Lecture 25: Smell

Question 1

What are the two chemical senses?

Answer 1

gustation (taste) and olfaction (smell)

Question 2

The sensation of taste and smell arise from what?

Answer 2

in response to contact with chemicals in the environment, either that we put in our mouth (tastes), or that we inhale (odours)

Question 3

The cells that detect these stimuli are called what?

Answer 3

chemoreceptors

Question 4

What do the chemoreceptors allow us to do?

Answer 4

detect chemicals in the environment that might help us survive and fulfil our biological destiny

Question 5

What is our biological destiny?

Answer 5

finding food and avoiding poisons

finding a compatible mate

Question 6

When we inhale odours through our nostrils, where do the chemicals reach?

Answer 6

the roof of the nasal cavity

Question 7

What is at the roof of the nasal cavity called?

Answer 7

olfactory epithelium

Question 8

Within the olfactory epithelium, there is the what plate?

Answer 8

the cribriform plate

Question 9

There are holes in the olfactory epithelium. What is located here?

Answer 9

there are olfactory receptor cells

Question 10

What type of cells are olfactory receptor cells?

Answer 10

they are bipolar nerve cells

Question 11

Describe the olfactory mucosa cells

Answer 11

these have 10-14 non-motile cilia that project through the holes into a large layer of mucous (to the “outside”)

Question 12

Describe what happens when we inhale gas?

Answer 12

the chemicals diffuse through the mucus layer and interact with receptors on the cilia which will stimulate smell transduction pathwyas

Question 13

Bipolar nerve cells have axons that go where?

Answer 13

these head out of the olfactory epithelium up to the olfactory nerve and reach the olfactory bulb

Question 14

Where does the olfactory bulb sit?

What is it responsible for?

Answer 14

underneath the frontal cortex and it is responsible for the sensation of smell

Question 15

Describe the olfactory signal transduction

Answer 15

The gases come in to the nose and diffuse into the mucus. This has got the cilia in it. The odourant interacts with specific G-protein coupled receptors on the membrane of the cilia. This interaction activates adenylyl cyclase and this causes the breakdown of ATP to cyclic AMP. This increase in cAMP open cation channels so there is an influx of Na+ and Ca2+ into the cilia so Cl- channels open so Cl- leave. This causes a depolarisation which causes a graded receptor potential which causes a wave to go through the olfactory mucosa cell to reach the axon hillock. If this is is a large enough depolarisation, an action potential will be fired from the axon hillock to the olfactory bulb

Question 16

are olfactory axons myelinated or unmyelinated?

Answer 16

unmyelinated

Question 17

Olfactory axons form what?

Answer 17

the first cranial nerve

Question 18

Where do the olfactory axons synapse?

Answer 18

in the olfactory bulbs on the underside of the frontal lobes

Question 19

Where is the olfactory cortex located?

Answer 19

in the somatosensory cortex

Question 20

How many different receptor types are there?

Answer 20

1000

Question 20

How many different smells can our 1000 different receptor types detect?

Answer 20

10,000 - 50,000

Question 20

The olfactory bulb axons pass to what?

Answer 20

the olfactory cortex (in the limbic system - associated with emotional, food-getting and avoiding, sexual behaviour)

Question 20

Why can we detect so many more smells than there are receptor types?

Answer 20

because to detect different odours, there is relative activation of different receptor types

Question 20

How did COVID-19 affect our sense of smell?

Answer 20

COVID-19 invades the supporting cells around the olfactory receptor cells

Question 21

What are retinal ganglion cells influenced by?

Answer 21

a network of photoreceptors and interneurons

Question 22

What is anosmia?

Answer 22

partial or complete loss of smell

Question 23

What causes anosmia? Give examples

Answer 23

swelling or blockage in the nose
caused by:
- irritation’s to the nose’s mucus membrane (eg. due to hay-fever or a cold)
- blockage of the nasal passage (eg. due to tumors)
- brain or nerve damage (eg. due to a stroke)

Question 24

What excites ganglion cells?

Answer 24

different patterns of light falling on the centre and surround of their receptive field

Question 25

What are the two types of receptive fields?

Answer 25

ones that are excited about light falling on the centre of the receptive field and those that are excited by light falling on the surround of the receptive field

Question 26

Retinal ganglion cells being influenced by less photoreceptors and interneurons sit where?

Answer 26

in the fovea (smaller receptive field so higher visual acuity)

Question 27

True or false? Odourants are chemicals that cause direct depolarisation of olfactory neurons by diffusing into them?

Answer 27

false - odourants diffuse in the mucous around the cilia and then interact with receptors on the cilia

Question 28

What are the two types of receptive fields?

Answer 28

ones that are excited about light falling on the centre of the receptive field and those that are excited by light falling on the surround of the receptive field

Question 29

How many taste receptors make up a taste bud?

Answer 29

50-100

Question 30

What are the balls of CaCo3 called?

Answer 30

otoconia

Question 31

The amount of displacement of the basilar membrane depends on what?

Answer 31

the amplitude of the sound