TASTE AND SMELL
Taste (gustation) and smell (olfaction) are often referred to as the chemical senses.
They are mediated in the first instance by receptors that are stimulated by chemical substances
– “chemoreceptors”
Taste: chemicals dissolve in our mouth (must be water soluble) and stimulate the taste buds in
the oral cavity (tongue, soft palate, cheek) etc.
Smell: Volatile (gaseous) chemicals are inhaled into the nasal passage or mouth where olfractory
receptors line the membranes.
Taste and Smell are closely linked in that they are both usually involved in activities like food
seeking and sampling.
- Smell conveys important non-nutritive information – presence of prey, predators and mates
- Taste aids in the regulation of nutrients and enables the organism to test substances prior to
ingestion.
TASTE
For humans, there are at least 4 basic taste qualities and sensations (Henning,1916)
- Sweet
- Sour
- Salty
- Bitter
The relationship between a substance taste and chemical composition is not straightforward,
but in general:
- Sweet taste: carbohydrates and amino acids (glucose)
- Sour taste: acidic substances (vinegar)
- Salty taste: organic salts
- Bitter taste: alkaloids often poisonous (quinine – tonic water, strychnine- rat poison)
As nutritious substances tend to taste sweet and poisonous ones bitter – evolutionary
But specifying the adequate stimulus for evoking a primary taste sensation is very difficult in
practise – taste quality depends on factors such as substance concentration .e.g. lithium
chloride changes from sweet to sour as concentration increases.
Average human has 10,000 taste buds.
- Found in 3 types of little bumps (papillae) on human tongue
- Each papillae has between several hundred to one taste bud
- Life span of a taste bud is 10 days
- Chemicals dissolved in saliva are in direct contact with microvilli (finger like structures) of
receptor cells.
- Follate Circumvallate (back of tongue)
- Fungiform (middle)
- Filoform (tip, no taste buds- just abrades food).
Taste stimuli interact with receptor sites and ion channels on the microvilli
, There are several different types of transduction mechanisms that convert chemical stimulation
into neural responses
3 sets of afferent nerve fibres carry taste information derived from the taste buds in the tongue
and oral cavity
- Chorda tympani: front of the tongue
- Glossopharyngeal: back region of the tongue
- Vagus: throat, pharynx and larynx
Afferent fibres travel to nuclei in the brainstem and then via the thalamus to the primary taste
area in the parietal lobe of the cortex (near the somatosensory cortex)
Some fibres also project to the orbito-frontal cortex. Involved in the behavioural
significance/reward value of food and perhaps the degree of pleasantness of stimuli.
Most receptor cells respond to some extent to all 4 basic kinds of taste, although with different
sensitivity
Many taste responsive cells in the thalamus also respond to all tastes
The Cross fibre theory (Plaffman, Erickson) says how the brain differentiates between different
substances
- Supported by electrophysical recordings from individual taste sensitive cells in hamster, rat
and primate
- The pattern of firing across different neurons to a particular stimulus is thus different
- Consequently information about taste quality/identity can be coded in the pattern of
activity within an ensemble or group of neurons.
Limits
Detection thresholds for taste depend on:
- Substance tested, temperature, mouth region tested, viscosity and presence of other
substances.
- Taste sensitivity greatest (threshold lowest) between 22 degrees – 32 degrees regardless of
taste quality
- Not all papillae are equally responsive and sensitivity to specific substances varies over the
tongues surface
- Front: sweet and bitter, back sides: sour and front sides: salt
- In addition soft palate maximally sensitive to bitter substances
Electrophysiological recordings from taste fibres innervating the front of the tongue (chorda
tympani) were made during surgery (Borg et al, 1967)
- Patient made magnitude estimate of taste substances
- Very close correspondence between estimates and recordings
- Responses of taste nerve and subject both increased with concentration
- Neural code for taste intensity appears to be the overall activity evoked by a stimulus.
- Subjects can discriminate intensity differences of 15-25% for sucrose.
Nontasters, tasters and supertasters
Individuals can differ dramatically in the ability to taste certain substances
Two such substances are the intensely bitter taster compounds phenylthiocarbamide (PTC) and
6-n-propylthiouracil (PROP)
Taste (gustation) and smell (olfaction) are often referred to as the chemical senses.
They are mediated in the first instance by receptors that are stimulated by chemical substances
– “chemoreceptors”
Taste: chemicals dissolve in our mouth (must be water soluble) and stimulate the taste buds in
the oral cavity (tongue, soft palate, cheek) etc.
Smell: Volatile (gaseous) chemicals are inhaled into the nasal passage or mouth where olfractory
receptors line the membranes.
Taste and Smell are closely linked in that they are both usually involved in activities like food
seeking and sampling.
- Smell conveys important non-nutritive information – presence of prey, predators and mates
- Taste aids in the regulation of nutrients and enables the organism to test substances prior to
ingestion.
TASTE
For humans, there are at least 4 basic taste qualities and sensations (Henning,1916)
- Sweet
- Sour
- Salty
- Bitter
The relationship between a substance taste and chemical composition is not straightforward,
but in general:
- Sweet taste: carbohydrates and amino acids (glucose)
- Sour taste: acidic substances (vinegar)
- Salty taste: organic salts
- Bitter taste: alkaloids often poisonous (quinine – tonic water, strychnine- rat poison)
As nutritious substances tend to taste sweet and poisonous ones bitter – evolutionary
But specifying the adequate stimulus for evoking a primary taste sensation is very difficult in
practise – taste quality depends on factors such as substance concentration .e.g. lithium
chloride changes from sweet to sour as concentration increases.
Average human has 10,000 taste buds.
- Found in 3 types of little bumps (papillae) on human tongue
- Each papillae has between several hundred to one taste bud
- Life span of a taste bud is 10 days
- Chemicals dissolved in saliva are in direct contact with microvilli (finger like structures) of
receptor cells.
- Follate Circumvallate (back of tongue)
- Fungiform (middle)
- Filoform (tip, no taste buds- just abrades food).
Taste stimuli interact with receptor sites and ion channels on the microvilli
, There are several different types of transduction mechanisms that convert chemical stimulation
into neural responses
3 sets of afferent nerve fibres carry taste information derived from the taste buds in the tongue
and oral cavity
- Chorda tympani: front of the tongue
- Glossopharyngeal: back region of the tongue
- Vagus: throat, pharynx and larynx
Afferent fibres travel to nuclei in the brainstem and then via the thalamus to the primary taste
area in the parietal lobe of the cortex (near the somatosensory cortex)
Some fibres also project to the orbito-frontal cortex. Involved in the behavioural
significance/reward value of food and perhaps the degree of pleasantness of stimuli.
Most receptor cells respond to some extent to all 4 basic kinds of taste, although with different
sensitivity
Many taste responsive cells in the thalamus also respond to all tastes
The Cross fibre theory (Plaffman, Erickson) says how the brain differentiates between different
substances
- Supported by electrophysical recordings from individual taste sensitive cells in hamster, rat
and primate
- The pattern of firing across different neurons to a particular stimulus is thus different
- Consequently information about taste quality/identity can be coded in the pattern of
activity within an ensemble or group of neurons.
Limits
Detection thresholds for taste depend on:
- Substance tested, temperature, mouth region tested, viscosity and presence of other
substances.
- Taste sensitivity greatest (threshold lowest) between 22 degrees – 32 degrees regardless of
taste quality
- Not all papillae are equally responsive and sensitivity to specific substances varies over the
tongues surface
- Front: sweet and bitter, back sides: sour and front sides: salt
- In addition soft palate maximally sensitive to bitter substances
Electrophysiological recordings from taste fibres innervating the front of the tongue (chorda
tympani) were made during surgery (Borg et al, 1967)
- Patient made magnitude estimate of taste substances
- Very close correspondence between estimates and recordings
- Responses of taste nerve and subject both increased with concentration
- Neural code for taste intensity appears to be the overall activity evoked by a stimulus.
- Subjects can discriminate intensity differences of 15-25% for sucrose.
Nontasters, tasters and supertasters
Individuals can differ dramatically in the ability to taste certain substances
Two such substances are the intensely bitter taster compounds phenylthiocarbamide (PTC) and
6-n-propylthiouracil (PROP)
