Neurophysiology/special senses/smell and taste
Lect. Dr. Zahid M. kadhim
Smell and taste sensation/ objectives of the lecture
■ Describe the basic features of the neural elements in the olfactory epithelium
and olfactory bulb.
■ Outline the pathway by which impulses generated in the olfactory epithelium
reach the olfactory cortex.
■ Describe the location and cellular composition of taste buds.
■ Name the five major taste receptors and signal transduction mechanisms in
these receptors.
■ Outline the pathways by which impulses generated in taste receptors reach the
insular cortex.
Introduction
Smell (olfaction) and taste (gustation) are generally classified as visceral senses
because of their close association with gastrointestinal function. Physiologically,
they are related to each other. The flavors of various foods are in large part a
combination of their taste and smell. Consequently, food may taste “different” if
one has a cold that depresses the sense of smell. Both smell and taste receptors
are chemoreceptors that are stimulated by molecules dissolved in mucus of the
nose and saliva of the mouth.
Olfactory epithelium and olfactory bulbs (figure 1: a & b)
Olfactory sensory neurons which are specialized cells of the smell, are located in
a specialized portion of the nasal mucosa, the yellowish pigmented olfactory
epithelium. In humans, it covers an area of 10 cm2 in the roof of the nasal cavity
near the septum. The human olfactory epithelium contains about 50 million
bipolar olfactory sensory neurons interspersed with glia-like supporting
(sustentacular) cells and basal stem cells. New olfactory sensory neurons are
generated by basal stem cells as needed to replace those damaged by exposure
to the environment. The olfactory epithelium is covered by a thin layer of mucus
secreted by the supporting cells and Bowman glands, which lie beneath the
epithelium.
Neurophysiology/special senses/smell and taste
Lect. Dr. Zahid M. kadhim
Each olfactory sensory neuron has a short, thick dendrite that projects into the
nasal cavity where it terminates in a knob containing 6–12 cilia and protrude into
the mucus overlying the olfactory epithelium. Odorant molecules (chemicals)
dissolve in the mucus and bind to odorant receptors on the cilia of olfactory
sensory neurons. The mucus provides the appropriate molecular and ionic
environment for odor detection.
The axons of the olfactory sensory neurons pass through the cribriform plate of
the ethmoid bone and enter the olfactory bulbs. In the olfactory bulbs, the axons
of the olfactory sensory neurons contact the primary dendrites of the mitral cells
and tufted cells to form anatomically discrete synaptic units called olfactory
glomeruli. Free endings of many trigeminal pain fibers are found in the olfactory
epithelium. They are stimulated by irritating substances, which leads to the
characteristic “odor” of such substances as peppermint, menthol, and chlorine.
Activation of these endings by nasal irritants also initiates sneezing, lacrimation,
respiratory inhibition, and other reflexes.
Figure 1-a: olfactory epithelium
Neurophysiology/special senses/smell and taste
Lect. Dr. Zahid M. kadhim
Figure 1-b: showing olfactory bulb
Olfactory cortex (figure 2)
The axons of the mitral and tufted cells pass posteriorly to terminate on apical
dendrites of pyramidal cells in five regions of the olfactory cortex: anterior
olfactory nucleus, olfactory tubercle, piriform cortex, amygdala, and entorhinal
cortex.
From these regions, information travels directly to the frontal cortex or via the
thalamus to the orbitofrontal cortex. Conscious discrimination of odors is
dependent on the pathway to the orbitofrontal cortex. The orbitofrontal
activation is generally greater on the right side than the left; thus, cortical
representation of olfaction is asymmetric. The pathway to the amygdala is
probably involved with the emotional responses to olfactory stimuli, and the
pathway to the entorhinal cortex is concerned with olfactory memories.
Neurophysiology/special senses/smell and taste
Lect. Dr. Zahid M. kadhim
Figure 2: olfactory cortex
Odorant receptors
The odorant receptors are G protein coupled receptors. When an odorant
molecule binds to its receptor, the G protein subunits (α, β, γ) dissociate. The αsubunit activates adenylate cyclase to catalyze the production of cAMP, which
acts as a second messenger to open cation channels, increasing the permeability
to Na+, K-, and Ca2+. The net effect is an inward-directed Ca2+ current which
produces the graded receptor potential.
Odor detection threshold
Odor-producing molecules (odorants) are generally small, containing from 3 to 20
carbon atoms; and molecules with the same number of carbon atoms but
different structural configurations have different odors. Relatively high water and
lipid solubility are characteristic of substances with strong odors.
The odor detection thresholds are the lowest concentration of a chemical that
can be detected. The wide range of thresholds illustrates the remarkable
sensitivity of the odorant receptors.
Neurophysiology/special senses/smell and taste
Lect. Dr. Zahid M. kadhim
Determination of differences in the intensity of any given odor is poor. The
concentration of an odor-producing substance must be changed by about 30%
before a difference can be detected.
Adaptation
It is common knowledge that when one is continuously exposed to even the most
disagreeable odor, perception of the odor decreases and eventually ceases. This
sometimes beneficent phenomenon is due to the fairly rapid adaptation, or
desensitization.
TASTE SENSE
Taste buds (figure 3 &4)
The specialized sense organ for taste (gustation) consists of approximately 10,000
taste buds, which are ovoid bodies measuring 50–70 μm. There are 2
morphologically distinct types of cells within each taste bud: basal cells and taste
cells which found at various stages of development. Taste cells are the sensory
neurons that respond to taste stimuli or tastants. Each taste bud has between 50
and 100 taste cells.
The apical ends of taste cells have microvilli that project into the taste pore, a
small opening on the dorsal surface of the tongue where tastes cells are exposed
to the oral contents. Each taste bud is innervated by about 50 nerve fibers, and
conversely, each nerve fiber receives input from an average of five taste buds.
The basal cells arise from the epithelial cells surrounding the taste bud. They
differentiate into new taste cells, and the old cells are continuously replaced with
a half-time of about 10 days.
If the sensory nerve is cut, the taste buds it innervates degenerate and eventually
disappear.
In humans, the taste buds are located in the mucosa of the epiglottis, palate, and
pharynx in the walls of papillae of the tongue. The fungiform papillae are
rounded structures most numerous near the tip of the tongue; the circumvallate
papillae are prominent structures arranged in a V on the back of the tongue; the
foliate papillae are on the posterior edge of the tongue.
Neurophysiology/special senses/smell and taste
Lect. Dr. Zahid M. kadhim
The von Ebner’s glands (also called gustatory glands or serous glands) secrete
saliva into the cleft around the circumvallate and foliate papillae. Secretions from
these glands may function to cleanse the mouth to prepare the taste receptors
for a new stimulant.
Figure 3: taste bud
Figure 4: papillae of the tongue
Taste pathways (figure 5)
The sensory nerve fibers from the taste buds on the anterior two-thirds of the
tongue travel in the chorda tympani branch of the facial nerve , and those from
the posterior third of the tongue reach the brain stem via the glossopharyngeal
Neurophysiology/special senses/smell and taste
Lect. Dr. Zahid M. kadhim
nerve . The fibers from areas other than the tongue (eg, pharynx) reach the brain
stem via the vagus nerve. On each side, taste fibers in these three nerves unite in
the gustatory portion of the nucleus of the tractus solitarius (NTS) in the medulla
oblongata. From there, axons of second-order neurons ascend in the ipsilateral
medial lemniscus and project directly to the ventral posteromedial nucleus of the
thalamus. From the thalamus, the axons of the third-order neurons pass to
neurons in the anterior insula and the frontal operculum in the ipsilateral
cerebral cortex. This region is rostral to the face area of the postcentral gyrus,
which is probably the area that mediates conscious perception of taste.
Figure 5: taste pathway
Taste modalities, receptors & transduction
Humans have five established basic tastes: sweet, sour, bitter, salt, and umami.
The umami taste was added to the four classic tastes relatively recently. Its
receptor triggered particularly by the monosodium glutamate used so extensively
in Asian cooking. The taste is pleasant and sweet but differs from the standard
sweet taste.
Although for many years it was thought that the surface of the tongue had special
areas for each of the first four of these sensations, it is now known that all
tastants are sensed from all parts of the tongue and adjacent structures. Afferent
Neurophysiology/special senses/smell and taste
Lect. Dr. Zahid M. kadhim
nerves to the nucleus of tractus solitarius contain fibers from all types of taste
receptors, without any clear localization of types.
Salt and sour tastes are triggered by activation of ionotropic receptors; sour,
bitter, and umami tastes are triggered by activation of G-protein coupled
receptor.
Taste thresholds & intensity discrimination
The ability of humans to discriminate differences in the intensity of tastes, like
intensity discrimination in olfaction, is relatively crude. A 30% change in the
concentration of the substance being tasted is necessary before an intensity
difference can be detected. Taste threshold refers to the minimum concentration
at which a substance can be perceived. The threshold concentrations of
substances to which the taste buds respond vary with the particular substance.
Bitter substances tend to have the lowest threshold. Some toxic substances such
as strychnine have a bitter taste at very low concentrations, preventing accidental
ingestion of this chemical, which causes fatal convulsions.
CHAPTER SUMMARY
■ Olfactory sensory neurons, supporting (sustentacular) cells, and basal stem cells
are located in the olfactory epithelium within the upper portion of the nasal
cavity.
■ The cilia located on the dendritic knob of the olfactory sensory neuron contain
odorant receptors that are coupled to G proteins. Axons of olfactory sensory
neurons contact the dendrites of mitral and tufted cells in the olfactory bulbs to
form olfactory glomeruli.
■ Information from the olfactory bulb travels via the lateral olfactory stria directly
to the olfactory cortex, including the anterior olfactory nucleus, olfactory
tubercle, piriform cortex, amygdala, and entorhinal cortex.
■ Taste buds are the specialized sense organs for taste and are comprised of basal
stem cells and three types of taste cells. Taste buds are located in the mucosa of
the epiglottis, palate, and pharynx and in the walls of papillae of the tongue.
Neurophysiology/special senses/smell and taste
Lect. Dr. Zahid M. kadhim
■ There are taste receptors for sweet, sour, bitter, salt, and umami. Signal
transduction mechanisms include passage through ion channels, binding to and
blocking ion channels, and GPCR requiring second messenger systems.
■ The afferents from taste buds in the tongue travel via the seventh, ninth, and
tenth cranial nerves to synapse in the nucleus of the tractus solitarius. From
there, axons ascend via the ipsilateral medial lemniscus to the ventral
posteromedial nucleus of the thalamus, and on to the anterior insula and frontal
operculum in the ipsilateral cerebral cortex.
MULTIPLE CHOICE QUESTIONS
For all questions, select the single best answer unless otherwise directed.
1. A young boy was diagnosed with congenital anosmia, a rare disorder in which
an individual is born without the ability to smell. Odorant receptors are
A. located in the olfactory bulb.
B. located on dendrites of mitral and tufted cells.
C. located on neurons that project directly to the olfactory cortex.
D. located on neurons in the olfactory epithelium that project to mitral cells and
from there directly to the olfactory cortex.
E. located on sustentacular cells that project to the olfactory bulb.
2. A 37-year-old female was diagnosed with multiple sclerosis. One of the
potential consequences of this disorder is diminished taste sensitivity. Taste
receptors
A. for sweet, sour, bitter, salt, and umami are spatially separated on the surface
of the tongue.
B. are synonymous with taste buds.
C. are a type of chemoreceptor.
D. are innervated by afferents in the facial, trigeminal, and glossopharyngeal
nerves.
E. all of the above
3. Which of the following does not increase the ability to discriminate many
different odors?
Neurophysiology/special senses/smell and taste
Lect. Dr. Zahid M. kadhim
A. Many different receptors.
B. Pattern of olfactory receptors activated by a given odorant.
C. Projection of different mitral cell axons to different parts of the brain.
D. High β-arrestin content in olfactory neurons.
E. Sniffing.
4. As a result of an automobile accident, a 10-year-old boy suffered damage to
the brain including the periamygdaloid, piriform, and entorhinal cortices. Which
of the following sensory deficits is he most likely to experience?
A. Visual disturbance
B. Hyperosmia
C. Auditory problems
D. Taste and odor abnormalities
E. No major sensory deficits
5. Which of the following are incorrectly paired?
A. ENaC : Sour taste
B. Gustducin : Bitter taste
C. T1R3 family of GPCR : Sweet taste
D. Heschel sulcus : Smell
E. Ebner glands : Taste acuity
6. A 9-year-old boy had frequent episodes of uncontrollable nose bleeds. At the
advice of his physician, he underwent surgery to correct a problem in his nasal
septum. A few days after the surgery, he told his mother he could not smell the
cinnamon rolls she was baking in the oven. Which of the following is true about
olfactory transmission?
A. An olfactory sensory neuron expresses a wide range of odorant receptors.
B. Lateral inhibition within the olfactory glomeruli reduces the ability to
distinguish between different types of odorant receptors.
C. Conscious discrimination of odors is dependent on the pathway to the
orbitofrontal cortex.
D. Olfaction is closely related to gustation because odorant and gustatory
receptors use the same central pathways.
Neurophysiology/special senses/smell and taste
Lect. Dr. Zahid M. kadhim
E. All of the above.
7. A 31-year-old female is a smoker who has had poor oral hygiene for most of her
life. In the past few years she has noticed a reduced sensitivity to the flavors in
various foods which she used to enjoy eating. Which of the following is not true
about gustatory sensation?
A. The sensory nerve fibers from the taste buds on the anterior two-thirds of the
tongue travel in the chorda tympani branch of the facial nerve.
B. The sensory nerve fibers from the taste buds on the posterior third of the
tongue travel in the petrosal branch of the glossopharyngeal nerve.
C. The pathway from taste buds on the left side of the tongue is transmitted
ipsilaterally to the cerebral cortex.
D. Sustentacular cells in the taste buds serve as stem cells to permit growth of
new taste buds.
E. The pathway from taste receptors includes synapses in the nucleus of the
tractus solitarius in the brain stem and ventral posterior medial nucleus in the
thalamus.
8. A 20-year-old woman was diagnosed with Bell palsy (damage to facial nerve).
Which of the following symptoms is she likely to exhibit?
A. Loss of sense of taste
B. Facial twitching
C. Droopy eyelid
D. Ipsilateral facial paralysis
E. All of the above