C12- Smell

Olfactory Mucosa and Odorants

• Odorants, the molecules responsible for smells, interact with chemoreceptors, extensions of the brain.
• Olfactory cells, bipolar nerve cells originally from the CNS, number approximately 100 million in the olfactory epithelium.
• Olfactory cilia (hairs) project into mucus lining the nasal cavity, crucial for odor detection.
• Bowman glands secrete mucus, aiding odor molecule binding.
• Dendrites are present in olfactory cells, as receptors for odorants are neurons.
• Axons of olfactory neurons pass through the cribriform plate to glomeruli in the olfactory bulb.
• Mitral cells, output of the glomeruli, transmit signals in the olfactory tract.
• Air turbulence, created by nasal conchae, increases odorant contact with the olfactory mucosa.
• Mucus, secreted by submucosal and goblet cells, increases odorant binding and protects the lower airways.
• Two types of mucus exist: watery (superficial) and viscous (covering cilia).

Olfactory Receptor Neurons (ORNs)

• ORNs, bipolar cells defining the neural portion of the olfactory epithelium, have a single dendritic process.
• Microvilli, also known as olfactory cilia on the apical surface, extend into mucus, containing odorant receptors.
• ORNs are directly exposed to inhaled air, putting them at risk from pollutants, allergens, and microorganisms.
• Regeneration of ORNs is crucial, supported by neural stem cells in the basal cells of the olfactory epithelium.
• Odorant receptors are structured in four rings linked by gap junctions.
• Multiple neurons responding to stimulation are needed for a relevant signal transmission.
• Receptor action occurs at the cilia level, when odorants bind, generating an inward current.

Olfactory Sense

• Olfaction relies on lock-and-key interactions between odorants and receptor molecules.
• Unlike taste, olfaction receptors lack channels; they react based on the shape of odorant molecules.
• Receptors show selectivity for different odorants, depending on their structure.
• The shape of the nose plays a role in olfaction through its impact on air turbulence.
• Odorants bind to receptor molecules in cilia membranes, activating G-protein coupled receptors.
• This activation amplifies the signal by increasing cAMP production, which modulates ion channels.
• Receptor proteins have 7 transmembrane domains, a variable cell surface region, and a cytoplasmic tail for G-protein interaction.
• Over 1000 genes code for odorant receptors, each detecting a unique set of odorants.
• Stimulation intensity of receptors, varies from strong to mild to no response.
• Each receptor is selective to a particular set of substrates.
• Receptors are distributed throughout the nasal mucosa.
• Axons of receptors project to olfactory bulbs.
• In the olfactory bulb, receptor families cluster, forming a matrix.
• Mitral cells transmit the stimuli to the olfactory tract to different brain regions.
• Olfactory bulb activity can be measured optically.
• Receptor sensitivity shifts with increasing odorant concentrations.

Olfactory Bulb Organization and Brain Regions

• The olfactory bulb is organized into zones sensitive to different odorant families.
• Mitral cells project to various brain areas, including the contralateral olfactory bulb, olfactory tubercle, thalamus, orbito-frontal cortex, frontal cortex, enthorinal and pyriform cortex, amygdala, and hypothalamus.

Olfactory Exploration and Memory

• Animals use olfaction for environmental exploration, food searching, and danger detection.
• The frontal cortex is linked to decision-making in primates and humans.
• Olfactory memory, including recalling scents, is connected to the hippocampus.
• Sensitivity to smell can decline with age.

Smell and Taste Deficits

• Covid-19 can cause taste and smell loss (ageusia and anosmia).
• Diminished abilities are termed hypogeusia and hyposmia.
• Increased sensitivity is termed hypergeusia and hyperosmia.
• Pregnancy can alter perceptions of some smells.
• Distorted sensory perceptions are called dysgeusia and dysosmia.
• Difficulty recognizing smells or tastes is called agnosia.
• Tumors in the insular lobe can cause seizures, preceded by unusual smell perception and headache.