Olfaction and Gustation

Question 1

Olfactory epithelium and olfactory bulb

Answer 1

-the olfactory cells synapse with secondary sensory neurons in the olfactory bulb. Axons of the olfactory cells form the olfactory nerve (first cranial nerve)

Question 2

Olfactory epithelium

Answer 2

-Olfactory receptor cells are sensory neurons.
-Basal cells are stem cells that can regenerate sensory cells
-supporting cells provide structure and release chemicals to block toxins
-Bowmans gland produces mucus layer surrounding cilia
-odor detecting GPCRs are located in the cilia

Question 3

Olfactory Ensheathing Cells Aid Axon Guidance in Olfactory Neurons

Answer 3

-The axons of olfactory receptor neurons are myelinated to speed conduction
-The axons are also surrounded by olfactory ensheathing cells that assist in forming a path for the axons going to the olfactory bulb
-Transplants of olfactory ensheathing cells have been used experimentally to repair severed axons in the spinal cord

Question 4

Glomeruli Receptor and odor specificity

Answer 4

-each odorant receptor type (GPCR) is expressed in ~2000 olfactory cells and innervate two discrete glomeruli. Therefore, a glomerulus codes for a specific GPCR

Question 5

How many olfactory receptors are there? Glomeruli? olfactory cells? why?

Answer 5

-400 distinct olfactory receptor GPCRs
-800 glomeruli on each side of the brain
-2 glomeruli for each type of GPCR
-5-10 million olfactory cells
-1000 olfactory cell axons per glomerulus

Using these 400 pair of glomeruli, humans can detect ~trillion scents. Each GPCR can respond to a spectrum of odorants. One odorant can stimulate several GPCRs. Result us a huge combinatorial coding.

Question 6

Combinatorial coding of odorants

Answer 6

-400 receptors can detect a trillion scents

Question 7

Anosmia

Answer 7

loss of smell

Question 8

Parosmia

Answer 8

distorted sense of smell.

Question 9

what innervates the taste buds

Answer 9

-taste buds are innervated by 3 cranial nerves
-Chorda tympani–> branch of the facial nerve (CNVII) innervates anterior 2/3 of tongue
-Glossopharyngeal nerve–> CN IX innervates the taste buds on posterior 1/3 of the tongue
-Vagus nerve (CN X) innervates a few taste buds in the larynx and upper esophagus

Question 10

Taste Bud

Answer 10

Taste bud contains multiple taste receptor cells, all sensitive to the same tastant, plus supporting cells and basal cells
-taste buds are found in the tongue, palate, esophagus and epiglottis
-lifespan of a taste receptor cell is ~1 week

Question 11

Taste Receptors subtypes

Answer 11

-Sour: Acid detector, spoiled food
-Salt: Sodium channel
-Sweet: Carbohydrate detector
-Bitter: most natural poisons are bitter
-Umami: Modified glutamate receptor GPCR, protein detector

Sour and salt receptors are channels (channel mediated transduction) and the others are GPCRs

Question 12

T1R1 and T1R2

Answer 12

-GPCRs for Umami and sweet taste transduction
-both receptors are dimers with T1R3

Question 13

T2R

Answer 13

-A large family of bitter taste GPCRs
-bitter taste receptors are also found in gut and lung, mediating responses to toxins and bacteria

Question 14

Gustatory Central Pathways

Answer 14

-the pathway for conscious taste detection includes 3 cranial nerves that go to the solitary nucleus in the medulla, then to the thalamus, and then to the gustatory cortex (MTC–> medulla, thalamus, gustatory cortex )

-There are additional gustatory pathways going from thalamus to limbic system (hippocampus and amygdala)

Question 15

Gustatory agnosia

Answer 15

Loss of taste sensitivity

Question 16

Hypergeusia

Answer 16

Increase in taste sensitivity

Question 17

Dysguesia

Answer 17

Distortion in taste detection

Question 18

Taste with age

Answer 18

there is a loss of taste buds, fungiform papillae, and a decrease in taste sensitivity with age