Neurology of Smell and Taste

Question 1

Describe the embryogenesis of the face

Answer 1

As the face develops the complex morphogenesis involves bilateral maxillary and mandibular processes.
The development ot the optic cups (from lens placode), and two olfactory pits (from olfactory placode).

Question 2

What is the basic embryogenesis of the olfactory epithelium?

Answer 2

Develops from the olfactory placode - a region of thickened ectodermal tissue.
Are neurogenic - delimaintes to help form CN1.

Question 3

Describe the key anatomy of the olfactory system within the nose/skull.

Answer 3

Primary olfactory sensory neurons sit in the olfactory epithelium in the roof of the nasl cavity
Olfactory sensory neurons are bipolar
They have axons penetrating through the cribriform plate forming the olfactory nerve to project to the olfactory bulb.
They have dendrites extending into the mucosa, giving rise to olfactory cilia which respond to oderants.

Question 4

What is the broad process of action potential generation in the olfactory system?

Answer 4

Receptor potentials are generated in the olfactory cilia.
These are summated and if depolarisation passes the threshold will generate an action potential which can be fired down the cell body (of Primary Olfactory Sensory Neurons) down the axon.
Medaited by opening of voltage-gated cation channel opening.

Question 5

What are the key features of olfactory receptors?

Answer 5

Olfactory receptors are GPCRs
There are around 400 active olfactory receptor genes in humans
Each primary olfactory sensory neuron will only expressed one type of olfactory receptor.

Question 6

What is the evidence underpinning the evolutionary diminishment of smell in humans?

Answer 6

There are 950 recognisxable olfactory genetic sequences in the human genome>
However only 400 are active olfactory receptor genes
The rest are pseudogenes, no longer transcribed and translated due to accumulated mutations.
Therefore human smell has diminished evolutionary as these mutations occurred.

Question 7

How does the pattern of olfactory recepots to primary olfactory sensory neurons influence smell perception?

Answer 7

Each primary olfactory neuron will only express one olfactory receptor gene.
Therefore each olfactory receptor has a unique response to different odourant molecules, in terms of yes or no response but also degree of activity.
The combination of patterns of responses from different olfactory receptors enables us to detect a range of different smells - more than the 400 receptors we posses.

Question 8

What are the different variation in anatomy that give some species a better sense of smell than other?
Links to evolution

Answer 8

The no of olfactory receptor genes
The number of active olfactory genes expressed.
The epithelial surface for olfactory neurons (hence the amount)
The brain power for the processing of sense of smell (relative size of brain region)

Question 9

What is the molecular signalling involved in olfactory signal transduction (generation of receptor potential)?

Answer 9

Oderant molecule binds to olfactory receptor in cell membrane of olfactory cilia.
Is a GPCR
G protein alpha subunit (Golf) exhcnages GDP for GTP is now active
Activates adenylayse cyclase, which converts ATP to cAMP
cAMP acts as a second messenger to activate Na+/Ca2+ cAMP gated channels.
Na+ and Ca2+ influx into the cytoplasm
Inc Ca2+ opens Ca2+ gated Cl- channels for Cl- efflux from the cytoplasm
Results in depolarisation of the cell.

Question 10

What is the pattern of primary olfactory neuron projection into the olfactory bulb?

Answer 10

OSNs project axon into mulitfarious synapses in the olfactory bulb called glomeruli - this is an example of convergence.
The glomerulus will be made of synpases of primary olfactory neurons all containing the same receptor.
All converge on one secondary order olfactory neuron.
This increases sensitivity of smell (detect faint odours) by amplifying signals to increase detection capacity.

Question 11

What are the two different secondary order olfactory neurons?

Answer 11

Mitral cells
Tufted cells

Question 12

What is the purpose of secondary order olfactory neuron cells?

Answer 12

Includes mitral and tufted cells
Project from the glomeruli down the olfactory tract to the brain.

Question 13

What regulates the sense of smell in the olfactory bulb?

Answer 13

Lateral inhibition is provided by the periglomerular and the granule cells in the olfactory bulb.
These are GABAergic, provide ‘competition’ between perception of smell.
Similar to the idea of central surround organisation in the retina - but no map of location base of smell (not topographic).

Question 14

Where in the brain due secondary order olfactory neurons project to?

Answer 14

Mitral and tufted cells project to multiple targets within the cortex.
Olfactory (pyriform) cortex - to process and code olfactory information.
Limbic areas - amygdala and the entorhinal cortex - link odor perception to memory with emotional and motivational responses, and spatial environment.
(Note does NOT project to the thalamus)

Question 15

What is unique about the pyriform cortex (olfactory cortex) structure?

Answer 15

No topographic
Can’t locate smell - it just exists.

Question 16

What are the five different principle tastes that a taste bud can detect?

Answer 16

Bitter
Sweet
Salty
Umami
Sour

Question 17

Where is taste detected?

Answer 17

Mainly on the tongue
Taste buds - contain many cells that open onto the tongue surface through a taste pore.
Collections of tasted buds in a raised surface are called papillae.

Question 18

What are the three different taste pupillae and what is the distribution of taste buds like?

Answer 18

The circumvallate - towards base of tongue - contains hunders of taste buds
The foliate - bilateral towards back - contains hundreds of taste buds
The fungiform - front - contains tens of taste buds

do not worry about location

Question 19

What makes up a taste bud?

Answer 19

Composed of four main cell types (I-IV).
Consists of less than 100 cells in total

Question 20

What is the basic role of different cell types in the tase bud?

Answer 20

Type 1 - glial like cells (support cells)
Type 2 - receptor like for bitter,umani, sweet, salty
Type 3 - receptor-like for sour
Type 4 - located basolateral act as progenitor cells.

Question 21

What are the different taste receptors for each modality of taste?

Answer 21

Sour - H+ ion channel
Salt - Na+ ion channel
Sweet - T1R2 and T1R3 (GPCRS)
Umami - T1R1 and T1R3 (GPCRs)
Bitter - 2X~25T2Rs (GPCRs)

Question 22

Describe the process of taste transduction at type II cells in the taste buds.

Answer 22

Salt - Na+ faciliated diffusion into cytoplasm via amiloride sensitive Na+ channel, Na+ influx causes depolarsiation.
Umami, sweet, bitter - bind to respective GCRPs , alpha subunit exchanges GDP for GTP and becomes active, activates PLCbeta, results in Ca2+ release from ER, activates TRMP5 channel causing cation influx - depolarsiation
Depolarisation is reinforced through opening of voltage gated Na+ channels.
ATP is produced by specialised mitochondria and released via ATP channel, binds to ATP receptor on gustatory afferent axon to trigger and action potential.

Question 23

Describe the process of taste transduction at type III cells in the tase buds.

Answer 23

Is responsible for sour tastes
H+ enters the cell cytoplasm via hydrogen/OTOP1 ion channels.
Inc H+ conc inhibits K+ channels, this decreases K+ efflux.
Combined effect results in depolarisation of the cell, further reinforced by the opening of voltage gated Na+ channels.
Results in opening of voltage-gated Ca2+ channels.
Causes fusion of seratonon containing vesciesl with cell membrane, seratonon binfs to receptors on gustatory afferent neruons - results in action potential generation.

Question 24

What is important from a human evolution perspective about the GPCRs for bitter taste sensation?

Answer 24

25 different GPCRS
As arranged in a heterodimerous structure - over 33 million different possible receptor complexes
Able to detect a range of poisoning molecules - is a safety mechanism.

Question 25

What is the central processing of gustatory information?

Answer 25

Afferent information from three different cranial nerves:
Facial nerve - anterior 2/3 of tongue
Glassopharangyeal - posterior 1/3 of the tongue
Vagus - pharnx and upper oesophagus
Projects to the solitary nucleus in the brainstem (synapse), project to VPM of the thalamus (synapse), project to the gustatory cortex in the insular and frontal lobe.
Also has input to and from the amygdala and hypothalamus in the limbic system.
Therefore taste influences decisions and emotions

Question 26

What are the three key sensory brainstem nuclei?
What is their pattern of synapses?

Answer 26

Cochlear and vestibular nuclei - input from CN8 - bilateral
Trigeminal sensory nucleus - input from CN5 - contralateral
Solitary nucleus - CN8,9,10 - ipsilateral.

Question 27

What is unique to the gustatory cortex compared to other sensory cortexes?

Answer 27

Has no topographical map - no geographic location to taste, just exists.