Physiology - Chemical Senses

Sense of smell and taste

Are examples of visceral senses due to their close association with gastrointestinal (GI) function. Also related to each other as the flavor of food is a combination of the two senses

Chemoreceptors

Stimulated by chemical molecules and solution in mucus from the nose and saliva from the mouth

Sense of taste

Primarily a function of the taste buds.

Dorsal tongue surface

There are ~5,000 taste buds located on the papilla of the?

Fungiform papillae

Prominent near the tip of the tongue. Contains ~5 taste buds each

Foliate papillae

Papillae found posterior edge of tongue

Circumvallate papillae

Papillae arranged in a V shape at the back of the tongue

Vagus nerve (CN X)

Taste buds send signals to the brainstem via the?

Chorda tympani

Innervates taste buds of the anterior ⅔ of the tongue

Lingual branch of the glossopharyngeal nerve (CN IX)

Innervates taste buds of the posterior ⅓ of the tongue

Basal stem cells, Taste cells, Taste pore

What are the components of taste buds?

Taste cells

Extend from the base to the taste pore

Taste pore

Where microvilli interact with tastants dissolved from saliva and mucus

Tactual senses

Detect food texture; found in the mouth

Taste sensation

The texture of food and certain substances that stimulate pain endings can greatly alter the taste experience. The importance of taste lies in allowing the selection of food in accord with desires and, often, with the body's metabolic needs for specific substances.

Sour

What taste sensation has the primary stimulus of acids, hydrogen ions?

Salty

What taste sensation has the primary stimulus of ionized salts, sodium ions?

Sweet

What taste sensation has the primary stimulus of mostly organic chemicals?

Bitter

What taste sensation has the primary stimulus of nitrogen-containing substances, alkaloids?

Umami

What taste sensation has the primary stimulus of L-glutamate, ingestion of proteins?

Sweetness

Usually caused by organic chemicals; slight changes in chemical structure can shift sweetness to bitterness.

Bitter taste

Also caused by organic chemicals, specifically nitrogen-containing long-chain substances and alkaloids (common in drugs and plant toxins).

Bitter taste

Much more sensitive and provides protective function against many dangerous toxins.

Hydrochloric acid (HCl)

What is the reference substance for sour taste?

Quinine

What is the reference substance for bitter taste?

Sucrose

What is the reference substance for sweet taste?

Sodium Chloride (NaCl) or Calcium Chloride (CaCl2)

What is the reference substance for salty taste?

Phenylthiocarbamide (PTC)

Some people exhibit taste blindness, particularly for thiourea compounds like?

Taste bud

Made up of epithelial cells, including: Sustentacular (supporting cells), Taste cells

Taste cells

Average lifespan: ~10 days. Replaced continuously by mitosis from surrounding epithelium. Outer tips are arranged around a taste pore. Each cell has microvilli (taste hairs) protruding from the tip of each cell

Superior and inferior salivatory nuclei

Transmit signals to salivary glands that regulate saliva secretion during ingestion and digestion

Central Nervous System

Final degree of adaptation occurs in the?

Ligand-gated channels (ionotropic receptors), G-protein coupled receptors (metabotropic receptors)

What are the two main types of taste receptors?

Epithelial sodium channel (ENaC)

Mediate salt and sour tastes. Composed of alpha (𝛼) and gamma (𝛾) subunits. Salt: sensed via sodium (Na⁺) movement. Sour: mediated by hydrogen (H⁺) movement

G-protein coupled receptors (GPCRs)

Detect sweet, bitter, and umami tastes. Span the membrane 7 times. Sweet: Detected by T1R2 + T1R3 family. Bitter: T2R receptor family. Umami: mGluR4 receptor (metabotropic glutamate receptor 4)

Taste Preference

Primarily a central nervous system (CNS) phenomenon.

Ageusia

Complete absence of taste. Lingual or glossopharyngeal nerve damage. Poor oral hygiene. Side effects of cisplatin or captopril.Vitamin B3 and zinc deficiency

Hypogeusia

Decreased sensitivity to taste. Lingual or glossopharyngeal nerve damage

Dysgeusia (Parageusia)

Difficulty in tasting. Unpleasant or distorted taste perception. Linked to neurotransmitter disturbances. Common in anxiety and depression

Supertasters

About 25% of the population. Heightened sensitivity to taste, particularly bitterness. Possibly due to the increased number of fungiform papillae on the tongue

Olfactory membrane

Superior portion of the nasal cavity. 5-10cm2 surface area. Area in the body where the nervous system is closest to the world

Olfaction

Poorly understood, subjective, and underdeveloped in humans in comparison to animals

Olfactory Cells

Receptor, bipolar nerve cells. The mucosal end has 4–25 cilia (non-motile olfactory hairs) that project into the mucus, forming a mat. Cilia react to odors in the air and stimulate the olfactory cells. ~100 million olfactory cells are arranged among sustentacular or supporting cells

Granule Cells

Special olfactory inhibitory cells in the olfactory bulb

Glutamate

What is the neurotransmitter for mitral and tufted cells?

GABA

What is the neurotransmitter for granule cells?

Methylmercaptan

Detected with 25 trillionth of a gram per mL of air. Added with natural gas in pipelines for the detection of leaks. Even if it is a small amount (even molecules), a person will detect it already

Odor blindness

Lack of appropriate receptor protein for a particular substance

Limbic system

Basal brain structure that controls emotion and behavior. It was originally thought to be part of the brain that processes olfaction

Cribriform plate

The olfactory bulb lies over the?

Olfactory Nerve (CN I)

Bundles of sensory nerve fibers extending from the nasal mucosa to the olfactory bulb

Glomeruli

Short axons from olfactory cells terminate in multiple globular structures called?

Mitral and tufted cells

Relay signals via the olfactory tract to higher brain regions of the CNS

Medial Olfactory Area

Part of the primitive olfactory system. Involved in basic olfactory reflexes and emotional responses

Lateral Olfactory Area

Input to a less old and newer system and pathway to CNS. Part of the paleocortex. Receives input for more complex odor perception and processing

Septal nuclei

Midline; feed into the hypothalamus and other parts of the limbic system. An area that is most concerned with basic behavior

Lateral Area

Thalamus → Dorsomedial thalamic nuclei → Lateroposterior quadrant of orbitofrontale cortex. Aid in conscious odor analysis and perception

Anosmia

Inability to smell. May be due to nasal congestion or polyps, and prolonged decongestant use (mucosal drying).

Hyposmia

Decreased sensitivity to smell. May be due to nasal congestion or polyps, and prolonged decongestant use (mucosal drying). May be a sign of a more serious problem, such as cribriform plate fracture, trauma, tumors, or respiratory tract infections. Cannot detect harmful odors, fire, or spoiled food

Congenital anosmia

A rare disorder where an individual is born without the ability to smell

Hyperosmia

Enhanced ability to smell. Less common than anosmia. More common in pregnant women become oversensitive to smell

Dyosmia

Distorted sense of smell. Sinus infection, partial nerve damage, and poor dental hygiene

Uncinate seizure

Aura of disagreeable odor similar to burning rubber before the seizure

Spicy

NOT a taste sensation. It instead activates the pain pathway of the Trigeminal nerve (CN V). This makes it a trigeminal sensation, not a taste sensation.

Salty

Na+ stimulus. Mediated by Epithelial Na Channel (ENAC)

Sour taste

H+ ions stimulus. Stimulates the TRPP3 channel

TRPP3 channel

A non-selective cation channel of the transient receptor potential family of ion channels

Gustation (taste) and Olfaction (smell)

What are the two visceral chemical senses?

Mucus and saliva

Which body fluids stimulate chemoreceptors involved in taste and smell?

Chemical senses

To detect harmful (lethal/in-edible) vs. beneficial (nutritious/edible) substances

Gastrointestinal (GI) function

Which physiological function are taste and smell most associated with?

Combination of taste and smell

What creates the perception of flavor in food?

True

T/F: The chemical senses include gustation and olfaction.

False

T/F: Chemoreceptors are only stimulated by airborne chemicals.

False

T/F: Smell plays no role in the perception of food flavor.

True

T/F: Chemical senses help us identify potentially harmful substances.

False

T/F: Taste and smell are considered somatic senses.

Epithelial Na Channel (ENaC)

Which type of channel mediates salty taste transduction?

TRPM5 channel

What is the name of the non-selective cation channel involved in sweet, umami, and bitter transduction?

Sweet, Umami, Bitter

Which taste stimuli are detected via G-protein coupled receptors (GPCRs)?

IP3 (Inositol Triphosphate)

What second messenger is involved in GPCR taste transduction leading to Ca²⁺ release?

Sour taste

Which taste is detected via the TRPP3 channel?

True

T/F: Salty and sour tastes use ligand-gated ion channels.

True

T/F: Bitter taste is mediated by TR2 family GPCRs.

False

T/F: Umami taste transduction involves the ENaC channel.

True

T/F: TRPM5 is specific to taste receptor cells

False

T/F: Each taste bud detects multiple types of stimuli.

Roof of the nasal cavity near the septum

Where is the olfactory epithelium located in humans?

Basal cells

What type of cells regenerate olfactory receptor neurons?

~48 days

What is the lifespan of an olfactory receptor cell?

Bowman’s glands

What type of cell releases mucus in the olfactory epithelium?

Inhibitory feedback (using GABA to refine signals)

What type of feedback do granule cells provide in olfaction?

False

T/F: Olfactory receptor cells are modified epithelial cells.

False

T/F: Each olfactory neuron expresses multiple odorant receptors.

True

T/F: Bowman’s glands secrete mucus to dissolve odorants.

True

T/F: The mitral and tufted cells receive input from olfactory neurons.

True

T/F: Chloride efflux contributes to further depolarization in olfactory transduction.