Smell and taste

Question 1

Where is the olfactory epithelium located?

Answer 1

Dorsal nasal cavity

Question 2

What does olfactory epithelium consist of?

Answer 2

10 ^8 bipolar olfactory receptor neurons and supporting cells.

Question 3

What is the structure of the dendrites of the bipolar olfactory receptor neurons?

Answer 3

A dendrite emerges from the ORN and extends to the surface of the epithelium to form a cluster of 6–12 immobile olfactory cilia. These extend into a layer of mucus secreted by
the supporting cells.

Question 4

What is the pathway of the axon of the olfactory receptor neuron?

Answer 4

The centrally directed unmyelinated axon of the ORN runs through the olfactory (cranial I) nerve to synapse with cells in the olfactory bulb.

Question 5

What are the properties of odor molecules?

Answer 5

usually small (Mr < 200 Da), lipid soluble, and volatile.

Question 6

What happens to odor molecules in the nasal cavity?

Answer 6

Initially they bind to odor-binding proteins in the mucus which concentrate the odor molecules in the vicinity of the cilia. Odor molecules are recognized by odorant receptors in the cilia plasma membrane. A given odor molecule can bind two to six different odorant receptors.

Question 7

What are the properties of odorant receptors?

Answer 7

They are G-protein coupled receptors and over 1000 have identified. They are relatively nonspecific, binding a range of related odor molecules, so although an individual ORN expresses just a single subtype of odorant receptor it responds to several odors.

Question 8

How is the mammalian nervous system able to discriminate odors?

Answer 8

It is is able to discrimi-
nate some 10 000 distinct odors on the basis of precisely which array of odorant receptors (and hence sensory neurons) are stimulated, and with what relative intensities.

Question 9

What effect does the binding of odor molecules have on odorant receptors?

Answer 9

Odorant receptors are coupled to Golf proteins (related to Gs proteins) which usually stimulate adenylyl cyclase, and binding of an odor molecule causes a rise in cAMP in about 50 ms. This activates a cyclic-nucleotide-gated (CNG) channel, a nonspecific cation conductance permeable to Na+, K+, and Ca2+ ions

Question 10

What signals the concentration of an odor molecule?

Answer 10

The resulting depolarisation after the opening of CNG channels in odorant receptors is graded with an amplitude that signals the concentration of the odor molecule. However, maximal response is produced by the opening of only a small fraction (3–4%) of the CNG channels available. This means that the concentration range that
can be signaled by firing of an ORN is narrow, about a 10-fold difference.

Question 11

What does high odor concentration or prolonged odor result in?

Answer 11

This allows a high Ca2+
influx through the CNG channels. This ion has a number of modulatory effects. It activates heme oxygenase 2, an enzyme that synthesizes carbon monoxide (CO) which can activate guanylyl cyclase (GC).
Because Ca2+ also inhibits GC, there is no overall activation of the cyclase in the target ORN. However, CO
is freely diffusible so it can activate GC in adjacent unstimulated ORNs, producing cyclic
guanosine monophosphate (cGMP) which binds to and opens the CNG channels. In this way odorant excitation spreads to a cluster of ORNs. Since neighboring ORNs respond to the same odors this increases sensitivity without loss of specificity.

Question 12

How do ORNs dhow adaptation to protracted stimulation?

Answer 12

Ca2+ binds to calmodulin (CaM) which can then bind to CNG channels, reducing the efficacy with which the cyclic nucleotides can open them. Hence Ca2+ attenuates the size of the generator potential.

Question 13

Which cells do ORNs synapse with?

Answer 13

They make excitatory synapses on the dendrites of mitral cells or tufted cells (M/T) and short axon inhibitory periglomerular cells in the olfactory glomeruli of the olfactory bulb.

Question 14

What are olfactory glomeruli?

Answer 14

They are spherical zones some 150 μm across. The olfactory bulb contains about 2000 glomeruli, each receiving the terminals of 25 000 ORNs that respond to the same odors. Hence,
glomeruli are odor-specific functional units. Each one has dendrites from about 75 M/T cells/

Question 15

What is the response in the glomeruli for a low concentration vs high concentration odor?

Answer 15

Low concentrations of a given odor molecule activate cells in the single glomerulus which gets input from the ORNs bearing odorant receptors with the highest affinity for the molecule. At higher concentrations, cells in other glomeruli are activated as their ORN odorant receptors’ low-affinity binding sites for the molecule are occupied.

Question 16

What is the effect of M/T cells on odor signals?

Answer 16

The M/T cells integrate weak inputs from a large number of ORNs within a glomerulus to generate a strong signal.

Question 17

What does lateral inhibition in the olfactory system achieve?

Answer 17

It inhibition dampens responses from glomeruli with slightly different odor specificities so as to heighten odor discrimination.

Question 18

How is lateral inhibition in the olfactory system brought about?

Answer 18

by reciprocal den-
drodendritic synapses between M/T cells and inhibitory interneurons termed granule cells. Via these synapses, M/T cells excite granule cells, which then inhibit the same, and
adjacent, M/T cells.

Question 19

Where do M/T cells send their axons?

Answer 19

Via the olfactory tract to the olfactory cortex

Question 20

What is the topographical organisation of the fibers of the olfactory nerve and their
projections to the olfactory bulb?

Answer 20

Thin strips of olfactory epithelium running in anteroposterior direction go to neighboring glomeruli. A given odor excites a particular array of glomeruli across the olfactory bulbs, an odor image. The higher the concentration of the odor molecule the bigger the area activated.

Question 21

In what two aspects is the olfactory cortex unnusual?

Answer 21

it is palaeocortex (old cortex) having only three layers. Secondly, it is the only cortex to receive sensory input directly rather than via the
thalamus.

Question 22

What are the five regions of the olfactory cortex, and their distinct connections?

Answer 22

The anterior olfactory nucleus project axons
across the midline in the anterior commissure to the contralateral olfactory bulb. The anterior perforated substance (olfactory tubercle) sends output to the posterior
hypothalamus. The entorhinal cortex, which sends its entire output to the hippocampus. The pyriform cortex projects, via the
dorsal thalamus, to the orbitofrontal cortex

Question 23

What is the function of the olfactory tubercle ->posterior thalamus pathway?

Answer 23

This pathway, together with that to the corticomedial amygdala, which then projects to the medial hypothalamus, is concerned with the affective and motivational aspects of odors.

Question 24

What is the function of the entorhinal cortex > hippocampus pathway?

Answer 24

presumably encodes olfactory components
of episodic memories.

Question 25

What is the function of the pyriform cortex?

Answer 25

Concerned with olfactory discrimination.

Question 26

What is the function of the orbitofrontal cortex?

Answer 26

Mediates the conscious perception of smell.

Question 27

Other than ORNs, where do the olfactory bulbs get their input from?

Answer 27

receives inputs from noradrenergic and serotonergic neurons in the brainstem and cholinergic neurons in the forebrain.

Question 28

Other than from the olfactory tract, what other input does the anterior perforated substance get?

Answer 28

receives a projection from the brainstem dopaminergic system.

Question 29

What is the implication of the outside input into the olfactory system?

Answer 29

implicated in olfactory learning and modifying olfaction on the basis of appetite and arousal. Thus,
mitral cell responses to food odors are modulated depending on whether an animal is hungry or sated.

Question 30

What is the sense of taste called?

Answer 30

Gustation

Question 31

what are the 5 well-defined tastes?

Answer 31

salty, sour, sweet, bitter, and umami (due to monosodium glutamate)—on the basis that no cross adaptation occurs between them.

Question 32

What is the significance of a bitter taste?

Answer 32

Plant alkaloids, some of which are toxic in high concentrations, are extremely bitter.

Question 33

What is the significance of sour taste?

Answer 33

A sour taste may signify a food degraded by microbiological action.

Question 34

What is the significance of sweet taste?

Answer 34

a sweet food has a high content of sugars and so a readily available supply of metabolic energy.

Question 35

The sensory experience produced by having food in the mouth is called flavor perception, and relies on several sensory modalities, what are they?

Answer 35

Apart from smell and taste, information about food texture is provided by mechanoreceptors and proprioceptors in the mouth and jaw innervated by trigeminal afferents.

Question 36

What is the importance of flavor perception?

Answer 36

important in triggering or modifying autonomic responses to feeding, for example, salivation, gastric secretion, and changes to
gastrointestinal motility.

Question 37

What is the structure of taste buds?

Answer 37

Formed by gustatory receptor cells (epithelial cells but are excitable), which are organized into small clusters of 50–100 along with supporting cells.

Question 38

Where are taste buds present?

Answer 38

They are located in the epithelium of the tongue, palate, pharynx, epiglottis, and the upper part of
the esophagus. In the tongue they are present in small projections, papillae.

Question 39

How do the microvilli on the apical border of gustatory receptor cell come into contact with the contents of the mouth?

Answer 39

They project through taste pores in the gustatory epithelium, they are the taste organelles

Question 40

What do the gustatory receptor cells synapse with?

Answer 40

with primary gustatory afferents, each of which
branches to synapse with receptor cells in more than one taste bud. The primary afferent axons from tongue, palate, and pharynx enter the facial (VII) and glossopharyngeal (IX)
nerves. The few afferent in the epiglottis and esophagus are innervated by the vagus (X) nerve.

Question 41

What are the properties of the ions or molecules responsible for taste sensation?

Answer 41

Hydrophilic and freely diffusible. Those which are hydrophobic include plant alkaloids which may bind to proteins in the saliva, equivalent to odorant-binding proteins, for presentation to gustatory receptor cells.

Question 42

What does taste transduction involve?

Answer 42

It involves changes in membrane conductance which causes a depolarizing generator potential, triggering action potentials and release of neurotransmitter which excites the gustatory primary afferents.

Question 43

What is salt taste caused by?

Answer 43

It is caused by Na+ ions. Salt transduction occurs by the influx of Na+ through an amiloride-sensitive Na+ channel.

Question 44

How is a sour taste transduced?

Answer 44

H+ ions responsible for sour (acid) sensation causes a generator potential by blocking voltage-dependent K+ channels in the apical membrane which at rest carries an outward, hyperpolarizing current.

Question 45

How are sweet sensations produced by sugars, some amino acids and some proteins?

Answer 45

Produced by interacting
with G-protein-linked receptors coupled to second messengers. Sugars activate adenylyl cyclase and the consequent rise in cAMP produces depolarization by closing a K+ channel. Some compounds responsible for sweetness (e.g., artificial sweeteners) increase inositol trisphosphate (IP3and mobilize Ca2+ within receptor cells.

Question 46

Multiple pathways mediate bitter taste transduction, what is the significance of this?

Answer 46

This reflects the wide
diversity of molecules that are bitter flavored.

Question 47

How do divalent salts and quinine mediate bitter taste?

Answer 47

block K+channels and so depolarize by reducing an outward potassium current.

Question 48

What is the mechanism of mediating bitter taste that is similar to phototransduction?

Answer 48

some bitter tasting agents bind metabotropic
receptors coupled to transducin (Gt) activating a phosphodiesterase that hydrolyzes cAMP. The fall in cAMP concentration causes cAMP to dissociate from a cyclic-nucleotide-gated (CNG) cation conductance, allowing influx of Na+ and Ca2+ and so depolarization.

Question 49

What is the mechanism of the umami taste sensation produced by l-glutamate?

Answer 49

seems to involve metabotropic glutamate (mGluR4) receptors coupled via Gi proteins to the inhibition of adenylyl cyclase

Question 50

Where are the cell bodies of the gustatory primary afferents?

Answer 50

CN VII- geniculate ganglion
CN IX- petrosal ganglion
CN X- nodose ganglion

Question 51

Where do the gustatory afferents’ axons project to?

Answer 51

in the rostral portion of the nucleus of the solitary tract (NST) which lies in the dorsal medulla

Question 52

What do taste primary afferents secrete?

Answer 52

glutamate and substance P.

Question 53

Where do nucleus of the solitary tract cells project to?

Answer 53

Some NST cells project to the lateral hypothalamus which organizes autonomic responses to feeding. Other NST gustatory neurons project via the central tegmental tract to the ipsilateral ventral posterior medial nucleus (VPMpc) of the thalamus, terminating on a population of small cells distinct from those receiving somatosensory input from the tongue or pharynx.

Question 54

Where do the cells for taste of the VPMpc send their axons to?

Answer 54

to the ipsilateral cortex. Taste area I for conscious perception of taste is located in the lateral sulcus adjacent to the somatotopic mapping of the tongue. Taste area II is in the insula and is concerned with the affective aspects of taste.

Question 55

How do the gustatory primary afferents differ in the different cranial nerves?

Answer 55

Afferents in the VII nerve commonly exhibit preferences for either salty or sweet stimuli, whereas most of those in the IX nerve, supplied by the posterior tongue, are tuned to acids (sour) or bitter stimuli. Many vagal (X) afferents respond to distilled water. These neurons have their lowest firing rate in 154 mM NaCl, and increase firing as salt concentration increases or decreases from this value

Question 56

What do vagal afferents seem to measure?

Answer 56

to what extent the pharyngeal contents differ in ionic concentration from extracellular fluid.

Question 57

Where do distinctive taste sensations arise from?

Answer 57

from neurons with opponent receptive fields that compare the out- puts of differently tuned populations of afferents. This is analogous to how color vision arises from opponent processing that compares output from just three populations of cone photoreceptors.