Olfactory and gustatory systems

Question 1

Describe the circuitry and pathway of the olfactory system

Answer 1

Odorants presented to the cilia of the olfactory receptor neurons get processed in the olfactory bulb
The olfactory bulb sends signals to the temporal lobe via the olfactory tract, and then onto the forebrain. Lastly, olfactory information is processed in the pyriform cortex

Question 2

Describe the circuitry for the gustatory system

Answer 2

Taste buds (modified epithelial cells) detect molecules on the tongue. Inside the taste buds are between 50-100 taste cells. Three cranial nerves synapse from here to the brain, cranial nerve VIII, cranial nerve IX and cranial nerve X. 
Taste cells synapse with primary sensory axons and send information to the gustatory nucleus in the medulla. Axons from the gustatory nucleus send projections to the thalamus and then the neocortex, hypothalamus and amygdala.

Question 3

What is the difference between the olfactory sensory pathway and the gustatory sensory pathway?

Answer 3

Olfactory pathway does not relay via the thalamus but the gustatory pathway does

Question 4

Describe the mechanism of transduction in the olfactory system

Answer 4

When a molecule binds to the cilia on the olfactory receptor cells, it causes a conformational change in an olfactory G-protein, activating adenylate cyclase (ACIII). This in turn generates cAMP, which causes the influx of sodium and calcium through cation selective ion channels. When the intracellular calcium concentration increases, Chloride channels open and this causes depolarisation.

Question 5

How does transduction occur in the taste buds?

Answer 5

Transduction occurs in different ways for each of the different taste senses.
For salty tastes- transduction occurs via a sodium ion channel
For sweet and umami tastes- transduction occurs via heterodimeric GPCRs.
For bitter tastes- transduction occurs via a homodimeric GPCR
For sour taste- transduction occurs via (H+) proton TRP channels

Question 6

What is the main difference between olfactory and gustatory transduction

Answer 6

Different smells are encoded in the pattern of stimulation rather than by specific differences in molecules