CBNS 120 Sensory Transduction II: Chemoreceptors, Thermoreceptors & Nociceptors

The three cell types in the olfactory epithelium are:

Olfactory sensory neurons, supporting cells, and basal cells (which regenerate olfactory neurons).

Olfactory transduction occurs in:

The cilia of the olfactory sensory neuron.

Olfactory transduction pathway:

Odorant binds GPCR → activates G(olf) → stimulates adenylyl cyclase III → increases cAMP → opens cAMP-gated cation channels (Na+/Ca2+ influx) → depolarization.

In olfactory transduction, Ca2+ also opens:

Cl- channels (Cl- exits because intracellular Cl- is high), causing further depolarization.

Dogs have a more sensitive sense of smell than humans because:

Up to 300 million olfactory neurons (vs. 6 million), and 40x larger olfactory brain area.

The five basic taste modalities are:

Salt, sour, sweet, bitter, and umami (savory).

The three types of papillae containing taste buds are:

Fungiform (anterior 2/3), foliate (sides), and circumvallate (back of tongue).

Filiform papillae do NOT contain:

Taste receptors.

Salt taste transduction involves:

Amiloride-sensitive ENaC (epithelial sodium channel) → Na+ enters and depolarizes the cell.

Sour taste transduction involves:

H+ entering via the TRP channel PKD2L1 (H+ may also block K+ channels, causing depolarization).

Sweet taste transduction involves:

T1R2-T1R3 heterodimeric GPCRs → activate gustducin → increases cAMP → closes K+ channels → depolarization.

Most bitter taste transduction involves:

T2R GPCRs (detect thousands of bitter, often toxic compounds).

Umami (savory) taste transduction involves:

T1R1-T1R3 heterodimeric GPCRs or metabotropic glutamate receptors (mGluR1/mGluR4).

Hot chili flavor (capsaicin) is detected by:

TRPV1 (heat and capsaicin receptor).

Carbonation taste is a combination of:

Acid (H+) stimulation of sour receptors and tactile stimulation by bursting bubbles.

Cold receptors in mammalian skin are activated by:

Temperatures 1-20°C below normal skin temperature (~34°C). They are innervated by Aδ and C fibers.

Warm receptors in mammalian skin are activated by:

Temperatures from 34-45°C. They are innervated exclusively by C fibers.

"Paradoxical cold" sensation (burning cold) is caused by:

Activation of cold receptors by high (burning) temperatures (Dodd & Zotterman, 1952).

Menthol activates:

Cold receptors (CMR1 = TRPM8, a cold- and menthol-sensitive TRP channel).

TRPV1 is activated by:

Noxious heat (>43°C) and capsaicin.

TRPA1 is activated by:

Extreme cold. Co-expression with TRPV1 may explain "paradoxical cold."

Pit organs in snakes (e.g., rattlesnakes) detect:

Infrared radiation (heat) via trigeminal nerve endings in the pit membrane.

Vampire bats and Melanophila beetles (fire beetles) also have:

Infrared sensors (for detecting blood-rich skin or forest fires).

Nociceptors are:

Free nerve endings (small myelinated Aδ fibers or unmyelinated C fibers) that detect painful stimuli.

Aδ fibers (nociceptors) mediate:

Sharp, prickling, well-localized pain (conduction velocity ~20 m/s).

C fibers (nociceptors) mediate:

Dull ache, diffuse pain (conduction velocity 0.3-3.0 m/s). Most are polymodal.

Polymodal nociceptors respond to:

Intense mechanical, thermal (>43°C), and chemical irritants.

Chemicals that activate or sensitize nociceptors include:

Histamine, bradykinin, prostaglandin E2 (PGE2), and leukotriene C4 (LTC4).

Allodynia is:

Pain produced by a normally non-noxious stimulus (e.g., light touch).

Stinging nettle (Urtica) contains:

Histamine, acetylcholine, and 5-HT (serotonin) that activate nociceptors.

Gympie-gympie (Australian stinging tree) produces:

Gympietide peptides that block inactivation of NaV1.7 channels, causing extreme pain lasting months to years.