Taste and smell Flashcards
List the 3 major intra- oral taste receptors and 2 extra- oral taste receptors
Major taste receptors
• Tongue
• Soft palate
• Epiglottis
Extra-oral taste receptors
• GIT
• Airway
Describe the location of the various papilla on the tongue
- Filiform: Dorsal surface of the tongue. NO taste buds.
- Fungiform: Tip/ apex of tongue. Has taste buds.
- Vallate: Posterior surface of the tongue. Has taste buds.
- Foliate: Posterior lateral surface of the tongue. Has taste buds.
Describe the histological structure of a taste bud/ gustatory cell
- It consists of a taste pore
- The receptor cells are known as gustatory cells
- The gustatory cells have projections called taste hairs
- Each gustatory cell is associated with nerve fibres
- There may be basal cells, whose role is to replenish dead receptor cells (these run out with age. That’s why there is decreased taste with age)
There are three types of gustatory cells, I, II and II
• Type I: salty tastes
• Type II: sweet, umami and bitter tastes
• Type III: sour tastes
Describe the stimuli responsible for activating the 5 taste sensations
Salt:
• Responds to Na+concentration in the saliva
Sour:
• Responds to the H+concentration released from acidic substances
Sweet:
• Responds to the glucose
• Molecules that are similar in structure to glucose will have a similar effect on the sensation of sweetness
Bitter:
• Responds to alkaloids/ quinine
Umami:
• Responds to amino acids such as glutamate (proteins)
Describe the signal transduction methods for the 5 taste sensations
Salt:
• Na+concentration becomes high outside the taste cells, a strong concentration gradient drives their diffusion into the cells
• This depolarizes the cells, leading them to release neurotransmitter
Sour:
• As the concentration of the hydrogen ions increases, the depolarization of specific taste cells increases
Sweet:
• Signal transduction occurs through the G- protein cell receptor
• This is when the tastant binds to the receptor, triggering a G protein signal within the receptor that ultimately leads to depolarization of the gustatory cell
Bitter:
• Signal transduction occurs through the G- protein cell receptor
Umami:
• Signal transduction occurs through the G- protein cell receptor
• Known as a savoury taste
Explain why there are differences in the concentrations of different receptors for the various types of taste modalities
- There are more bitter taste receptors for evolutionary purposes: because many poisons have a bitter taste
- Moreover, taste buds from all regions of the oral cavity contain cells that respond to the five basic modalities.
- However, some taste receptors are more active at certain regions than they are at different areas
Explain how taste is perceived neurologically and the innervation of the tongue through cranial nerves
How taste it perceived:
• Certain chemicals cause the receptor cells to release neurotransmitters
• Neurotransmitter from the gustatory cells can activate the sensory neurons in the facial and glossopharyngeal cranial nerves
Innervation:
• Facial nerve: The chorda tympani of the facial nerve connects to the taste buds in the anterior third of the tongue
• Glossopharyngeal nerve: connects to the taste buds in the posterior third of the tongue
• Vagus nerve: connects to the taste buds in the extreme posterior third of the tongue, mostly sensitive to bitter tastes (epiglottis)
Describe the entire process of how taste is perceived neurologically by the brain, afferently
- Axons from the three cranial nerves carrying taste information travel to the medulla
- From there most of the information is carried to the thalamus and then routed to the primary gustatory cortex which interprets the sensation of taste
- The primary gustatory cortex may also receive some information about the smell and texture of food, contributing to the overall taste experience
- The nuclei in the medulla also send projections to the pons/ limbic system, where the hypothalamus and amygdala are. These are involved in autonomic reflexes such as gagging and salivation
Explain how the spicy sensation and minty taste is actually felt
- The active ingredient in chilli peppers is capsaicin
- Its bind to vanilloid receptors (V1 receptors)
- These receptors detect heat and send a signal to the brain about temperature, burning, itching
- So it tricks the brain into responding to spicy food as though it were hot
- Similarly, menthol from mints bind to temperature-sensitive members which activates cold sensors
Describe the basic features of the olfactory epithelium and olfactory bulb
Olfactory epithelium:
• The olfactory receptor neurons are located in a small region within the superior nasal cavity (direct neurons, unlike taste receptors which eventually connect to the neurons)
• This region is referred to as theolfactory epitheliumand contains bipolar sensory neurons
• Olfactory bulb
Describe how smell results in nerve conduction
- As airborne molecules are inhaled through the nose, they pass over the olfactory epithelial region and dissolve into the mucus
- The cilia of the sensory neurons are immersed in a layer of mucus.
- Odorantmolecules (molecules that we can smell) dissolve in the mucus and bind to receptorson the cilia
- Binding of the odorant activates a G proteincoupled to the receptor on its cytoplasmic side.
- This activatesadenylyl cyclase, which then catalyses the conversion of ATPto the “second messenger” cyclic AMP in the cytosol
- cAMP opens up ligan gated sodium channel, and the influx of Na+results in depolarisation, and thus an action potential = nerve impulse
- The brain evaluates this and other olfactory signals reaching it as a particular odour
Explain the pathways involved in how the brain interprets smell, and relate this to why smells are related to memories/ emotions
- Collectively the axons that pass through the cribriform plate are called the olfactory nerve
- From the olfactory bulb, axons connect with structures within the limbic cortex, including the cortex in the temporal lobe associated with long-term memory formation
- Smell can often be a potent trigger for memories because of this intimate connection of the olfactory system with the limbic system (hypothalamus and amygdala)
Explain the shape- pattern theory of olfaction, which explains how we can discern smells
The way the odorants bind to the olfactory epithelium produces specific firing patterns of neurons in the olfactory bulb, which then determine the particular scent we perceive
Define hypogeusia, ageusia, dysgeusia, hyposmia, anosmia and dysomia
- Hypogeusia: reduced ability to taste
- Ageusia: inability to taste
- Dysgeusia: altered sense of taste
- Hyposmia: reduced ability to smell
- Anosmia: inability to smell
- Dysomia: altered sense of smell
Explain why head and neck radiation causes taste disorders (4)
It kills the salivary glands, and saliva is essential for:
• Transport taste substances
• As a solvent for taste substances
• Chemically interact with taste substances
• Protect taste receptors
