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Question 1

describe the evolution underlying chemoreception

Answer 1

• 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 the olfactory receptors located?

Answer 2

in the olfactory epithelium

Question 3

describe the location of the olfactory epithelium

Answer 3

lies at the top of the nasal cavity, covering 3cm2 in each of the 2 sides

Question 4

describe the contents of the olfactory epithelium

Answer 4

• contains ~10 million receptor cells in total
• pigmented: no one knows why, but the richness of its color correlates with olfactory sensitivity (darker = more sensitive)

Question 5

the olfactory receptor neurons are —- neurons

Answer 5

ciliated

Question 6

describe the structure of the olfactory receptor neurons

Answer 6

• each cell has a single dendrite that extents 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 odourant molecules

Question 7

describe the specificity of receptor neurons

Answer 7

each receptor cell has (many copies of) one type of odourant receptor molecule on its membrane. we have ~400 kinds of receptor cell (ie 400 primary odours)

Question 8

are GPCRs common?

Answer 8

yes; 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 expressed in humans)

Question 9

what happens when an odourant molecule binds its GPCR?

Answer 9

• it activates a G protein, Golf, which increases the local concentration of cAMP
• cAMP-gated cation channels open, depolarising the receptor neurons and triggering an action potential that travels along the cell’s axon to the olfactory bulb

Question 10

describe receptor sensitivity in humans

Answer 10

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

Question 11

what three unusual properties do olfactory receptor cells have?

Answer 11

• pinocytotic: continually sipping in fluid and sending it along the nerves into the brain (we don’t know why)
• short-lived: degenerate after a month or 2, to be replaced by new ones from below
• send their axons into the brain through tiny holes in the cribriform (‘sieve-like’) plate - the bone at the base of the cranial cavity

Question 12

where do receptor cells project to?

Answer 12

the olfactory bulb

Question 13

describe the olfactory bulb

Answer 13

an extension of the cerebrum, and lies on the underside of the frontal lobes

Question 14

olfactory nerve

Answer 14

cranial nerve I - the projection from the receptors to the bulb

Question 15

describe convergence in the olfactory sense

Answer 15

• many receptor cells converge on each bulb neuron
• this arrangement enhances sensitivity but discards spatial information

Question 16

where does the olfactory bulb project to?

Answer 16

• the olfactory cortex, bypassing the thalamus
• the limbic system

Question 17

limbic system

Answer 17

old group of brain regions concerned with motivation and emotion

Question 18

where is the olfactory cortex located?

Answer 18

in the frontal and temporal lobes

Question 19

what is the significance of the olfactory bulb projecting to the limbic system?

Answer 19

• 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 but they are still handled by old olfactory areas (hence why doors call up emotional memories)

Question 20

does olfaction adapt? explain

Answer 20

yes; slowly but completely

Question 21

define pheromones

Answer 21

chemicals released by an animal (rodents and maybe humans) into the environment which affect the physiology or behaviour of other members of its species

Question 22

describe the response of rodents to sex pheromones

Answer 22

rodents have an olfactory structure in the nasal cavity called the vomeronasal organ (VNO), which is involved in their behavioural responses to sex pheromones

Question 23

do humans have a VNO? explain

Answer 23

the VNO disappears during fetal development, but we do respond to airborne chemical signals

Question 24

where are our main taste receptor cells located?

Answer 24

clustered in taste buts

Question 25

how many taste buds do we have and where?

Answer 25

- we have ~5000 taste buds, mainly on the top of the tongue but also on the soft palate, epiglottis, and upper oesophagus - babies have 10,000

Question 26

lifespan of a taste buds

Answer 26

10 days

Question 27

describe the structure of taste buds

Answer 27

contains ~100 receptor cells, which are epithelial cells (not neurons) arranged like petals

Question 28

how do taste buds contact the oral cavity?

Answer 28

through a small opening, the taste pore

Question 29

a typical taste bud contains at least --- kinds of receptor cell. what do each of these receptor cells do?

Answer 29

5; each kind of receptor cell detects one flavour, and all 5 have clear biological roles

Question 30

state the 5 kinds of receptor cell

Answer 30

sweet umami bitter salty sour

Question 31

sweet receptor cells

Answer 31

detect sugar (energy)

Question 32

umami receptor cells

Answer 32

detect the amino acid glutamate (indicating protein)

Question 33

bitter receptor cells

Answer 33

detect poison

Question 34

salty and sour receptor cells

Answer 34

detect Na+ and H+ - 2 important ions

Question 35

what is a 6th type of receptor thought to be present in the tongue?

Answer 35

fatty acids receptors

Question 36

how are receptors distributed on the tongue? what implications does this pose?

Answer 36

- there are receptor cells of all 5 kinds all over the top of the tongue - but different areas of the tongue do vary slightly in their thresholds for different flavours

Question 37

taste receptor cells are grouped into 3 types:

Answer 37

type I cells: may sense salt type II cells: sense sweet, bitter, and umami type III cells: sense sour

Question 38

how do type I/II/III cells cause a response?

Answer 38

- only type III cells form synapses with sensory neurons, activating them with serotonin - type II cells release ATP, which acts on neurons and type IIIs

Question 39

describe the membrane proteins employed by cells for sweet, umami, and bitter

Answer 39

have receptor molecules coupled to a G protein called gustrducin, which activates signal pathways, increasing intracellular [Ca2+] and triggering release of ATP

Question 40

how does detection of salt differ from the mechanism of cells for sweet, umami, and bitter?

Answer 40

detection of salt and sour involves ion channels which are not linked with G proteins

Question 41

what sensors does our experience of food depend on - besides the taste buds?

Answer 41

smell, temperature, pain, texture, crunch, appearance, and cognition

Question 42

why are our mouths also sensitive to temperature and chemicals?

Answer 42

nerve endings in the walls of the mouth have TRP channels sensitive to temperature and chemicals - eg vanilloid receptors and TRPM8 channels

Question 43

where - other than the mouth - may chemoreceptors be found in the digestive system? what function do these have?

Answer 43

chemoreceptors in our stomach and intestines monitor their contents; some of these receptors resemble ones on the tongue (eg sweet and umami)

Question 44

describe the paths taken by taste signals to the brain

Answer 44

- receptor cells in the taste buds excite fibres of cranial nerves VII (facial), IX (glossopharyngeal), and X (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