Week 11 - Smell and taste

Question 1

Chemoreception

Answer 1

smell and taste are a form of chemoreception
Chemoreception is evolutionarily old: bacteria use it to guide their
movements; animals without brains use it to find food and mates.
Chemoreception may have evolved into chemical synaptic communication.

Question 2

where are olfactory receptors located

Answer 2

olfactory epithelium
The epithelium lies at the topf of the nasal cavity, cover 3cm^2 in each of the sides and it contains 10 mil receptors in total.
The epithelium is pigmented. the richness of its color correlates with olfactory sensitivity.

Question 3

receptor neurons

Answer 3

the receptor neurons are ciliated
each cell has a single dendrite that extends into the olfactory epithelium. there it branches to form nonmotile cilia that increase the surface area of the cell, so it has a greater chance of catching odorant.

Question 4

G-protein coupled receptor cells in olfactory molecules

Answer 4

The genes for these receptor molecules form the largest known
gene family in vertebrates — 1000 genes, or ~3–5% of the genome
— though only ~400 are expressed in humans.
When an odorant molecule binds its receptor, it activates a G
protein, Golf , which increases the local concentration of cAMP.
cAMP-gated cation channels open, depolarizing the receptor neurons and triggering an action potential that travels along the cell’s axon to the olfactory bulb.

Question 5

unusual properties of olfactory cells

Answer 5

They are pinocytotic, continually sipping in fluid and sending it
along the nerves into the brain. We don’t know why.
They are short-lived, degenerating after a month or 2, to be re-
placed by new ones from below.
They send their axons into the brain through tiny holes in the cribriform ( sievelike”) plate — the bone at the base of the cranial cavity.

Question 5

where do olfactroy receptor cells project

Answer 5

to the olfactory bulb
the bulb is an extension of the cerebrum, and lies on the underside of the frontal lobes.
the projection from the receptors to the bulb is called the olfactory nerve, or cranial nerve I

Question 5

sensitivity of receptor cells

Answer 5

Some of them can detect a single molecule of their preferred
chemical, though ~40 cells must react before we experience a smell.

Question 6

where does the olfactory bulb project

Answer 6

directly to olfactory cortex, bypassing thalamus

Question 7

What happens when many receptor cells converge on each bulb neuron

Answer 7

enhances sensitivity but discards spatial information

Question 8

which lobe is the olfactory cortex in

Answer 8

frontal and temporal lobes
limbic system

Question 9

limbic system

Answer 9

This is an old group of brain regions concerned with motivation
and emotion. For early animals, motivation was tightly linked to smell:
they used their noses to identify food and poisons, mates and predators.
Our emotions are no longer so smell-related (e.g. we like money)
but they are still handled by these old olfactory areas. Maybe that is
why odors call up emotional memories

Question 10

does olfaction adapt ?

Answer 10

slowly but completely
eg. people are unaware of their body odors

Question 11

pheromones

Answer 11

chemicals released by an animal into the environment which affect the physiology or behavior of other
members of its species.
Rodents have an olfactory structure in the nasal cavity called the
vomeronasal organ (VNO), which is involved in their behavioral
responses to sex pheromones.
In humans, the VNO disappears during fetal development, but we
do respond to airborne chemical signals.

Question 12

where are our main taste receptor cells

Answer 12

clustered in taste buds
we have 5000 taste buds, mainly on the top of the tongue but also on the epiglottis, soft palate and upper esophagus. babies have 10000
A taste bud only lives about 10 days
Each taste bud contains about 100 receptor cells, which are epithelial cells. they contact the oral cavity through a small opening, the taste pore.

Question 13

receptor cells in taste buds

Answer 13

Sweet and umami receptor cells detect sugar (energy) and the
amino acid glutamate (indicating protein), respectively.
Bitter receptor cells detect poison.
Salty and sour receptor cells detect Na+ and H+ — 2 important ions.
The tongue may also have receptors for fatty acids.

Question 14

3 groups of taste receptor cells

Answer 14

Type I may sense salt
Type II sense sweet, bitter and umami. they release ATP whidh acts on neurons and type III
Type III sense sour. they form synapses with sensory neurons, activating them with serotonin

Question 15

membrane proteins employed by sweet, umami and bitter

Answer 15

receptor molecules coupled to a G protein called gustducin, which activates signal pathways, increasing intracellular [Ca 2+] and triggering the release of ATP.

Question 16

membrane proteins employed by salt and sour

Answer 16

they involve ion channels which are not linked with G proteins

Question 17

what does our experience of food depend on

Answer 17

It depends on smell, temperature, pain, texture, crunch, appearance, and cognition — if I tell you some lousy food is a delicacy then
you like it better.
Nerve endings in the walls of the mouth have TRP channels sensitive to temperature and chemicals, e.g. vanilloid receptors respond
to heat and to capsaicin; TRPM8 channels respond to cold and to menthol.
Chemoreceptors in our stomach and intestines monitor their
contents; some of these receptors resemble ones on the tongue, e.g.
for sweet and umami.

Question 18

taste pathways to brain

Answer 18

Receptor cells in the taste buds excite fibers of cranial nerves VII,
IX, and X, the facial, glossopharyngeal, and vagus nerves. These pathways synapse in the medulla and thalamus en route to the cortex. TRP receptors in the walls of the mouth excite cranial nerve V, the trigeminal.