2.4 Taste, Smell & Somatosensation

2.4 Taste, Smell & Somatosensation

MCAT Foundational Concept: Chemical molecules and cutaneous mechanical/thermal forces trigger specialized chemoreceptors and mechanoreceptors, routing through distinct cranial and spinal pathways to primary sensory cortices and limbic networks.

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Gustation (Taste)

Gustation is the chemical sense detecting dissolved molecules (tastants) on the tongue.

Organization of Lingual Papillae & Taste Buds

Taste buds are specialized neuroepithelial structures located within three types of lingual papillae:

• Fungiform Papillae: Mushroom-shaped, located primarily on the anterior two-thirds of the tongue.
• Foliate Papillae: Leaf-like folds located on the posterolateral borders of the tongue.
• Circumvallate Papillae: Large dome-shaped papillae arranged in an inverted V-shape on the posterior one-third of the tongue.
• (Note: Filiform papillae cover the tongue surface but contain zero taste buds; they provide mechanical friction).

The 5 Primary Taste Modalities & Transduction

[Dissolved Tastants]
       │
       ├─► Salty (Na+)  ──► ENaC Channel Entry ───────────────┐
       ├─► Sour (H+)   ──► OTOP1 Channel Entry / K+ Block ───┼─► Depolarization ──► Ca2+ Influx ──► Transmitter Release
       ├─► Sweet       ──► GPCR (T1R2 + T1R3) ───────────────┤
       ├─► Umami       ──► GPCR (T1R1 + T1R3) ───────────────┤
       └─► Bitter      ──► GPCR (T2R Family) ────────────────┘

Gustatory Neural Pathway

1. Anterior 2/3 of Tongue: Innervated by Facial Nerve (CN VII, chorda tympani branch).
2. Posterior 1/3 of Tongue: Innervated by Glossopharyngeal Nerve (CN IX).
3. Epiglottis & Pharynx: Innervated by Vagus Nerve (CN X).
4. Primary afferents synapse in the Nucleus of the Solitary Tract (NST) in the medulla $\rightarrow$ project to the Ventral Posteromedial (VPM) nucleus of the Thalamus $\rightarrow$ Primary Gustatory Cortex (insula and frontal operculum).

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Olfaction (Smell)

Olfaction is the chemical sense detecting volatile airborne odorant molecules.

Olfactory Epithelium & Neurogenesis

Housed in the superior nasal cavity. Olfactory Receptor Neurons (ORNs) are true bipolar neurons.

• High-Yield MCAT Fact: ORNs undergo continuous neurogenesis throughout adulthood, regenerating from basal stem cells every 4-8 weeks—a rare exception to the general rule that CNS neurons do not divide.

Molecular Phototransduction & Transduction Cascade

1. Airborne odorants bind specific GPCRs on non-motile cilia of ORNs.
2. Receptor activates olfactory-specific G-protein $\text{G}_{\text{olf}}$.
3. $\text{G}_{\text{olf}}$ activates Adenylyl Cyclase III $\rightarrow$ converts ATP to cAMP.
4. cAMP opens cAMP-gated cation channels $\rightarrow$ influx of $\text{Na}^+$ and $\text{Ca}^{2+}$.
5. Elevated intracellular $\text{Ca}^{2+}$ opens $\text{Ca}^{2+}$-activated $\text{Cl}^-$ channels.
6. Ion Inversion: ORNs maintain an unusually high intracellular $\text{Cl}^-$ concentration; opening $\text{Cl}^-$ channels causes $\text{Cl}^-$ EFFLUX, producing robust membrane depolarization and action potential firing.

Central Olfactory Pathway: The Thalamic Bypass

[Airborne Odorant] ──► [ORN Cilia GPCR (Golf)] ──► [cAMP Rise & Cl- Efflux Depolarization]
                                                              │
                                                              ▼
[Olfactory Cortex (Pyriform)] ◄── [Olfactory Tract] ◄── [Mitral / Tufted Cells in Glomeruli]
         │
         ├─► [Amygdala] (Emotional Odor Memory)
         └─► [Hippocampus] (Contextual Memory)

• CRITICAL MCAT RULE: Olfaction is the ONLY primary sensory modality that bypasses the thalamus prior to reaching primary sensory cortex!
• ORN axons pass through the cribriform plate of the ethmoid bone to synapse in Glomeruli within the Olfactory Bulb.
• All ORNs expressing the exact same odorant receptor gene converge onto the same specific glomerulus, synapsing with Mitral and Tufted cells.
• Mitral/tufted axons form the Olfactory Tract, projecting directly to the Primary Olfactory Cortex (Pyriform Cortex) and limbic structures (Amygdala and Hippocampus). This direct connection explains why odors trigger vivid emotional memories.

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Somatosensation

Somatosensation encompasses four modalities: tactile (touch), thermal (temperature), nociception (pain), and proprioception (body position).

Cutaneous Mechanoreceptors

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Dual Ascending Somatosensory Pathways

Somatosensory information ascends to the brain along two distinct pathways:

                                    ┌──► Ipsilateral Dorsal Columns ──► Medulla Decussation ──► VPL ──► S1 (DCML)
[Peripheral Primary Afferents] ─────┤
                                    └──► Immediate Spinal Decussation ──► Anterolateral STT ──► VPL ──► S1 (STT)

Clinical MCAT Application: Brown-Séquard Syndrome

A spinal cord hemisection (e.g., transection of the right half of the spinal cord at T10) results in:

1. Ipsilateral loss of DCML sensory modalities (touch, vibration, proprioception) below T10, because DCML fibers ascend uncrossed until the medulla.
2. Contralateral loss of STT sensory modalities (pain and temperature) starting 1-2 segments below T10, because STT fibers decussate immediately upon entering the spinal cord.

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Pain Mechanisms & Gate Control Theory

Nociceptive Fiber Types

• $A\delta$ Fibers: Thinly myelinated, medium diameter ($2-5\text{ }\mu\text{m}$), fast conduction ($5-30\text{ m/s}$). Transmit sharp, acute, localized "first pain."
• $C$ Fibers: Unmyelinated, small diameter ($0.2-1.5\text{ }\mu\text{m}$), slow conduction ($0.5-2\text{ m/s}$). Transmit dull, aching, burning, diffuse "second pain."

Gate Control Theory of Pain (Melzack & Wall)

Gate control theory explains how non-painful tactile input modulates pain perception:

• The spinal cord dorsal horn contains inhibitory interneurons that synapse onto pain projection neurons.
• Activation of non-nociceptive $A\beta$ fibers (e.g., rubbing your skin after bumping your shin) sends collateral branches that excite inhibitory interneurons.
• Inhibitory interneurons release GABA/enkephalin to presynaptically inhibit incoming $A\delta$ and $C$ nociceptive afferents, "closing the gate" to pain transmission.

[A-beta Fiber (Touch)]  ──► (+) ──► [Inhibitory Interneuron] ──► (-) ──┐
                                                                         ├─► [Spinal Pain Projection Neuron] ──► Pain to Brain
[C / A-delta (Pain)]    ─────────────────────────────────────────► (+) ──┘

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Proprioception vs. Kinesthesia

• Proprioception: Subconscious and conscious perception of body position, posture, and spatial orientation. Key receptors include:
  • Muscle Spindles: Stretch receptors embedded in skeletal muscle; monitor muscle length and rate of stretch via Ia and II afferents.
  • Golgi Tendon Organs (GTOs): Tension receptors located at muscle-tendon junctions; monitor muscle contraction force/tension via Ib afferents.
• Kinesthesia: Conscious awareness of body movement and motion trajectory.