Lecture 3: Olfaction

Question 1

Which sense is first active in newborns and what experiment has shown this?

Answer 1

Smell - it is operational with first respiratory movements. An fMRI study has shown differential patterns of activation to different odorants such as cabbage and eucalyptus.

Question 2

Which sense as the most direct route to affect/emotion?

Answer 2

Olfaction

Question 3

Where are olfactory receptors located?

Answer 3

High in the nasal cavity - in the superior turbinate

Question 4

What about the structure of the nose enhances our ability to smell?

Answer 4

Bony labyrinths (nasal conchae/turbinates) of the nose results in inspired air to move in eddy currents, such that it exposes olfactory mucosa as much as possible to the odorant.

Question 5

Which receptors are able to regenerate

Answer 5

Olfactory receptors and taste receptors are the only sensory receptors with the capacity to regenerate

Question 6

Describe the structure of olfactory receptors.

Answer 6

Olfactory receptors are bipolar neurons with 3-50 cilia (this increases the surface area of the receptor surface). Each cell contains one type of receptor protein (their axons are unmyelinated). The cells are pseudo-stratified columnar epithelium. The axonal ending sends information to the CNS via penetrating the cribriform plate of the ethmoid bone. Each receptor is a receptor and first order neuron.

Question 7

How many receptors exist in each nostril of a human - how does this compare to a rat?

Answer 7

5-6 million (10-12 million total) compared to 50 million per nostril in rats.

Question 8

What are the first order neurons of the olfactory system?

Answer 8

Olfactory receptors are the first order neurons which synapse on the olfactory bulb

Question 9

How many different olfactory receptor proteins exist in humans?

Answer 9

500 - and only one type exists on each receptor cell.

Question 10

Describe how odorants enter the CNS and draw a diagram

Answer 10

Odour molecules first need to dissolve so they become accessible to the receptor surface. The mucous layer of olfactory epithelium transfers odorants from the air phase to aqueous phase. When the odorant becomes liquid they meet receptor proteins on the ciliated surface of olfactory receptors. Each olfactory cell only has one type of receptor protein that responds to multiple odorants (accepts binding to multiple odorants. Binding of the odorant to the receptor protein results in G protein coupled.

This activates adenyl cyclase (an enzyme embedded in the plasma membrane of cilia) which catalyses the conversion of ATP to cyclic AMP. cAMP opens ligand gated sodium channels resulting in an influx of Na+ and if depolarisation reaches threshold, results in an action potential

Question 11

What type of glands secrete mucous?

Answer 11

Bowmans glands

Question 12

What is the function of mucous?

Answer 12

To liquify odorants and later detoxify and degrade odorants.

Question 13

Where is the olfactory bulb derived from?

Answer 13

telencephalon

Question 14

What is contained in the olfactory bulb?

Answer 14

Neurons, afferent and efferent nerve fibres, interneurons, microglia, astrocytes and blood vessels

Question 15

Which meninges cover the olfactory bulb?

Answer 15

Pia mater and arachnoid mater

Question 16

Where are the first synapses in the olfactory pathway?

Answer 16

olfactory bulb.

Question 17

How many laters are there in the olfactory bulb and what are there names and functions?.

Answer 17

There are 6 layers

1. olfactory nerve layer: containing axons of olfactory receptor first order neurons organised into fascicles.
2. glomerular layer: the axons of olfactory receptor neurons terminate in the glomerular layer, where they synapse with the dendrites of mitral cells and tufted cells, forming spherical glomeruli. This is the initial sites for synaptic processing of odours. Each glomerulus receives input from olfactory receptors containing one type of receptor protein. For example all cells that respond to citrus go to a single glomerulus. This causes a differential pattern of activation for each odorant. This layer of cells also contains interneurons allowing lateral inhibition
3. external plexiform later
4. mitral cell layer
5. internal plexiform layer
6. granule cell layer: shapes the output of the olfactory bulb via inhibition, for example, by GABAergic granule cells

Question 18

What type of cells are the major outputs from the olfactory bulb?

Answer 18

Mitral cells and tufted cells

Question 19

Which layer of the olfactory bulb do the olfactory receptors synapse?

Answer 19

Glomerular layer

Question 20

Which is the largest cell layer of the olfactory bulb?

Answer 20

Granule cell layer

Question 21

Where do medial fibres of the olfactory bulb project to?

Answer 21

Septal area and anterior olfactory nucleus (which projects back to contralateral olfactory bulb.

Some medial fibres project directly to the olfactory tubercle.

Question 22

What is the function of the olfactory tubercle and what are its inputs and outputs.

Answer 22

The olfactory tubercle plays a crucial role in odour guided behaviour, towards things which are rewarding and away from experiences that are unpleasant/negative.

Receives direct input from olfactory bulb, as well as the dopaminergic system from the ventral tegmental area and substantia nigra. The olfactory tubercle then projects onto the mediodorsal thalamus and onto the orbitofrontal and insular cortex (Where temporal contiguity occurs to make decisions about what you are going to do based on previous experience and able to predict what might happen.

Question 23

Structure function Q:

What are the inputs and outputs of the anterior perforated substance

Answer 23

Inputs: olfactory bulb (direct mitral cell collaterals), anterior olfactory nucleus, amygdala and temporal cortex.

Outputs: stria medullaris (onto habenular nuclei) and medial forebrain bundle (to septal nuclei)

Question 24

What is the function of the anterior perforated substance in relation to olfaction?

Answer 24

Receives input from amygdala and temporal cortex, areas which encode intense emotional experience. and via projections through stria medullaris -> habenular nuclei –> down to brain regions that control behaviour directly (such as interpeduncular neuclei and dorsal tegmental nucleus.

Question 25

What is the function of septal nuclei and what are its inputs and outputs?

Answer 25

Not thought to play a role in sense of smell, but rather in guiding behaviours related to reward and reinforcement. It receives direct input from medial fibres and projects to hypothalamus.

Question 26

What makes up the primary olfactory cortex and secondary olfactory cortex?

Answer 26

pre-piriform cortex, piriform cortex, periamygdaloid cortex. The entorhinal cortex is often known as the secondary olfactory cortex.

Question 27

Where does the piriform cortex project?

Answer 27

entorhinal cortex, amygdala, anterior olfactory nucleus and mediodorsal thalamus.

Question 28

What are the functions and anatomical differentiation of these functions of the piriform cortex

Answer 28

There are two parts of the piriform cortex. The anterior half encodes chemical detection - substances with similar structures cause activation in broad patches of similar neurons.

The posterior part is involved in odour categorisation - neurons fire in response to perceptual similarity (things that smell the same) for example, mint and chewing gum - may have different chemicals but similar smells.

Question 29

What is the role of the amygdala in the olfactory system

Answer 29

Involved in the emotional processing of olfactory stimuli - involved in smells that produce strong emotional responses . eg. smells that produce fear

Question 30

Where does the entorhinal cortex project

Answer 30

hippocampus, insular cortex, orbitofrontal cortex, hypothalamus (role in feeding and sex) and amygdala