Taste and somatosensory

Which cranial nerves carry taste information and what functions does taste serve?

Taste via CN VII, IX, X. Functions: pleasure/aversion, food-seeking, satiety.

Where are taste buds located and how are they organized?

In papillae; each papilla has 1–hundreds of taste buds; each bud houses 50–150 TRCs with microvilli projecting into taste pore.

What is CD36 and what taste modality does it support?

Lipid/fat taste receptor; enriched in circumvallate & foliate papillae; detects long-chain fatty acids.

What are the threshold concentrations for sour, salty, sweet, bitter?

Sour 0.0009M; salty 0.01M; sweet 0.01M; bitter 0.000008M.

What health issues are linked to erythritol?

Associated with higher platelet reactivity, thrombosis, increased MACE risk.

How are salty and sour tastes transduced?

Salty: Na+ via ENaC. Sour: H+ blocks K+ channels or activates PKD2L1, causing depolarization.

Which GPCRs detect sweet, umami, and bitter taste?

Sweet T1R2+T1R3; Umami T1R1+T1R3; Bitter ~30 T2Rs.

What is the GPCR signaling cascade for sweet/umami/bitter?

Tastant→GPCR→Gustducin→PLCβ2→IP3→Ca2+ release→TRPM5→Na+ influx→CALHM1 releases ATP.

What are the 3 types of taste cells?

Type I glial-like; Type II detect sweet/umami/bitter; Type III detect sour/salty.

What is labeled-line vs population coding in taste?

Labeled-line = specific pathway for each taste; population coding = pattern-based; taste uses both.

What is the central taste pathway?

TRCs→VII/IX/X→solitary nucleus→VPM thalamus→insular cortex.

What cells are in the olfactory epithelium?

ORNs (true neurons), supporting cells, basal stem cells.

How does olfactory transduction occur?

Odor→GPCR→Golf→↑cAMP→cation channels→Ca2+ opens Cl− channels→depolarization→AP.

How many OR genes per neuron?

Each ORN expresses exactly one OR gene.

What is a glomerulus?

Convergence site where ORNs with same receptor synapse onto mitral/tufted cells.

How are odors encoded?

Population coding of spatially distributed glomerular activity + temporal firing patterns.

What brain regions receive olfactory input?

Piriform cortex, amygdala, then orbitofrontal cortex.

How does COVID-19 cause anosmia?

Infects ACE2/TMPRSS2+ support cells → inflammation → reduced OR protein expression.

What are the 4 major somatic sensations?

Touch, pain, temperature, proprioception.

What are the 4 main receptor types in somatosensation?

Mechanoreceptors, nociceptors, thermoreceptors, proprioceptors.

What is a generator potential?

Graded depolarization created by receptor stimulus; may trigger AP.

What are Merkel, Meissner, Pacinian, and Ruffini receptors?

Merkel: pressure/texture; Meissner: 30–50Hz vibration; Pacinian: 250–350Hz vibration; Ruffini: stretch.

Why are fingertips highly sensitive?

High receptor density, small receptive fields, large cortical representation.

What is a dermatome?

Skin area supplied by a single spinal segment.

Describe the dorsal column–medial lemniscal pathway.

Touch/proprioception: DRG→dorsal column→nuclei→cross→medial lemniscus→VPL→S1.

How is face touch transmitted?

CN V→trigeminal nucleus→VPM→S1.

What is cortical somatotopy?

Body surface mapped on S1; proportional to receptor density, not body size.

What is cortical map plasticity?

Experience/injury reshapes S1; basis of phantom limb.

What are nociceptors and what do they detect?

Free nerve endings detecting mechanical, thermal, chemical damage.

What causes hyperalgesia?

Bradykinin, prostaglandins, Substance P (histamine release).

What is first vs second pain?

First = sharp Aδ; second = dull C fibers.

What is referred pain?

Visceral + somatic afferents converge on same dorsal horn neurons.

Describe the spinothalamic pathway.

Pain/temperature: DRG→dorsal horn→immediate crossing→ascend→VPL→S1.

What is gate theory of pain?

Aβ touch fibers inhibit pain transmission in dorsal horn.

How does descending pain modulation work?

PAG→raphe→endorphins/enkephalins inhibit pain neurons and glutamate release.

What receptor detects heat and capsaicin?

TRPV1, activated by >43°C and chili peppers.

How do thermoreceptors work?

Cold (10–35°C) and warm (30–45°C) receptors; pathway parallels spinothalamic tract.