3. Special senses: Taste and smell Flashcards
Where are taste buds found?
Taste buds found on our tongue, palate, tonsilar pillars, epiglottis and a few on the proximal oesophagus.
What is the distribution of taste buds in the mouth?
Over 3 types of papillae of the tongue:
- Circumvallate
- Fungiform
- Foliate
Describe the distribution of taste buds in the mouth:
- Circumvallate
- Fungiform
- Foliate
They a distributed differentially over three types of papillae of the tongue, these are:
1. Circumvallate (located at the posterior of the tongue in a V shape), large numbers
of taste buds are found on the walls of these papillae
2. Fungiform (found at the anterior surface of the tongue), a moderate number of
taste buds are found here, and
3. Foliate (located in the folds on the lateral margins of the tongue), a moderate number are found
Structure of a taste bud?
Taste pore and microvilli projecting from it
Taste cell and support cells
Synpases –> Sensory neurones
What chemicals do taste buds detect?
Via 13 receptors: 2 potassium 2 sodium 1 chloride 1 adenosine 1 inosine 2 sweet 2 bitter 1 glutamate 1 H+
Receptor depolarise taste cells either via ionotropic or metabotropic pathways
What are the 5 categories of taste buds?
- Sour
- Salty
- Sweet
- Bitter
- Umami
What chemical cause stimulation of the “sour” category of taste buds?
H+ ions
What chemical cause stimulation of the “salty” category of taste buds?
Mainly Na+ ions
Also K+ ions
What chemical cause stimulation of the “sweet” category of taste buds?
SUGAR and a wide variety of other chemicals like:
- Glycols
- Alcohols
- Ketones
What chemical cause stimulation of the “bitter” category of taste buds?
Same as sweet but also ALKALOIDS found in toxic plants.
Note sweet= SUGAR and a wide variety of other chemicals like: -Glycols -Alcohols -Ketones
What chemical cause stimulation of the “umami” category of taste buds?
L glutamate, found in meats and aged cheese
How does taste threshold vary and why
Salty taste from NaCl - 0.01M (Low)
Sweet taste from sucrose- 0.01M (Low)
Sour taste for H ions- 0.0009M (Mid)
Bitter taste from quinine- 0.000008M (High)
Because bitter tastes are generally indicative of a more dangerous substance, sensitivity needs to be high
What is the neuronal pathway for taste?
Via CN VII= Ant 2/3 tongue and soft palate CN IX- Post 1/3 tongue CN X- Vallucula --> Solitary nucleus --> VPM of thalamus --> Cortical areas of taste
Remember that gustation has a limbic component via the thalamus , and can activate brainstem nuclei for salivation, or vomiting
What is the cortical area of taste?
Most inf portion of the post-central gyrus extending into the insula
The two main sensory inputs to recognition of a foodstuff are..
Olfaction and gustation
What makes up the olfaction system?
Olfactory epithelium
Receptor cells (bipolar neurons)
Axons that project through the base of the skull to the olfactory bulb
Neuronal tract to multiple olfactory destinations in the brain
Stages of odorant receptor activation?
- An odorant molecule binds to the receptor of the primary olfactory neurone
- The activated receptor activates a G protein, which stimulates adenylyl cyclase to convert ATP to cAMP
- cAMP activates a cation channel, making it permeable to Na+ and Ca2+
Result of odorant receptor activation?
Activation of the receptors cause the cells to depolarise as generator potentials.
Sufficient depolarisation leads to action potentials, and the frequency of AP generation is proportional to the Log concentration of the odorant
Process of odour detection?
- The lower end of the bipolar neurons extends into a layer of mucus secreted from the Bowman gland in the olfactory epithelium, forming and olfactory knob.
- From this develops olfactory cilia which detect the odorants in the nasal cavity
- The bipolar neurons continually develop and replace each other over time
Journey from mouth/nose to the olfactory bulb
High in the nasal cavity is the 2.5cm squared area of olfactory epithelium
The odorant detectors are bipolar neurons originally derived from the CNS but now only connected by central processes
These central processes project up through the cribriform plate into the olfactory bulb of CN I from the olfactory epithelium
Olfactory bulb to Cortex:
- Olfactory information is transferred from the bipolar cells to ________ of mitral cells found in the neuropil of the glomeruli in the bulb
- There is a massive convergence at this point which helps ensure _______
- Also synapsing in the glomeruli are Tufted cells which are thought to provide a form of ‘_______ inhibition’ for signal enhancement
- Within the bulb, it is generally thought that specific glomeruli respond to particular odorants in a dose dependent manner.
Olfactory bulb to Cortex
- Olfactory information is transferred from the bipolar cells to dendrites of mitral cells found in the neuropil of the glomeruli in the bulb
- There is a massive convergence at this point which helps ensure sensitivity
- Also synapsing in the glomeruli are Tufted cells which are thought to provide a form of ‘lateral inhibition’ for signal enhancement
- Within the bulb, it is generally thought that specific glomeruli respond to particular odorants in a dose dependent manner.
What are the mechanisms of lateral inhibition in olfaction?
- Synapsing in the glomeruli are TUFTED CELLS which are thought to provide a form of ‘lateral inhibition’
- There is a second layer of lateral inhibition where GRANULE CELLS cells in the bulb make dendritic connections between mitral cells.
Odours are grouped into the following categories (these are not primary odours, but formed of a spectrum of odours in specific proportions), name the 7 groups?
- Camphoraceous 2. Musky
- Floral
- Pepperminty
- Ethereal 6. Pungent 7. Putrid
Odour selectivity:
There are over 300 genes in humans for different odorant receptors, and it seems that each bipolar neuron expresses _____ of these genes per neuron.
The implication is that there are a large number of chemicals that can be detected by the olfactory epithelium, which is complicated by each receptor potentially responding to a number of odorants.
Each smell we recognise is certainly a pattern of ______ receptor cells and glomeruli ______ responding in a proportional way to the odorants.
Odour selectivity
There are over 300 genes in humans for different odorant receptors, and it seems that each bipolar neuron expresses one of these genes per neuron.
The implication is that there are a large number of chemicals that can be detected by the olfactory epithelium, which is complicated by each receptor potentially responding to a number of odorants.
Each smell we recognise is certainly a pattern of activated receptor cells and glomeruli circuitry responding in a proportional way to the odorants.
