Taste and smell (Chapter 13)

Question 1

Name the different categories of chemical senses.

Answer 1

Visceral chemoreceptors, gustatory receptors, olfactory receptor neurons, chemical sense receptors (chemosensitive endings).

Question 2

Which cranial nerves are involved in taste?

Answer 2

Facial (taste buds, salivary glands)
Glossopharyngeal (taste buds on posterior tongue, parotid glands)
Vagus (taste buds on base of tongue/epiglottis)

Question 3

What are the three types of taste bud papillae?

Answer 3

Vallate, foliate, fungiform

Question 4

What is the purpose of taste pores?

Answer 4

Taste pores are small openings that allow microvillar processes extending from the taste receptor cell to be exposed directly to food (interacting directly with chemical stimuli).

Question 5

How is flavor created?

Answer 5

Inputs from direct chemical stimulation of taste buds, stimulation of olfactory receptors, stimulation of chemosensitive and somatosensory endings (latter: temperature, spiciness, pungency of food).

Question 6

What brain region is the major site where multiple inputs are combined into overall sensation about food?

Answer 6

OFC.

Question 7

Name the different categories of tastants.

Answer 7

Salty, sweet, bitter, sour, umami (glutamate).

Question 8

Does gustation depend on which taste receptors are being activated?

Answer 8

No. Taste depends on the pattern of activity in a large population of neurons across a large population of taste buds.

Question 9

Describe the major branches in the taste pathway.

Answer 9

Taste receptors on tongue
↓
Taste buds
↓
Glossopharyngeal nerve
↓
Solitary nucleus
↓
Thalamus (ventral posteromedial nucleus)
↓
Gustatory cortex (insula, frontal operculum)
↓↓
OFC (combined w/olfactory info); AMY and other limbic structures.

• 1st order neurons (ganglia)
• Rostral medulla (2nd order connections to solitary nucleus)
  
  • Projections to vagus motor nucleus (gag, swallowing, coughing), reticular formation
• Rostral pons via central tegmental tract
• Thalamus
  
  • Medial VPM to insula and frontal operculum
• OFC: taste + smell
• Amygdala
• Hypothalamus

Question 10

Describe the major branches in the olfactory pathway.

Answer 10

Olfactory receptors (on olfactory epithelium)
↓
Cribiform plate
↓
Glomeruli
↓
Mitral cells
↓
Olfactory tract:
- Olfactory tubercule
- Anterior olfactory nucleus → anterior commissure → contralateral olfactory nucleus
- Piriform cortex (main cortex for olfactory info)
(also: entorhinal cortex, periamygdaloid cortex, anterior parahippocampal gyrus, amygdala)

Question 11

What are glomeruli and what is their function?

Answer 11

Glomeruli are olfactory receptor axonal endings stratfied by chemical type. Glomeruli are spherical structures where synapses between the olfactory nerve axons and mitral cell dendrites form. They code for the presence and amount of a chemical.

Question 12

What is the mechanism for olfactory transduction, and how can we detect so many different scents?

Answer 12

Olfactory transduction occurs via coupled G proteins with second messengers. We can detect thousands of scents because we have ~300 different kinds of olfactory receptor neurons. Each olfactory receptor neuron binds a wide array of odorants that overlaps but differs from the arrays bound by other receptor neurons.

Question 13

Why do smells tend to linger?

Answer 13

Smells tend to linger because the unmyelinated axons of olfactory receptors are very thin and slow conducting: they are slow to adapt, but remain adapted for longer periods.

Question 14

Why can’t we smell unilaterally?

Answer 14

We can’t smell unilaterally because olfactory nerve fibers synapse in the anterior olfactory nucleus, where they cross the midline through the anterior commissure and inputs are sent back to the contralateral olfactory bulb.