the chemical senses

Question 1

anosmia

Answer 1

Loss of the sense of smell; can be total or restricted to a single odorant.

Question 2

G-olf

Answer 2

A G-protein found uniquely in olfactory receptor neurons.

Question 3

glomeruli

Answer 3

Characteristic collections of neuropil in the olfactory bulb; formed by dendrites of mitral cells and terminals of olfactory receptor cells, as well as processes from local interneurons.

Question 4

insula

Answer 4

The portion of the cerebral cortex that is buried within the depths of the lateral fissure. Also called insular cortex.

Question 5

lateral olfactory tract

Answer 5

The projection from the olfactory bulb to higher olfactory centers.

Question 6

mitral cells

Answer 6

The major output neurons of the olfactory bulb.

Question 7

odorants

Answer 7

Molecules capable of eliciting responses from receptors in the olfactory mucosa.

Question 8

olfactory bulb

Answer 8

Olfactory relay station that receives axons from cranial nerve I and transmits this information via the olfactory tract to higher centers.

Question 9

olfactory epithelium

Answer 9

Pseudostratified epithelium that contains olfactory receptor cells, supporting cells, and mucus-secreting glands.

Question 10

olfactory receptor neurons (ORNs)

Answer 10

Bipolar neurons in olfactory epithelium that contain receptors for odorants.

Question 11

pheromones

Answer 11

Species-specific odorants that play important roles in behavior in some animals, including many mammals.

Question 12

pyriform cortex

Answer 12

Component of cerebral cortex in the temporal lobe pertinent to olfaction; so named because of its pearlike shape.

Question 13

taste buds

Answer 13

Onion-shaped structures in the mouth and pharynx that contain taste cells.

Question 14

ventral posterior medial nucleus

Answer 14

Component of the ventral posterior complex of thalamic nuclei that receives brainstem projections carrying somatic sensory information from the face.

Question 15

What roles do the trigeminal sensory system, the olfactory system and the gustatory system play in taste?

Answer 15

The experience of taste is actually a combination and integration of trigeminal, gustatory and olfactory stimuli. The texture and temperature of food, along with the presence of chemicals (such as capsaicin in spicy foods) are trigeminal. Taste buds identify one of five specific characteristics of foods dissolved in the mouth (sweet, salty, sour, bitter and umami). Olfaction actually provides much of the sensory experience of food (try eating while holding your nose). All of these sensations are integrated in the cortex.

Question 16

What roles does the trigeminal sensory system and TRP receptors play in the mouth and nose?

Answer 16

The trigeminal sensory system in the mouth and nose plays two roles. It is mostly protective of the oral and nasal passages, potentially triggering reflexes of gagging, coughing and sneezing. However, in the mouth, it also participates in taste. The peripheral end of nerves contain TRP receptors which are activated by pain, temperature and mechanical stimuli, but also by certain tastants, such as capsaicin (the ingredient in hot peppers).

Question 17

What is the anatomical and functional organization of the olfactory sensory system?

Answer 17

Olfactory receptors exist in the upper nasal cavity (which is why sniffing enhances the presentation of odorants to receptor cells). The epithelium has sensory cells which have cilia that express GPCRs that are the receptors. Hair cells are supported by “supporting cells” and are replaced about every two months from basal cells. They are covered with mucous that traps and presents odorants. It is unmyelinated processes of these cells that form the olfactory “nerve”, which is actually a series of filaments traversing the cribriform plate to the olfactory bulb.

Question 18

What are the molecular mechanisms involved in development of odorant specificity and signaling in the olfactory receptor cell?

Answer 18

Olfactory receptors are GPCR’s that are part of the largest family of genes (around a thousand humans, with at least half being expressed). There are two main pathways for olfactory receptors to activate the cell. These GPCRs can activate adenyl cyclase, which produces cAMP that opens cation channels directly, or it can activate phospholipase C which opens calcium channels through the action of inositol triphosphate. Note that these are both common mechanisms for GPCRs. Both of these pathways activate the olfactory receptor cells when the receptor recognizes its specific odorant.

Question 19

What are the roles of glomeruli and mitral cells in the olfactory bulb and how do they participate in olfactory signaling?

Answer 19

All of the olfactory receptor cells terminating in a particular glomerulus in the olfactory bulb express the same odorant receptor. The receptor axons terminate on dendrites of mitral cells in the glomeruli. The axons of the mitral cells, in turn, comprise the olfactory tract that conveys olfactory stimulation to the brain.

Question 20

How do glomeruli participate in identification of odors?

Answer 20

Individual odors generally activate more than one specific receptor, but they do so in differing proportions. It is the ability to recognize the pattern of activation of glomeruli that allows discrimination of more odors than there are individual receptor types. This also means that a single odorant may smell different based on its concentration and also if that odorant is presented in combination with another odorant.

Question 21

How can sensitivity to odors be changed based on physiologic state? ?

Answer 21

Noradrenergic axons (from locus ceruleus of the brain stem) can enhance olfactory sensitivity by acting on transmission in the olfactory bulb when you are hungry or seeking food. Epinephrine can enhance olfactory receptor sensitivity directly and can enhance the sense of smell when you are afraid or excited.

Question 22

How and where do central connections from the olfactory bulb terminate?

Answer 22

The olfactory tract splits into a lateral and medial olfactory stria. The medial stria terminates in the bed nucleus of the olfactory tubercle and also extends to the contralateral side of the brain. The lateral olfactory stria (the larger of the two) terminates in the anteriormost part of the parahippocampal gyrus (piriform cortex), and the amygdala. This anteromedial temporal lobe is the primary olfactory cortex.

Question 23

Where are taste buds located and how are they different over the tongue?

Answer 23

Taste buds are located at the base of papilla on the tongue. They consist of a cluster of sensory cells with a small opening to the surface of the tongue (taste pore). They have different response properties in different regions of the tongue, with salty and sweet being anterior on the tongue and bitter and sour being more posterior. They are also connected to different cranial nerves with anteriorly-located taste receptor cells activating (via serotonin) peripheral taste sensory axons of the facial nerve (via the chorda tympani nerve). Taste receptors on the posterior third of the tongue mostly synapse with the ends of glossopharyngeal nerve fibers; and vagal nerve fibers only connect with some very posterior receptors (bitter).

Question 24

What are the mechanisms involved in gustatory cells that sense sweet, salty, sour, bitter and umami?

Answer 24

Taste buds detecting sour and salty (which are both detecting ions: hydrogen for sour and mostly sodium for salty) are ion channels. Taste buds for bitter, sweet and umami (savory) are second messenger receptors (GPCRs).

Question 25

How does taste get to the brain, and where does relay, and terminate in the CNS?

Answer 25

Afferent axons of CNs VII, IX, and X project to the solitary nucleus of the medulla. Projections from here go to several places. There are projections to other parts of the brain stem to control salivation, coughing, and gagging in response to tastes. For conscious interpretation of taste, the solitary nucleus projects to the ventral posterior medial (VPM) nucleus of the thalamus. This relays test to the gustatory cortex to the insular cortex, just below the area that responds to somatosensory information from tongue, mouth, and throat. This allows trigeminal sensation to be combined with the chemical sense of taste.

Question 26

How can taste sensitivity be altered based on physiological states?

Answer 26

Various hormones and neurotransmitters can enhance sensitivity of taste receptor cells. Both serotonin and cholecystokinin (a GI hormone) enhance taste sensitivity. Also, aldosterone and vasopressin (which are often released in response to dehydration and volume depletion) enhance taste response to salt.

Question 27

Fungiform papillae are…

Answer 27

mushroom-shaped structures located on the tip and sides of the tongue, which contain taste buds.

Question 28

Foliate papillae are..

Answer 28

folded structures at the back of the tongue on both sides, which contain taste buds.

Question 29

Circumvallate papillae are..

Answer 29

flat mound structures that are found at the back of the tongue and contain taste buds.

Question 30

Filiform papillae..

Answer 30

do not contain taste buds and exist all over the tongue. The center of the tongue contains only filiform papillae. This is why stimulation of the center of the tongue does not cause a taste sensation, while the back and perimeter produce a broad range of taste sensations.

Question 31

What is the chorda tympani?

Answer 31

The chorda tympani is a branch of the 7th cranial nerve and transmits information from receptors in the anterior region of the tongue to the brain.

Question 32

Which cranial nerves transmit information from the posterior region of the tongue and the throat to the brain.

Answer 32

The glossopharyngeal and vagus nerves

Question 33

Suppose that a patient’s chorda tympani is anaesthetized. What would be the consequences for the perception of taste?

Answer 33

The patient would lose the ability to taste with the anterior two-thirds of the tongue, but would still be able to taste with the posterior third of the tongue and the throat because these signals would still be carried by the glossopharyngeal and vagus nerves

Question 34

What is the difference between the pathways for taste and the pathways for vision, hearing, and touch?

Answer 34

Taste is perceived from the ipsilateral side, while vision, hearing, and touch input is mostly contralateral.

Question 35

What is a distinct feature of olfaction?

Answer 35

Olfaction, unlike the other senses, does not synapse in the thalamus before higher processing.

The pathway for olfaction goes from the olfactory bulb to either the amygdala or the piriform cortex. From there the signal is transmitted to the orbitofrontal cortex.

Question 36

Where is the first place in the brain where the olfaction and gustation systems integrate?

Answer 36

Orbitofrontal cortex

Question 37

Describe the binding of salty tastants

Answer 37

Salty tastants bind to salt receptors which detect the presence of sodium ions and should not be chosen by the researchers.

Question 38

Describe the binding of sour tastants

Answer 38

Sour tastants bind to sourness receptors that react with hydrogen cations (H⁺). Once H⁺ binds to the receptor, it closes potassium channels.

Question 39

Desribe the binding of sweet, bitter, and umami tastants

Answer 39

Sweet, bitter, and umami tastants bind to G-coupled protein receptors.

Question 40

Stimulus-coupled depolarization for bitter tastes and amino acids (umami) relies on the same PLCβ2/IP3/TRPM5 dependent mechanism. How do we then distinguish between umami and bitter tastes?

Answer 40

Differentiation occurs at the receptor level and separate channels for the relay of this information to the brain. The bitter tastes use T2R receptors while umami uses T1R1 and T1R3.

Question 41

There are 3 papillae on the surface of the tongue. Describe their location on the tongue and the taste or tastes that dominate for the papillae. What cranial nerve innervates these different regions of the tongue?

Answer 41

Circumvallate, bitter, 48%, glossopharyngeal (IX); Foliate, sour, 28%, glossopharyngeal(IX); Fungiform, sweet and salty, 24%, facial nerve (VII)