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The Chemical Senses
Taste and Smell
The senses of taste and smell allow us to
separate undesirable or even lethal foods
from those that are pleasant to and
Sense of Taste
A.Stimulus – Sapid substance or substances with
Primary Sensations of Taste
1. Sour Taste – caused by acid (hydrogen ion concentration.
Intensity of sensation is proportional to the logarithm of the
hydrogen ion concentration
2. Salty taste – elicited by ionized salts, mainly by sodium ion
concentration .The cations of salt especially sodium cations, are
mainly responsible for the salty
3. Sweet Taste – is not caused by any single class of
chemicals. Includes sugars, glycols, alcohols, aldehydes,
ketones, amines, esters, some amino acids etc.
4. Bitter Taste – Like bitter taste is not caused by any single
type of chemical agent. Long chain organic substances that
contain nitrogen and alkaloids are likely to causes bitter taste
5. Umami Taste – Japanese work (meaning “delicious”)
designating a pleasant taste sensation . It is dominant in food
containing L-glutamate
A. Receptors - Taste Buds ( Sustentacular cells and
Taste cells)
Location of Taste Buds – found on three types of papillae in
the tongue (Guyton)
1. Walls of the troughs that surrounds the Circumvallate papilla
which form V line on the posterior tongue
2. Fungiform papillae over the flat anterior surface of the
tongue (moderate number)
3. Foliate papillae located in the folds along the lateral
surfaces of the tongue (moderate number)
Additional taste buds are located on the palate, tonsillar
pillars, epiglottis and proximal esophagus
1. Fungiform papilla – numerous at the tip of the
2. Vallate (circumvalate ) papilla – arrange on the
back of the tongue
3. Filiform papilla – cover the dorsum of the
tongue , do not usually contain taste buds
Also in the mucosa of the epiglottis, palate, and
Specificity of Taste Buds for a Primary Taste
Each taste bud usually responds mostly to one of the
five primary taste stimuli when taste substance is in
low concentration. At high concentration, most buds
can be excited by two or more of the primary taste
Mechanisms of Stimulation of Taste Buds
Receptor Potential
* For sodium ions and hydrogen ions, which elicit salty and
sour sensations, respectively, the receptor proteins open
specific ion channels in the apical membranes of the taste
cells, thereby activating the receptors
* For sweet and bitter taste sensation the receptor protein
molecules activate the second -messenger transmitter
substances inside the taste cells and these second
messengers cause intracellular chemical changes that elicit
the taste signals
Transmission of Signals into the Central Nervous
System (Conducting Pathway)
* Taste buds from the anterior 2/3 of the tongue > Chorda
tympani branch of Facial Nerve.
* From posterior third of the tongue > Glossopharyngeal nerve
* From tonsillar areas and back of the tongue > Vagus Nerve
Tractus Solitarius (medulla oblongata) > Medial Lemniscus
> Thalamus
Center – Postcentral Gyrus
Adaptation of Taste – Extreme degree of adaptation that
occurs in the sensation of taste almost certainly occurs in the
central nervous system
Abnormalities of Taste
1. Ageusia – absence of taste sensitivity
2. Hypogeusia – diminished taste sensitivity
3. Dysgeusia – disturbed sense of taste
Smell is the least understood of our senses.
A. Stimulus – Odorant or Odoriferous substances
Physical Factors that affect the degree of stimulation
1. only volatile substances that can be sniffed into the nostril
can be smelled.
2. the stimulating substances must be slightly water soluble.
3. substances must be at least slightly lipid soluble.
Receptors – Olfactory Cells ( olfactory hairs or cilia)
Guyton Fig. 53-3
, Fig. 10-2
Stimulation of the Olfactory Cells
Mechanism of Excitation of the Olfactory Cells
1. Activation of the receptor protein by the odorant
substance activates the G-protein complex
2. This, in turn activates multiple molecules of adenylyl cyclase
inside the olfactory cell membrane
3. This causes the formation of many times more of molecules
of cAMP
4. cAMP opens still many times more sodium channels
Membrane Potentials and Action Potentials in Olfactory
Adaptation – Large numbers of centrifugal nerve fibers pass
from the olfactory regions of the brain backward along the
olfactory tract and terminate on special inhibitory cell s in the
olfactory bulb, the granule cells
Primary Sensations of Smell
1. Camphoraceous
2. Musky
3. Floral
4. Pepperminty
5. Ethereal
6. Pungent
7. Putrid
C. Transmission of Smell signals into the Central Nervous
System (Conducting Pathway)
Guyton Fig. 53-4
Olfactory Cells > Glomerulus > Mitral Cells > Olfactory
Bulb > Olfactory Tract
D. Center
Olfactory Area
1. Medial Olfactory Area – Very Old Olfactory System
*subserves the basic olfactory reflexes such as licking of the
lips, salivation, and other feeding responses caused by smell
of food
2. Lateral Olfactory Area –
A.. Less Old Olfactory system
provides automatic but partially learned control of food
intake and aversion to toxic and unhealthy foods
B. Newer Pathway
Used for conscious perception and analysis of olfaction
Adaptation (Centrifugal Control of Activity in the Olfactory
Bulb by the Central Nervous System)
Abnormalities of Olfaction
1. Anosmia – absence of the sense of smell
2. Hyposmia – diminished olfactory sensitivity
3. Dysosmia – distorted sense of smell
Modified Source: Victoria DiGano