for various tastes in the different regions.
The tip of the tongue is most responsive to which taste(s)?
The tip of the tongue is most responsive to sweet, umami, and salty compounds. This is thought to be because these are pleasurable and can activate feeding behaviour early.
The receptor proteins for salty taste react to which molecules?
Most likely Na+ from salt. These are ionotropic receptors.
The receptor proteins for sour taste react to which molecules?
Most likely H+ in the acids.
How do taste cells for salt and sour generate an action potential?
The cells are depolarised by the influx of either Na+ (in salt-receptors) or H+ (in sour-receptors). This leads to activation of voltage-gated Na+ channels in the base of the taste cell. This additional depolarisation leads to the activation of voltage-gated Ca2+ channels, which lead to the release of neurotransmitter.
What kind of receptors are the protein receptors on taste cells that are sensitive to sweet and umami?
They are G-protein-coupled receptors.
The receptors for sweet taste react to which molecules?
They react to sugars or sweet stimuli.
The receptors for umami taste react to which molecules?
They react to amino acids.
Observations suggest that sweet and amino acid transduction, representation and processing remain segregated from the time of encoding at peripheral receptors through central processing. Which observations?
Molecular genetic experiments in mice that featured mice that had the genes for these receptors inactivated. They were not able to respond to sweet or umami tastes at all.
What do we know about central processing of tastes?
Not a whole lot. We know that the tastes are segregated in processing from as early on as the receptors, and it is thus likely that the segregation is kept in more central structures.
The trigeminal chemoreceptors are typically activated at which concentrations in the human body?
The peripheral receptor neurons and their associated endings are typically activated by relatively high concentrations of irritating chemicals that come into direct contact with the mucous membranes of the head, including the mouth, nose, and eyes.