Sensory: chemosensory

Question 1

Describe the human olfactory system.

Answer 1

The olfactory epithelium is found in the upper part of the nasal cavity. The olfactory epithelium is lined with the dendrites of sensory neurons containing olfactory receptors. The sensory neurons terminates in the olfactory bulb located right above the nasal cavity. Here the signal is transmitted to the brain via NI.

Question 2

Describe the general signal transduction in vertebrate olfactory signaling.

Answer 2

Odorant binds to GPCR –> G-alfa-olf dissociates from G-beta-gamma and activates ADCY3 –> increase of [cAMP] –> cAMP gates CNGA2 –> influx of Ca^2+ and Na^+ –> depolarization –> AP

Question 3

Describe the organisation of the vertebrate taste bud. What structures and cells are
present, what are their functions?

Answer 3

Gustatory cells (chemoreceptive cells with microvilli), basal cells (replacing the gustatory cells) and supporting cells (secreting cells, creates the microenvironment)

Question 4

Describe the three types of taste papillae.

Answer 4

1: Fungiform - most abundant, front and lateral region of the tounge
2: Foliate - posterolateral region
3: Circumvallate - posterior borders

Question 5

How is an odour perceived?

Answer 5

By combinatorial coding:
ORs bind to several similar molecules but with different affinity –> different strength of AP. All ORNs with the same receptor project to the same glomerulus. One odour targets several receptortypes –> activates several glomeruli. Odour molecules have multiple OR binding sites (=odotypes)
=> each odor has a unique activation pattern in the olfactory bulb (across glomeruli/fiber pattern)

Question 6

Where are the olfactory bulbs located in humans?

Answer 6

Rostral, part of the limbic system

Question 7

How does no sense of smell affect a person?

Answer 7

Loss of appetite, decreased libido, emotional blunting and depression

Question 8

How many odorants is it estimated that humans are able to detect and discriminate?

Answer 8

> 100 000

Question 9

What is the difference between odorant and olfaction?

Answer 9

Odorant: a molecule that can reach the olfactory epithelium and trigger a signal conduction
Olfaction: synthetic sense - what are perceived as a specific smell are a blend of multiple molecules

Question 10

What are the receptors responsible for detecting odorants called, and what type of receptors are they?

Answer 10

Odorant receptors (ORs), GPCRs

Question 11

How does the rodent olfactory system differ from the human?

Answer 11

They contain glomeruli, a functional unit where OSN’s axons synapse on mitral cells

Question 12

Describe pattern separation.

Answer 12

Decorrelation of overlapping glomerular inputs into less overlapping mitral cells output, partly mediated by inhibitory granule cells in the olfactory bulb.

Question 13

What stuctures are part of the olfactory cortex?

Answer 13

Pyriform cortx, olfactory tubercle, amygdala and entorhinal cortex

Question 14

What is unique about the sense of smell?

Answer 14

It goes directly to the limbic system and cerebral cortex without going through the thalamus = privileged link between odors and memory/emotions

Question 15

What are some non-classical olfactory systems?

Answer 15

Vomeronasal receptors, trace-amine associated receptors and two guanylyl cyclases

Question 16

What is a tastant?

Answer 16

Water soluble chemicals that activate receptors in the taste buds

Question 17

How does tastants differ from odorants?

Answer 17

Generally detected at higher concentrations, except for bitter

Question 18

How is the topology for the 5 basic taste receptors?

Answer 18

Umami + sweet: dimeric GPCR (T1R1/2 + T1R3) - family C
Bitter: GPCR, app. 30, each bitter cell express all 30
Salt: Na^+ gated ion channel, trimer, each monomer has 2 TMs - DEG/ENaC family
Sour: H^+ gated ion channel, dimer, each monomer har 12 TMs - otop 1 family

Question 19

Descripe the signal transduction of sweet/umami/bitter.

Answer 19

Tastant binds and activates GPCR. G-q-beta-gamma activates PLC, which cleaves PIP2 into IP3 and DAG. IP3 gates a Ca^2+ channel on ER, Ca^2+ gates TRPM5 inducing Na^+ influx, resulting in depolarization triggering voltage gated Na^+ channels to open -> AP. The action potential results in the opening of voltage gated ATP channels (pannexin). ATP binds to P2X3 on afferent neuron, and the neuron is depolarized and an actionpotential is initiated -> signal travels to brain

Question 20

Describe the signal transduction of salt/sour.

Answer 20

Na^+/H^+ binds to ion channel, resulting in Na^+/H^+ influx, causing depolarization resulting in AP. Voltage gated Ca^2+ channels open, and stimulate the release of serotonine. Serotonine binds to 5HT3R on afferent neuron, causing Na^+ influx and AP -> signal travels to brain.

Question 21

Which receptors mediates the tasting of spices?

Answer 21

TRP channels, e.g. TRPV1: capsaisin (heat/pain) and TRPM8: menthol (cold)

Question 22

Describe the route of gustatory signals to the brain.

Answer 22

First info are collected by afferents from gustatory sensory neurons (cell bodies in ganglia) –> nucleus of the solitary tract in the brain stem –> VPM thalamic nucleus –> insular cortex –> amygdala

Question 23

How many cranial nerves are involved in taste?

Answer 23

3: VII, IX and X

Question 24

How are taste quality encoded?

Answer 24

Labeled line: each taste cell express only one type of receptor, and distinct taste receptor cells encode each of the taste quality

Question 25

True or false: induvidual “bitter” taste receptor cells express many or most of the T2Rs.

Answer 25

True: they are generelist poison detectors