Sensations of smell and taste
The Sensation of Smell
Olfactory Epithelium and Olfactory Neurons
Contains the olfactory sensory neurons.
Located on the dorsal and posterior nasal cavity.
In quiet breathing, only a small fraction of air reaches the olfactory epithelium.
Active sniffing increases the fraction of air that reaches it.
The surface of the nasal cavity has conchae, which are highly vascular organs with erectile tissue that can engorge with blood, reducing airflow to the olfactory epithelium during cold weather.
Odorant Receptors and Transduction Mechanism
Olfactory sensory neurons possess cilia where specific odorant receptors are located.
Cilia are embedded in a layer of lipid-rich mucus secreted by the Bowman’s gland, making lipid solubility a requirement for substances to produce an odor.
The receptors open cyclic nucleotide-gated channels, allowing Na$^+$ and Ca$^{2+}$ influx, leading to depolarization (receptor potential).
Continuous exposure to an odor leads to adaptation, where the receptors become desensitized.
Coding of Different Odors
The olfactory sensory neurons express one and only one type of odorant receptor, which belong to a superfamily with ~400 functional members in humans.
Amino acid changes (polymorphisms) in a single receptor contribute to individual differences in odor perception.
Odorant receptors are G-protein coupled receptors with 7 transmembrane domains, allowing them to recognize different chemical features of odorants.
The Combinatorial Code Hypothesis states that the odorant molecule is encoded by a combination of activated odorant receptors rather than by a single receptor.
Different odors activate neurons in varied patterns at the level of the glomeruli in the olfactory bulb.
Central Processing of Smell
Olfactory information is transmitted to primary olfactory cortical areas for emotions related to scent and to secondary areas for conscious odor discrimination.
The olfactory bulb's mitral cells convey signals to multiple cortical and subcortical areas, enabling both recognition and emotional responses.
Common Disorders of Smell Sensation
Anosmia: Absence of sense of smell
Hyposmia: Diminished olfactory sensitivity
Dysosmia: Distorted sense of smell
Common causes include the absence or disrupted function of odorant receptors, increasing olfactory threshold with age, and neurodegenerative diseases like Parkinson's and Alzheimer’s.
The Sensation of Taste
Taste Papillae and Taste Buds
Taste buds are located in three types of papillae distributed across the human tongue.
The anterior two-thirds are innervated by the chorda tympani nerve, and the posterior one-third is innervated by the glossopharyngeal nerve.
Taste buds are also found in the oral mucosa of the palate and epiglottis.
The Taste Buds and Taste Cells
Each taste bud contains 50-150 taste cells with microvilli in contact with dissolved tastants.
Saliva is crucial for delivering food molecules to the taste cells.
Like olfactory neurons, taste cells have a limited lifespan (9-10 days) and are replaced by basal stem cells.
Types and Meanings of Different Tastes
Sweetness: Indicates sources of metabolic energy, e.g., sugars.
Bitterness: Often signals poison; should be avoided.
Sourness: Relates to acidity; signals that fruit may not be ripe or indicates bacterial activity.
Saltiness: Primarily indicates NaCl, an important electrolyte.
Umami: Indicates the presence of glutamate, which signals protein sources.
There are four basic taste modalities: sweet, salty, bitter, sour; umami and fatty acids could be additional types.
Specific regions of the tongue show varying sensitivity to different taste modalities, although not absolute sensitivity.
Diverse Transduction Mechanisms of Taste
Unlike olfaction, the mechanisms of chemo-electric transduction for taste stimuli vary:
Saltiness: Na$^+$ passes through Na-selective channels directly.
Sourness: H$^+$ either enters directly through Na channels or blocks K$^+$ channels.
Bitterness: Some bitter molecules block K$^+$ channels while others use G-protein coupled receptors to generate intracellular signaling affecting membrane excitability.
Sweetness: Certain GPCRs bind specific sweet molecules like glucose, leading to their effects.
Central Gustatory Pathways
Gustatory signals are projected ipsilaterally to the secondary gustatory cortical area in the orbitofrontal cortex.
Gustatory and olfactory signals converge in the orbitofrontal cortex, which may help integrate sensory perceptions into the final taste sensation.
Common Disorders of Taste Sensation
Ageusia: Absence of taste
Hypogeusia: Diminished taste sensitivity
Dysgeusia: Distorted sense of taste.
Facial nerve palsy (e.g., Bell’s palsy) can impair taste sensation.
The perception of taste is actually the result of a combination of taste, olfaction, temperature, and somatosensation, including pain and texture.
Diminished smell during colds or flu significantly reduces the sensation of taste.
Recognition of Contributions in Olfactory Science
Nobel Prize in Physiology or Medicine 2004
Awarded to Linda Buck and Richard Axel for their discoveries of odorant receptors and the organization of the olfactory system, regarded as one of the last major uncollected prizes in sensory science.