Taste and smell are closely linked senses that involve different receptors but overlap in central processing. Taste is detected by taste receptor cells located in taste buds in the tongue, soft palate, and pharynx, which respond to five primary tastes: salty, sour, sweet, bitter, and umami. Smell is detected by olfactory receptor neurons located in the nasal cavity that transmit signals through the olfactory bulbs and to the brain. Both senses play an important role in food intake and can influence appetite.

1. Chemical Senses
Taste & smell: Both determine the flavour of food
Taste and smell are closely linked even though they
involve different receptors and receptive processes.
This suggests an overlap in central processing.
Receptors are chemoreceptors
In association with food intake, influence flow of
digestive juices and affect appetite
Stimulation of receptors induces pleasurable or
objectionable sensations and signals presence of
something to seek or to avoid
Taste (Gustation)
Chemoreceptors housed in taste buds
Present in oral cavity and throat
Taste bud consists of
 Taste pore
Opening through which fluids in mouth come into contact with
surface of receptor cells
 Taste receptor cells
 Modified epithelial cells with surface folds called microvilli
 Plasma membrane of microvilli contain receptorsites
that bind selectively with chemical molecules
Sensation of Taste
Located in taste buds in:
 Tongue
 Epiglottis
 Soft Palate
 Pharynx
Anatomy of Taste Buds.
10,000 taste buds found on tongue, soft palate & pharynx
Taste buds consist of:
 ~50 receptorcells (type 3) surrounded by supporting cells
 Basal cells (type 1 &2) develop into supporting cells then receptor cells
 Gustatory hairs project through the taste pore
 Life span of 10 days

2. Taste
Binding of tastant (taste-provoking chemical) with receptor cell alters cell’s ionic channels to produce depolarizing
receptor potential
Receptor potential initiates action potentials within terminal endings of afferent nerve fibers with which receptor cell
synapses
Terminal afferent endings of several cranial nerves synapse with taste buds in various regions of mouth
Signals conveyed via synaptic stops in brain stem and thalamus to cortical gustatory area
Taste Pathway
Taste information is send to the CNS by the cranial nerves # 7, 9 and 10 the taste nucleus (n. tractus solitarius)
thalamus primary gustatory cortex insular cortex
Taste Perception
Influenced by information derived from other receptors, especially odor
Temperature and texture of food influence taste
Psychological experiences associated with past experiences with food
influence taste
How cortex accomplishes perceptual processing of taste sensation is
currently unknown
Responses of Taste buds:
Each taste bud responds strongly to one type of taste But they also respond to other tastes as well
5 primary tastes
a) Salty Stimulated by chemical salts, especially NaCl
b) Sour Caused by acids which contain a free hydrogen ion, H+
c) Sweet Evoked by configuration of glucose
d) Bitter Brought about by more chemically diverse group of taste substances. Examples – alkaloids,
toxic plant derivatives, poisonous substances
e) Umami Meaty or savory taste/ pleasant taste
Physiology of taste
Sour ... Acidity by {H+
} – HCL
Salt … Sodium chloride
Sweet… Sucrose, glucose, Saccharin
Bitter… hydrochloride, Quinine sulphates, alkaloids
Mechanism of stimulation of taste sensations:-
By sodium and hydrogen ions respectively. Saltiness and sourness are transduced directly. The transduction process for
sweetness and bitterness involve second messengers.
Sour: Acids (H+
), Blocks K+
channels
Salt taste Na+
, Depolarization
Sweet G protein activation of adenyl cyclase c-AMP K conductance
Bitter G protein Activation. Of Phospholipase C IC-insitol(PO4)3 Ca2
release
Ion channels
Saltiness or sodium receptors allow sodium ions to cross the membrane, thereby causing depolarization.
Sourness receptors operate by closing potassium channels, which allows a positive charge to build up, thereby causing
depolarization of the cell.

3. Taste modifier Miraculin (a glycoprotein extracxted from miracle fruit): When applied to tongue makes acids taste
sweet
Clinical considerations
 Ageusia: Absence of sense of taste
 Dysgeusia: Disturbed sense of taste
 Hypogeusia: Diminshed sense of taste
 Hypergeusia: increased sense of taste
The Sense of Smell
Olfactory Mucous Membrane
-Olfactory receptor cells
-Area of 5cm2
in roof of nasal cavity near the septum
-10 to 20 million receptor cells
-Each olfactory receptor is a neuron
-Olfactory mucous membrane is the place in body where NS is closest to
external world
Composition of Olfactory epithelium
-Each neuron has a thick dendrite with an expanded end called olfactory rod
-From rods cilia project to the mucous surface
-Each receptor neuron has 10-20 cilia
-Axons of olfactory receptor neurons pierce cribriform plate of ethmoid bone
and enter olfactory bulbs

4. Mucus producing Glands
-Olfactory mucous membrane is constantly covered by mucus
-Mucus is produced by Bowman’s glands, placed just under the basal lamina of
the membrane
Olfactory Bulbs
-Axons of receptorscontactthe primary dendrites of mitral cells and tufted cells.
-Forming complex globular synapses called olfactory glomeruli.
-Periglomerular cells are inhibitory neurons connecting one glomerulus to another
-Granule cells have no axons and make reciprocal synapses with lateral dendrites of tufted and mitral cells
-Mitral and Tufted – excite granule releasing glutamate and granule cell in turn inhibits both by releasing GABA
Stimulation of Olfactory cells
G-protein is stimulated -triggers activation of Adenyl cyclase (enzyme speeds up the conversion of ATP to cAMP –
cAMP then binds to action channels in membrane of cilia- this causes channels to open and Ca ions to enter cilia –
influx of Ca ions activates Cl channels to open and Cl leaves. Membrane becomes depolarized and AP is created. The
action potential travels down the axon of olfactory receptor cell eventually meets with the other axons

5. Transmission of smell signals to CNS
Olfactory thresholds and discrimination
Olfactory receptors respond only to substances in contact with olfactory epithelium
and need to be dissolved in mucus
Methyl marcaptan one of the substances in garlic can be smelled at very low
concentration showing the remarkable sensitivity of olfactory receptors
Humans can recognize more than 10.000 different odors
However determination of intensity of odor is poor
Vomeronasal Organ
Organ is not well developed in humans very well developed in rodents
This organ is concerned with perception of odors that act as pheromones
There is evidence of pheromones in humans and a close relationship between smell
and sexual function
Role of Pain Fibers in the Nose
Many trigeminal pain fibers are found in olfactory membrane
They are stimulated by irritating substances
Are responsible for initiating sneezing, lacrimation and other reflex responses.
Abnormalities
 Anosmia – absence of sense of smell
 Hyposmia – diminished olfactory sensitivity
 Dysosmia – distorted sense of smell
 More than 75% of humans over the age of 80 have an impaired ability to identify smells