Glossary of Taste and Smell Disorders

Tastant molecule

any substance capable of stimulating the sense of taste

Papillae

The majority of taste buds on the tongue sit on raised protrusions of the tongue surface called papillae. There are four types of papillae present in the human tongue.

Filiform papillae

these are thin, long papillae 'V'-shaped cones that don't contain taste buds but are the most numerous. These papillae are mechanical and not involved in gustation.

Fungiform papillae

these are slightly mushroom-shaped if looked at in section. These are present mostly at the apex (tip) of the tongue, as well as at the sides.

Foliate papillae

these are ridges and grooves towards the posterior part of the tongue found on lateral margins.

Circumvallate papillae

there are only about 3-14 of these papillae on most people, and they are present at the back of the oral part of the tongue. They are arranged in a circular-shaped row just in front of the sulcus terminalis of the tongue.

Taste bud

structure on the tongue that contains several taste receptor cells. A young tongue contains ~10,000 taste buds.

Taste receptor cells

provide taste information. They are located throughout the tongue in the taste buds, have areas of higher sensitivity, and have a very short life span (i.e., they are replaced frequently with new taste cells).

Microvilli

microscopic cellular membrane protrusions that increase the surface area of cells and minimize any increase in volume and are involved in a wide variety of functions.

Taste pore

any of numerous spherical clusters of receptor cells found mainly in the epithelium of the tongue and constituting the end organs of the sense of taste.

Supertasters

people who have a genetic mutation producing extra fungiform papillae and an increase of gustatory nerve fibers per papilla. They therefore experience a greater intensity of taste than normal, and may be especially sensitive to bitter tastes and the texture of fat.

Gustatory nerves

nerve fibers at each taste bud that receive information from the taste receptor cells.

Cranial Nerves

Their axons join three different cranial nerves to carry taste information to cortex (which cranial nerve depends on location in tongue and pharynx/throat).

Anterior insula & frontal operculum

Adjacent cortical regions involved in taste processing; considered to be primary gustatory cortex. Plays a primary role in taste identification and evaluation of taste intensity.

Orbitofrontal cortex

Ventral region of the frontal lobes that is located just above the orbits of the eyes. Contains secondary gustatory cortex, which is involved in identifying taste, determining the reward value (e.g., pleasantness) of a taste, combining multisensory information to determine flavor, and signaling satiety (fullness).

Flavor

The perception of flavor is a multisensory experience, which relies on the integration of cues from most of the human senses. The process of flavor perception involves the complex interaction of taste with the smell, texture (from S1), and sight of food. In addition, flavor perception changes over time during a meal to signal satiety.

Gustatory map

Cortical representation of the different taste modalities; recent research shows a gustatory map in the right insula.

Ageusia

The complete loss of taste; patients will not be able to discriminate differences among sweetness, sourness, bitterness, saltiness, and umami.

Hypogeusia

Partial loss of taste; taste sensitivity is reduced.

Dysgeusia

Distortion or alteration of taste; food tastes abnormal.

Causes of ageusia, hypogeusia, dysgeusia

Medication side effects, vitamin deficiencies, and respiratory illnesses with clogged nasal sinuses are common causes of dysgeusia and hypogeusia. Trauma is a common cause of ageusia.

Geographical tongue

An inflammatory condition of the mucous membrane of the tongue in which the loss of fungiform papillae produce smooth, red, painful regions that change location over time; may result from other infections or medications like antibiotics.

Fissured tongue

Benign condition characterized by deep grooves (fissures) in the top surface of the tongue; associated with geographical tongue, aging, environmental factors, and genetic conditions like Down's syndrome.

Odorant molecule

any substance capable of stimulating the sense of smell by binding to an olfactory receptor; substance must be volatile (able to vaporize)

Olfactory epithelium

a sheet of cells that contains the olfactory receptors and that lines the upper part of the nasal passages. The epithelium is covered by a mucous layer through which odorants must be absorbed before activating the olfactory receptors.

Olfactory receptors

are expressed in the dendrites of the olfactory receptor neurons and are responsible for the detection of odorant molecules. Rather than binding only one specific odorant, olfactory receptors can bind to a range of odorant molecules with different degrees of activation, and, conversely, a single odorant molecule may bind to a number of olfactory receptors with varying affinities.

Olfactory receptor neurons

bipolar neurons with dendrites facing the nasal cavity (in the olfactory epithelium) and axons that pass through the openings in the cribriform plate (bone) to synapse in the olfactory bulb. Olfactory receptors are located along the dendrites and lie across the olfactory epithelium within the mucus layer. These neurons make up the 'olfactory nerve' - the first cranial nerve, and are one of only ~3 structures in the brain that have been found to undergo continuing neurogenesis in adult mammals.

Olfactory nerve

the first cranial nerve (CN I) is actually the many small nerve fascicles of the olfactory receptor neurons. The olfactory nerve is unique among cranial nerves, because it is capable of some regeneration if damaged.

Cribriform plate

a section of the bone that separates the nasal cavity from the brain. The cribriform plate contains many small holes through which the olfactory receptor neurons project axons. Clinical significance: (i) a fractured cribriform plate can result in leaking of CSF into the nose and loss of sense of smell. (ii) The tiny holes of the cribriform plate can become the entry point for a pathogenic amoeba (Naegleria fowleri). This amoeba destroys the olfactory bulb and the adjacent inferior surface of the frontal lobe of the brain before fatally spreading to the rest of the brain and CSF.

Trigeminal sense

sensation of touch, pressure, pain, temperature (hot chili peppers!) in mouth, eye, nasal cavity that is carried by the trigeminal nerve (the fifth cranial nerve, CN V). ~70% of odorants co-activate both the olfactory nerve and the trigeminal nerve.

Olfactory bulb

is a multi-layered structure located on the ventral surface of the brain that receives inputs from olfactory receptor neurons and sends output to cortex via mitral cell axons. The olfactory-receptor-neuron axons that form synapses in olfactory bulb glomeruli are also capable of regeneration following regrowth of an olfactory receptor neuron in the olfactory epithelium. The neural circuitry here may play a role in identification of odor type and concentration.

Olfactory glomerulus

Spherical structures located in the olfactory bulb where synapses form between the axon terminals of the olfactory nerve and the dendrites of mitral cells.

Mitral cells

Neurons located in the olfactory bulb that receive inputs from the olfactory receptor neurons within the glomeruli and project axons to several cortical areas.

Olfactory tract

The bundle of axons including those from the mitral cells that connects the olfactory bulb to several target regions in the brain.

Olfactory tubercle and piriform cortex

Ventral regions considered 'primary' olfactory cortex, involved in identification of odor type and concentration.

Orbitofrontal cortex for olfaction

Ventral region of the frontal lobes involved in odor identification and determining the reward value of an odor.

Limbic system

Includes several interconnected regions in the medial temporal lobe that play important roles in processing emotion and memory.

Sniffing

Key for olfactory perception: odor localization, odor identification and olfactory attention.

Anosmia

Lack of ability to smell; associated with trauma to olfactory neurons projecting through the cribriform plate bone.

Hyposmia

Decreased ability to smell; often associated with clogged nasal sinuses or from nerve damage from trauma/infection.

Dysosmia

Things smell differently than they should; often associated with hormonal changes like pregnancy or with nerve damage.

Hyperosmia

An abnormally acute sense of smell; often associated with hormonal changes like pregnancy.

Phantosmia

"Hallucinated smell", often unpleasant; commonly seen as an aura of a seizure in temporal lobe epilepsy.