153. Physiology of Smell – Quiz

📌 Professor’s Notes: Quick Revision

High-yield revision notes for rapid review before university examinations.

• Olfaction is the sense of smell and is closely related to taste, appetite, memory, emotion and autonomic responses. It also helps detect spoiled food, smoke and harmful chemicals. Unlike most sensory systems, primary olfactory information reaches the cerebral cortex without an initial thalamic relay.
• Olfactory receptors are located in the olfactory epithelium over the roof of the nasal cavity, superior nasal concha and upper nasal septum. It covers about 2.5 square centimetres in each nasal cavity and contains approximately 10–20 million receptor neurons.
• Olfactory receptor cells are bipolar neurons with cilia projecting into the mucus. The cilia contain odorant receptors. Mucus dissolves odorants, while odorant-binding proteins help transport poorly water-soluble odorants to the receptor surface.
• Olfactory receptor neurons can regenerate throughout life. New receptor neurons arise from basal cells approximately every 30–60 days. Sniffing improves smell by directing more air towards the olfactory epithelium.
• The olfactory pathway is: olfactory receptor neurons → olfactory nerve filaments → olfactory bulb → olfactory tract → primary olfactory cortical areas. The unmyelinated olfactory nerve fibres pass through the cribriform plate to reach the olfactory bulb.
• In the olfactory bulb, receptor neurons synapse with mitral and tufted cells in olfactory glomeruli. Periglomerular and granule cells provide inhibitory control, improving contrast and discrimination between different odors.
• The lateral olfactory stria projects to the piriform cortex, amygdala, entorhinal cortex, anterior olfactory nucleus and olfactory tubercle. The piriform cortex is the principal area for odor processing, the amygdala links odors with emotions, and the entorhinal cortex connects odors with learning and memory.
• Primary olfactory areas project through the thalamus to the orbitofrontal cortex, where conscious odor recognition and discrimination occur. The orbitofrontal cortex also combines smell and taste to contribute to flavor perception.
• The medial olfactory stria projects towards the septal region and hypothalamus and influences autonomic and instinctive responses. Smell of appetizing food can increase salivary, gastric and pancreatic secretion during the cephalic phase of digestion.
• In olfactory transduction, an odorant binds to a G protein-coupled receptor → activates a G protein → stimulates adenylyl cyclase → increases cyclic adenosine monophosphate → opens cyclic nucleotide-gated cation channels → sodium and calcium enter → calcium activates chloride channels → chloride leaves the cell → depolarization → action potential.
• Receptor neurons expressing the same receptor type project to the same glomerulus. Different odors activate different combinations of glomeruli, allowing discrimination of thousands of odors. Continuous exposure produces rapid olfactory adaptation, partly through calcium-dependent reduction in receptor responsiveness.
• Factors affecting olfaction include odorant volatility and solubility, receptor sensitivity, age and hormonal influences. Olfactory sensitivity generally decreases with advancing age.
• Irritating chemicals mainly stimulate the trigeminal nerve, producing burning, sneezing, lacrimation, coughing or brief respiratory inhibition. Offensive odors may also produce nausea and vomiting.
• Hyposmia means reduced smell, anosmia means complete loss, and parosmia means distortion of a familiar odor. Causes include infections, aging, chronic rhinitis, cribriform plate trauma and olfactory nerve or bulb injury. Kallmann syndrome combines anosmia with hypogonadotropic hypogonadism.
• Clinical testing uses familiar, non-irritating odors presented separately to each nostril. Olfactometry quantitatively measures odor detection threshold and sensitivity.