NSCI 320 Ch. 8 Chemical Senses

Papillae

Rough bumps on the tongue each has several hundred taste buds Contain basal cells and lingual stem cells

Threshold concentration for a chemical

the amount of exposure of a single papilla to detect taste

Every chemical has a different threshold

Taste bud

Has elongated taste receptor cells which have preferences for categories of chemicals that can be tested

Taste receptor cells (taste cells)

Apical ends-microvilli->taste pore

These cells process the preliminary information, which is further expanded upon as this information progresses through brain (central) circuits

Interact via electrical synapses via transmitters

Constant growth cycle with lifespan of 2-24 days

Lingual stem cells

Regenerate taste receptor cells every 2-24 days

Taste receptor potential

Voltage shift usually a depolarization of taste cell membrane

Most fire one or more fast, sodium-dependent action potentials

Transmitter diffuses across synapses, carrying signals from taste cells to gustatory axons

Specialist taste receptor cells

Respond exclusively to one of five basic tastes

Generalist taste receptor cells

Respond to two or more basic tastes

Transduction

environmental stimulus causes electrical or chemical responses in sensory receptor cell

Taste stimuli (tastants) may

Bind to G-protein-coupled receptors and activate second messenger to open ion channels

Pass directly through ion channels

Bind to and block ion channels

Bitter, sweet, and umami taste receptor proteins

Subtypes are G-protein-coupled receptors

Use the same second messenger pathway and synaptic transmitter (ATP) to carry signals to afferent axons

Dimers

Two proteins tightly affixed to one another

The two combined make the flavor

Transduction mechanisms for bitter, sweet and umami tastants

1. Tastants bind directly to G protein-coupled membrane receptors

2. Which activate phospholipase C

3. Which increases the synthesis of IP3.

4. IP3 then triggers the release of Ca2+ from internal storage sites.

5. Ca2+ opens a taste-specific channel (TRPM5), leading to a depolarizing receptor potential

6. Which opens voltage-gated Na+ channels that generate action potentials,

7. Which in turn cause ATP-permeable CALHM1/3 channels to open

8. Allow the transmitter, ATP, to diffuse from the taste cell to purinergic receptors on the afferent axon terminals.

Bitter receptors

Two different or similar TAS2R proteins

Monomer

Sweet receptors

Dimers of G-protein-coupled receptors

TAS1R2+TAS1R3 (both required)

Expressed in different taste cells from similar receptor

Dimer

Salt taste

Special Na+-selective channel: ENaC; detects low concentrations

Blocked by the drug amiloride

Transduction mechanism for saltiness

1. In the salty cell, inward Na+ current through EnaC channels generates membrane depolarization

2. Which activates voltage-gated sodium channels

3. Generates action potentials

4. Which in turn open ATPpermeable CALHM1/3 channels and release the transmitter ATP

Sour taste

high acidity (low pH)

Protons: causative agents of acidity and sourness

H+- permeable channels (OTOP1)

Transduction mechanism for sourness

1. In the sour cell H+ current through OTOP1 channels generates a receptor potential

2. Which activates voltage-gated sodium and calcium channels on the basal membrane

3. Leading to action potentials and an increase of intracellular [Ca2+]

4. Triggers transmitter (serotonin) release via exocytosis

What three cranial nerves carry primary gustatory axons and bring taste information to the brain?

Cranial nerve 7-facial nerve

Cranial nerve 9-glossopharyngeal nerve

Cranial nerve 10-vagus nerve

How is taste transmitted to the brain?

Taste is transmitted to the brain via the nucleus of the solitary tract, which is in the medulla

From here, information passes through the thalamus and to the primary gustatory cortex

Each flavor of taste is represented in a slightly different portion of the primary gustatory cortex (mapping)

Population coding

Roughly labeled (not discrete) lines differentiate sweet, sour, bitter, and salty

Large numbers of broadly tuned neurons

Additional contribution of temperature, olfactory, and textural features of food

Pheromones

Chemical signals of reproductive behavior, territorial boundaries, identification of individuals, and signal aggression or submission

Non-volatile chemicals that are important for communication in many species

Olfactory receptor cells

On the olfactory epithelium

The sites of transduction

Continually exposed to pollutants, viruses, and bacteria

Regularly regenerate across lifespan

Supporting cells (sustentacular cells)

resemble glia

Basal stem cells

source of new receptor cells

Odorants

are airborne, volatile chemicals detected by specialized receptors in the olfactory epithelium.

Cribriform plate

thin sheet of bone through which small clusters of axons penetrate, coursing to the olfactory bulb

The axons of the olfactory receptors project directly to the brain through this

Broad tuning of single olfactory receptor cells

Each receptor cell expresses a single odorant receptor protein and different cells are randomly scattered within a region of the epithelium

Microelectrode recordings from three different cells show that each one responds to many different odors, but with differing preferences. By measuring responses from all three cells, each of the four odors can be clearly distinguished

Olfactory epithelium

Our 6 million olfactory receptors are located within just two patches of mucous membrane

Presentation of olfactory stimuli causes activation of

Oribitofrontal cortex which is an area associated with reward

The piriform cortex (aka the olfactory cortex) and olfactory bulbs themselves (not surprisingly) and

The amygdala, and area associated with emotion

Olfactory Transduction

Odorant molecules dissolve in the nasal mucus and bind to specific G protein-coupled receptors on the cilia

The receptor activates an olfactory-specific G protein (Golf)

The G protein stimulates adenylyl cyclase type III, which converts ATP into cyclic AMP (cAMP)

cAMP opens cyclic nucleotide-gated (CNG) ion channels, allowing Na⁺ and Ca²⁺ ions to flow into the cell

Entering Ca²⁺ ions open calcium-activated chloride channels, causing Cl⁻ to flow out, which further depolarizes the cell membrane

The resulting electrical signal travels down the axon through the cribriform plate to the olfactory bulb in the brain

Spatial Maps

Neurons response to particular odors organized into these

Each odor represented by activity of large population of neuron

Olfactory population coding

the way the brain identifies and processes smells using the combined, distributed activity patterns of a large group of neurons rather than relying on a single cell

Sweetness

preferred, most of these foods are safe to eat

Salty

preferred, is an important ion for our bodies and so we crave it.

Umami

preferred, monosodium glutamate (glutamate) is an amino acid and is associated with protein which is an important nutrient

Sour

less preferred, when food spoils, it can become acidic because of bacterial activity, so we tend to avoid it.

Bitter

less preferred, many poisonous plants produce these substances, which prevent them from being eaten and thus we have learned to generally avoid these things

ions like K+ and Mg2+, quinine, and caffeine

Fat

preferred, also provides a lot of calories = nutrients

Flavor

a different combination of taste receptors

also includes: texture, temperature, and sensation

Umami receptors

TAS1R1 + TAS1R3 receptors

Detect amino acids

Anosmia

inability to smell

Humans are

weak smellers compared to many animals

due to small surface area of olfactory epithelium

have fewer than 400 odorant receptor genes

Odorant receptor protein families

G-protein-coupled receptors: Golf and Cyclic AMP (cAMP)

V1R, V2r, and TAAR

Labeled-lined hypothesis

individual taste receptor cells for stimuli, but in reality, most neurons are broadly tuned