Olfaction and taste

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.

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.

Tastant molecule

any substance capable of stimulating the sense of taste

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. These neurons make up the 'olfactory nerve' - the first cranial nerve.

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

Gustatory nerves

nerve fibers at each taste bud that receive information from the taste receptor cells. Their axons join three different cranial nerves to carry taste information to cortex (which cranial nerve depends on location in tongue and pharynx/throat).

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 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. Each glomerulus receives input from olfactory receptor neurons expressing only one type of olfactory receptor. The glomerular activation patterns within the olfactory bulb are thought to represent the odor being detected. Specifically, the glomeruli layer represents a spatial odor map organized by chemical structure of odorants.

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 and regions in the limbic system)

Ageusia

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

Dysgeusia

distortion or alteration of taste; food tastes abnormal

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 - or fullness.

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

Hyposmia

decreased ability to smell; often associated with clogged nasal sinuses (e.g., stuffiness from a head cold can block odorants from reaching olfactory receptors), or from nerve damage from trauma/infection

Olfactory nerve

he 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 (see olfactory bulb below).

Limbic system

includes several interconnected regions in the medial temporal lobe that play important roles in processing emotion and memory. One function is higher-order olfactory processing - tying odors to emotion and memory.

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 bulb is one of only three structures in the brain that has been found to undergo continuing neurogenesis in adult mammals. 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.

Anosmia

lack of ability to smell; associated with trauma to olfactory neurons projecting through the cribriform plate bone. Damage to one olfactory bulb can cause unilateral (one-sided) anosmia, which is only noticeable if nostrils are tested separately.

Mitral cells

neurons located in the olfactory bulb that receive inputs from the olfactory receptor neurons within the glomeruli, along with inputs from modulatory cells, and then project axons to several cortical areas including the olfactory tubercle and piriform cortex and regions in the limbic system (the part of cortex involved in emotions and memory). May encode odor concentration in timing of firing.

Supertasters

people who have extra fungiform papillae and therefore experience a greater intensity of taste than normal

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).

Taste bud

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

Papillae

structures on the surface of the tongue that contain the taste buds. Types of papillae include circumvallate, folate, and fungiform papillae, which differ by shape, location on tongue, and location of taste buds within a papilla.

Dysosmia

things smell differently than they should; often associated with hormonal changes like pregnancy, with clogged sinuses (e.g., stuffiness from a head cold can block odorants from reaching olfactory receptors) as seen in colds, or with nerve damage from trauma, infection

Orbitofrontal cortex for taste

ventral region of the frontal lobes that is located just above the orbits of the eyes. Contains secondary gustatory cortex (among other functions; see above for olfaction), 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).

Orbitofrontal cortex for olfaction

ventral region of the frontal lobes that is located just above the orbits of the eyes. Contains secondary olfactory cortex (among other functions; see below for taste), which is involved in odor identification and determining the reward value (e.g., pleasantness) of an odor.

Olfactory tubercle and piriform cortex

ventral regions that together are considered 'primary' olfactory cortex. The definition of 'primary' olfactory cortex is not as clear as for other senses - note that olfactory information does not pass through the thalamus before reaching these cortical regions, unlike all other senses. Both regions are involved in identification of odor type and concentration.

Phantosmia

"hallucinated smell", often unpleasant; commonly seen as an aura (i.e., early symptom) of a temporal lobe seizure

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

Odorant molecule

Any substance capable of stimulationg the sense of smell by binding to an olfsctory recpetor

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 o,factory receptors with varying affinities

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.

Hyperosmia

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

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

Hypogeusia

partial loss of taste; taste sensitivity is reduced