Lecture 28: Chemical Senses, Taste and Smell Flashcards
Where are the primary sensory neurons located?
Olfactory receptor cells or Olfactory Sensory neurons (OSN) are located in the neuroepithelium in the nose
-sends axons up through the cribriform plate and synapse with olfactory bulb neurons (second order sensory neurons of smell)
Where do second order neurons in olfactory bulb go?
Via the olfactory tracts, to the following places:
- Pyriform cortex: uncus
- important for smell perception
- Amygdala: inside temporal lobe
- important for emotional perception of smell
- located within the temporal lobe
Once second order neurons reach pyriform cortex and amygdala, where do they go next?
- frontal cortex
- hippocampus (critical for memory consolidation
- hypothalamus (integrates function of autonomic nervous system and hormone release)
Do humans have a functional vomeronasal organ (VNO)?
No…other species have VNO that play a role in sniffing out pheromones
-vestigial in humans
What role do supporting cells play in the neuroepithelium of nose?
- Supporting cells = offers glial like support
2. Basal cells = are the stem cells of olfaction…self renew and create new olfactory sensory neurons
What are basal cells?
the stem cells of olfaction…self renew and create new olfactory sensory neurons
What is the process of smell signal transduction?
i. Odorants bind to odorant receptor
ii. odorant receptor is a GPCR…activates cAMP
iii. camp opens Na/Ca cells so that cell is depolarized
iv. depolarization by Na/Ca influx will open up Cl- cells and have a net EFFLUX of Cl-…leading to further depolarization
What is labeled line coding?
Integrity of information maintained from receptor to cortex along non-integrated paths
Example: vision
-primary afferent from receptor cells responding to the same stimuli synapses on nucleus which project to dorso-medial thalamus primary sensory cortex if olfaction
What is population coding?
Involves reading out population activity of MANY DIFFERENT neurons along the pathway
Example: taste and olfaction is processed
How are odor quality and intensity encoded?
Via population coding
-encoded by combinations of multiple receptors/neurons
One odorant = multiple receptors
One receptor = multiple odorants
What are the three types of cells found in the olfactory bulb?
i. Mitral/Tufted cells
ii. Periglomerular cells
iii. Granule cells
What are glomeruli and why don’t we consider them one of the three types of cells found in olfactory bulbs?
Glomeruli = glial wrappings that synapse on the distal end of dendrites from 8-10 mitral cells
Glomeruli are the anatomical and functional units in the olfactory bulbe
-each glomeruli receives a few thousand OSN inputs with the same olfactory receptor
-each glomeruli send ouputs out to ~25 mitral/tufted cells that then project to the olfactory cortex
What are mitral/tufted cells?
- receive input from OSN
- send axons to higher cortical regions
What are periglomerular cells?
Modulate INPUT to mitral/tufted axons
-located adjacent to the OSN/glomeruli synapse
What are Granule cells?
modulate OUTPUT from mitral/tufted axons
located adjacent to the output axons of mitral/tufted cells
How many receptors does one odorant activate?
- An odorant activates a small subset of glomeruli
- Remember different odorants activate unique but dispersed ensembles of cortical neurons
- Odor representation is distributed WITHOUT apparent spatial preference
What is smell dysfunction a potential precursor for?
-could be a precursor to AD and neurodegenerative disorders
What are age-related changes in smell?
- olfactory bulbs show atrophy with age
- diminished olfactory performance results in a reduced perception of flavor, food recognition and food preference
What are the five taste modalities (flavors)?
- salt: Na+
- sour: vinegar, citric acid
- bitter: poisons, aversive compounds, also coffee
- sweets
- umami: amino acids (taste of meat)…MSG is an example
What are the three different types of papillae (types of bumps on the tongue)?
- Foliate
- on the lateral dorsal portion
- circumvaliate
- papillae in the dorsal medial side
- fungiform
- the papillae in the anterior tongue
What is a Taste bud? How long is its lifespan?
A taste receptor, which are located n each of the three papillae)
2 week lifespan
What are the afferent taste fibers? Where do they send their nuclei to?
CN 7 (chorda tympani, anterior 2/3), 9 (posterior 1/3) and 10 (for the throat taste buds)
Pathway
1. Solitary tract to the nucleus of the solitary tract
2. ventral posterior medial nucleus (VPM) of the thalamus
3. gustatory cortex located in the insula (which is what you see if you separate temporal lobe from frontal/parietal
What are the two types of signal transduction in taste
i. ionotropic receptors (ion channels) for salt and sour (salt channels ninja)
ii. metabotropic receptors (Gprotein) for sweet, umami and bitter)
- called T1R or T2R
- significance is that it allows for plasticity…allows for you to distinguish these taste modalities with only a few hundred receptors
What type of coding is involved in taste fibers?
Labeled-line Coding: each modality is detected by distinct, non-overlapping receptor cells.
-the brain has specialized areas for sensing each modality
What are 4 mechanisms that lead to gustatory dysfunction?
a. disorder of oral cavity
b. disorders of peripheral neural pathways (CN 7, 9, 10 and even 5…for pain and shit)
c. disorders of CNS
d. special circumstances like pregnancy
Ageusia
absence of taste sensation
Hypoageusia
decreased sensation to tastants
Dysgeusia
distortion of taste sensation
Gustatory agnosia
inability to recognize a taste sensation (rare)
What are age-related changes in taste?
- salivary secretions decrease with age
- ability to detect salt decreases
- older persons require greater concentrations of taste substances except for sweets
How do we sense flavors?
Sense of flavors comes from an INTEGRATION of
- olfactory (70-80% of the information)
- gustatory
- somatosensory organ
How common is total taste loss? More commonly, loss of taste is due to?
Very rare; usually due to loss of smell
