Smell and Taste- Heinbockel Flashcards
What are the two chemical senses?
- olfaction
- gustation
Both taste and smell chemoreceptors respond to chemicals in what?
aqueous solutions
Taste – to substances dissolved in saliva
Smell – to substances dissolved in fluids of the nasal membranes
The olfactory system deals with volatile or nonvolatile substances? What about the gustatory system?
olfactory system: general odors, VOLATILES, pheromones (food, predators, mating and aggression cues, individual identity)
Gustatory system – NONVOLATILES (sweet,
bitter, salty, sour, umami tasting substances)
What do the chemical senses need to do?
- Detect all relevant chemicals in the environment (including novel ones)
- Encode both QUALITY and QUANTITY of individual chemicals in the context of complex chemical mixtures; when you smell smoke it is not just one type of chemical but a mixture of different chemicals
- Decode and interpret this sensory information in the context of experience and other sensory input so that the nervous system can generate an appropriate behavioral response
What factors do the chemical senses encode in the chemicals?
both quality and quantity
From the Nose to the
Olfactory Bulb and on to
Higher Olfactory Centers
Olfactory receptor neurons (ORNs), typical bipolar neurons, are found in an epithelial sheet in the nasal epithelium
ORNs project axons (CN I) to the ipsilateral olfactory bulb
Synaptic interactions occur within glomeruli in the bulb.
Olfactory bulb neurons interact with central neurons (mitral and tufted cells) which send axons to olfactory cortex and other structures in the limbic system
axons travel through nasal epithelium to cribriform plate to olfactory bulb
Where is the olfactory epithelium?
on the superior concha in the nasal cavity
Describe the olfactory pathway.
Air and odorant molecules come in and go to the back of the nose where they activate olfactory receptor neurons through cilia (has membrane proteins) binding the molecules
the axons all together form CN I and go through the cribriform plate to the olfactory bulb where they synapse on central neurons like the mitral cells that will go to higher olfactory centers (olfactory cortex or limbic system)
ORNs project to the olfactory bulbs and synapse with mitral cells
Glomerular mitral cells process odor signals
Mitral cells project axons and send impulses to the olfactory cortex (anterior olfactory nucleus, piriform cortex, olfactory tubercle), amygdala, and entorhinal cortex
Secondary projections go to the thalamus, hypothalamus, and hippocampus
Where does olfactory transduction takes place where?
on the cilium at the end of dendrites of the olfactory receptor neurons (has specific membrane proteins and is bathed in serous fluid)
What are the two types of cells in the nasal epithelium?
- supporting cells (sustentacular cells)
- stem cell population of basal cells; need them due to the rapid turnover rate of olfactory receptor neurons (live only for months or two)
While olfactory receptor neurons have cilia, what do the supporting cells in the nasal epithelium have?
microvilli
Describe olfactory receptor neurons.
-odors interact with olfactory receptor neurons (ORNs) at the dendritic cilia
- these cilia contain the molecular components of the sensory transduction cascade
- ORNs are bipolar neurons with cilia
- ORNs send axonal projections to the olfactory bulb-supporting cells have microvilli
Why do olfactory receptor neurons have a short lifespan of 30-60 days?
constantly bombarded with air and odor molecules
How are ORN regenerated?
new cells generated by the stem cell population (basal cells) in the nasal epithelium
basal cells contain progenitor and stem cell populations–migrate, differentiate and replace ORNs and other OE cell types
What is interesting about the new ORN generated by the basal cells in the nasal epithelium?
the “new” ORN forms new connections in the olfactory bulb
they produce axons that project to the olfactory bulb; they know exactly where to go
Where does the watery fluid that bathes the cilia of ORNs come from?
secreted by olfactory (Bowman’s gland)
What is the arrangement of cilia in the respiratory system vs those in the olfactory system? Why is this arrangement important?
few nonmotile cilia in the olfactory system (just there to bind to air and odor molecules)
9 plus 2 arrangement (motile) cilia in the lower respiratory system (trachea and bronchi) important for moving material out of the airway
We have a Large Family of
Odorant Receptors that are of what type?
g protein coupled receptors
Each receptor consists of a protein chain that traverses the cell membrane seven times. When an odorant substance attaches to an olfactory receptor, the shape of the receptor protein is altered, leading to a G protein activation. An electric signal is triggered in the olfactory receptor neuron and sent to the brain via nerve processes.
What is the unexpected finding in terms of the expression of the olfactory receptors?
Every single olfactory receptor cell expresses one and only one gene of all the genes that code for olfactory receptor molecules.
Each olfactory receptor cell expresses one type of odorant receptor, and each receptor can detect a limited number of odorant substances.
Explain why the olfactory receptors are triggered by different odorant molecules.
All odorant receptors are related proteins and differ only in some amino acid residues. The subtle differences in the protein chains explain why the receptors are triggered by different odorant molecules.
How are the different olfactory receptors arranged in the nasal epithelium?
There are separate zones of the olfactory epithelium. A different set of odorant receptor genes is expressed in each zone.
How many different odorants can be distinguished?
350