17.5 Somatosensation, Pain & Chemoreception

17.5 Somatosensation, Pain & Chemoreception

Somatosensation & The Neurobiology of Pain

Somatosensation encompasses touch, vibration, temperature, and pain across somatic tissues.

Cutaneous Mechanoreceptors

• Meissner Corpuscles: Located in dermal papillae of glabrous skin; rapidly adapting; small receptive fields; detect light flutter, slip, and low-frequency vibration (fine touch).
• Pacinian Corpuscles: Located deep in dermis and subcutaneous tissue; rapidly adapting; large receptive fields with concentric fluid lamellae; detect high-frequency vibration and transient pressure.
• Merkel Discs: Located in basal epidermis; slowly adapting; small receptive fields; detect static pressure, edges, texture, and form.
• Ruffini Endings: Located deep in dermis; slowly adapting; large receptive fields; respond to skin stretch and joint rotation.

Pain Transmission: $A\delta$ versus $C$ Fibers

Nociceptive signals travel via two distinct primary afferent populations:

1. $A\delta$ (A-delta) Fibers: Medium diameter, lightly myelinated fibers that conduct rapidly (5 to 30 m/s). Transmit first pain—sharp, localized, acute, pricking pain and cold thermal sensations.
2. $C$ Fibers: Small diameter, unmyelinated fibers that conduct very slowly (0.5 to 2.0 m/s). Transmit second pain—dull, throbbing, diffuse, burning, chronic aching pain, warmth, and pruritus (itch).

The Gate Control Theory of Pain (Melzack & Wall)

Why does rubbing an injured elbow alleviate the sensation of pain? Melzack and Wall proposed that a neural "gate" in the substantia gelatinosa (lamina II of the spinal dorsal horn) regulates nociceptive flow to ascending projection neurons:

• Nociceptive $C$ and $A\delta$ fibers stimulate ascending projection neurons (second-order spinothalamic neurons) while simultaneously inhibiting local inhibitory interneurons, keeping the gate open.
• Non-nociceptive tactile mechanoreceptors ($A\beta$ fibers) send collateral branches into the dorsal horn that excite the inhibitory interneurons.
• When activated by rubbing, the inhibitory interneuron fires, releasing GABA and enkephalins that presynaptically inhibit the terminals of the nociceptive fibers and postsynaptically inhibit the projection neuron. This closes the gate, suppressing pain transmission before it ascends to the brain.

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Chemoreception: Olfaction vs. Gustation

Olfaction (Smell): Direct Cortical Access

Olfaction is phylogenetically ancient and exhibits a structural organization unique among sensory systems:

1. Olfactory Receptor Neurons (ORNs): Bipolar neuroepithelial cells residing in the olfactory epithelium in the superior nasal cavity. Their apical dendrites terminate in cilia bathed in mucus, expressing G-protein coupled olfactory receptors (GPCRs, coupled to $G_{\text{olf}}$). Odorant binding activates adenylyl cyclase, generating cAMP, which opens cyclic nucleotide-gated cation channels ($Na^+/Ca^{2+}$ influx), driving calcium-activated chloride channels to efflux $Cl^-$ and depolarize the cell.
2. Cribriform Plate Passage: Axons of ORNs form small bundles (filia olfactoria, Cranial Nerve I) that pass through perforations in the cribriform plate of the ethmoid bone into the anterior cranial fossa.
3. Olfactory Bulb Glomeruli: ORN axons synapse in spherical neuropil structures called glomeruli within the olfactory bulb. All ORNs expressing the same specific receptor gene converge onto the same 2 to 4 glomeruli, creating an organized spatial odor map. Glomeruli synapse onto secondary projection neurons: mitral cells and tufted cells.
4. THE THALAMIC BYPASS: Mitral and tufted cell axons form the lateral olfactory tract and project directly to the primary olfactory cortex (piriform cortex) and limbic structures (amygdala, entorhinal cortex) WITHOUT first synapsing in the thalamus. This unique direct anatomical connection to the limbic system explains why scents evoke immediate, visceral, and emotional autobiographical memories (the Proustian effect).

Gustation (Taste): The Classical Pathway

Gustation recognizes five basic taste modalities:

• Sweet, Umami, Bitter: Transduced via GPCRs coupled to the specialized G-protein gustducin. Sweet (T1R2 + T1R3 heterodimer), Umami (T1R1 + T1R3 heterodimer), Bitter (~25 distinct T2R monomeric receptors). Activation stimulates phospholipase C (PLC$\beta2$), generating $IP_3$ and DAG, releasing internal $Ca^{2+}$ and opening TRPM5 cation channels, releasing ATP as a non-vesicular neurotransmitter.
• Salty & Sour: Transduced directly through ion channels. Salty tastants ($Na^+$) enter epithelial sodium channels (ENaC), directly depolarizing the cell. Sour tastants ($H^+$ ions) enter via proton-selective channels (OTOP1) and block leak potassium channels, causing depolarization.
• Innervation: Taste buds across the tongue and pharynx are innervated by three cranial nerves:
  • Facial Nerve (CN VII): Anterior 2/3 of the tongue (via chorda tympani).
  • Glossopharyngeal Nerve (CN IX): Posterior 1/3 of the tongue.
  • Vagus Nerve (CN X): Epiglottis and pharynx.
• Pathway: Axons converge on the solitary nucleus (nucleus tractus solitarii) in the medulla $\rightarrow$ ascend to the Ventral Posteromedial (VPM) nucleus of the thalamus $\rightarrow$ project to the Primary Gustatory Cortex in the anterior insula and frontal operculum.

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Comparative Profiles of the Major Sensory Pathways