Olfaction and the Human Olfactory System

Olfaction: The Oldest Evolutionary System

• Chemotaxis: Bacteria sense their environment by detecting chemicals like glucose via chemoreceptors initiating an intracellular cascade. Plants similarly recognize other plants, likely via similar chemical recognition.

• Human Olfactory System: Odorants in the air are filtered by nasal hairs and cavities before reaching sensory neurons in the olfactory epithelium. Olfactory sensory neurons are short-lived, and replaced by basal cells. Olfactory receptor neurons within this epithelium have cilia that detect odorants. Each neuron expresses only one type of odorant receptor, and neurons with the same receptor type are located in one zone within the epithelium.

• Olfactory System Organization: Olfactory information is processed through several brain regions including the amygdala (emotions), entorhinal cortex (contextual/spatial memories), orbitofrontal cortex (behavior/planning), hypothalamus (hormones), and pyriform cortex. The thalamus is a secondary structure in the pathway.

• Sensitivity and Differences: The human olfactory system can detect trace amounts of odorants, and distinguishing intensity, odorants and pleasantness from slight differences in chemical structure. Rats have more olfactory bulb proportion but fewer receptors than humans

• Olfactory Epithelium and Anosmia: Olfactory cells are a component of epithelium which regenerate monthly. Anosmia is the loss of smell and can arise from damage to the epithelium like in COVID-19 cases.

• Olfactory Receptors: Receptors are located on cilia on olfactory cells, not the cell body, and generate membrane currents. They're G protein-coupled receptors, not ion channels. They possess 7 transmembrane domains and their activation triggers intracellular cascades. Approximately 1000 genes are dedicated to olfactory receptors, representing a significant portion of the human genome.

• Olfactory Signalling Cascade: Odorants bind to specific receptors on olfactory neurons. This binding triggers a cascade including G-protein activation, cAMP production, which opens ion channels, allowing Na+ and Ca2+ to flow into the neuron and depolarizing it which releases neurotransmitters.

• Olfactory Bulb: Olfactory neurons project axons to the glomeruli in the olfactory bulb. Glomeruli gather input from thousands of neurons and process the signals further. Mitral and tufted cell dendrites within the glomeruli process the signals and granule cells, which are inhibitory cells, inhibit or refine the signals they receive.

• Olfactory Information Processing: The processing of olfactory information extends to regions in the cerebral cortex including the pyriform cortex, which does not show a topographic organization but receives input from a single glomerulus.

Taste: Influenced by Smell

• Taste Buds: Taste buds are densely packed structures located on different tongue papillae (circumvallate, foliate, and fungiform). Taste receptors within the taste buds are epithelial cells, not neurons, and aren't localized to different tongue areas/receptors

• Taste Types: Taste sensations include: sour (H+), bitter (potentially poisonous), salt (NaCl), sweet (energy, sugar), and umami (protein, amino acids).

• Sensory Transduction in Taste Cells:

  • Salt and Sour: Transduction involves ion channels. Salt relies on amiloride-sensitive sodium channels, while sour utilizes H+ sensitive cation channels.
  • Sweet, Umami, and Bitter: These tastes utilize G-protein coupled receptors (GPCRs). Sweet/Umami are GPCRs, and bitter involves TR2 receptors. They trigger intracellular cascades culminating in the depolarization of the gustatory cells and neurotransmitter release (like serotonin).

• Chemosensation and Trigeminal Nerve: Free nerve endings in the trigeminal nerve sense sensations of "heat/cold" and detect chemicals like capsaicin (heat) and menthol (cold) via TRP receptors.

• Transport to the Brain: Taste information is carried to the brain via cranial nerves (facial, glossopharyngeal, vagus and trigeminal). The nucleus of the solitary tract combines this signal with visceral information before information eventually passes through the thalamus and then ultimately to the gustatory cortex, which is situated within the insula.

• Gustatory Cortex: Topographic maps exist in the gustatory cortex, representing different taste sensations. Techniques like optogenetics allow scientists to manipulate sensory experiences and perception.