OLFACTION + TASTE EXTRA DETAILS

What is another name for olfactory receptor cells?

Olfactory sensory neurons

What type of epithelium is the olfactory epithelium?

pseudostratified columnar epithelium which contains olfactory sensory neurons

How often do olfactory sensory neurons renew?

Olfactory sensory neurons are renewed every 60 days by basal cell division

The olfactory mucosa is the…

olfactory epithelium + the lamina propria

What glands are found in the lamina propria and what does it secrete?

Bowman’s glands which secrete serous fluid containing odorant-binding proteins (OBPs) that helps dissolve odorants.

What is the chemical nature of odorants?

Most are hydrophobic

What does it mean that odorants are hydrophobic?

They are unable to dissolve in the mucosa because it is a wet surface.

What do OBP’s do?

OBPs reversibly bind to odorants and transport them to the cilia of the olfactory sensory neurons

What is distinct about the olfactory system of insect to humans?

OBPs have been studied in insects. Rather than a cartilage turbinate containing olfactory mucosa, insects have hair-like structures called sensilla that are found in olfactory sensory neurons which are filled with fluid called sensillia lymph which contains OBPs. The insect olfactory sensory neurone is contained within the sensila.

Where is the olfactory mucosa located?

The olfactory mucosa is only located on the superior nasal turbinate (concha + meatus)​

Where does olfactory transduction occur?

Transduction occurs in the olfactory cilia of the olfactory sensory neurons.

How many cilia are found on each OSN?

Each olfactory sensory neurone has around 10-30 cilia.​

How many olfactory GPCR’s are present in humans?

In humans there are around 400 different olfactory GPCRs, each which bind in a complementary way to different odorants .

What is the specificity feature of OSN?

Each olfactory sensory neurone expresses ONLY ONE type of GPCR- this means that each olfactory sensory neurone is specific at identifying a SINGLE odorant in theory.

How can some OSN’s respond to more than one odorant molecule?

This is because GPCRs have binding pockets which can interact with parts ododorant molecules that share similar structural (ie chain length, functional groups) or chemical features (ie. Polarity). So each GPCR recognises a range of molecular shapes with varying affinities.

This feature maximises sensory diversity

What affects the strength of activation of OSN?

The strength of activation (how much cAMP is produced) depends on how well the odorant binds.

Compare GPCR’s in olfactory system to other sensory GPCR’s?

GPCRs used in precisely regulated systems such as B2 adrenergic receptor only binds to a few catecholamines with high specificity- this prevents cross-talk and ensures signal fidelity. ​

How does the brain interpret odors?

Each odorant can partially fit and consequently activate multiple GPCRs- thus activating a unique combination of receptors- ‘pattern’ of activation. This is how the brain interprets odor: by interpreting patterns of receptor activation rather than just 1 odor.

Different odorants evoke distinct but overlapping patterns of activity in the olfactory bulb. This means the brain interprets odorants using combinatorial coding.

How does the combinatorial code of olfaction interpretation be advantageous?

• 1 receptor per odor is not required ​

• With around 400 receptor types, thousands of odors can be represented through combinations ​

• Similar odors have similar but not identical patterns – the brain can detect both similarity and difference ​

What happens when receptor current increases?

The receptor current increases with odorant concentration,increasing spike firing. ​

What does prolonged stimulation cause?

Adaptation

How does adaptation work?

Ca2+ binds to calmodulin (CaM), which binds to the CNG channel and reduces its sensitivity to cAMP- meaning theCNG channels require more cAMP to stay open. This causes partial closure of the CNG channels.​

Ca2+-CaM also activates phosphodiesterase PDE1C, breaking down cAMP and activates Ca2+-CaM dependent protein kinase II (CAMKII) which phosphorylates ACIII and reduces cAMP production.

What is the role of the olfactory bulb?

Olfactory receptor axons pass through the cribriform plate of the ethmoid bone to the olfactory bulb.

There, they excite mitral cells and smaller tufted cells in the olfactory glomeruli ​

Afferents from olfactory sensory neurons expressing the same type of GPCR selectively converge on TWO glomeruli in each bulb- one on the medial side of the bulb and one on the lateral side of the bulb. ​

The convergence is important because it amplifies weak signals, increases specificity (each glomeruli represents one receptor type) and creates a spatial map (symmetry)-introduces redundancy.

What are mitral cells?

Principal output neurons of olfactory bulb ​

What are tufted cells?

Smaller excitatory output neurons​

What are periglomerular cells?

Small inhibitory interneurons which regulate synaptic activity within and between glomeruli.

They provide intra and inter-glomerular inhibition, which helps to control how strongly each glomeruli responds to odor input and suppresses neighboring glomeruli. ​

What type of synapses do PG cells form?

They form dendro-dendritic synapses with mitral/tufted cell dendrites and olfactory nerve terminals within the glomerulus.

What are granule cells?

Inhibitory interneurons deeper in the olfactory bulb: they connect to mitral and tufted cells via dendro-dendritic synapses.

When mitral/ tufted cells excite a granule cell, they release GABA back onto them or onto neighboring mitral/tufted cells​.

What is the importance of granule and PG cells?

This sharpens odor tuning: without inhibition, many mitral cells may respond weakly to similar odors- responses overlap too much. With lateral inhibition, only the most strongly stimulated mitral cells remain active. And there is sharper, more distinct patterns of activation for each odor.

The inhibitions acts like contrast enhancement- similar to retina sharpening visual edges. ​

How do mitral cells leave the olfactory bulb?

Leave the bulb in the lateral olfactory tract ​

What does the AON do?

The anterior olfactory nucleus (AON) mediates inhibition between the two bulbs via the anterior commissure.This means it can inhibit activity in the opposite bulb. The inter-bulb inhibition helps balance the activity between the two bulbs, enhances the contrast between the left and right odor inputs and airs in odor localization (identifying which nostrildetects the smell most strongly)​

What are the higher olfactory processing pathways?

The lateral olfactory tract synapse on neurons in 5 regions of the olfactory cortex:​

-Anterior olfactory nucleus (AON)

-Olfactory tubercle

-Pyriform cortex

-Amygdala

-Entorhinal cortex

How does the Anterior olfactory nucleus (AON) work?

cross communication and modulation of olfactory signals between the two bulbs.

How does the Olfactory tubercle work?

(located at base of forebrain)receives direct input from the LOT. It projects to the medial dorsal (MD) nucleus of the thalamus. TheMD thalamus then projects to the orbitofrontal cortex (OFC)​

Olfactory tubercle -> MD thalamus -> OFC=conscious odor perception pathway​

How does the Pyriform cortex work?

The largest olfactory cortical area,this is where the brain first creates a pattern-based representation of odors. ​

How does the amygdala work?

Receives input from the lateral olfactory tract. Important for assigning emotional value to odors ​

How does the Entorhinal cortex work?

Connects smell with memories.Sends olfactory information to the hippocampus.