Chapter 13, 14 & 15 Definitions
• olfaction: The sense of smell.
• gustation: The sense of taste.
• odor: A general smell sensation of a particular quality. For example, “The cake had a
chocolate odor.” By contrast, when referring to a specific chemical entity, the term
odorant should be used.
• odorant: Any specific aromatic chemical. For example, “You were given the odorant
menthol to smell.”
• olfactory cleft: A narrow space at the back of the nose into which air flows, where the
main olfactory epithelium is located.
• olfactory epithelium: A secretory mucosa in the human nose whose primary function is
to detect odorants in the inspired air. Located on both sides of the upper portion of
the nasal cavity and the olfactory clefts, the olfactory epithelium contains three types
of cells: olfactory sensory neurons, basal cells, and supporting cells.
• supporting cells: One of the three types of cells in the olfactory epithelium. This cell type
provides metabolic and physical support for the OSNs.
• basal cells: One of the three types of cells in the olfactory epithelium. The basal cells are
precursor cells to olfactory sensory neurons.
• olfactory sensory neurons (OSNs): The main cell type in the olfactory epithelium. OSNs
are small neurons located beneath a watery mucous layer in the epithelium. The cilia
on the OSN dendrites contain the receptor sites for odorant molecules.
• cilia: Hairlike protrusions on the dendrites of olfactory sensory neurons. The receptor sites
for odorant molecules are on the cilia, which are the first structures involved in
olfactory signal transduction.
• olfactory receptor (OR): The region on the cilia of olfactory sensory neurons where
odorant molecules bind.
• cribriform plate: A bony structure riddled with tiny holes, at the level of the eyebrows, that
separates the nose from the brain. The axons from the olfactory sensory neurons
pass through the tiny holes of the cribriform plate to enter the brain.
• anosmia: The total inability to smell, most often resulting from sinus illness or head
• olfactory (I) nerves: The first pair of cranial nerves. The axons of the olfactory sensory
neurons bundle together after passing through the cribriform plate to form the
• olfactory bulb: The blueberry-sized extension of the brain just above the nose, where
olfactory information is first processed. There are two olfactory bulbs, one in each
brain hemisphere, corresponding to the right and left nostrils.
• ipsilateral: Referring to the same side of the body (or brain).
• mitral cells: The main projective output neurons in the olfactory bulbs.
• tufted cells: A secondary class of output neurons in the olfactory bulbs.
• glomeruli: Spherical conglomerates containing the incoming axons of the olfactory
sensory neurons. Each OSN converges onto two glomeruli (one medial, one lateral). • primary olfactory cortex: The neural area where olfactory information is first processed,
which includes the amygdala–hippocampal complex and the entorhinal cortex.
• amygdala-hippocampal complex: The conjoined regions of the amygdala and
hippocampus, which are key structures in the limbic system. This complex is critical
for the unique emotional and associative properties of olfactory cognition.
• entorhinal cortex: A phylogenetically old cortical region that provides the major sensory
association input into the hippocampus. The entorhinal cortex also receives direct
projections from olfactory regions.
• limbic system: The encompassing group of neural structures that includes the olfactory
cortex, the amygdala, the hippocampus, the piriform cortex, and the entorhinal
cortex. The limbic system is involved in many aspects of emotion and memory.
Olfaction is unique among the senses for its direct and intimate connection to the
• trigeminal (V) nerves: The fifth pair of cranial nerves, which transmit information about
the “feel” of an odorant (e.g., menthol feels cool, and cinnamon feels warm), as well
as pain and irritation sensations (e.g., ammonia feels burning).
• shape-pattern theory: The current dominant biochemical theory for how chemicals come
to be perceived as specific odorants. Shape-pattern theory contends that different
scents—as a function of odorant-shape to OR-shape fit—activate different arrays of
olfactory receptors in the olfactory epithelia. These various arrays produce specific
firing patterns of neurons in the olfactory bulb, which then determine the particular
scent we perceive.
• specific anosmia: The inability to smell one specific compound amid otherwise normal
• stereoisomers: Isomers (molecules that can exist in different structural forms) in which
the spatial arrangement of the atoms are mirror-image rotations of one another, like
a right and left hand. Also called optical isomers.
• psychophysics: The science of defining quantitative relationships between physical and
psychological (subjective) events.
• staircase method: A psychophysical method for determining the concentration of a
stimulus required for detection at the threshold level. A stimulus (e.g., odorant) is
presented in an ascending concentration sequence until detection is indicated, and
then the concentration is shifted to a descending sequence until the response
changes to “no detection.” This ascending and descending sequence is typically
repeated several times, and the concentrations at which reversals occur are
averaged to determine the threshold detection level of that odorant for a given
individual. Also called reverse staircase method.
• triangle test: A test in which a participant is given three odors to smell, of which two are
the same and one is different. The participant is required to state which is the odd
odor out. The order of the three odors given (e.g., same, same, different; different,
same, same; same, different, same) is typically manipulated and the test repeated
several times for greater accuracy.
• tip-of-the-nose phenomenon: The inability to name an odorant, even though it is very
familiar. Contrary to the tip-of-the-tongue phenomenon, one has no lexical access to the name of the odorant, such as first letter, rhyme, number of syllables, and so on,
when in the tip-of-the-nose state. This is one example of how language and olfactory
perception are deeply disconnected.
• G protein-coupled rectors (GPCRs): The class