Physiology of olfaction

• Odorants are chemicals that are released by the body and are detected by the nose.
• Olfaction is made possible by paired olfactory organs (bulbs) and the olfactory membrane (OM).
• The olfactory membrane is made up of two layers; the olfactory epithelium and lamina propria.
• The olfactory epithelium is a yellowish-pigmented epithelium in the roof of the nasal cavity near the septum.
• Smell is detected by olfactory chemoreceptors which are specialized endings of afferent (bipolar) neurons that convert olfactory stimuli (chemicals in gaseous state) into nerve impulses (action potentials).
• Receptor cells have no axon but synapse directly onto mitral cells in the olfactory bulb.
• Each receptor cell has one dendrite with many microvilli covered with odorant binding proteins (OBPs), which bind to specific odorants.
• The olfactory epithelium contains 1. Olfactory sensory neurons (Receptors) 2. Supporting cells (support the olfactory epithelium, and have numerous microvilli and secretory granules, which empty their contents onto the mucosal surface). 3. Basal Stem Cells (continually give rise to the olfactory receptor cells)
• The olfactory epithelium covers the inferior surface of the cribriform plate, the superior portion of the perpendicular plate and the superior nasal conchae of the ethmoid bone.
• Lamina propria is the innermost layer of the mucosa. It is composed of connective tissue and contains blood vessels and nerves.
• Lamina propria contains areolar tissue, blood vessels, nerves and olfactory (Bowmans) glands.
• Secretions of the olfactory glands absorb water and form a thick, pigmented mucus and coat surfaces of olfactory organs.
• Receptor are bipolar nerve cells derived originally from the CNS. Unlike the other Receptors it found in olfactory mucosa. Bipolar nerve cells Lifecycle of 4-8 week.
• Olfactory receptor cell has dendrites that extend into the lamina propria where they come into contact with odorant molecules dissolved in the mucous covering the epithelial surface.
• Dendrite endings contain thousands of hairlike projections called cilia
• Cilia are covered by an extension of the plasma membrane known as the olfactory vesicle
• A single axon pass through the cribriform plate of the ethamoid bone and enter the olfactory bulbs.
• Olfactory Discrimination – Can distinguish thousands of chemical stimuli.
• Olfactory discrimination is a pre-processing centre that sorts the signals before they travel along the olfactory tract to the brain.
• Olfactory reception occurs on the surface membranes of the olfactory cilia.
• Odorants interact with receptors called odorant-binding proteins on the membrane surface.
• Axons leaving olfactory epithelium – Collect into 20 or more bundles – Penetrate cribriform plate of ethmoid bone – Reach olfactory bulbs of cerebrum where first synapse occurs in structures called Glomeruli.
• Mitral cell axons leave the olfactory bulb as part of the lateral olfactory striae.
• Travel along the olfactory tract to reach the olfactory cortex, hypothalamus, and portions of the limbic system.