Lecture 15

Gustation and Olfaction

• Gustation (taste) and olfaction (smell) are chemical senses.

• Both systems involve receptors that detect specific molecules.

The Olfactory System

• The olfactory system begins in the olfactory epithelium, located in the nasal cavity.

• Olfactory sensory neurons (OSNs) are bipolar neurons within the olfactory epithelium.

• Olfactory Epithelium:

  • Contains olfactory sensory neurons, supporting cells, and basal cells.

  • Olfactory Sensory Neurons: Have cilia that detect odors. These neurons' axons project to the olfactory bulb through the cribriform plate.

  • Supporting Cells: Provide metabolic and physical support to the OSNs.

  • Basal Cells: Act as stem cells, capable of differentiating into new olfactory sensory neurons, allowing for neuron replacement.

• Each olfactory neuron expresses only one type of olfactory receptor.

• Odorant Receptors:

  • Mammals have a large family of odorant receptors.

  • These receptors use a standard G protein-coupled receptor (GPCR) transduction mechanism, leading to the activation of cation channels.

  • Different combinations of receptors encode different odorants.

• Olfactory Transduction Pathway:

  • Odorant molecules bind to specific receptors on the cilia of olfactory sensory neurons.

  • This activates a G protein, which in turn activates adenylyl cyclase.

  • Adenylyl cyclase increases the level of cAMP (cyclic adenosine monophosphate), which opens cation channels, allowing influx of $Na^+$ and $Ca^{2+}$ ions.

  • The influx of cations depolarizes the neuron, generating an action potential.

• Olfactory Bulb:

  • Axons of olfactory sensory neurons converge on glomeruli in the olfactory bulb.

  • Sensory inputs in the olfactory bulb are arranged by receptor type.

  • Neurons expressing the same odorant receptor converge on a few glomeruli.

  • One odorant can activate many glomeruli.

  • The olfactory bulb has a precise map of odorant receptor inputs.

  • Mitral and tufted cells within the olfactory bulb receive input from the glomeruli and transmit information to the olfactory cortex.

• Olfactory Cortex:

  • The olfactory cortex includes several regions such as the piriform cortex, anterior olfactory nucleus, olfactory tubercle, amygdala, entorhinal cortex, and others.

  • The olfactory bulb transmits information to the olfactory cortex via mitral and tufted cells.

• Olfaction involves a direct connection to the amygdala and orbitofrontal cortex (OFC), which contributes to the activation of feelings and emotions based on smell.

• Olfactory bulb → Piriform cortex → Orbitofrontal cortex

The Gustatory System

• The gustatory system controls the sense of taste.

• Taste has five submodalities:

  • Sweet: To ensure adequate intake of carbohydrates for energy.

  • Sour: To prevent intake of toxic substances and detect ripeness.

  • Salty: To ensure adequate intake of salt for osmoregulation.

  • Bitter: To prevent intake of poisonous substances.

  • Umami: To ensure adequate intake of proteins for growth and maintenance.

• Taste buds are clustered in papillae on the tongue.

• Papillae Types:

  • Circumvallate papillae: Located at the back of the tongue.

  • Foliate papillae: Located on the lateral sides of the tongue.

  • Fungiform papillae: Located all over the tongue.

• Each taste modality is detected by distinct sensory receptors and cells.

• Sensory Transduction in Taste Cells:

  • Salty: Detected by $Na^+$ channels (ENaC – epithelial sodium channel).

  • Sour: Detected by $H^+$ channels (Otop1).

  • Bitter: Detected by T2Rs (G protein-coupled receptors).

  • Sweet: Detected by T1R2 + T1R3 (G protein-coupled receptors).

  • Umami: Detected by T1R1 + T1R3 (G protein-coupled receptors).

• Gustatory information is relayed from the periphery to the gustatory cortex.

• Gustatory Pathway:

  • Tongue → Cranial nerves (VII, IX, X) → Brainstem (Nucleus of the solitary tract) → Thalamus (Ventral posterior medial nucleus) → Gustatory cortex (anterior insula-frontal operculum).

• Cranial nerves involved:

  • Chorda tympani nerve (VII) - Facial Nerve.

  • Glossopharyngeal nerve (IX).

  • Vagus nerve (X).

Perception of Flavor

• Perception of flavor depends on gustatory, olfactory, and somatosensory inputs.

• Inputs Contributing to Flavor:

  • Taste: Sweet, umami, salt, sour, bitter.

  • Smell: Odor patterns.

  • Somatosensory: Temperature, pain/pungency, touch/texture/creaminess, astringency.

  • Vision: Color and shape.

  • Gut: Autonomic and metabolic properties.

• Flavor perception involves neocortex (conscious circuits), hippocampus, olfactory/limbic system (limbic subconscious memory systems), and amygdala (emotion/motivation/craving circuits).

• Taste is a sensation, a change in the environment. Flavor is a perception and is subjective.