Download Gustatory and Olfactory Systems - Dr. Costanzo

Gustatory and Olfactory Systems
Richard M. Costanzo, Ph.D.
OBJECTIVES
After studying the material of this lecture, the student should be able to:
1. Describe the location and morphological characteristics of the sensory organs
responsible for taste and smell.
2. Describe the innervation for each of the cranial nerves that mediate:
a. Taste sensations
b. Smell sensations
3. Describe the spatial distribution of taste sensations across the different regions of
the tongue.
4. Describe a mechanism by which olfactory stimuli might be encoded by the
nervous system.
5. Identify clinical disorders that could result in impairment of gustatory or olfactory
function.
6. Compare and contrast the features of the gustatory and olfactory systems with
those of other sensory systems.
I.
GUSTATION (THE CHEMICAL SENSE OF TASTE)
A.
TASTE BUDS
The receptor cells for taste stimuli are located in structures called TASTE
BUDS. These taste buds are found primarily distributed over the surface
of the tongue in bulges or projections called PAPILLAE. Some taste buds
are also found on the soft palate, epiglottis and lower oral pharynx.
Figure 1: Structure of the taste bud
(from Costanzo, 2006)
Individual taste buds are made up of specialized epithelial cells
(approximately 40-50 cells per bud) which form a barrel-shaped structure.
At the surface of each taste bud is a small fluid-filled opening in the
epithelium called the taste pore. It is through this pore that chemical
stimuli reach the taste receptor cells.
There are three cell types within the taste buds:
1. BASAL CELLS
2. SUPPORTING CELLS
3. SENSORY CELLS (also called TASTE RECEPTOR CELLS)
The BASAL CELLS are undifferentiated, stem cells located near the base
of the taste bud. In the normal adult, basal cells undergo a process of
continuous regeneration. Every 10 days a new cell is produced which
migrates toward the center of the taste bud where it differentiates into
either a sensory or supporting cell. Basal cells replace old cells which die
off and presumably pass through the taste pore to the surface of the
tongue. This unique capacity to replace sensory receptor cells appears
to be limited to the chemical senses. As you will see later, a similar
process takes place in the olfactory epithelium.
The SUPPORTING CELLS are found distributed among the receptor
cells within the taste bud. They are similar in appearance to receptor cells
but do not respond to taste stimuli. The function of the supporting cells is
not known.
The SENSORY CELLS or TASTE RECEPTOR CELLS are covered
with numerous microvilli. These microvilli are found at the apical surface
of the cell and project into the taste pore. Because of their location and
large surface area, microvilli provide ideal sites for the transduction of
chemical stimuli into electrical currents (receptor potentials). It should be
emphasized that taste receptor cells are not neurons. They do not have
axons and they do not transmit action potentials to the central nervous
system. The cell bodies of the first order taste neurons are located outside
the gustatory epithelium, and only their nerve fibers enter the taste bud
where they make synaptic connections with the sensory receptor cells. A
single taste nerve fiber may innervate several taste buds, and within a
given taste bud may innervate several taste receptor cells.
The integrity of cells within a given taste bud depends very much upon the
innervation of that bud by the taste fibers. If these nerve fibers are cut and
allowed to degenerate, cells within the taste bud will also degenerate and
the taste bud will soon disappear. If, however, the nerve is allowed to
regenerate and grow back into the epithelium, a new taste bud will
develop in the region where the nerve ending terminates.
B. PAPILLAE
Taste buds located on the tongue are always associated with specialized
papillae. There are four types of papillae in man. These are the
FUNGIFORM, FILIFORM, FOLIATE and CIRCUMVALLATE
(VALLATE) papillae.
Figure 2: Types of papillae found on the tongue
FUNGIFORM papillae are scattered across the dorsal surface of the
tongue. They are most numerous near the anterior tip. These mushroomshaped bulges contain 3-5 taste buds which are located on the dorsal
surface. Fungiform papillae can be easily identified as red spots on the
surface on the tongue. This is due to the rich blood supply which is located
just beneath the taste buds. The FOLIATE papillae are located on the
lateral border of the tongue. In this case, the taste buds are in folds on the
sides of the papillae.
CIRCUMVALLATE papillae are the largest of all papillae. They are few
in number (only 10-12 in man) and are arranged in rows near the base of
the tongue. Each circumvallate papillae is surrounded by a circular furrow
or trench and numerous taste buds are located in the sides of these
furrows. In fact, nearly half of the taste buds in the tongue are located on
the circumvallate papillae (it has been estimated that there are as many as
250 buds per circumvallate papillae).
The FILIFORM papillae make up the fourth type. They are leaf-shaped,
2-3 mm long and do not contain taste buds. In spite of the absence of
taste buds, the filiform are the most abundant type of papillae found on the
surface on the tongue.
C. GUSTATORY NERVES
The cranial nerves involved in taste pathways are:
1. FACIAL (VII)
2. GLOSSOPHARYNGEAL (IX)
3. VAGUS (X)
The FACIAL NERVE (VII) subserves taste over the anterior two-thirds
on the tongue. Fibers from the anterior tongue run through the lingual
nerve, chorda tympani and facial nerve to the geniculate ganglion. The cell
bodies of these taste fibers are in the GENICULATE GANGLION. The
GLOSSOPHARYNGEAL NERVE (IX) innervates the posterior third of
the tongue and its fibers have cell bodies originating in the PETROSAL
GANGLION. Finally, taste fibers in the VAGUS NERVE (X) innervate
the epiglottis and lower pharynx. Cell bodies of the vagus nerve are
located in the NODOSE GANGLION.
Figure 3: Nerves innervating the tongue
The TRIGEMINAL NERVE (V) also innervates the tongue but is not
considered to be involved in gustation. This nerve travels together with the
chorda tympani fibers in the lingual nerve to innervate the anterior twothirds of the tongue. The cell bodies are located in the GASSERIAN
GANGLION and are involved in touch, temperature and pain.
D. CENTRAL TASTE PATHWAYS
Figure 4: Central taste pathways
The nerve fibers mediating taste (VII, IX and X) enter the medulla and
ascend together in a bundle called the SOLITARY FASCICULUS or
TRACT. These nerves terminate on the second order taste cells located in
the rostral portion of the SOLITARY NUCLEUS. Taste cells in the
SOLITARY NUCLEUS project primarily ipsilaterally to the VENTRAL
POSTEROMEDIAL NUCLEUS (VPM) of the thalamus.
Fibers projecting to the thalamus (VPM) probably travel in the central
tegmental tract and not the medial lemniscus. From the thalamus, there are
projections to cells in a region of the neocortex known as the
"CORTICAL TASTE AREA". This region includes the LATERAL
PORTION of the POSTCENTRAL GYRUS (the face-tongue area of SI)
and the OPERCULAR INSULAR CORTEX.
Gustatory information also reaches areas of the BASAL FOREBRAIN
including the hypothalamus, amygdaloid complex and the bed nucleus of
the stria terminalis. It has been suggested that projections to the basal
forebrain may play a role in the regulation and control of food intake and
drinking behavior.
E. BASIC TASTE SENSATIONS (TASTE QUALITIES)
The four basic taste sensations are usually identified as SALTY,
SWEET, SOUR AND BITTER. A fifth taste called UMAMI is used to
describe the flavor-enhancing taste of monosodium glutamate (MSG).
Several transduction mechanisms are involved in mediating taste
sensations. Salt and sour sensations are mediated by a direct interaction of
chemicals with ion channels. For example the taste of table salt (NaCl) is
mediated by an Amiloride sensitive sodium channel located on taste bud
receptor cells. Sour (H+) taste is mediated by inhibition of a K+ voltage
sensitive channel. Sweet and bitter sensations involve second messenger
systems that lead to depolarization of the receptor cells. Depolarization of
the taste receptor cells cause release of neurotransmitters and activation of
the taste nerves.
Figure 5: Taste transduction mechanisms
(from Costanzo, 2006)
F. NEURAL CODING
Recordings from individual single taste nerve fibers has led to the across
fiber pattern code for taste. These recordings show that no two taste
fibers have identical response characteristics. Each fiber responds to all
four taste qualities and there are differences in the magnitude of response
to each of the different stimuli. In the across fiber pattern code, a single
fiber alone does not encode stimulus quality, rather the response pattern
across many fibers at the same time is used to discriminate a particular
stimulus.
G. TASTE DISORDERS
Taste disorders are not considered life threatening, however patients with
a taste loss often develop problems related to diet and nutritional status,
and there is an increased risk of accidental food poisoning or ingestion of
toxic substances. Disorders of taste may occur after: damage to taste
nerves, lesions in the cortical taste centers (head injury, stroke, and brain
tumors), drug treatment, radiation of the head and neck, diseases of the
oral cavity and normal aging.
1. Quantitative
a. HYPOGEUSIA - a decrease in taste sensitivity
b. AGEUSIA - absence of the sense of taste
c. HYPERGEUSIA - an increase in taste sensitivity
2. Qualitative
a. DYSGEUSIA - impairment or distortion of taste
1. CACOGEUSIA - a bad or foul sense of taste
2. PARAGEUSIA - a taste sensation in the absence of
the appropriate stimuli
II.
OLFACTION (THE SENSE OF SMELL)
A.
NASAL CAVITY
Odor stimuli, or smells, reach the olfactory receptors by way of the nasal
cavity. During normal breathing, air enters the external nares (nostrils),
passes across the nasal cavity and exits into the nasopharynx.
Figure 6: The human nasal cavity
Within the nasal cavity are a series of structures called the
TURBINATES or CONCHAE (SUPERIOR, MIDDLE AND
INFERIOR). These structures act as baffles or deflectors, causing airflow
to become turbulent or nonlinear in nature. The result is that some of the
air stream, is diverted to the upper regions where the olfactory receptor
cells are located. The OLFACTORY EPITHELIUM lines the entire
SUPERIOR TURBINATE, part of the MIDDLE TURBINATE and
the upper part of the NASAL SEPTUM. This region is yellowish brown
in color and occupies an area of about 2-5 cm2. Some 100 million (108)
olfactory receptor cells are concentrated in this region. The rest of the
nasal cavity contains RESPIRATORY EPITHELIUM which in contrast
to the olfactory epithelium is reddish pink in color. This region contains
ciliated epithelial cells which beat rhythmically and aid in the movement
of mucous across the surface of the epithelium.
Figure 7 Netter Presenter III.84
B. OLFACTORY EPITHELIUM
The olfactory epithelium consists of three basic cell types:
1. BASAL CELLS
2. SUPPORTING CELLS
3. OLFACTORY RECEPTOR CELLS
Figure 8: The olfactory epithelium
BASAL CELLS, located at the base of the epithelium, are
undifferentiated stem cells which give rise to olfactory receptor cells.
These stem cells undergo mitosis producing a continuous turnover and a
supply of new supporting and receptor cells. This process takes place
throughout the adult life. Continuous neurogenesis of olfactory receptor
cells is similar to that which occurs in the gustatory epithelium. However,
there are few important differences. First, the time it takes a basal cell in
the olfactory epithelium to develop and emerge as a mature receptor cell
(25-30 days) is somewhat longer than in the gustatory epithelium.
Secondly, unlike taste receptor cells, olfactory receptor cells are true
nerve cells (neurons). They have a dendritic rod, an axon, and conduct
nerve impulses (action potentials) into the central nervous system. This is
quite remarkable since these cells are neurons, and neurons in the adult
mammalian nervous system do not replace themselves.
The SUPPORTING CELLS (sometimes called SUSTENTACULARCELLS) are COLUMNAR EPITHELIAL CELLS. They are located at
the border of the epithelium and are distributed homogeneously among
receptor cells. They have numerous microvilli which extend into the
olfactory mucosa and contain secretory granules which are located near
the surface and empty their contents into the mucosal layer. It may be that
this secretory process is an important function of the supporting cells.
The RECEPTOR CELLS are flask-shaped cells and have relatively large
cell bodies (5-8 microns) and a thin dendritic rod of about 1-2 microns
wide. At the mucosal surface, this rod forms a swelling called the
olfactory vesicle or knob. The olfactory vesicle contains a number of cilia
(10-15) which extend out into the mucous layer for distances up to 100
microns. These cilia have a typical 9 plus 2 pattern of filaments, and
because of their close proximity to the air-mucosal surface, they are
thought to be the site for transduction of odor molecules.
The basal end of the receptor cells give rise to a thin unmyelinated fiber
which passes out of the epithelium and travels centrally to the olfactory
bulb. These olfactory nerve fibers are unique for two reasons. First, they
are among the smallest fibers in the nervous system ranging in diameter
from 0.1-0.3 microns and as expected they have extremely slow
conduction velocities of 0.2-0.3 M/sec (very slow). The second unique
property of the olfactory nerves is that within a single Schwann cell
invagination, these unmyelinated fibers run together in bundles and
apparently are not insulated from one another.
It should be pointed out that in addition to innervation by the olfactory
nerve, the olfactory epithelium is also innervated by branches of the
TRIGEMINAL (V) NERVE. They are the principle detectors for
noxious and caustic chemicals such as ammonia, formaldehyde and other
stinging and sometimes painful odors. A basic distinction should be made
between the detection of noxious stimuli by the V nerve and "olfaction"
which is the discrimination and detection of odor stimuli and is by
definition mediated by the olfactory nerves (1st cranial nerves). This
distinction has clinical importance. For example in head injury the delicate
olfactory nerves that pass through the cribriform plate of the ethmoid
bone can be completely severed making the olfactory system incapable of
function. However, if tested with inappropriate stimuli (i.e., alcohol,
smelling salts of ammonia, vicks), patients will report that they can still
smell. This phenomenon is the result of stimulating the trigeminal nerve
endings within the nasal cavity and is not an olfactory sensation.
Evaluation of the olfactory nerves (1st cranial nerves) requires the use of
appropriate odor stimuli. Mild odors such as chocolate, peanut butter,
aroma of a cup of coffee or the scent of a mild perfume are good examples
of appropriate stimuli.
C. OLFACTORY BULB
Olfactory nerve fibers from the nasal epithelium, pass through the
cribriform plate and project directly into the olfactory bulb. The
olfactory bulb is a highly organized structure consisting of several distinct
layers. They are: the glomerular layer, the outer plexiform layer, mitral
cell body layer, inner plexiform layer and granule cell layer. The most
important cell type is the MITRAL CELL. Mitral cells have apical
dendrites that receive direct synaptic input from the olfactory nerve fibers.
Their axons join together to form the OLFACTORY TRACT, which is
the principle output of the bulb. The processing of olfactory information
that takes place within the olfactory bulb is a consequence of the
interconnections between different cells types.
Figure 9. Netter Presenter, 2004, III.85
In the outermost layer of the bulb, the first order olfactory nerve fibers
form clusters of synaptic connections with the dendrites of the second
order mitral cells. These clusters are called GLOMERULI. There are
approximately 50,000,000 olfactory nerve fibers that project to 45,000
mitral cells in each olfactory bulb. This means that there is a convergence
of approximately 1,000:1 onto the second order mitral cells. The cell
bodies of mitral cells are arranged in a thin layer (mitral cell body layer).
They have a single apical dendrite, and several lateral dendrites which
enter the external plexiform layer. Here they interact with the dendrites of
other cell types such as the GRANULE CELLS.
In the outermost glomerular layer there are cells which make connections
between glomeruli. These are the PERIGLOMERULAR (PG) CELLS
which are inhibitory interneurons. They provide for lateral inhibition of
neighboring glomeruli. The deeper regions of the bulb (granule cell layer)
contain short axon cells and the more significant granule cells. Granule
cells consist of a cell body and a dendritic process with spines. They do
not have axons. Most of the granule cell dendritic fields are located in the
external plexiform layer where they interact with mitral cells by means of
a specialized synapse called the dendrodendritic or reciprocal synapse.
Under the electron microscope one can see two areas where Synaptic
vesicles are in close opposition to the membrane. Vesicles on the granule
cell side are more irregular and oval shaped indicating an inhibitory
synapse from granule to mitral cell dendrites. Vesicles on the mitral cell
side are more regular and spherical shaped indicating an excitatory
synapse from mitral to granule cell. Thus, mitral cells excite; granule
cell dendrites and granule cells inhibit mitral cell dendrites.
Dendrodendritic synapses have been observed both in the glomerular layer
(between mitral and periglomerular cells) and in the external plexiform
layer (between mitral and granule cells). In each case, these synapses are
excitatory in one direction and inhibitory in the opposite direction.
Dendrodendritic connections in the olfactory bulb appear to serve two
functions: SELF-INHIBITION and LATERAL INHIBITION. Selfinhibition limits the amount of excitability within a single mitral cell after
the initial onset of the stimulus. This type of inhibition could explain why
cells in the olfactory system adapt so rapidly to continuous odor stimuli.
Lateral inhibition provides a means for inhibiting neighboring mitral cells
via inhibitory periglomerular cells in the glomerular layer or via inhibitory
granule cells in the external plexiform layer. The information coming
through a particular mitral cell would be sharpened perhaps by eliminating
background activity in neighboring mitral cells. If different mitral cells
encoded different odors, then lateral inhibition could play an important
role in improving the discrimination between odors.
D. CENTRAL OLFACTORY PATHWAYS
Figure 10: Central olfactory pathways
The second order (mitral) cells in the olfactory bulb project to central
regions of the brain via the olfactory tract. A medial division of this tract
(the medial olfactory stria) sends connections to the ANTERIOR
OLFACTORY NUCLEUS and the SEPTAL AREA (subcallosal and
paraolfactory areas). There are also projections to the
CONTRALATERAL OLFACTORY BULB by way of the
ANTERIOR COMMISSURE. Fibers in the lateral division (lateral
olfactory stria) project directly to the PRIMARY OLFACTORY
CORTEX (Prepyriform cortex), PERIAMYGDALOID CORTEX
(ENTORHINAL CORTEX) and the PREPYRIFORM CORTEX. In
man there is intermediate branch of the olfactory tract (intermediate stria)
that penetrates the anterior perforated substance of the basal forebrain to
terminate in its nuclei. Like in other sensory systems, there are projections
to the NEOCORTEX (orbital frontal cortex) via a thalamic relay
(DORSAL MEDIAL THALAMUS, DM). Olfactory information reaches
both the hypothalamus (feeding and reproduction) and the limbic system
(emotion and sexual drive).
E. NEURAL CODING
The basic transduction mechanism for olfactory receptors involves a Gprotein second messenger system. Individual olfactory receptor cells
respond to a wide range of different chemicals (are broadly tuned). This
suggests that a single receptor cell alone may not provide sufficient
information to encode odors. However, if one looks at the response profile
across many receptors at the same time (across fiber pattern code), there
appears to be different patterns of response for different odors. Thus each
odor stimulus could produce a unique pattern of activity across the
population of receptors and that pattern could encode that particular
stimulus. This is known as the ACROSS FIBER PATTERN THEORY
of olfaction.
Figure 11: Coding of olfactory stimuli
F. OLFACTION DISORDERS
It has been estimated that between 1.5 and 2 million Americans experience
some type of olfactory dysfunction. Certain pathological conditions result
in anosmia (absence of smell sensation), hyposmia (decreased sensation)
and dysosmia (distortion of smell sensation). These include head injury,
trauma, upper respiratory infections, tumors of the anterior fossa, exposure
to toxic chemicals, and diseases and anomalies of the nasal cavity. In
addition, anosmia is symptomatic of certain endocrine dysfunctions such
as Kallman's syndrome (hypogonadism). Detecting smell deficits can help
to make a positive diagnosis. Olfactory disorders, while not typically life
threatening, create significant disabilities that are frequently overlooked.
For example, smell disorders can result in loss of appetite, weight loss,
and in some cases malnutrition. Persons with smell problems are also
unable to detect life threatening situations such as gas leaks, fires, and
toxic substances.
1. QUANTITATIVE
a. HYPOSMIA - a decrease in smell sensitivity
b. ANOSMIA - absence of sense of smell
c. HYPEROSMIA - an increase in smell sensitivity
2. QUALITATIVE
a. DYSOSMIA - impairment or distortion of the sense of
smell
1. CACOSMIA - a bad or foul sense of smell
2. PAROSMIA - a smell sensation in the absence of
the appropriate odor stimulus
REFERENCE
Costanzo, L.S., Physiology, 3rd Edition, Saunders Elseveir, 2006, pp. 92-96.
A Self-Assessment is available for this lecture.
* Netter Presenter Image Copyright 2004 Icon Learning Systems. All rights
reserved.