Chapter 15- The Special Senses
- Sense of smell occurs in response to odours that stimulate receptors located In the extreme superior
region of the nasal cavity.
- Only a small superior part of the nasal cavity is dedicated to olfaction.
- Olfactory epithelium is the nasal epithelium in the olfactory region.
Olfactory Epithelium and Bulb
- 10 million olfactory neurons are present within the olfactory epithelium.
- Axons of these bipolar neurons project to the olfactory bulbs where they synapse with secondary
- Olfactory tracts project from the bulbs to the cerebral cortex.
- Dendrites of olfactory neurons extend to the epithelial surface of the nasal cavity, and their ends are
modified into bulbous enlargements called olfactory vesicles.
- These vesicles possess cilia called olfactory hairs .
- Airborne molecules enter the nasal cavity and are dissolved in the fluid covering the olfactory epithelium.
- Some of these odorants bind to transmembrane odorant receptor molecules of the olfactory hair
- A G protein, associate with each odorant receptor, is activated by the binding of the odorant.
- The a subunit of the activated G protein binds to and activates adenylate cyclase, which in turn catalyzes
the formation of cyclic AMP from ATP, Camp in these cells causes na+ and ca2+ channels to open.
- The influx of ions into the olfactory hairs results in depolarization ad the production of action potentials in
the olfactory neurons.
- Odor receptors are composed of seven transmembrane polypeptide subunits produced by a large family
- Combinations of subunits from approximately 1000 different odorant receptors can be produced.
- These receptors can react to odorants of different sizes, shapes and functional groups.
- These capabilities, together with multiple intracellular pathways involving G proteins, adenylate cyclase
and ion channels allow for a wide variety of detectable smells, which is about 4000 for the avg. person.
- Seven primary classes have been proposed: camphoraceous(eg. Moth balls), musky, floral, pepperminty,
ethereal (eg. Fresh pears), pungent and putrid.
- Threshold for the detection of odors is very low, so very few odorant molecules are required to trigger the
- Low specificity- a given receptor may react to more than one type of odorant.
- Odorant receptors may become saturated with odorants and no longer respond to odorant molecules.
- Makes people less sensitive to as odor after beong exposed to it for a short time.
- The primary olfactory neurons have the most exposed nerve ending of any neurons, and they are
constantly being replaced.
- The entire olfactory epithelium, including the neurosensory cells, is lost about every 2 months as the
olfactory epithelium degenerates and is lost from the surface.
- Basal cells in the olfactory epithelium replace lost olfactory cells.
- Unique among neurons.
Neuronal Pathways for Olfaction
- Axons from the olfactory neurons form the olfactory nerves, which enter the olfactory bulbs where they
synapse with mitral cells or tufted cells.
- These cells relay olfactory info to the brain through the olfactory tracts and synapse with granule cells in
the olfactory bulb.
- Olfactory bulb neurons also receive input from nerve cell processes entering the olfactory bulb from the
brain. - As a result of input from both mitral cells and the brain, olfactory bulb neurons can modify olfactory info
before it leaves the olfactory bulb.
- This enhances the accommodation occurring in the odorant receptors.
- Olfaction is the only major sensation that is relayed directly to the cerebral cortex without first passing
through the thalamus.
- Each olfactory tract terminates in an area of the brain called the olfactory cortex.
- The olfactory cortex is in the frontal and temporal lobes, within the lateral fissure of the cerebrum, and it
can be divied structurally and functionally into three areas: lateral, medial and intermediate.
- The lateral area is involved in the conscious perception of smell.
- The medial olfactory area is responsible for visceral and emotional reactions to odors and has connections
to the limbic system, through which it connects to the hypothalamus.
- Axons extend from the intermediate olfactory area along the olfactory tract to the bulb and synapse with
the olfactory bulb neurons, thus constituting a major mechanism by which sensory information is
modulated within the olfactory bulb.
- The feedback from the intermediate olfactory is mostly inhibitory, further enhancing the rapid
accommodation of the olfactory system.
- Accommodation is what makes the olfactory system insensitive to an odorant after a brief exposure.
- Sensory system that detect gustatory or taste stimuli are the taste buds.
- Associated with specialized portions of the tongue called papillae.
- Also located on other areas of the tongue, the palate and even the lips and throat, especially in children.
- The four major types of papillae according to their shape are: vallate, fungiform, foliate, and filiform.
- Taste buds are associated with vallate, fungiform and foliate papillae.
- Filiform are the most numerous but have no taste buds.
- They provide a rough surface on the tongue.
- Vallate are the largest but least numerous.
- 8-12 of these form a v-shaped row along the border between the anterior and posterior parts of the
- Fungiform are scattered irregularly over the entire superior surface of the tongue, appearing as small, red
dots interspersed among the far more numerous filiform papillae.
- Foliate papillae are distributed in folds on the sides of the tongue and contain the most sensitive of the
- They are most numerous in young children and decrease with age.
- They are located mostly posteriorly in adults.
Histology of Taste Byds
- Taste buds are oval structures embedded in the epithelium of the tongue and mouth.
- Each of the 10 000 taste buds on a person’s tongue consists of three major types of specialized epithelial
- The sensory cells of each taste bud consist of about 50 taste or gustatory cells.
- The remaining two cell types which are nonsensory, are basal cells and supporting cells.
- The cells are replaced continuously.
- Having a lifespan of about 10 days.
- Each taste cell has several microvilli called gustatory hairs, extending from its apex into a tiny opening in
the epithelium called the taste or gustatory, pore.
Function of Taste
- Substances called tastants dissolved in saliva, enter the taste pores and by various mechanisms, cause the
taste cells to depolarize.
- These cells do not have classic axons but have short connections with secondary sensory neurons. - These connections have some characteristics of chemical synapses.
- Neurotransmitters( including ATP) are released from the taste cells and stimulate action potentials in the
axons of sensory neurons associated with them.
- The taste of salt results when na+ diffuse through na+ channels of the gustatory hairs or other cell
surfaces of taste cells, resulting in depolarization of the cells.
- Hydrogen ions of acids, which result in sour taste can cause depolarization of taste cells by one of three
mechanisms: they can enter the cell through h+ channels, they can bind to ligand-gated k+ channels and
block the exit of k+ from the cell, or they can open ligand-gated channels for other positive ions and allow
them to diffuse into the cell.
- Sweet and bitter tastants bind to receptors on the gustatory hairs of taste cells and cause depolarization
through a G protein mechanism.
- A taste called umami results when amino acids such as glutamate bind the receptors on gustatory hairs of
taste cells and cause depolarization through a G protein mechanism.
- Texture of food also affects the perception of taste
- Hot or cold food may interfere with the ability of the taste buds to function in tasting food.
- If a cold fluid is held in the mouth, the body warms the fluid and the taste becomes enhanced.
- Adaptation is very rapid for taste.
- Adaptation occurs at the level of the taste bud and within the CNS.
- Adaptation may occur 1 or 2 sec after a taste sensation is perceived, and complete adaptation may occur
within 5 minutes.
- Humans can perceive a fairly large number of different tastes
- Many of the sensations thought of as being taste are strongly influenced by olfactory sensations.
- Thresholds vary for the five primary tastes.
- Sensitivity for bitter substances is the highest.
- Sweet and salty is the lowest.
- Many alkaloids are poisonous, thus the high sensitivity for bitter tastes may be protective.
- On the other hand, humans tend to crave sweet, salty and umami tastes, perhaps in response the the
body’s need for sugars, carbs, proteins and minerals.
Neuronal Pathways for Taste
- Taste from the anterior two thirds of the tongue except from the circumvallate papillae is carried by the
means of the facial nerve called the chorda tympani.
- It crosses over the surface of the tympanic membrane of the middle ear.
- Taste from the posterior one thi